High Aspect-Ratio vs. Low Aspect-Ratio

[John Frazer]

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]

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...

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]

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]

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

1. Jet-engines were coming online: Some saw propellers as obsolete. This was one official explanation
2. 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.
3. 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.