Pfeil's cousins for your airforce

[tomo pauk]

After, perhaps, seeing a push-pull layout of Do-18, a design has started to build a 1-2 place combat plane, with engines in such a layout and crew in between. Depending on the country, what should be finesses of such a plane (engines, armament etc), and what it should do better than planes of 'ordinary' twin-engine configuration? Or, a single-engined one; maybe even serving side-by-side with other twin-engined planes.
The project should start in early 1936, with 1st planes in service by 1939.

 

[davebender]

We don't need no stinking front engine!

The American P-39 employs prop shaft technology similiar to the Do-335 rear engine. It has the potential to be a world beater. Build it without all the historical flaws, which requires additional funding for research and development.

 

[tomo pauk]

Sorry, Dave - it's 2 engines or bust

 

[GregP]

How about the Martin XB-51?

Two engines in front and one in the back, though definitely not a fighter.

I don't think it owes anything to the Do.335, being a jet with swept wings and a T-tail, but has front and rear engine locations in common.

 

[Shortround6]

I am still puzzled as to why the Dornier Do-18 (or actually the WAL) gets such credit for for the push pull arrangement. Tandem engines had been used as early as 1914 and by 1919/20 you can find perhaps a half dozen examples. The HP 1500 being probably the most produced.

 

[GrauGeist]

I am still puzzled as to why the Dornier Do-18 (or actually the WAL) gets such credit for for the push pull arrangement...

The "Wal" (meaning Whale) was actually the family of flying boats Dornier manufactured starting with the Do15: Do18, Do22, Do24/Do318, Do26, etc...

 

[vanir]

The first design compromise is putting a thrust producer right in the propwash of another. This leads me to the abstract likelihood that one engine or the other will perform as a primary thrust producer in practise, that higher/maximum engines settings will be marginally removed from each other and the overall characteristics will be a performance disadvantage over other layouts, although an increased reliability and it will fly more like a civilian research plane than a military warplane meant for combat.

I think Dornier faced these problems and accomplished solving most of them which is what makes the Do335 so special, but I think the layout is nevertheless less than ideal as a starting basis for a tactical multirole in a 1945 combat environment. And the RLM apparently agreed, the competition was between the Ta152C and the Do335 for the Me410 replacement and the Ta152C had its production tooling built by the end of the war, the Do335 didn't. I don't believe the Dornier was actually going to enter mainstream service, just continue development testing and maybe limited production in special roles like nightfighter, tactical reconnaissance, etc.

 

[Shortround6]

It was pretty well established by the very late 30s that the normal tandem engine arrangement (2 engines in one long nacelle) while cutting down on drag didn't offer the same propulsive efficiency as two tractor engines. The nacelles were always too short and the rear propeller was always operating in disturbed airflow. The Do 335 does offer, by far, the greatest distance between propellers of any of the tandem aircraft so perhaps it was less of a problem.

 

[tomo pauk]

Why Do-18, but not others?
The layout of Do-18 engines is just so pleasing to my eyes (seems like well streamlined?), so let's say someone decided to make a combat plane of similar layout.

One engine is enough?
While in 1944/45 one engine was capable to offer/support great performance, punch combat range, back in late 1930's that was not the case - two were needed (in Me-110, Whirlwind, P-38, even in MiG-5 etc). So, for sake of discussion, lets review possible designs for different airforces, in push-pull layout, akin Do-335.

 

[Shortround6]

Part of the problem in the 30s is that constant speed propellers are brand new. For the Do 335 layout to work you need an extension shaft, you need a rigid rear fuselage that weighs more. You need tricycle landing gear (that weighs more), etc.
You have to make a heavier, more complicated, more expensive airplane. Are the performance gains going to be worth it?

 

[tomo pauk]

P-39s have had both (ext shaft, tricycle), yet it was lighter than P-40 P-51. Plus it have had the armament that weighted more. It was able to outperform almost anything under 12000ft?
P-38s have had tricycle, 'almost' 3 hulls, but it was capable to outperform any twin-engined plane in service, from Mosquito down. But not the Do-335? Lightning was as expensive as perhaps two P-40s - being a twin-boom plane, with 2 engines 2 turbo instalations, it's no wonder price was high. Reckon a single-hull plane would be just a tad less expensive?
Hence I think we could've snatched some performance from our push-puller

Eg. Potez making such a plane, instead of its model 630.
Two HS-12Y - almost 1800 HP, empty weight up to 7500-8000 lbs (vs. 6900 of real 63.11), wing area 350 ft sq - not far away from Hellcat?

 

[GregP]

Things that might be considered to be related to the DO.335 Pfeil. The Pfiel first flew in October 1943.

1) Ryan FR-1 Fireball: Navy fighter. One piston in front, one jet in back. First flew in 1944, and so is unlikely to actually be related to the Do-335.

2) Ryan XF2R Darkshark: One turboprop in front and one jet in back. Related to FR-1. The piston was replaced by a turboprop.

3) Ryan XF2R-1: One turboprop in front and one jet in back. Related to above, but a revised aircraft with a different canopy and vertical tail, plus a few assorted other differences.

4) Convair XP-81: One turboprop in front and one jet in back. First flew in 1945. Prototypes were ordered on 11 Feb 1944, and the effort was related to the availability of General Electric TG-100 turboprop engines as much as anything else.

5) Grumman XTBF-1 Guardian: Navy sub hunter. One piston in front and one jet in back. First flew in 1953. Could be related to Do-335, but much more likely related to similar American research and the availability of R-2800 engines.

6) McDonnell XF-88B: Air Force fighter prototype. One turboprop in front and one jet in back. Probably related to the XP-81 as much as anything.

7) Fokker D.XXIII: Prototype fighter, with a piston engine on both ends. Flew in 1939, 4 years before the Do-335.

8) A. S. Moskalev SAM-13: Piston in both ends, but flew in 1940, 3 years before the Do-335.

9) Sukhoi Su-5: Piston in front and jet in back. First flew in 1945.

 

[Shortround6]

P-39s have had both (ext shaft, tricycle), yet it was lighter than P-40 P-51. Plus it have had the armament that weighted more. It was able to outperform almost anything under 12000ft?

It could outperform almost anything under 12000ft when? after if got 100/130 fuel and a WER rating? in 1940-41 it couldn't climb for sour apples. And if it needs another fighter to give it top cover just cheap is it? One P-39 + One top fighter = what price?
It also had crap for range. Short field performance was little lacking too. It gave up certain areas of performance to get high performance in others.

P-38s have had tricycle, 'almost' 3 hulls, but it was capable to outperform any twin-engined plane in service, from Mosquito down. But not the Do-335? Lightning was as expensive as perhaps two P-40s - being a twin-boom plane, with 2 engines 2 turbo instalations, it's no wonder price was high. Reckon a single-hull plane would be just a tad less expensive?

Hence I think we could've snatched some performance from our push-puller [/QUOTE]

why would the single hull be cheaper? a few less rivets and sheet metal? P-38 tail booms got real small real quick. The Push-Pull Do 335 style and not Fokker D.XXIII style needs a fat rear fuselage to hide the engine and a stiff fuselage to hold the propeller shaft in alignment. 1/2 degree of "flex" equals 1 in in 10 feet 25mm in 3 meters?). P-38s central pod wan't all that big either. There are real advantages and real disadvantages to the Push pull layout, it was a more difficult engineering problem in the late 30s than a conventional twin.

Eg. Potez making such a plane, instead of its model 630.
Two HS-12Y - almost 1800 HP, empty weight up to 7500-8000 lbs (vs. 6900 of real 63.11), wing area 350 ft sq - not far away from Hellcat?

Question is can you actually do it? replace 900lb air cooled engines with 1100lb liquid cooled ones plus radiators and coolant. Fit bigger propellers plus one extension shaft. More power plant weight than the Hellcat, less power, better streamlining??? Heavier, more complicated landing gear, than the Potez 631. French had two choices for guns. The 20mm Hispano and the 7.5mm machine gun Hispano was drum feed. Potez 630 could carry two underneath. With a Push pull Do 335 style you can have one in the engine. What is curious is I don't think the French ever synchronized the 7.5mm machinegun, could be wrong about that but that means any extra guns of either type go out in/under the wings.

Without structural beefing up you are rapidly cutting into your stress limits and those bigger engines are going to need more fuel if you use the extra power. Potez 631s had neither armor or self sealing tanks and while nobody else did in the 30s it is something to consider when trying to figure out how a 1936-40 design would fair if it was kept in production after that. Vulnerable or reduced performance.

As an idea of the difference between a single and twin it was thought in the early design/concept stage of the P-38 that the required performance could be gotten with either twin 1000hp engines or a single 1500hp engine. Since no 1500hp engine existed at the time they went for the twin.

 

[GregP]

I vastly prefere the engines near the CG, not out on the ends as ion the Do 335. With engines on the far ends, the maneuverability had to be reduced realtive to an aircraft without engines on the ends. If I had to lose one axis, I'd rather keep the pitch axis crisp and sacrifice the roll to twin engines much nearer the CG, as in the P-38, Mosquito, Me110/210/410, many Japanes fighrters, Fokker G.I, and a lot of others. Bailing out would also have been tough, especially iof the explosive charges somehow got combat damage ... a likely event of the Do 335 was ever shot at from the rear quarter.

The Pfeil was unsual, but that didn't make it successful. It was fast, and that didn't either. We'll never know since they only made 11 and the combat encounters add up to one or two at best. The first combat encounter saw the Do 335 running away succesfully at high speed and low altitude. Successful evasion, but hardly a good combat result for either side.

It definitely IS interesting and unusual, both of which attract my notice.

 

[Shortround6]

you can look up the Fokker D. XXIII: http://www.dutch-aviation.nl/index5/Military/index5-1 D23.html

And get an Idea of the push pull fighter in 1939. The Wiki entry is wrong in that the engines were not liquid cooled but air cooled. The Walter engines were 18.4 liters, weighed 385kg and gave 440hp for take off and 540hp at 4000 meters.
Granted the D. XXIII was something of a proof of concept plane as built with more powerful engines and perhaps a different wing on production versions (had they happened) but a plane with more powerful liquid cooled engines would have gained weight rapidly. The proposed twin Merlin version would have gained 220kg per engine plus radiators and coolant, bigger oil systems, bigger propellers and so on. Fuel for 560 miles or so of range with the smaller engines would need to increased or range is no better (if as good) as single engine planes with 1000-1200hp. The Armament of two 7.9mm mgs and two 13.2mm mgs is not bad for a 1000hp fighter in 1939 but with twin 1000hp engines it would seem a poor return. Armament for the higher powered versions is not given in what I have seen so far. Speed for the twin Merlin version is given as 385 mph estimated which is rather good but it is 25mph better than a Spitfire I while using twice the power. It is also not much different than a Fw 187 with twin DB engines of lower power, or P-38s with their considerably heavier armament or even the later Mosquito (which, while it had more powerful engines was also a much bigger airplane).

This is just one design and it could have been more streamlined but a plane with twin 1000hp liquid cooled engines would have had to have been somewhat larger.

 

[tomo pauk]

It could outperform almost anything under 12000ft when? after if got 100/130 fuel and a WER rating?

That makes, what, 95% of all P-39s produced really being excellent performers under 12K?

in 1940-41 it couldn't climb for sour apples. And if it needs another fighter to give it top cover just cheap is it? One P-39 + One top fighter = what price?

P-40 was in the need for top cover too. IIRC P-39s were providing top cover for P-40s in VVS service.

It also had crap for range. Short field performance was little lacking too. It gave up certain areas of performance to get high performance in others.

Blaming tricycle extension shaft for those shortcomings is exaggeration.

why would the single hull be cheaper? a few less rivets and sheet metal? P-38 tail booms got real small real quick.

Think that Joe (FLYBOYJ) mentioned that more than once (multi-hull more expensive than single hulls). Joe, please correct me here if I'm wrong.

The Push-Pull Do 335 style and not Fokker D.XXIII style needs a fat rear fuselage to hide the engine and a stiff fuselage to hold the propeller shaft in alignment. 1/2 degree of "flex" equals 1 in in 10 feet 25mm in 3 meters?). P-38s central pod wan't all that big either. There are real advantages and real disadvantages to the Push pull layout, it was a more difficult engineering problem in the late 30s than a conventional twin.

The hull might've been fat, but that's far away from what was required for F6F, Fw-190 co. Engineers across the globe were solving stuff wrt. to aircraft design, so I don't see anything so special about push-puller. Even the 'small' countries were manufacturing their own designs by late 1930s (Romania, Czechoslovakia, Yugoslavia, Poland, Holland) - again no problem for a major player to develop something different/more complicated. France was flying 4-engined planes prior 1940's?

Question is can you actually do it? replace 900lb air cooled engines with 1100lb liquid cooled ones plus radiators and coolant. Fit bigger propellers plus one extension shaft. More power plant weight than the Hellcat, less power, better streamlining??? Heavier, more complicated landing gear, than the Potez 631. French had two choices for guns. The 20mm Hispano and the 7.5mm machine gun Hispano was drum feed. Potez 630 could carry two underneath. With a Push pull Do 335 style you can have one in the engine. What is curious is I don't think the French ever synchronized the 7.5mm machinegun, could be wrong about that but that means any extra guns of either type go out in/under the wings.

Powerplant for F6F weighted 3900 lbs (prop, cowling, supercharging included) - makes 1700 lbs allowance for 2 cooling systems, props, ext. shaft cowling. Landing gear of P-38 weighted 840-890 lbs - for 50% heavier plane. So perhaps it's 100 lbs more weight for 'tricycled' Potez. Armament can be engine-mounted 20mm + 6 LMGs in wings.

Without structural beefing up you are rapidly cutting into your stress limits and those bigger engines are going to need more fuel if you use the extra power.

Why would I cut into structural limits, with plane being stressed for that from day one? We don't need more fuel - range was 930 mi for Potez 63.11, and we cut on drag here.

Potez 631s had neither armor or self sealing tanks and while nobody else did in the 30s it is something to consider when trying to figure out how a 1936-40 design would fair if it was kept in production after that. Vulnerable or reduced performance.

So it's in the same position as P-38 - no self-sealing tanks, with 2 x 1150 HP. If France stays in war, it receives both SS tanks and stronger engines, and performance increases. If not, too bad.

As an idea of the difference between a single and twin it was thought in the early design/concept stage of the P-38 that the required performance could be gotten with either twin 1000hp engines or a single 1500hp engine. Since no 1500hp engine existed at the time they went for the twin.

Identical thing for France - since we have no 1300-1350 HP engine available, lets use two with 880-900 each.

 

[Shortround6]

That makes, what, 95% of all P-39s produced really being excellent performers under 12K?

Some of them were really great at up to 5,000ft in 1943, 1580hp at 2500ft WER in 1943. 1500hp at 5,000ft, 1360hp at 9,500ft an 1150-1200hp at 11,800ft. These are the the K,L and a few Ms. There is no WER rating for most of 1942.

Blaming tricycle extension shaft for those shortcomings is exaggeration.

We can also blame the small fuselage, small wing (for an American plane) and high wing loading. The point is you can't take a particular plane and just take the good points.

The next question is what would you/we do in 1938-1940 knowing what they knew then or we talking about what you/we would have done KNOWING what we know NOW. What improved fuels were going to show up and when, limits on engines or how far some could be pushed. WHat weapons would become available 2-4 years after an air frame was designed?

Think that Joe (FLYBOYJ) mentioned that more than once (multi-hull more expensive than single hulls). Joe, please correct me here if I'm wrong.

It all rather depends on the what you mean by a "hull" and how big they are. A small diameter short pod and two skinny booms or a long and extra stiff large fuselage? The P-82 had two full fuselages. The P-38 did not really have three full fuselages and neither did a Mosquito.

The hull might've been fat, but that's far away from what was required for F6F, Fw-190 co. Engineers across the globe were solving stuff wrt. to aircraft design, so I don't see anything so special about push-puller. Even the 'small' countries were manufacturing their own designs by late 1930s (Romania, Czechoslovakia, Yugoslavia, Poland, Holland) - again no problem for a major player to develop something different/more complicated. France was flying 4-engined planes prior 1940's?

It is not quite that simple and actually it is the other way around. Small countries like Romania, Czechoslovakia, Yugoslavia, Poland, Holland and others could design and build their own designs in the 1920s because the engineering was not really that complicated. Going into the 30s things got a bit more difficult and by the late thirties the engineering needed for a first class design was rapidly increasing. Structures were more complicated, the forces acting on the wings going up with the square of the speed. Systems like retracting landing gear and flaps needed actuators, hydraulic or electric or pneumatic. It took the Douglas company over 4 years and well over 1 million 1940 dollars to build the B-19 bomber.
Building multi-engine planes was no great trick either. It is what kind of multi engine plane. Igor Sikorsky had built a 4 engine plane in 1913/4. The French were actually building 6 engine flying boats in 1935/36. Latecoere 521 - passenger flying boat with some engines mounted in tandem.
But mounting engines in tandem in a nacelle is a different problem than trying the Dornier trick of using the extension shaft. Allison in the P-39 was building on it's experience of designing and building extension shafts and reduction gears for airships in the late 20s and early 30s. Allison having more experience at this than any other US engine company. B-36s and other postwar pusher aircraft had their fair share of extension shaft troubles even without trying fighter like maneuvers

Powerplant for F6F weighted 3900 lbs (prop, cowling, supercharging included) - makes 1700 lbs allowance for 2 cooling systems, props, ext. shaft cowling. Landing gear of P-38 weighted 840-890 lbs - for 50% heavier plane. So perhaps it's 100 lbs more weight for 'tricycled' Potez. Armament can be engine-mounted 20mm + 6 LMGs in wings.

The last part kind of gets to the point of it all. With MS 410s and D 520s carrying a single 20mm and four 7.5mm mgs. using a second engine to carry another two 7.5mm mgs seems a bit of waste.

Why would I cut into structural limits, with plane being stressed for that from day one? We don't need more fuel - range was 930 mi for Potez 63.11, and we cut on drag here.

your weight estimate would cut structural limits. If a plane has an 8G limit at 8000lbs it has a 6.4G limit at 10,000lbs, and that is if everything is laid out just right. A concentration of weight in the wrong place could lower that limit. You can get the 8G rating back but it calls for increased structural weight, which would be designed in when working on the re-engined plane but it is not weight free. As far as range goes I guess it depends on what you think a good range would be and how fast you want to cruise. A later Version of the Potez 631 (the 63.11 was teh recon variant), the Potez 671 carried 220imp gallons of fuel for slightly more powerful engines. The P-38 was designed for 330imp gallons before self sealing tanks for it's 1150 engines. Building twin engine aircraft with little more armament and little more range than single engine aircraft doesn't seem like a good return.

 

[tomo pauk]

Some of them were really great at up to 5,000ft in 1943, 1580hp at 2500ft WER in 1943. 1500hp at 5,000ft, 1360hp at 9,500ft an 1150-1200hp at 11,800ft. These are the the K,L and a few Ms. There is no WER rating for most of 1942.

We can also blame the small fuselage, small wing (for an American plane) and high wing loading. The point is you can't take a particular plane and just take the good points.

The next question is what would you/we do in 1938-1940 knowing what they knew then or we talking about what you/we would have done KNOWING what we know NOW. What improved fuels were going to show up and when, limits on engines or how far some could be pushed. WHat weapons would become available 2-4 years after an air frame was designed?

While all what you've posted here is valid, it hardly points into any perceived shortcoming a plane having a tricycle and extension shaft might have.

It all rather depends on the what you mean by a "hull" and how big they are. A small diameter short pod and two skinny booms or a long and extra stiff large fuselage? The P-82 had two full fuselages. The P-38 did not really have three full fuselages and neither did a Mosquito.

I won't press anymore about the price of an airframe needed to house two engines.

It is not quite that simple and actually it is the other way around. Small countries like Romania, Czechoslovakia, Yugoslavia, Poland, Holland and others could design and build their own designs in the 1920s because the engineering was not really that complicated. Going into the 30s things got a bit more difficult and by the late thirties the engineering needed for a first class design was rapidly increasing. Structures were more complicated, the forces acting on the wings going up with the square of the speed. Systems like retracting landing gear and flaps needed actuators, hydraulic or electric or pneumatic. It took the Douglas company over 4 years and well over 1 million 1940 dollars to build the B-19 bomber.

By 1938-40, Poland was building a bomber with laminar flow wing (PZL-38), while Romania, Yugoslavia and CZ were building modern fighter planes, along with licensed bombers.

Building multi-engine planes was no great trick either. It is what kind of multi engine plane. Igor Sikorsky had built a 4 engine plane in 1913/4. The French were actually building 6 engine flying boats in 1935/36. Latecoere 521 - passenger flying boat with some engines mounted in tandem.
But mounting engines in tandem in a nacelle is a different problem than trying the Dornier trick of using the extension shaft. Allison in the P-39 was building on it's experience of designing and building extension shafts and reduction gears for airships in the late 20s and early 30s. Allison having more experience at this than any other US engine company. B-36s and other postwar pusher aircraft had their fair share of extension shaft troubles even without trying fighter like maneuvers

Thanks for the info. The HS-12 will have to been tested, started in 1936, so we can have bugs out by 1938/39.

The last part kind of gets to the point of it all. With MS 410s and D 520s carrying a single 20mm and four 7.5mm mgs. using a second engine to carry another two 7.5mm mgs seems a bit of waste.

Same goes for Bf-109 vs. 110, or P-39/40 vs. P-38.

your weight estimate would cut structural limits. If a plane has an 8G limit at 8000lbs it has a 6.4G limit at 10,000lbs, and that is if everything is laid out just right. A concentration of weight in the wrong place could lower that limit. You can get the 8G rating back but it calls for increased structural weight, which would be designed in when working on the re-engined plane but it is not weight free.

Again, 'my' Potez will share all shortcomings benefits experienced by all planes that received any upgrades.

As far as range goes I guess it depends on what you think a good range would be and how fast you want to cruise. A later Version of the Potez 631 (the 63.11 was teh recon variant), the Potez 671 carried 220imp gallons of fuel for slightly more powerful engines. The P-38 was designed for 330imp gallons before self sealing tanks for it's 1150 engines. Building twin engine aircraft with little more armament and little more range than single engine aircraft doesn't seem like a good return.

220 imp gals means 264 US gals. F6F was managing 1300-1350 mi with 250 gals - we can do 1400, maybe more, since we're better streamlined? The in-line engines don't need fuel to cool - another 10%?
The main reason for the 'French Dornier' was to provide speed anyway - don't claim 400 mph, but some 370 seems okay. Along with 50% more firepower than D.520, once the cannon is out of amo.

 

[Shortround6]

Same goes for Bf-109 vs. 110, or P-39/40 vs. P-38.

Not quite. Bf-109 two 20mm guns with 60rpg. Bf 110 two 20mm guns with 180 rpg. 109 two 7.9mm mgs with 1000rpg. 110 four 7.9mm mgs with 1000rpg.

Early P-40 two .50s and four .30s. in late 1941 they had shifted to six fifties but with 1/2 the ammo the P-38 carried.

220 imp gals means 264 US gals. F6F was managing 1300-1350 mi with 250 gals - we can do 1400, maybe more, since we're better streamlined? The in-line engines don't need fuel to cool - another 10%?
The main reason for the 'French Dornier' was to provide speed anyway - don't claim 400 mph, but some 370 seems okay. Along with 50% more firepower than D.520, once the cannon is out of amo.

Ranges for US navy planes, while "possible" were for speeds and altitudes that would be fatal in European flying. You can some rather amazing ranges from Spitfires and 109s if you fly them at 180mph at 5,000ft

Air-cooled engines do not use fuel for cooling while cruising. Their SFC is quite comparable to liquid cooled engines when running lean mixture.

Since the 20mm cannon was providing about 70% of the D.520's fire power until the ammo runs out swapping one 20mm for two 7.5mm's hardly seems like a good trade. I don't know if the French planes allowed for reloading in flight. That is what the back seater did in the Bf 109 and earlier Beaufighters. In addition to other duties he was part of the loading system, swapping empty drums for full ones. using an engine mounted cannon rules that possibility out.

 

[tomo pauk]

I don't advice to install only 6 LMGs, but a cannon + 6 LMGs.

Now, a Soviet Pfeil, perhaps 1st as a fast bomber, than a heavy fighter (built 'stead of Pe-2/2i/3?). Two VK-105s (pre-serial planes having M-100s) , hull recess for a 1000 kg bomb (perhaps a full-fledged bomb bay for all the bombs?), with 250 kg bombs under wings, one Shvak (plus two-three in belly gun pack of fighter version), dive brakes.