"Stretch or not strech" of aircraft designs

[spicmart]

Gentlemen

I don't know if it is possible to assess in advance how much "stretch" capability aka develoment potential and adaptability a design initially has.
A plane which was literally "stretched" from a stocky brute of a plane to an elegant slim longwinged bird is the Fw 190 to become the Ta 152.
A plane which reached his pinnacle early and could not developed much further without growing disadvantages was the Me 109.
There are many examples for both to be found in history.
Some designers/companies were renowned to create planes which wouldn't age or at least age well, like Ed Heinemann.
Others were less succesful in that.

What do you think about this?

 

[PAT303]

I would have liked the Spitfire stretched like the FW190 for no other reason than to give it more internal fuel capacity.

 

[Ivan1GFP]

Hello Spicmark,

This is a pretty interesting subject and one that I have also wondered about.
First of all, I disagree with you a bit about the Messerschmitt 109. It had some inherent flaws pretty early in the series and the later versions just made those flaws more obvious because of increased speeds, but I believe it stretched pretty well from the pre-war versions through the last K series that were over 100 MPH faster.

I believe there are a couple important factors to consider:
Engine type and location: If your initial design has an engine that has a great potential for improvement and increased power such as the Merlin and DB 601/605 series did, then things are likely to go well; As the aircraft grows, the engine power can maintain or even improve performance. The alternative would be something like the Whirlwind with the Peregrine engine. When the Peregrine went out of production, there really was no suitable replacement. I believe the Wildcat had nearly the same situation.
The engine location is also important because more powerful versions are likely to be bigger even if only slightly and also heavier.
In the case of the Airacobra, there was no room to put in a better supercharger when that became available.
With Wildcat and Whirlwind, higher powered replacement engines would have required a major airframe redesign in order to maintain a proper Center of Gravity.

Another factor required for "stretch" is either a high fuel capacity in the original design or the space to add internal fuel capacity. As engine power and weight grow, fuel consumption also increases. Many fighters ended up with additional fuel tanks in non-optimal places because without them they could not fly the required missions. The Airacobra and Messerschmitt 109 were two that had no room for increasing fuel capacity, but both ended up in theaters in which range was not a big factor. The Warhawk was another aircraft that didn't have room for additional fuel

Yet another factor is of course the capability for armament changes. The FW 190 and Spitfire accepted new weapons pretty well. The Me 109 and Airacobra did not. Having to mount additional weapons externally in pods is really not the best way to do things.

These are pretty obvious things. A lot can be gathered by observing where the major components of the aircraft are and determining what would happen if they were to increase in size and weight as they inevitably do. Along with this comes another question of how the maximum G Load would change with the new weight of the aircraft.

- Ivan.

 

[Shortround6]

Trying to assess how much stretch an airplane had very early in it's career is just about impossible.

The more powerful engines it will use later in it's career don't exist. In a lot of WW II aircraft the fuel they used late in their careers didn't exist early in their careers.
P-51D running at 9lbs (48in) boost anyone?

The Warhawk was actually this aircraft after a lot of stretching.

The P-36 was this aircraft on it's 3rd or 4th engine change let alone sticking the Allison on it.

Ed Heinemann's A-4 Skyhawk first flew months if not a year before the engine that would power the later versions was first run on a test stand. Engine power went from 7000lbs thrust on the early planes to over 11,000lbs thrust on the later ones (and they may have gotten better fuel consumption) now perhaps Mr. Heinemann designed the engine bay with a little extra room to allow for a possible engine change even if he didn't know exactly what was coming?

The Bf 109 of WW II was stretched from this.

They did find room for extra fuel and more/larger guns but when you start with a 700hp engine there is only so much you can do. You either design an oversized airframe to allow for stretch and that doesn't perform very well with existing engines or you design a smaller airframe that performs better with the existing engines and hope better engines become available that will still fit in the airframe.

Spitfire with RR Vulture engine running on 87 octane anyone?
That is why the Hurricanes successors were so large, using late 1930s fuel and engines require a large airframe f you were going to get anywhere near a 2000hp engine.

 

[spicmart]

German planes were mostly conceived with a minimum of surface (wing) area and had to have the wings enlarged/fuselage lengthened during their evolution whereas the Western Allied planes had often plenty of wing and big fuselages from the get-go.
I wonder if resources were a reason for the Germans to keep their planes' dimensions smaller. They were generally not lighter though than their a Allied counterparts, often even heavier.

Just going with feel I have the impression that the planes of Kurt Tank were designed with a bit more foresight than Messerschmitt's as those were to reach a certain performance goal as fast as possible with less thought on later development. At least it seems so.
But I could be wrong.
Messerschmitt liked to use underwing radiators which required large cut-outs which increased aero-elasticity which decreased manoeuverability and high speeds. And they took away potential space for wing tanks while not being the aerodynamically most sound solution for radiator placement.
He preferred the ellipsoid shape for airframes to be the one with least resistance but shot it with draggy radiator installations.

Not sure about it but could it be that wings of one-spar construction were less able to mount wing tanks?

The later US fighters were roomy and had the propulsion to give them cutting edge performance.

 

[Ivan1GFP]

Trying to assess how much stretch an airplane had very early in it's career is just about impossible.

The more powerful engines it will use later in it's career don't exist. In a lot of WW II aircraft the fuel they used late in their careers didn't exist early in their careers.
P-51D running at 9lbs (48in) boost anyone?

Hello Shortround6,

For the most part, I agree with you. It is hard to tell whether an engine will have a lot of development potential or prove to be a dead end. It is also hard to guess what suitable replacement engines would become available during the aeroplane's production life.

In the case of the Mustang, I believe your example is not a good illustration. The Mustang started off life with a fairly low powered Allison engine with a boost pressure pretty close to +9 pounds. The basic design of the airframe DID have a fair amount of stretch to accommodate the heavier two-stage Merlin and the external radiator location allowed for a great increase in size without too many issues.
The same would not have worked with the Warhawk. The heavier engine and cooling system were all up front and did not balance each other out. Perhaps relocating the cooling system aft would have been a solution to the balance problem.
The cooling system space limitations also affected designs such as the Airacobra and King Cobra.

The +9 pounds sounds pretty low, but that also works out to 1.65 ATA or +465 mm boost for the Japanese and many of THEIR engines never exceeded that level of supercharging.

German planes were mostly conceived with a minimum of surface (wing) area and had to have the wings enlarged/fuselage lengthened during their evolution whereas the Western Allied planes had often plenty of wing and big fuselages from the get-go.
I wonder if resources were a reason for the Germans to keep their planes' dimensions smaller. They were generally not lighter though than their a Allied counterparts, often even heavier.

Hello Spicmart,

I don't believe the wing area of either the FW 190 or Me 109 changed much once they were past their prototype stage.
I believe the size of the wing was mostly a result of design philosophy. Larger wings cause more drag in general and the idea was to use the smallest wing that would not greatly compromise handling. This can be seen in the prototypes of the FW 190.
The original small winged aircraft were a bit faster but had worse handling. The wing size was increased and was probably reasonable for the weights of the early aircraft but I am fairly convinced that the wings were too small when the aircraft became heavier in later versions.
The Russians also had a tendency to use relatively small wings for their fighters as compared to American or British aircraft.

Messerschmitt liked to use underwing radiators which required large cut-outs which increased aero-elasticity which decreased manoeuverability and high speeds. And they took away potential space for wing tanks while not being the aerodynamically most sound solution for radiator placement.

I am not sure there is a connection between wing mounted radiators and maneuverability and high speed. From one article describing the radiator system of the Me 109, it sounded like a fairly well thought out system.
Basically, any radiator location has its costs.

Wing tanks also may not be such a great idea. As Shortround6 has mentioned a few times, a flat and wide fuel tank tends to have a lot of surface area relative to volume which means it is heavier. Location in the wings also makes for a fairly large target.
Another aspect of wing tanks is that they often (as in Hayabusa and Yakovlev / Lavochkin fighters) prevent the installation of wing armament. All armament mounted in the fuselage sounds like a pretty serious limitation, but some designers have managed to work around it with three or four guns around the cowl.

- Ivan.

 

[spicmart]

I really agree with you that the wings of later German fighters were way too small for their weight. The Spitfire Mk. XIV weighed about 3900 kg compared a D-9's about 4300 kg while featuring a 22.5 sqm wing in contrast to the Dora's 18.3 sqm wing. The Ta 152C weighed in at over 5300 kg with 19.5 sqm wing area.
It must have flown like a truck. Same for the Me 309 (16 sqm for 4600 kg).
Imo the only Focke Wulfs capable of tangling with the latest nimble Allied fighters were the late hotrod Doras, especially thanks to their superior roll rate and the Ta 152H thanks to its turn rate. The heavy fighter versions B and C were practically single-engined destroyers.
Most German panes were to receive larger wings in one form or the other. Other nations much less so.


Wing mounted radiators and manoeuverability do not have a connection I think but high speed does. The British performed postwar tests on drag of various radiator installations. Chin radiators were the worst, followed by underwing radiators iirc. Ventral were the best. Annular/drum installations were surprisingly good.

Focke Wulf preferred large ailerons and fuselage mounted tanks to lessen the inertia in a roll to keep a high roll rate,, though with the latest Ta 152 they used a shortened, 250 kg heavier steel main bar to incorporate additional wing tanks. This should have made them even less capable as dogfighters.

I wonder how Soviet fighters could have been developed. They had a high-altitude fighter (project) with the MiG-3 even though that machine looks like a low-to-medium alt fighter.

Japanes planes had rather large surfaces as they had to carry a lot of fuel to cover the vast distances of the Pacific Ocean and to provide superior turn performance.

 

[spicmart]

Light stick forces and control harmony were also important with increasing speeds.
Some designers seem to put more emphasis on that than others.

 

[pinsog]

Early P47 with relatively small amount of fuel and toothpick prop, then went to more fuel, drop tanks, more hp and paddle prop, then went to P47N with even more power, lots and lots of fuel finally finishing with the 'wish they would have built it' XP72 with a mind blowing amount of power.

 

[pinehilljoe]

You could say the P-51 was stretched into the F-82.

For the P-58 its a stretch to say it was stretched from the P-38, it followed the design concept.

 

[Shortround6]

The use of the Mustang as an example was to show that even if they had gone to the two stage supercharger as they did, IF they had not had improved fuel that was better than 1940 fuel then the P-51D might have been limited to around 9lbs of boost (depends on whos 100 octane fuel you are using). The Mustang would still have been a fast plane, just not as fast, but climb and sustained turn would have been dismal.

The thing about the fuel is that allowed some engines, like the Merlin, to make the same power at less weight than a new larger engine would using the old fuel. Some engines could not make as good a use of the new fuels. And yes, many nations didn't have access to 100/130 fuel and were forced to use larger, heavier engines in order to get the required power.

Some aircraft that tried to get a little too tricky with low drag/size/radiator placement did wind up have limited growth potential.
The Mustang may have benefited from the initial armament fit. Two .50 cal in the cowl and a single .50 and a pair of .30 cal guns was fairly heavy armament (weight wise) in 1940 and also required a fair amount of volume.

 

[pinsog]

The use of the Mustang as an example was to show that even if they had gone to the two stage supercharger as they did, IF they had not had improved fuel that was better than 1940 fuel then the P-51D might have been limited to around 9lbs of boost (depends on whos 100 octane fuel you are using). The Mustang would still have been a fast plane, just not as fast, but climb and sustained turn would have been dismal.

The thing about the fuel is that allowed some engines, like the Merlin, to make the same power at less weight than a new larger engine would using the old fuel. Some engines could not make as good a use of the new fuels. And yes, many nations didn't have access to 100/130 fuel and were forced to use larger, heavier engines in order to get the required power.

Some aircraft that tried to get a little too tricky with low drag/size/radiator placement did wind up have limited growth potential.
The Mustang may have benefited from the initial armament fit. Two .50 cal in the cowl and a single .50 and a pair of .30 cal guns was fairly heavy armament (weight wise) in 1940 and also required a fair amount of volume.

How did the fuel affect, specifically, the turbocharged radials used in the B17, B24 and P43?

 

[pinsog]

P35 with 1 30 and 1 50, that rediculous landing gear and top speed of 275 ish stretched to the P43, turbocharged, 4 50's, top speed 350+ at 25,000 feet. Too bad it didnt have real fuel tanks that didn't leak, the few they built could have been very useful against the Japanese

 

[Ivan1GFP]

I really agree with you that the wings of later German fighters were way too small for their weight. The Spitfire Mk. XIV weighed about 3900 kg compared a D-9's about 4300 kg while featuring a 22.5 sqm wing in contrast to the Dora's 18.3 sqm wing. The Ta 152C weighed in at over 5300 kg with 19.5 sqm wing area.
It must have flown like a truck. Same for the Me 309 (16 sqm for 4600 kg).
Imo the only Focke Wulfs capable of tangling with the latest nimble Allied fighters were the late hotrod Doras, especially thanks to their superior roll rate and the Ta 152H thanks to its turn rate. The heavy fighter versions B and C were practically single-engined destroyers.

Hello Spicmart,

The real heavyweight before the Ta 152 was actually the FW 190A-8 / A-9. The D-9 actually weighed less but for some reason apparently lost some of its roll rate as well. I don't believe the B and C models ever got past the prototype stage to make a difference.

Wing mounted radiators and manoeuverability do not have a connection I think but high speed does. The British performed postwar tests on drag of various radiator installations. Chin radiators were the worst, followed by underwing radiators iirc. Ventral were the best. Annular/drum installations were surprisingly good.

The Spitfires and late Me 109 were able to achieve some pretty good speed with underwing radiators. It may not have been the best setup, but seemed to work well enough.
I believe the cleanest setup from a drag point of view is the buried cooling system as found in the Airacobra and King Cobra. The problem was that it wasn't very effective and limited the amount of power that could be installed.

Focke Wulf preferred large ailerons and fuselage mounted tanks to lessen the inertia in a roll to keep a high roll rate,, though with the latest Ta 152 they used a shortened, 250 kg heavier steel main bar to incorporate additional wing tanks. This should have made them even less capable as dogfighters.

I don't believe the FW 190 actually had particularly large ailerons. I believe their effectiveness had more to do with the actuating mechanisms which were rigid rods and the lack of friction in the system. Although the fuel location was better for inertia, there was typically a fairly heavy wing armament which would have increased the moment of inertia as well.

Japanes planes had rather large surfaces as they had to carry a lot of fuel to cover the vast distances of the Pacific Ocean and to provide superior turn performance.

I don't believe this is actually an accurate summary. Aircraft such as the A6M and Ki 43 really didn't carry all that much fuel. They were just very economical in their use of fuel. The A6M2 for example only had 535 liters of internal fuel.
Later Japanese aircraft in general didn't carry a particularly large amount of fuel either.

The use of the Mustang as an example was to show that even if they had gone to the two stage supercharger as they did, IF they had not had improved fuel that was better than 1940 fuel then the P-51D might have been limited to around 9lbs of boost (depends on whos 100 octane fuel you are using). The Mustang would still have been a fast plane, just not as fast, but climb and sustained turn would have been dismal.

Hello Shortround6,

If the Merlin had been limited to +9 pounds boost, it might have been a better idea to keep using the Allison engines. The speed down low was better and the altitude performance was only slightly worse than the typical German fighter.

- Ivan.

 

[PAT303]

The use of the Mustang as an example was to show that even if they had gone to the two stage supercharger as they did, IF they had not had improved fuel that was better than 1940 fuel then the P-51D might have been limited to around 9lbs of boost (depends on whos 100 octane fuel you are using). The Mustang would still have been a fast plane, just not as fast, but climb and sustained turn would have been dismal.

The thing about the fuel is that allowed some engines, like the Merlin, to make the same power at less weight than a new larger engine would using the old fuel. Some engines could not make as good a use of the new fuels. And yes, many nations didn't have access to 100/130 fuel and were forced to use larger, heavier engines in order to get the required power.

Some aircraft that tried to get a little too tricky with low drag/size/radiator placement did wind up have limited growth potential.
The Mustang may have benefited from the initial armament fit. Two .50 cal in the cowl and a single .50 and a pair of .30 cal guns was fairly heavy armament (weight wise) in 1940 and also required a fair amount of volume.

I think the question is whether the airframe can be modified/upgraded to suit the later war high H/P engines successfully, the Spitfire P51 FW190 P47 are examples that did.

 

[PAT303]

I don't believe this is actually an accurate summary. Aircraft such as the A6M and Ki 43 really didn't carry all that much fuel. They were just very economical in their use of fuel. The A6M2 for example only had 535 liters of internal fuel.
Later Japanese aircraft in general didn't carry a particularly large amount of fuel either.

Japanese planes sacrificed everything for range and cruised at 120mph over open ocean to reach those distances, you won't last long doing that in the ETO.

 

[Ivan1GFP]

Japanese planes sacrificed everything for range and cruised at 120mph over open ocean to reach those distances, you won't last long doing that in the ETO.

Hello Pat303,

The A6M and Ki 43 are about the only two fighters that this description really applied to.
The Japanese manufactured and fielded quite a few other designs for which this description did not apply.
The Ki 44 and J2M would be examples of just the opposite kind of thing.

- Ivan.

 

[PAT303]

The Japanese designers were limited by the engines and fuel they had, Japanese aircraft were very good but when you are 300+ hp down there's only so much you can do.

 

[spicmart]

Hello Ivan

The real heavyweight before the Ta 152 was actually the FW 190A-8 / A-9. The D-9 actually weighed less but for some reason apparently lost some of its roll rate as well. I don't believe the B and C models ever got past the prototype stage to make a difference.

The Jumo 213 weighed less than the BMW 801. I'm not sure if it really lost that much roll rate as the effective increase of inertia would still be at the centerline , so it should not have been to great. But I'm so physicist. But the Dora turned better than the Anton due to better power-to-weight ratio and aerodynamics. It's right hand turn at speed almost matched the Spitfire XIV's.



The Spitfires and late Me 109 were able to achieve some pretty good speed with underwing radiators. It may not have been the best setup, but seemed to work well enough.
I believe the cleanest setup from a drag point of view is the buried cooling system as found in the Airacobra and King Cobra. The problem was that it wasn't very effective and limited the amount of power that could be installed.

The underwing raidators actually caused more drag than the annular/drum radiator installation of late war German planes.
Most effective setup apparently was the Mustang's which was effective and produced the most additional thrust.


I don't believe the FW 190 actually had particularly large ailerons. I believe their effectiveness had more to do with the actuating mechanisms which were rigid rods and the lack of friction in the system. Although the fuel location was better for inertia, there was typically a fairly heavy wing armament which would have increased the moment of inertia as well.

The wing root, basic, armament is still near the centerline and many pilots removed the outer wing cannons, when there wer some, in order to make the plane more agile
The Russian fighter also featured cowling cannons to have place for fuel tanks in the wings. The late Yaks and Las had three or four cowl cannons, thanks for the Berezin B-20 20 mm being such a compact design.

 

[spicmart]

Would it have been possible to stretch the Me 109 design like the Fw 190? Lengthening the fuselage to carry more armament and fuel, enlarging the wings to carry the weight. Obviously the undercarriage would have to change to a broad one, too. They would have had to strenghten the structure considerably as the 109 was not nearly as rugged as the 190.

Why did they not develop the A6M Zero further to be competitive?