# High Aspect-Ratio vs. Low Aspect-Ratio



## Zipper730 (Aug 11, 2017)

As I understand it, high aspect-ratio wings (as a rule of thumb)

Produce more lift at low angle of attack
Produce less drag due to vortex-generation at the tips (wing body interference might theoretically be lower as well due to less wing being affected)
Produce more lift for their wing-area
Are more subject to flexing loads and have higher T/C ratios
As I understand it, low aspect-ratio wings (as a rule of thumb)

Operate better at high airspeed
Operate better at higher angle of attack
Experience less gust-response at low-speeds

Often require less flexibility
Often can be designed more easily with a low T/C-ratio
I've noticed, however that there are cases were there were exceptions to these rules such as the following

High Altitude

Flight at altitudes of 25000 to 35000 feet, at Mach numbers around 0.45 to 0.65 seems to favor a wing of moderate aspect-ratio with a high taper-ratio than a high aspect-ratio wing: The Spitfire, Mosquito, Hornet, and P-38 Lightning* seem to be good examples of this.
Turning-performance at speeds from 0.75 to 0.90, and altitudes of around 40000-50000-feet seems to favor large wings with low/moderate aspect-ratio, often with moderate to substantial taper-ratio: The Canberra, Valiant, Vulcan, Victor, F-102, and F-106 all seem to conform to this.
I would guess the difference has to do with the fact that taper-ratios play a role in vortex strength as does aspect-ratio, lower aspect-ratio wings are thinner which tends to favor high-mach operation, and the longer root-chord might facilitate wing-body blending schemes.

Am I onto something?


----------



## pbehn (Aug 11, 2017)

Zipper730 said:


> Am I onto something?


Possibly, I think a little more reading is required.


----------



## Zipper730 (Nov 2, 2017)

You know, sometimes it's nice to know where to go to be told where to get information: However, if a person has information, it's preferable to just give the person the information rather than relaying a person in circles.


----------



## pbehn (Nov 2, 2017)

Zipper730 said:


> You know, sometimes it's nice to know where to go to be told where to get information: However, if a person has information, it's preferable to just give the person the information rather than relaying a person in circles.


Zipper your endless quest for a simple almost one word answer never stops. Ask yourself if the turning performance was high on the list of priorities for the Valiant Victor Vulcan and Canberra.

You cannot take one out of the hundreds of different parameters of air plane design and then choose WW2 piston engined fighters and bombers plus post war jet fighters and bombers to prove or disprove your theory.

To answer your question you need to be expert on all aspects of aircraft design AND how changes in knowledge and technology and materials have affected each era of design. Some of todays frontline fighters can cruise at supersonic speeds and are fundamentally unstable requiring a computer to keep them flying. This could not possibly be conceived by the designers of the Spitfire Mosquito and P38 who were just running into problems of compressibility, mach tuck etc.


----------



## Zipper730 (Nov 2, 2017)

pbehn said:


> Zipper your endless quest for a simple almost one word answer never stops.


I wouldn't say a one word answer. I figure a few sentences, a paragraph or so is fine.


> Ask yourself if the turning performance was high on the list of priorities for the Valiant Victor Vulcan and Canberra.


Well, they were designed to maneuver well at high altitude. The Canberra was designed to be a jet-mosquito...


> You cannot take one out of the hundreds of different parameters of air plane design and then choose WW2 piston engined fighters and bombers plus post war jet fighters and bombers to prove or disprove your theory.


It wasn't exactly a theory... it was more a personal observation that seemed to have a touch of fact to it.

I was basically curious on why aspect ratio's benefit on low speed performance tapers off with altitude most of the time (the B-47 and U-2 are an exception to this).


----------



## pbehn (Nov 2, 2017)

Zipper730 said:


> I wouldn't say a one word answer. I figure a few sentences, a paragraph or so is fine.
> Well, they were designed to maneuver well at high altitude. The Canberra was designed to be a jet-mosquito...
> It wasn't exactly a theory... it was more a personal observation that seemed to have a touch of fact to it.
> 
> I was basically curious on why aspect ratio's benefit on low speed performance tapers off with altitude most of the time (the B-47 and U-2 are an exception to this).


To discuss all different parts of the f104 design and its positives and negatives would be a very long essay at least.
Jet Mosquito is a simplistic handle, the Canberra was a light medium bomber and high altitude recon aircraft. The U2 was so specialized it is was barely a practical aircraft, It operated at the very limits of flight where maximum speed and stall speed are almost the same


----------



## Zipper730 (Nov 2, 2017)

pbehn said:


> To discuss all different parts of the f104 design and its positives and negatives would be a very long essay at least.


I didn't say anything about the F-104...


> Jet Mosquito is a simplistic handle


It was simplistic, but the fact is the Canberra was kind of designed to do the high altitude bomber/reconnaissance role that the DH Mosquito was designed for: Presumably this would include a good g-load and good turning rate at altitudes


> The U2 was so specialized it is was barely a practical aircraft


That's true


----------



## pbehn (Nov 2, 2017)

Zipper730 said:


> I didn't say anything about the F-104...


My bad (tired eyes) but same for f102 and f106 delta wings solve some problems and uncover others


Zipper730 said:


> It was simplistic, but the fact is the Canberra was kind of designed to do the high altitude bomber/reconnaissance role that the DH Mosquito was designed for: Presumably this would include a good g-load and good turning rate at altitudes
> That's true


The defence of both the Mosquito and Canberra was based on height and speed you cannot bet on out turning a fighter he can just keep you turning until his mates turn up or you run out of fuel. By the time the Canberra was in service SAMs had entered the game too. I have read of a Mosquito being able to avoid an Fw190 by going into a shallow but high speed dive, at those speeds the Mosquito had slightly better control and was able to survive until the FW 190 ran out of fuel or ammo. That is desperation stuff though.


----------



## Zipper730 (Nov 2, 2017)

pbehn said:


> The defence of both the Mosquito and Canberra was based on height and speed you cannot bet on out turning a fighter he can just keep you turning until his mates turn up or you run out of fuel. By the time the Canberra was in service SAMs had entered the game too. I have read of a Mosquito being able to avoid an Fw190 by going into a shallow but high speed dive, at those speeds the Mosquito had slightly better control and was able to survive until the FW 190 ran out of fuel or ammo.


Getting into a protracted turning match with a fighter would be foolish, but a few quick moves would sometimes shake a plane lose high altitudes particularly when aircraft were still subsonic. The Mosquito could not only pick up speed faster, it actually could turn better at altitudes of 22,000 feet or better. As for dive performance, in a book on the Mosquito, there was a statement about traversing a remarkable distance while descending (if I did my math right I think it was over Mach 0.86, but they could have had a stiff tailwind).

As for SAMs: The USSR didn't seem to have a SAM until 1956, and the SA-2 until 1959


----------



## pbehn (Nov 2, 2017)

Zipper730 said:


> Getting into a protracted turning match with a fighter would be foolish, but a few quick moves would sometimes shake a plane lose high altitudes particularly when aircraft were still subsonic. The Mosquito could not only pick up speed faster, it actually could turn better at altitudes of 22,000 feet or better. As for dive performance, in a book on the Mosquito, there was a statement about traversing a remarkable distance while descending (if I did my math right I think it was over Mach 0.86, but they could have had a stiff tailwind).
> 
> As for SAMs: The USSR didn't seem to have a SAM until 1956, and the SA-2 until 1959


The first thing that happens when you turn is that you slow down and lose altitude. The second thing is that you start going back where you came from.


----------



## Shortround6 (Nov 2, 2017)

Zipper730 said:


> I wouldn't say a one word answer. I figure a few sentences, a paragraph or so is fine.



As Pbehn has noted and I am expanding on, some of these subjects have had entire books written on them, and often sizable chapters in books that try to cover more than one area or aircraft design. This is more than even a long essay. 
Trying to cut it to a short paragraph or a couple of sentences means leaving out a lot of "stuff" than can lead to wrong conclusions, or at least leave out a number of negative characteristics while highlighting a few positive ones. 

You also have to be able to actually build certain structures and not just theorize about them. 

One of the highest aspect ratio wings to see use on a powered aircraft 






the Hurel-Dubois HD-31 form about 1953, from some angles it can look rather distorted by the camera angle





Please note the extent of the under wing bracing as the wing was NOT stiff enough on it's own. Trying to play with limits of structural design at the same time as exploring aerodynamic limits can get you in trouble real quick. 

As one problem with high aspect ratio wings goes, they are slower in roll response than lower aspect ratio wings.
So even if your high aspect ratio wing gives you better turning ability _once _you have rolled to the desired bank angle the shorter aspect ratio plane can either see you starting to bank and roll into his turn quicker and cut the corner a bit or if being pursued can bank one way and then other before the slower rolling plane can follow. 

This is a generality, size and placement of ailerons, mechanical advantage in control system or powered ailerons can greatly affect actual results. One reason trying to compare jets and piston aircraft gets very complicated, only a few piston powered planes had powered flight controls (and like the late P-38s, often only one powered system) while jets were increasingly given powered flight controls for all three axis.

Reactions: Like Like:
1 | Winner Winner:
1 | Like List reactions


----------



## pbehn (Nov 3, 2017)

It should always be borne in mind that the aircraft has to do a job. The HD-31 was a prototype airliner. I flew one time on a similar plane the Short Skyvan. London to Leeds-Bradford then to Teesside. The total journey time was 2 hrs 45 mins and from descent to Leeds and take of then landing at Teesside the whole time was in cloud or turbulence the thing was all over the place and half the passengers were physically sick. The pilot did great job of two cross wind landings in about 45 minutes but it is a civilian plane, there were pensioners and women with children on board, absolute chaos. I myself had just had 3 months in Saudi Arabia and had enjoyed a few beers in a Heathrow bar, bad mistake.
air ecosse skyvan pics - Google Search:

The next time I took the train, which got me home an hour earlier in comfort with a nice breakfast.


----------



## XBe02Drvr (Mar 17, 2018)

pbehn said:


> I flew one time on a similar plane the Short Skyvan.


Our airline had a pair of Shorts SD30s, big brother of the Skyvan, and I rode in them occaisonaly, even getting an hour of stick time on a ferry flight. What a pig! Waddled like an obese goose, and had to replenish barf bag supplies after every leg. To make matters worse, the fuel tank vents were upstream of the cabin intakes, so there was the everpresent aroma of kerosene to add to the residual hint of stomach acid. I thought I had an iron stomach, but I almost lost it a couple times.
Cheers,
Wes

Reactions: Funny Funny:
1 | Like List reactions


----------



## swampyankee (Mar 17, 2018)

High aspect ratio will produce lower induced drag; this will tend to improve _sustained_ turn performance, rate of climb, ceiling, and lift/drag in comparison to a low-aspect ratio wing; a high aspect ratio will tend to be heavier, may have less internal volume, will be more responsive to gusts, and may have a poorer aileron response. If the wing is swept, high-aspect ratio wings tend to be more susceptible to pitch-up.

Picking an aspect ratio is a balancing act. For piston-engined fighters, it was a trade-off between structural weight, internal volume, sustained turn rate, service ceiling, and rate of climb.


----------



## John Frazer (Apr 21, 2018)

It seems to be a myth at least for very low aspect ratio (below 3-1) that they produce very high drag due to wing-tip vortices.
The Vought V-173 perpetuated this.
See the Arup S-2 and S-4 of the early-mid 1930s. Very near 1-1 aspect ratio, all-wing little round things with a bulge for the pilot and a tailfin (S-4 had a tail-plane up on the fin)




Both planes flew for several seasons on airshow circuits (see youtubes of them). Several pilot,. No accidents. A couple of versions each.
They did not exhibit high drag at "low-A" cruise. As little all-wings, they were slippery. Stable and responsive, very difficult to stall, impossible to spin.

As very low aspect ratio planes, they had the trick of being able to fly at silly slow speeds, and very high almost 35 degree "A". S-2 was ~780 pounds with a 37hp engine, 90+kts cruise, and astonishing 23kts landing speed.

See NASA studies of the '90s Wainfan "Facetmobile". Low aspect ratio unitary wing-body.
Low aspect ratio planes can stay aloft at such low speeds at very high "A", because of the enormous wing-tip vortices that develop at high-A. They wrap around and keep airflow from the leading edge over the top of the wing from detaching. Called "vortex lift", it is not present at normal flight.

Charles Zimmerman worked for NACA and was with the team that saw the S-2 perform an impresssive showing.
It was after this that his dream of a twin-rotor VTOL hovering toy gelled and took the low aspect-ratio discoid shape.
When the Navy decided to look into such planes for a STOL fighter, for some reason they let Zimmerman and Sikorsky & Vought build his silly flappy-prop parody of the Arup. For some reason, they were under the misapprehension that it would suffer high drag from the wing-tips, so he used his twin-rotor hovering design with the props counter-rotating to counter the wing-tip vortices.
Later tests with simple 2-blade normal props rotating the opposite direction (*with* the wing-tip vortices) showed almost no change in performance.
They don't have high drag at cruise, so it did not need the huge silly flapping props and the gearing system that killed the follow-on XF5U. In low-speed landing, high-A flight, you wouldn't want to counter the wing-tip vortex lift if you could.
We can call the V-173 and XF5U "Zimmerman's folly" or how the Navy threw away a fighter like the Bearcat but with greater range, speed and payload, and 40kt landing speed.

See also the Boeing model 396 proposal for a Navy flapjack fighter test plane. A simple straightforward study of the Arup planes, it would have led to their phenomenal fighter with the 28 cylinder 4-row radial engine and contra-props at the nose.





The advent of the jet age didn't kill the Navy flapjack. For one thing, they continued flying piston-prop planes until the '70s. The Skyraider and the S-2/C-1/E-1 series were mid-'40s designs.
If the Boeing flapjack had flown, the Arup planform would have taken over the fleet.

For the second thing, Vought had a design for a jet powered version that would have been awesome -better than the P-80 or maybe even better than the F-86




Sergei Sikorsky also worked on such a thing.

Hatfield worked on & flew the Arup planes, and in the '80s built the "Little Bird" planes which proved the Arup planform worked. In one youtube, he congratulates Rutan on the "Voyager" flight and then challenges him that if he'd used an Arup planform, it would have been stronger, faster, more room, and finished the 'round the world flight with fuel to spare.

That doesn't sound like a poor glider or cruising plane.

Reactions: Like Like:
1 | Informative Informative:
1 | Like List reactions


----------



## swampyankee (Apr 21, 2018)

There are advantages to low aspect ratio, but efficiency is not one of them. One rather good side-by-side comparison is the Mirage F1 (swept wing) and Mirage III (delta). Both used the same engine and had similar take-off weights. The former had longer range, shorter take off distance, and maintained energy better in air combat.

Reactions: Agree Agree:
1 | Like List reactions


----------



## John Frazer (Apr 21, 2018)

Hardly the same as an all-wing lifting body with very low aspect ratio. We'll never know what those Sikorsky discoids might have done.


----------



## swampyankee (Apr 21, 2018)

John Frazer said:


> Hardly the same as an all-wing lifting body with very low aspect ratio. We'll never know what those Sikorsky discoids might have done.



Not without enough information to do a bit of analysis. On the other hand, there is enough information to predict that they're unlikely to aerodynamically efficient. As I said, low aspect ratio has some advantages, but at any given lift coefficient, a low aspect wing _will_ have a larger induced drag than one with a high aspect ratio.


----------



## Zipper730 (Apr 21, 2018)

John Frazer said:


> It seems to be a myth at least for very low aspect ratio (below 3-1) that they produce very high drag due to wing-tip vortices.


As I understand it, they generally produce high-drag at low speed, and work well at high speed.

It seems taper-ratio also plays a role.


> See the Arup S-2 and S-4 of the early-mid 1930s. Very near 1-1 aspect ratio, all-wing little round things with a bulge for the pilot and a tailfin (S-4 had a tail-plane up on the fin)
> . . .
> Both planes flew for several seasons on airshow circuits (see youtubes of them). Several pilot,. No accidents. A couple of versions each.
> They did not exhibit high drag at "low-A" cruise.


From what I read, admittedly all of it on wikipedia (I'm not fond of quoting wikipedia, but it's a source that's easily available) their glide ratio is was 3:1 which is the same as L/D. It seems more a testament to the propeller having enough power to push such a design with such little horsepower 


> See NASA studies of the '90s Wainfan "Facetmobile". Low aspect ratio unitary wing-body.
> Low aspect ratio planes can stay aloft at such low speeds at very high "A", because of the enormous wing-tip vortices that develop at high-A. They wrap around and keep airflow from the leading edge over the top of the wing from detaching. Called "vortex lift", it is not present at normal flight.


That usually depends on a highly swept wing, that, or specially designed flaps.


> Charles Zimmerman worked for NACA and was with the team that saw the S-2 perform an impresssive showing.


Yeah, and that became the basis for the V-173. The V-173 had some advantages in theory with the propeller right at the tip, it would negate the vortices by spinning in the opposite direction of the vortex.


> For some reason, they were under the misapprehension that it would suffer high drag from the wing-tips


Because the S-2/S-4 had L/D of 3:1 which is quite poor. The Spitfire was around 13:1.


> Later tests with simple 2-blade normal props rotating the opposite direction (*with* the wing-tip vortices) showed almost no change in performance.


I've never heard anything about that...


> See also the Boeing model 396 proposal for a Navy flapjack fighter test plane. . . it would have led to their phenomenal fighter with the 28 cylinder 4-row radial engine


R-4360...


> For the second thing, Vought had a design for a jet powered version that would have been awesome -better than the P-80 or maybe even better than the F-86


Have you any performance data?


> Sergei Sikorsky also worked on such a thing.


That picture is one of the attachments you posted: It's a nice looking design, though I'm curious as to how it was projected to perform.


----------



## John Frazer (Apr 23, 2018)

A single mention with zero corroboration or sources for that L/D or glide ratio.

I'm looking around, but other than Hatfield's own story, not much to be found.

Reactions: Bacon Bacon:
1 | Like List reactions


----------



## John Frazer (Apr 24, 2018)

Apparently, NASA Langley did studies with a wind-tunnel model and found only changes were slight changes in longitudinal stability at high-"A", with the props spinning in the *same* direction. (not as if to counter the wing-tip vortices).
(reported by a commenter at the secretprojects forum)
I've requested a copy of that book from a local university to my local library 

Amazon product
_View: https://www.amazon.com/Radical-Wings-Wind-Tunnels-Advanced/dp/1580071163_


----------



## Zipper730 (Apr 30, 2018)

John Frazer said:


> Apparently, NASA Langley did studies with a wind-tunnel model and found only changes were slight changes in longitudinal stability at high-"A", with the props spinning in the *same* direction. (not as if to counter the wing-tip vortices).


Well, today's your lucky day: I know a person who's a member and they downloaded the file.

https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20050019375.pdf

I'd like to hear input from other people, particularly types such as drgondog, FLYBOYJ, KiwiBiggles, pbehn, Shortround6, swampyankee, wuzak, XBe02Drvr, and probably a few others I forgot for some reason regardless of their knowledge


----------



## John Frazer (Jun 1, 2018)

We still need to see better data on the Arup planes. Only one wikipedia mention of glide ratio, with zero citations... And NACA was impressed with the efficiency of the S-2, and Hatfield spoke very highly of their efficiency as well. Models of them are very good at soaring, and the originals were lightly loaded.

"Radical Wings and Wind Tunnels" is about Langley, and in the section of the V-173 it establishes that the planform was chosen to maximize wing-tip wash-around for the "parachute lift" it offers at low speed high *A*.
Tests with powered models in tunnels showed little effect from props spinning the other way -inward- at the tips, except more stability that way.
The enormous drag of the vortices is not present at cruise, and it's a mystery why they built the silly flappy props to counter what they'd specifically built to maximize, after establishing that it didn't help.

Photos added: scans from the book.

Reactions: Like Like:
1 | Like List reactions


----------



## Zipper730 (Jun 1, 2018)

John Frazer said:


> We still need to see better data on the Arup planes.


I can actually agree to that. Short of building a model flight simulator... anybody here got some variation of X-plane?


> Only one wikipedia mention of glide ratio, with zero citations...


Supposedly, the guy who led to the design of it was a podiatrist who somehow figured that a heel lift flew surprisingly well.


> And NACA was impressed with the efficiency of the S-2, and Hatfield spoke very highly of their efficiency as well. Models of them are very good at soaring, and the originals were lightly loaded.


Yeah it basically seems that if the aspect ratio is high you can get more lift for less wing area, but if the wing area be sufficiently obscene, you can make it fly decently well.


> "Radical Wings and Wind Tunnels" is about Langley, and in the section of the V-173 . . . Tests with powered models in tunnels showed little effect from props spinning the other way -inward- at the tips, except more stability that way.


Basically it seems to list two things

In the summary it says

The peak propulsive efficiency for β = 20-degrees and β = 30-degrees were increased 7 percent at CL ≅ 0.67 at 20 percent at CL ≅ 0.74, respectively, with the propellers rotating upward in the center than with the propellers rotating downward in the center. Indications are that the minimum forward-flight speed of the airplane for full-power operation at sea-level will be about 90 miles per hour.​
Later on under effects of propeller operation on lift, it says

The effects of propeller operation on the left of the model are presented in figure 42 at angles of attack ranging from about 0-degrees to 30-degrees. At angles of attack of -0.5 degrees and -0.6 degrees for propeller blade angles of 20-degrees and 30-degrees, respectively, increases in coefficient of lift amounting to between 0.2 and 0.3 were measured for the propeller advance-diameter ratio ranges investigated. This change in lift coefficient is caused principally by the change in the local angles of attack of the wing induced by the slipstream rotation.​
As the angle of attack is increased the change in lift-coefficient at a given propeller advance-diameter ratio increases. Calculations showed that about one-third to one-half the total increase in lift due to propeller operation at the high angles of attack results from the lift component of the propeller resultant force. Most of the remaining increase is attributed to the increased slipstream velocity over the wing.​
Depending on how I read this the rotation of the propellers increased lift 7% when rotating upward from center, and reduced 20% when rotating downward from center, or at high AoA about 33.3% - 50% of the lift is produced by the rotation of the propellers and the rest from slipstream. 



 drgondog
, I'm curious if this is correct...


> it's a mystery why they built the silly flappy props


The vibration if I recall was caused at high AoA by span wise airflow. This effect is not dissimilar to advancing/retreating propellers..


----------



## John Frazer (Aug 17, 2018)

Zipper730 said:


> The vibration if I recall was caused at high AoA by span wise airflow. This effect is not dissimilar to advancing/retreating propellers..



All sources say it was the power distribution system to allow either engine to take over and drive either prop, combined with the complexity of the flapping system. It seems US airplane & airplane engine makers were having bad luck designing prop extension shafts or distribution systems; maybe should have asked boat builders for help.

V-173 did nothing that the Arup hadn't demonstrated years before, without the consideration of the contra-props. 
It's a mystery because the Arup had extreme STOL ability and high lift throughout the speed range, which was good. There was no apparent reason to innovate such extreme measures, for to counter the vortex which the aspect ratio was specifically designed to maximize at low speed, and which wasn't present to be a hindrance during normal flight.
Could they have thought the XF5U could actually be VERTOL so the V-173 was worth going ahead with? That extreme prospect was worth abandoning everything the Arup showed as entirely workable and practically revolutionary in capabilities?


----------



## XBe02Drvr (Aug 17, 2018)

John Frazer said:


> All sources say it was the power distribution system to allow either engine to take over and drive either prop, combined with the complexity of the flapping system. It seems US airplane & airplane engine makers were having bad luck designing prop extension shafts or distribution systems; maybe should have asked boat builders for help.


Not just US designers, it's been a problem in aeronautics since the Wright brothers, linking a herky jerky reciprocating engine to a smoothly rotating propeller through a crank shaft extension, a driveshaft, or a gearbox light enough for aircraft use. The solutions that work in nautical applications are generally too heavy for aircraft use and are generally aimed at a lower RPM range. Aircraft structures tend to be more flexible than boat hulls and more likely to transmit rather than dampen vibration.


John Frazer said:


> There was no apparent reason to innovate such extreme measures


How about asymmetric thrust? The loss of one engine without an interconnect would render a multi engine flapjack uncontrollable. The only thing that keeps a conventional multi engine plane straight with one out is the power of the rudder and the arm through which it is working. This power diminishes abruptly as speed decreases. A flapjack flying slowly at high AOA is not going to be able to handle an asymmetric thrust situation.
Cheers,
Wes


----------



## Zipper730 (Aug 17, 2018)

John Frazer said:


> All sources say it was the power distribution system to allow either engine to take over and drive either prop, combined with the complexity of the flapping system.


The flapping system was designed to deal with the propellers being mounted on the tips


> V-173 did nothing that the Arup hadn't demonstrated years before, without the consideration of the contra-props.


Counter-rotating propellers spin in opposing directions, contra-props are coaxial propellers (two rows of props which spin in opposition). While I'm uncertain the exact degree the rotation affected lift since I'm not 100% sure as to what I'm reading, but it seems to be fairly minimal at high-speed and more significant at low speed however significant/insignificant. Low speed would be where it would count so it's a nice touch.

In addition to the XF5U being built to take off in short distance, there was a desire to climb vertically and that depended on a lot of power: The ways to this would have hinged upon having a big propeller, without two engines it'd needed to be quite massive. Contra-rotation seem to reduce the required diameter, but I'm not sure how much. The fact is you'd have probably needed two anyway, though you could have put them mid-span with less difficulty. Depending on the effect of vortex cancellation (however small), it could have been worth it.


> There was no apparent reason to innovate such extreme measures, for to counter the vortex which the aspect ratio was specifically designed to maximize at low speed


The Arup planes were designed for considerably lower speeds, and at least the S-2's wings were pretty thick in some areas. The V-173 was a demonstrator, so it had fixed-gears to verify the low speed of the equation (we could already build fast aircraft). The XF5U-1 was built for low and high speed, so a thinner wing would have been arguably more useful, and that could have affected the low-speed side of the equation.


> Could they have thought the XF5U could actually be VERTOL


They had thought of making it able to lift off and rocket vertically, or be able to land vertically if I recall. I don't remember the source so you might not wish to quote me


----------



## John Frazer (Aug 29, 2018)

> How about asymmetric thrust? The loss of one engine without an interconnect would render a multi engine flapjack uncontrollable. The only thing that keeps a conventional multi engine plane straight with one out is the power of the rudder and the arm through which it is working. This power diminishes abruptly as speed decreases. A flapjack flying slowly at high AOA is not going to be able to handle an asymmetric thrust situation.



Only if it uses twin props so far off the cener axis line -'way out at the tips. Many normal twin-prop planes don't suffer too badly, most are fly-able with one engine out. There's no reason to suspect that a more normal twin arrangement on an extremely low aspect ratio plane would be any different.

And irrelevant with center-line props.
Boeing's 390 little test plane would have done everything the V-173 did, and probably been faster.
The Navy let Vought & Zimmerman explore the wing-tip down-ward turning props, after seeing that it made negligible difference, while ignoring the remarkable performance which the extremely low aspect ratio integral wing/body would have offered.
A half-deck on a converted merchant ship could have 2 spots, for planes like a Hellcat with more climb-rate, more speed, range and payload, and <35kt landing speed. Build it with jets like the Sikorsky thing, and nothing else they built for another 15+ years could have matched it, and nothing else could have been STOL capable.

Today, people say that extremely low aspect ratio needed the outward-turning props for (phantom) extreme drag during cruise, and since they didn't work, the planform couldnt work. Not true at all.


----------



## Zipper730 (Aug 29, 2018)

John Frazer said:


> Only if it uses twin props so far off the cener axis line -'way out at the tips. Many normal twin-prop planes don't suffer too badly, most are fly-able with one engine out.


The V-173 and XF5U would not have crashed if one engine went out (if one propeller stopped spinning, that would be different): They were designed so that each engine can drive the other shaft. Should you lose an engine, the RPM of both props would slow down a lot, but you'd remain in control.

As for vortex cancellation on lift: Rotate the propellers the wrong way and lift drops by about 1/3 to 1/2 at high angles of attack which is where they would count most for lift. If you had no angular momentum to the propeller slip-stream, you would have an increase of 1/6 - 1/4 the amount of lift which is about 16.7% to 25%.


----------



## XBe02Drvr (Aug 29, 2018)

John Frazer said:


> Only if it uses twin props so far off the cener axis line -'way out at the tips.


Isn't that exactly the configuration of the "flapjack"?


John Frazer said:


> Many normal twin-prop planes don't suffer too badly, most are fly-able with one engine out. There's no reason to suspect that a more normal twin arrangement on an extremely low aspect ratio plane would be any different.


I've only got about 9,000 hours of twin prop time, so maybe I'm not qualified enough to comment, but here goes.
Yes "normal" twins fly quite well on one engine, AS LONG AS you keep the speed up. Get too slow, (like normal landing speed) and you don't have enough control authority to overcome the assymetric thrust. It's called VMC (Velocity Minimum Controllable), and in most twins, it's faster than normal touchdown speed. In most twins, single engine landings have to be accomplished with less flap and more speed, hence longer runways.
Now, what's the attraction of low aspect ratio aircraft like the flapjack?
Why, it's slow flight and STOL of course. Now put the props at the lateral limits of its discoid "wing", with the twin "micro" rudders aligned with the props, like the flapjack. Fail one prop at low airspeed, and how are you going to keep it straight? All features of your design point to a very high VMC. (Maximum assymetric thrust, minimum rudder effectiveness.)
Without a propeller drive "crosslink" (with all its friction losses), you're sunk. Also, to correct a mistaken idea perpetrated by Zipper, if you lost one engine in a "cross-linked" flapjack, you wouldn't lose prop RPM, (they're governed), but you would lose over half your thrust, and still have a controllable aircraft.
Cheers,
Wes

Reactions: Informative Informative:
1 | Like List reactions


----------



## Zipper730 (Aug 29, 2018)

XBe02Drvr said:


> Isn't that exactly the configuration of the "flapjack"?


His idea was focusing on the fact that part of the massive lift benefit of the V-173 & XF5U came from the massive propeller slipstream itself. Even at low-speeds where AoA is the highest and vortex-strength the most extreme, with the propellers designed to reverse the wrong way, you'd have a loss lift ranging from 33-1/3 to 50%, which is obviously quite substantial, but with no angular momentum (which is a true test of the effect since rotating them the wrong way actually would intensify the vortex, and rotating them the right way will neutralize the vortex), you would see half that loss -- 16-2/3% to 25% produced by vortex cancellation. This would result in 75% to 89-1/3% produced by simply blowing massive quantities of air over the area of the wing.

His contention was if the propellers were mounted further inward, this would reduce the amount of yaw produced by loss of thrust, increase in drag, and roll produced both by yaw and loss of slipstream over the wing affected. Vortex cancellation would be less to nonexistent, but engine-out would be better in theory.

Functionally, since the design worked heavily on the propellers blowing nearly the full span, the propellers would have to be huge regardless and the engines would have to be placed further out than the actual XF5U mounted them to cover that much of the wing, and the propellers would still require long shafts to avoid choosing the cockpit off (unless it was recessed further back). I assume that the production of roll-rate would be quite high if an engine failed with the sudden change in lift, as low aspect-ratio wings usually have good roll-rates, and yaw would probably be quite noteworthy as well with the large thrust produced


> Also, to correct a mistaken idea perpetrated by Zipper, if you lost one engine in a "cross-linked" flapjack, you wouldn't lose prop RPM, (they're governed), but you would lose over half your thrust, and still have a controllable aircraft.


Sorry about that


----------



## XBe02Drvr (Aug 30, 2018)

Zipper730 said:


> His contention was if the propellers were mounted further inward, this would reduce the amount of yaw produced by loss of thrust, increase in drag, and roll produced both by yaw and loss of slipstream over the wing affected.


You can't have it both ways. Move the props (rotors?) inward, and you limit their diameter and lose some of your full-span slipstream, for not much improvement in assymetric handling. Leave them out near the wingtips and you get better slipstream/span coverage at the cost of truly impossible single engine handling. Gotta have that propeller interconnect.
Cheers,
Wes


----------



## Zipper730 (Aug 30, 2018)

XBe02Drvr said:


> You can't have it both ways.


Plus with the existent diameter, you'd only be able to move it in a small ways, and even if you reduced the diameter a skosh, to cover the wing you'd still need a substantial diameter and would have to move the engine further outwards which would be difficult to do, would probably still have poor engine-out performance, and you'd probably need to increase the wingspan to make the cowl and airframe to blend right.


> Move the props (rotors?) inward, and you limit their diameter and lose some of your full-span slipstream, for not much improvement in assymetric handling. Leave them out near the wingtips and you get better slipstream/span coverage at the cost of truly impossible single engine handling. Gotta have that propeller interconnect.


The other possibility that John Frazer proposed was the Boeing 390 (he listed it as the Boeing 396, I couldn't find it in searches, so I just entered "Boeing Flapjack" and got the right answer) which was similar to the Arup S-2 and S-4 arrangement.

The S-2 has a listed stall speed of 20 knots, but there's no loaded weight figure. Using the Arup S-4 as an estimate which is around 1200 pounds fully loaded with a 273 square foot area, I get a wing-loading of 4.395. Assuming the stall-speed was the same (which is an assumption), the XF5U weighs 16722 lbs with 475 square feet of wing area: Wing-loading is 35.2, which produces an 8-fold increase in wing-loading and a stall speed increase of 2.83 which would be 56 knots. I'd say that's not bad considering the earlier F4F-3 had a stall speed of around 64.7 knots either lightly loaded or empty. However the XF5U offered a stall speed ranging of around 20-40 knots which seems a bit better (and probably owing to better coverage of the wing by slipstream).

The Boeing 390 I have no figures for in terms of dimensions and weight, 

 John Frazer
might have some figures here, but with an R-4360 powering it, the generally listed as 3400 hp WEP/3000 hp MIL/2500 hp Normal; the R-2000 rated at 1600/1350/1100 in respective order, with 2 of them you'd have 3200/2700/2200 which would show an advantage for the Boeing 390, and two engines do have higher installation drag so I guess you'd have a slight drag reduction on that.

The power loading is better for the Model 390 than the Arup S-4 which might indicate an improvement in slipstream based lift and with a contra-rotating propeller, there'd be no angular momentum to the flow which would probably result in a faster velocity over the wings. The coverage over the wing would be less so the question basically is which works better -- I have no answer.

That being said: I would say the biggest problem with the XF5U was that Vought didn't add the flapping propeller blades off the bat (something which Zimmerman wanted), and decided to add them later. Not sure how much time would have been spared, admittedly.


----------



## John Frazer (Jun 13, 2019)

Not true that it gained the extra lift at very low speed from slip-stream from the outward-rotating props. It's stated that 3-4x lift for such a speed, comes from the planform.
It's the same phenomena for the Arup, the Nemeth, the Little Bird, the Facetmobile. (all of which were stall-proof, also due to the "vortex parachute" it creates behind/above it while flying very slow with high *A)*
Wainfan put it into verified science, just as Zimmerman established that the very low aspect-ratio gave it the extra lift at low speed.

He was after something else. The very-slow speed for the clean all-wing-body, was due solely to the planform and type, not the props.


----------



## Zipper730 (Jun 13, 2019)

John Frazer said:


> Not true that it gained the extra lift at very low speed from slip-stream from the outward-rotating props.


From what the NACA report said, from what I remember, the biggest variable was the slipstream rather than the rotation of the propellers (by that I mean the velocity of the air blowing over the wing, not the direction of rotation) except potentially at low speed, where the result would vary by around 1/3 to 1/2.


> It's stated that 3-4x lift for such a speed, comes from the planform.


I could re-read it, but from what I remember at low speeds the amount would be around 1/3 to 1/2. At higher speeds the amount would likely be less significant.


> It's the same phenomena for the Arup, the Nemeth, the Little Bird, the Facetmobile. (all of which were stall-proof, also due to the "vortex parachute" it creates behind/above it while flying very slow with high *A)*


I assume you mean the trailing edge of the wing? From what I remember the variables that lead to vortex production are highly swept surfaces, and the trailing edge was more highly swept.


> Wainfan put it into verified science, just as Zimmerman established that the very low aspect-ratio gave it the extra lift at low speed.
> 
> He was after something else. The very-slow speed for the clean all-wing-body, was due solely to the planform and type, not the props.


But that airplane had a totally different wing-cross section...


----------



## John Frazer (Jun 19, 2019)

None of that's accurate.
The enhanced lift at super-slow speeds is due to the "parachute lift" created by the wing-tip wrap-around vortices of the low aspect ratio. The airstream from the props did not make the extra lift it had -other planes without that over-complex prop system have done the same thing.
The Zimmerman thing has one sort of shape, the Nemeth was circular, the Facetmobile was deltoid, but they all shared low aspect ratio, and the "parachute lift" which that planform allows them to use.


----------



## Zipper730 (Jun 19, 2019)

John Frazer said:


> None of that's accurate.


I read the document on the XF5U/V-173. They said the bulk of the performance wasn't due to the direction of the propeller's spinning, but instead, the function of the large slipstream produced by the propellers.


> The enhanced lift at super-slow speeds is due to the "parachute lift" created by the wing-tip wrap-around vortices of the low aspect ratio.


As I understand this, the problem would be that it would produce massive amounts of drag unless the wing loading was very very low.

I've did a little research and have found the following

*Arup S-2*
Weight: 780 lb.
Wing Area: 211 sq.ft.
Aspect Ratio: 1.7109
Wing Loading: 3.6967

*Arup S-4*
Weight: 1200 lb.
Wing Area: 273 sq.ft.
Aspect Ratio: 1.7729
Wing Loading: 4.3956

*Nemeth Parasol/Umbrella Wing*
Weight: 867-1300 lbs
Notes: I don't have actual figures, I looked, but made some guesses based on the Alliance Argo, which weighed 650 pounds; I assumed the wing was 1/3 the weight of the plane (which I'm not sure what the norm was but it's about twice what constituted the wing-weight of a WWII aircraft), and doubled that to produce what seems like a conservative figure; another figure just included me doubling the weight. The numbers I came up with were meant to be as conservative as possible.

Wing Area: 201 sq.ft.
Notes: The wingspan was around 16 feet, so I just used the mathematical formula for the area of a circle which comes out to 201.061929829746767 provided one uses 3.141592653589793 for pi. Since most aircraft wing-areas are rounded, I just went with 201.

Aspect Ratio: 1.2732
Wing Loading: 4.3210 - 6.4657
Notes: These wing loadings are based on the estimated figures for weigh


*Vought V-173*
Weight: 2258 lb.
Wing Area: 427 sq.ft.
Aspect Ratio: 1.2750
Wing Loading: 5.2881

*Wainfan FMX-4 Facetmobile*
Weight: 740 lb.
Wing Area: 214 sq.ft.
Aspect Ratio: 1.0514
Wing Loading: 3.4579

You'll notice that all these designs have wing-loading figures that are in the single-digits. The Nemeth design required a lot of guesswork, so if you have exact weight figures, I'd like to have that so I can produce a more accurate calculation. As the figures are, the wing-loading varies by a little under 1.5, which considering that if an airplane's weight doubles, the stall speed would increase, in theory, to the square root of 2, this would yield an increase in stall speed of 1.2233 x Vs

Getting to the XF5U on the other hand, the figures are as follows

Weight: 16722
Wing Area: 475
Aspect Ratio: 1.1462
Wing Loading: 35.2042
Note the wing-loading here, it's in the double-digits. It's the only one in the double digits.


----------



## XBe02Drvr (Jun 20, 2019)

Zipper730 said:


> Getting to the XF5U on the other hand, the figures are as follows
> 
> Weight: 16722
> Wing Area: 475
> ...


And that wing loading and aspect ratio are going to produce some bodeacious vortices!


----------



## swampyankee (Jun 20, 2019)

NASA's NTRS has at least two reports on the XF5U. The more useful is https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20050019375.pdf

The model lost about 40% of CLmax with the propellers off.

Reactions: Like Like:
1 | Like List reactions


----------



## Zipper730 (Jun 20, 2019)

XBe02Drvr said:


> And that wing loading and aspect ratio are going to produce some bodeacious vortices!


Bodacious vortexes, sure, but the higher the wing-loading the higher the AoA, and that will drive up the drag a lot.

The other designs worked because they had light wing-loading, so they would produce the parachute lift effect with low AoA.


----------



## John Frazer (Jul 12, 2019)

I wrote: "The enhanced lift at super-slow speeds is due to the "parachute lift" created by the wing-tip wrap-around vortices of the low aspect ratio."

Zipper730 wrote: "the problem would be that it would produce massive amounts of drag"

The effect disappears at low _A_ normal cruise or high-speed flight. They do not carry around with them always, the tremendous drag of the vortices.

This is the usual misconception about very low aspect-ratio.
We constantly see the confusion between the high drag they produce & use for low speed flight, and efficiency at cruise.
The parachute drag effect goes away.
The Zimmerman/Vought/Sikorsky design did not need the outward-turning props to counter it for cruise. They knew this, it's a misconception of everyone after who's looked at it.
The simple fact is that the other planes which have used various shapes of low aspect-ratio planform, achieved the same slow speed flight, without worry about shaping the slipstream. Zimmerman was after something else, not connected straightforward with the low aspect-ratio slow flight.

The Navy ignored the real promise which the Arup/Nemeth/Eshelman demonstrated, to let Zimmerman explore his quirky exaggerated props, and after the war continued on as if those other planes never happened to amply provably demonstrate superior flight qualities.

Everyone talks against low aspect-ratio because of extreme vortex drag. Misunderstanding the situation and ignoring that these other planes didn't demonstrate anything of a supposed horrible span efficiency. Horrible span efficiency which no one can demonstrate against them, or even describe adequately to account for these other little planes that demolished the rumors about low aspect ratio being a dog.
And these designs proved to be stall/spin proof.
Aviation has ignored it, tried any number of ways to make good STOL planes that weren't draggy.


----------



## Zipper730 (Jul 12, 2019)

John Frazer said:


> The effect disappears at low _A_ normal cruise or high-speed flight. They do not carry around with them always, the tremendous drag of the vortices.


I get that, which is why I said "unless the wing-loading is very light". That's why I also put a weight comparison (The Nemeth Parasol/Umbrella Wing, however, for reasons I explained, was largely guesswork).


> The Zimmerman/Vought/Sikorsky design did not need the outward-turning props to counter it for cruise. They knew this, it's a misconception of everyone after who's looked at it.
> The simple fact is that the other planes which have used various shapes of low aspect-ratio planform, achieved the same slow speed flight, without worry about shaping the slipstream.


There was an NACA report that was on Reply #22 in this thread, which covers the matter in decent detail.

Regardless of Charles Zimmerman's intentions, the outward-rotating props did, indeed, have little effect on cruise performance: They however, did have an effect at higher AoA and lower-speed (which is the point of this design). Basically, the NACA report states that the direction of propeller-rotation affects lift as much as 33-1/3% to 50% (which is quite substantial), with the remaining 50% to 66-2/3% coming from the velocity of the slip-stream alone.

Here's a quote from the report...



NACA RM No. L6I19 said:


> The effects of propeller operation on the left of the model are presented in figure 42 at angles of attack ranging from about 0-degrees to 30-degrees. At angles of attack of -0.5 degrees and -0.6 degrees for propeller blade angles of 20-degrees and 30-degrees, respectively, increases in coefficient of lift amounting to between 0.2 and 0.3 were measured for the propeller advance-diameter ratio ranges investigated. This change in lift coefficient is caused principally by the change in the local angles of attack of the wing induced by the slipstream rotation.
> 
> As the angle of attack is increased the change in lift-coefficient at a given propeller advance-diameter ratio increases. Calculations showed that about one-third to one-half the total increase in lift due to propeller operation at the high angles of attack results from the lift component of the propeller resultant force. Most of the remaining increase is attributed to the increased slipstream velocity over the wing.


I'm not pulling numbers out of my ass.


> Zimmerman was after something else, not connected straightforward with the low aspect-ratio slow flight.


I think he also had an idea of being able to have enough thrust to go vertical. Not sure if he succeeded in that department, but that was one of his goals. That would be desirable for any fighter-plane assuming fuel burn wasn't affected badly enough.


> The Navy ignored the real promise which the Arup/Nemeth/Eshelman demonstrated, to let Zimmerman explore his quirky exaggerated props, and after the war continued on as if those other planes never happened to amply provably demonstrate superior flight qualities.


While I don't have any data on Eshelman's Flying Flounder (NX28993 and NC22070), I do have data on the Eshelman FW-5.

Weight: 2650 lb. (GTOW)
Wing-Area: 232 ft^2
Aspect-Ratio: 3.8793
Wing-Loading: 11.4224
It's wing-loading is higher than the Arup S-2/S-4 (3.6967/4.3956), the Vought V-173 (5.2881), but so is its aspect ratio at 3.8793 (*Arup S-2*: 1.7109; *Arup S-4:* 1.7729; *Vought V-173:* 1.2750), and the somewhat lower aspect-ratio compensates to a degree for the heavier wing-loading.

The Arup S-2 (1.7109) and S-4 (1.7729); the V-173 (1.2750), and; the Wainfan FMX-4 Facetmobile (1.0514) all have aspect ratios under 2:1. The XF5U-1 also has such an aspect ratio in the same range (1.1462), but has a wing-loading that is in the double-digits (35.2042), whereas all the others are not.

If the V-173's wing-loading was brought as high as the XF5U-1, you would see stall speed go up by 2.58. So the V-173 would go from having an enviable stall-speed of 40 knots to the very unenviable 103 knots. Yet the stall-speed of the XF5U-1 was around 20-25 knots by some estimates, around 40 knots for others.


----------



## nuuumannn (Jul 12, 2019)

I've been reading through this thread and others on the XF5U in particular and the reason's beind its cancellation and although I have no evidence that confirms my theories as reasons why this aircraft was not proceeded with, my experiences, as limited as they are with working on modern turboprop aircraft give me a bit of an insight into systems that have an effect on an aircraft's service criteria.

Rught angle gearboxes are mechanically complex things that introduce inefficiencies into a system that cannot get the total amount of power and translate that into equivalent thrust, for mechanical reasons already known and discussed. I know from experience that high performance propeller systems introduce complexities that simply do not exist in pure jet gas turbines. Introducing right angle gearboxes is gonna increase not only complexities but also unforeseen maintenance into the mix. 

Any aircraft operator knows that maintenance is the opposite of operating efficiency because it means your aircraft are on the ground when they could be out doing their jobs. But maintenance is essential, so an airframer has to be smart about designing maintenance schedules, and introducing comlexities like weird gearbox layouts does introduce maintenance issues that previously did not exist in more conventional airframes. It would have been interesting to see just how much wear and tear such a gearbox layout might have been able to take in service when the aircraft is being pushed to performance limits, etc. Crapping out gearboxes is a serious issue that can have ramifications on your airframe; they are not just a line-replaceable-unit that can be dropped in and out. There's serious work, which equals time in doing that sort of stuff. This also increases operating costs, another factor that needs to be taken into consideration by designers and operators alike, whether civil or military.

Could that layout have been a curse in disguise for the Flapjack? What use is an aircraft that spends more time in the hangar being repaired than on the flight line as a result of induced complexities. How well would an aircraft with the complexities of the Flapjack handle the unique and punishing environment of aircraft carrier operations? Questions we'll never know the answer to. Comparing the Flapjack with, say the V-22 at this stage would be foolish, mind. The V-22 is designed wth ease of maintenance and operation in mind through judicious use of computerised systems, something that the Flapjack designers did not have access to. The V-22 takes advantage of modern technologies and materials that mean that it couldn't have been built at the time of the Flapjack.

The last thing is the assymetric flight issue. It's worth taking into consideration how modern large turboprops cope with this issue. If the Flapjack had an engine out situation, even an interconnect between the two rotors (there's that mechanical complexity leading to more unplanned maintenance compared to more conventional types on the flightline) would not be able to reduce the handling difficulties of the type. In a modern turboprop you have an engine management computer governing fuel and power inputs to the engine, and mechanical governors managing prop functions, not just normal operating pitch changes either. Most have overspeed governors that kick in if the engine is oversped to protect the prop. 

When an engine out condition occurs, the good engine increases power to compensate for the drop in power, although the amount of thrust cannot be regained. The prop that has now going into feather is not producing thust, but the working engine is able to operate and still produce thrust to compensate to a small degree by use of the governor to provide the best propeller pitch for thrust production depending on the condition. Now all of this is managed by the engine management system. We can do this now because of advances in computer design, an option they didn't have in the Flapjack. Problem is, this still doesn't mean lower maintenance. The number of engine runs I've been on where problems have arisen as a result fo some of these features going wrong...

The Flapjack might have been an aerodynamic wunderkind, but it's likely to have been a maintenance hog and a hangar queen, which is something you don't want on your flightline.


----------



## Zipper730 (Jul 12, 2019)

nuuumannn said:


> I've been reading through this thread and others on the XF5U in particular and the reason's beind its cancellation and although I have no evidence that confirms my theories as reasons why this aircraft was not proceeded with, my experiences, as limited as they are with working on modern turboprop aircraft give me a bit of an insight into systems that have an effect on an aircraft's service criteria.


You actually make very good points.

The best explanations I had up to this point was basically

Jet-engines were coming online: Some saw propellers as obsolete. This was one official explanation
The budget battles after WWII ended might also have played a role in the matter: It could be used to further the argument that carriers would not be needed if an aircraft could stall as low as 20-40 knots, and the USAAF/USAF wanted to scuttle the carriers.
The plane would not make a good CAS aircraft because it couldn't carry shitloads of rockets as there's only a small gap in between the props.


----------

