Tandems (and push pulls, maybe?)

[pigeon]

Something that has seen surprisingly little discussion even here.

Why was there no successful tandem or push pull twin? The only ones to ever reach production (sorta kinda) I can think of were the VB 10 and Do 335 respectively. Both pretty much pointless by the time they came out, Do 335's job taken over by 262s, and the VB getting sorted out about a year before Mig 15.

Now there's obviously a lot of very good reasons (financial, logistical, etc) to not make a twin engined "pure" fighter. What I'm more interested here is purely technical issues, and what, if anything, could have been done to remedy them.

So, maybe?

Tandems: routing the power from rear engine. Not enough room for driveshaft in the "Vee". Seen numbers like 70mm for the "cannon hole" in DB's. Hissos being the least cramped, and that's what the only serious experiments I know of were made with IIRC (the VB 10 and Bolkhovitinov Sparka). Rear engine needed gearing just to raise the driveline, and if you did the logical thing also having it be the actual reduction gear, you now had torque multiplication to deal with. Pretty much anything with remote drive had the shaft run at engine rpms with gearbox mounted at the prop, the only exception I can think of was Do 335 but that driveshaft was absolutely massive. Then again, Fiat AS6 somehow managed to run not one, but two, concentric, geared down driveshafts through it's front half...

Push pulls: all kinds of trouble with rear engine. Prop striking runway (or bailing pilot), crap getting thrown into prop by landing gear. Some aerodynamic inefficiency (how much? Fokker D23 did pretty well for it's size vs combined power). Central nacelle getting overcrowded in twin boom designs. Possible vibration issues for rear prop right behind the wing, especially when the pilot really hauls back on the stick with combat flaps out?

Cooling issues for both, along with all the serviceability crap that comes along with buried engines?

Maybe not strictly a topic for this thread, but why not a v16-v24 with central power takeoff? Chrysler made one, and it supposedly worked pretty well, just too late. The concept itself was certainly known (Bugatti U16).

 

[Marcel]

The D.23 had well known cooling issues that were never fully resolved. Another problem was that the pilot was pretty much trapped. They would be hit by the aft propellor when bailing out. Fokker had a solution for that, but it was never satisfactory.

 

[pigeon]

The D.23 had well known cooling issues that were never fully resolved. Another problem was that the pilot was pretty much trapped. They would be hit by the aft propellor when bailing out. Fokker had a solution for that, but it was never satisfactory.

Well it only flew with air cooled Walter Sagittas. Production versions were supposed to have Kestrels IIRC. Might have still been a problem, but solvable at least in theory.

 

[AMCKen]

Just about 3,000 of these built.

Cessna Skymaster - Wikipedia

en.wikipedia.org

 

[Shortround6]

Now there's obviously a lot of very good reasons (financial, logistical, etc) to not make a twin engined "pure" fighter. What I'm more interested here is purely technical issues, and what, if anything, could have been done to remedy them.

So, maybe?

Well, they violate the KISS principle.
Pretty much the only reason to resort to them is if you don't have a powerful enough single engine.
You are trying to use cleverness to overcome the drag of the normal twin tractor set up.

I am not sure were the two side by side engines geared together fall in here.

P-75.
It does show some of the problems with twin concept. You have to cool both engines and unless you are very very tricky that means twice the drag for the radiators and oil coolers.
Now for the US it also ran into the fact that by the time this thing flew in Nov 1943 (in cruder form) the P-47 was within a few weeks of being fitted with paddle blade props and water injection and drop tanks. R-2800 was giving about 2300hp compared to the 2600hp in the early V-3420 engine.

There was a tandem Soviet high speed bomber. But while skinny it was also freakishly long.

You get the small frontal area, You also have the crap view over the nose, and for a bomber, no good place to put the bombs.
In the picture one prop is turning, the other stationary.
By the time you fit a decent bomb-bay and enough fuel to actually go very far the drag has increase to the point where the tandem engine arrangement isn't getting you that much.

Now for a fighter you don't need a bomb bay and you need less fuel so maybe you fit that stuff in the wing. You do have a CG problem and there is no way to get around the poor view, unless you jack up the cockpit which kind of ruins the whole idea.
Kawasaki tried the cockpit between the engine trick like the VB 10 with the Ki-64
See

Kawasaki Ki-64 Experimental Fighter

The Kawasaki Ki-64 was an experimental fighter that employed tandem engines and evaporative cooling. More pressing priorities during WWII resulted in the aircraft not being repaired after it was da…

oldmachinepress.com

If you are not already aware of it. Japanese tried to get around the cooling problem by using evaporative cooling in the wing surfaces.
also see

Kawasaki Ki-64

Technical Data Manufacturer: Kawasaki Kokuki Kogyo KK (Kawasaki Aircraft Engineering Co Ltd). Type: Single-seat fighter with tandem-mounted engines. Crew (1): Pilot in enclosed cockpit. Powerplant: One Kawasaki [Ha-72] 11 (Ha-201) twenty-four...

ww2aircraft.net

for some drawings showing the layout.
Let's remember that the drive shaft in the P-39 weighed about 50lbs and they needed to make the fuselage about 50lbs heavier to keep the bending to about 1in the 10 ft and they needed a universal joint in the drive shaft.
Actually getting the front and rear engines to play well together was often a problem.

 

[Admiral Beez]

Just about 3,000 of these built.

Cessna Skymaster - Wikipedia

en.wikipedia.org

With kickass payload options too.

 

[AMCKen]

"With kickass payload options too."
But not all that at once.

 

[tomo pauk]

My remark in the brackets:

Well, they [tandems] violate the KISS principle.
Pretty much the only reason to resort to them is if you don't have a powerful enough single engine.
You are trying to use cleverness to overcome the drag of the normal twin tractor set up.

A lot of countries were in that boat: there is no powerful enough single engine. There was probably nothing bad in being clever, although the trick was not over-do it

It does show some of the problems with twin concept. You have to cool both engines and unless you are very very tricky that means twice the drag for the radiators and oil coolers.
Now for the US it also ran into the fact that by the time this thing flew in Nov 1943 (in cruder form) the P-47 was within a few weeks of being fitted with paddle blade props and water injection and drop tanks. R-2800 was giving about 2300hp compared to the 2600hp in the early V-3420 engine.

Drag of two V12s in tandem + two 'classic' cooling systems + two oil system radiators = drag of a big radial with it's oil system radiators (these were substantial on 2000 HP engines). If one's engineers devote some effort to make the radiators less protruding/draggy (perhaps 1st engine is cooled by an annular radiator?), we might even get a tad lower drag.

We can recall that Americans have had the working V-1710 by the Autumn of 1940, and the working 1-stage supercharged R-2800A a year later. The V-1710 will be making some 1070 HP at 14000 ft (obviuosly times two = 2140 HP) for by the time 1-stage R-2800 was good for 1500 HP there. So the advantage of two V-1710-33s (or the like) at 'normal' fighter altitudes will be about 40%, and that is a lot in my book. Yes, the double V-1710 installation will pay the price in weight, talk extra 300 lbs for the bare engine weight, another 300 for the cooling system, and perhaps another 100 for the two light props instead of one heavy? 700 lbs on a fighter that weights, equipped but without bombs, north of 12000 lbs, but has heaps of power does not look too shabby. Especially if expected to carry hefty battery of guns, as it was the case with what USAAC wanted.

Similar math can be applied for the Soviets, however the British and Germans would've have had the real over-performers with these kind of aircraft due to their V12s being better than the V-1710 or the M-105, and their 2000 HP engines took much more time than R-2800 to mature.

 

[Shortround6]

A lot of countries were in that boat: there is no powerful enough single engine. There was probably nothing bad in being clever, although the trick was not over-do it

Agreed. Unfortunately a lot of the non-standard twins crossed the line.

We can recall that Americans have had the working V-1710 by the Autumn of 1940, and the working 1-stage supercharged R-2800A a year later. The V-1710 will be making some 1070 HP at 14000 ft (obviuosly times two = 2140 HP) for by the time 1-stage R-2800 was good for 1500 HP there. So the advantage of two V-1710-33s (or the like) at 'normal' fighter altitudes will be about 40%, and that is a lot in my book. Yes, the double V-1710 installation will pay the price in weight, talk extra 300 lbs for the bare engine weight, another 300 for the cooling system, and perhaps another 100 for the two light props instead of one heavy? 700 lbs on a fighter that weights, equipped but without bombs, north of 12000 lbs, but has heaps of power does not look too shabby. Especially if expected to carry hefty battery of guns, as it was the case with what USAAC wanted.

To nit pick.
.....................................................double V-1710.............................R-2800..............................difference
engine weight......................1307 x 2=2614.................................2265....................................349
cooling system.....................294 x 2= 588.....................................0 (?).....................................588
oil system..................................61 x 2=122......................................118...........................................4
Starter sys.................................43 x 2= 86...........................................56........................................30
Prop/s......................................383 x 2= 767.......................................541....................................226
___________________________________________________________________________________1197

This is for the V-1710 as used in the P-40E and using the numbers from the P-47D (early).
The P-40C engines weighed a bit more(around 30lbs each?) but the props were a bit lighter (about 40lbs each ?)
Engine mounts and cowlings are bit of an unknown.
We can argue a bit over which version of the R-2800. The single stage bomber engine goes to 1600hp in late 1941 at 13500ft.
The two stage engine in the Corsair was already flying in late 1940 (in development ) and weighed about 2500lbs (without the intercoolers and ducting).
BTW the prop on an F6F-3 only weighed 485lbs. No idea how they did that one as the later P-40s about 418lbs.
YP-38 went from 692lb the pair to 827lbs for the P-38J for the pair.
The tandem Allison will probably beat the R-2800 but it is going to be a lot closer and more expensive than it seems. Is it worth it?

 

[don4331]

To the original question: Not only do you need an airframe manufacturer to look at alternative to the tractor twin, you need an engine manufacturer to work with you and probably propeller manufacturer.

de Havilland is making 16' 3 blade propellers for the Avro Manchester, but you're probably going to want something smaller for a tandem fighter = more blades and/or more area per blade.
If you're intending to have each of the engines run separate propellers, you need the rear propeller to have a larger hub e.g. #70 splines, so the front one e.g. #50 spline can pass through. And the constant speed mechanism for the rear propeller needs to be same as one with gun firing through the hub - not unique, but different. Different = expensive.

And almost by definition, its going to have to be a liquid cooled engine manufacturer.

Tractor tandem only solutions.

V16 - Chrysler understood there was a challenge in getting the correct amount of cooling to the correct location - to that end, they made individual cylinders for each of the 8 pistons per side... The result is an engine over 10' long. Almost 4' longer than Griffon of approximately same displacement.​

V-24 - Well, if you though the Chrysler was long, the Fiat AS.6 adds another 1'. Now, the Fiat does have the advantage, if there is damage to the front engine, the rear one will continue to run and get you home. Its also easier to start just the rear engine. It does have complex carburetor to allow running slow speed on just the rear, running slow speed on both and running full speed on both.​

Both engines have a major issue - they have a long, large diameter "pipe" filled with fuel/air under pressure....a backfire from any cylinder turns it into a bomb...not only did the resulting explosion blow the cowl off the MC.72, the engine is no longer receiving fuel and promptly stopped.​

2 - V-12s The Hispano-Suiza (Arsenal VB10) and Klimov (Bolkhovitinov Sparka) is basically same solution (Klimov starting as license copy).​

In the Sparka, you have a 2nd complete engine behind the first one, the length has now become >14'! The issue in the Sparka - the torque of the rear engine in one direction versus the torque of the other engine in opposite, caused twisting of the engines in the airframe. The twist caused the shaft from the back engine to bind - reducing power/breaking things... Latécoère 299A seems to have had just a little more space between engines and perhaps a different coupling solution​

In the VB10, the engines are spaced even further apart, which seems to an reduced (alleviated) the issue with the shaft between the engines. The splitting allows pilot to have visibility equal to any other V-12 fighter, with 2nd engine positioned ala P-39. Allison offered a similar solution to Bell and Fisher​

from Allison V-1710 & V-3420 Designs and Concepts​

Engines are each offset 6-3/8" from centerline. We will note the shaft from the rear engine attached to the back of the front engine steady it and allow airframe manufacturer to treat the reduction box and front engine a single package. tomo pauk , I think you could hit weight very close to the P-322 (~11,500lbs empty) with this solution. Allison also proposed a version of this with auxiliary 2nd stage supercharger.​

Allison also offered Fisher a solution with separate V-1710s instead of the DV-3420 - a little narrower but longer. I don't think any heavier as it doesn't have the combined crankcase/crankcase joining box. (I'm thinking Bugatti 100P scaled up to Allison size solution)​

A solution using 2 - Peregrines configured as in picture in alternate cannon fighter to Westland Whirlwind would be my British alternate. Probably easier to replace the 2 Peregrines with Merlins when RR cancels the small V-12. One just needs RR to take the risk of the remote gearbox.​

​

Push pull solutions:

Allison's installation in Bell XFM-1 added 77lbs to 'normal' C series engine to move the propeller ~6-1/2' back.​

Both Allison and DB left the reduction gear for their pusher solution at the engine - simplifies parts?​

​

For both Do.335 and Curtis-Wright XP-55, the rear propeller dictated tricycle gear. Probably not a bad thing, but different at the time and customers (gov't) need to have demonstrated advantage. Again for both aircraft, cooling the pusher engine was a problem.​

​

I was surprised in my looking at solution - Napier made a Lion with "splitter" gear box, allowing 2 tractor (or pusher) propellers. So, there was some historic precedence for Kelly Johnson's alternate P-38 configurations.

 

[don4331]

Agreed. Unfortunately a lot of the non-standard twins crossed the line.

To nit pick.
.....................................................double V-1710.............................R-2800..............................difference
engine weight......................1307 x 2=2614.................................2265....................................349
cooling system.....................294 x 2= 588.....................................0 (?).....................................588
oil system..................................61 x 2=122......................................118...........................................4
Starter sys.................................43 x 2= 86...........................................56........................................30
Prop/s......................................383 x 2= 767.......................................541....................................226
___________________________________________________________________________________1197

Alternate: 1938 timeframe

...............................................double Kestrel XVI...........................R-2800..............................difference
Engine power.........................773 x 2.............................................1,500.................................46
engine weight......................957 x 2=1914.................................2265....................................-351
cooling system.....................221 x 2= 442.....................................0 (?).....................................442
oil system..................................46 x 2=92......................................118...........................................-26
Starter sys.................................32 x 2= 64...........................................56........................................8
Prop/s......................................288 x 2= 576.......................................541....................................35
___________________________________________________________________________________112

As power is similar fuel requirements should favour the liquid cooled engine; just need to save 20 US gallons to be ahead

Big advantage - the 2nd engine can be turned off during cruise/loiter saving fuel so extending range/endurance/engine maintenance.

I'm thinking FAA in '38-41 timeframe - where RADAR isn't available, so RN needs continuous CAP. Tandem engines provide single line thrust for take off and landing security, power for climb. But while you're flying circuits, the fighter doesn't need 400mph performance. And altitude which FAA operated isn't so high that the 2nd engine is frozen and challenge to restart (although USAF managed it with R-4360s on B-36s)..​

Swapping the Kestrel's for Peregrine increases power to 1,770hp on 87 octane; over 2,000hp on 100 octane, but it will adds between 500 and 750lbs (heavier engine, increased cooling system, bigger props).
Better streamlining/lower cooling drag of liquid vs air cooled engines, should keep performance similar.

So, for basically same power in '38, we have the proven Kestrel vs experimental XR-2800 which is spitting crankshafts like baby spits dummy. Now, P&W will tame the R-2800 and with 2nd speed supercharger or turbocharger it will get better altitude performance. But the ~250lbs (plus ducting and intercooler) of auxiliary boost can be applied to the Peregrine as well. Again, for 2 engines there will be additional weight, but by this time I'd hope to be entrenched.

 

[Frog]

Not a fighter, but as long as the Allison V-1710 is concerned, the Douglas XA-42 Mixmaster evidenced some promising features.

 

[tomo pauk]

The tandem Allison will probably beat the R-2800 but it is going to be a lot closer and more expensive than it seems. Is it worth it?

USA had no problems in procuring even more complex and expensive P-38 (due to it being twin boom fighter, with a pair of turboes), so I see no problem with the price. Even the UK and France were eager to buy the Lightning versions, and were procuring 2-engined fighters in their country - everyone was aware of greater price of 2-engined fighters, while expecting that these are better than 1-engined fighters (not everytime that panned out, though). Japan was purchasing 2-engined fighters, indifferent as they were; Luftwaffe was buying 2-engined fighters by thousands. Heck, USAAC was paying for Aircaudas.
P-47 was as expensive as the expensive P-38 - again, price is not the problem.

Main advantage of a 2-engine V12 powered fighter is that one can have a fighter with a lot of power (1700-2000 HP), on 87 oct fuel, and without too much drag even before ww2 starts; if not in service by hundreds, than at least as flying prototypes. Not waiting until 1943 for a 2000 HP 1-engined fighter is huge. Even the US can pull it out by late 1940-early 1941.

So I'd say it is very much worth it, with the caveat that other, historical 2-engined 'day fighters' will pay the price (ie. either will be stillborn, or will be produced in far lower numbers). Or the 1-engined fighters that were either very expensive or were under-performing.