Mid-Engine layout

[gjs238]

Despite the seemingly much despised P-39, the later Fisher P-75 Eagle was also designed with a mid-engine with the propeller driven through an extension shaft.
If the mid-engine layout was so disliked, or at least if the P-39 was so disliked, why was this layout again chosen?

 

[FLYBOYJ]

Despite the seemingly much despised P-39, the later Fisher P-75 Eagle was also designed with a mid-engine with the propeller driven through an extension shaft.
If the mid-engine layout was so disliked, or at least if the P-39 was so disliked, why was this layout again chosen?

My guess would be because of room and cooling. The P-75 was built from components from other aircraft. Considering the size of it's engine and required cooling as well as other components, it seems the only place for the engine to go was in the middle of the fuselage.

 

[Thorlifter]

I may be wrong, but I don't believe it was the mid engine aspect of the P-39 that was disliked. It was the P-39's lack of a turbo supercharger, thus severely limiting its high altitude performance.

 

[Shortround6]

Like a lot of things, with every layout there are advantages and disadvantages. The mid engine layout is supposed to offer better maneuverability by putting the greatest mass of weight on the CG. The down side is that with the engine on the CG the spaces for consumables (fuel and ammo) can no longer be on the CG and so the aircraft trim will change as the load changes. Depending on the size of the plane and the planned armament the fuel situation can get a bit dicey. The P-39s failing was that it was too small. Great for drag, not so good for sticking in all the stuff that people wanted. A bit like 4 people trying to go camping with a Mini-Cooper. Not everything is going to fit inside and hanging it outside really starts to affect the performance.

The P-75 had the Allison's side by side and so had more fuselage volume and interior volume tends to go up with the cube of the dimensions so maybe they hoped that would solve the volume (fuel, guns and ammo) problem.

 

[fastmongrel]

Did the mid engine Bell layout leave any room for upgrading engines. The Spitfire for example had various models of Merlin/Griffon each one a bit bigger than before.

 

[Shortround6]

Depends what you mean by upgrading. The P-39 went from a 1000hp paper engine in the planning stages to a 1090-1150hp engine in the prototype stages and finally to a 1200-1325hp engine that could hit about 1500hp in WEP mode. A roughly 45-50% increase in power at low level. The P-39 was constrained by several things, one was the size of the plane and the other was the layout details. It is estimated that the P-39 fuselage was about 50 lbs heavier than a front engine fighter's due to the need to have a rigid fuselage between the engine and propeller. The P-39 did not have engine mounts as such, like the mounts in a 109 or P-40 but used more like car engine mounts to box sections in the fuselage.

For most planes the engine and propeller were one unit with the rest of the plane hanging on behind

P-39 had to make sure the propeller didn't get very far out of line from the engine, 1/2 degree over the 10 feet of propshaft was 1in (25mm) out of line. Bigger engines also may mean bigger reduction gears and props.

A bit exaggerated perspective but.....

 

[fastmongrel]

I was thinking more of the supercharger getting bigger similar to a RR Merlin and needing the intercooler and the like. I was just wondering if there was the room to fit a longer Allison engine like the Spit got a 40 or 60 series Merlin. For the Spit it seems to have been a relatively (I know it took a lot of work) painless job to fit longer and longer power plants.

 

[Edgar Brooks]

One thing RAF pilots were decidedly unimpressed with was the thought of half a ton of engine, poised behind their kidneys, and liable to break free in a forced landing.

 

[davebender]

The Fisher P-75 was not placed into mass production. That suggests the USAAC considered other aircraft designs to be superior.

 

[wuzak]

The Fisher P-75 was not placed into mass production. That suggests the USAAC considered other aircraft designs to be superior.

At the time it was proposed they were looking for a long range fighter. When they saw teh P-51 could do the job it was no longer required.

 

[wuzak]

The P-75 had the Allison's side by side and so had more fuselage volume and interior volume tends to go up with the cube of the dimensions so maybe they hoped that would solve the volume (fuel, guns and ammo) problem.

Allison - single. They P-75 had the V-3420.

 

[Shortround6]

Allison - single. They P-75 had the V-3420.

And the V-3420 was what?

two V-1710 power sections placed side by side with a common accessories section (supercharger) at the rear and geared together (or not?) at the front.

Yes it was a single powerplant but in size and volume it was two side by side engines.

 

[gjs238]

Fascinating photo. I didn't realize it used TWO driveshafts.

 

[wuzak]

Fascinating photo. I didn't realize it used TWO driveshafts.

There were two types of V-3420 - the A series and B series. One had both crankshafts rotating in the same direction an dth other had opposite turning crankshafts - as was used for the P-75. This made getting dual rotation props easier.

 

[Shortround6]

That may be a shortened drive shaft set up from a XB-42, I am not certain. But keeping to the Allison style of building in moduals there were different front cases/gear units for different applications.

 

[wuzak]

And the V-3420 was what?

two V-1710 power sections placed side by side with a common accessories section (supercharger) at the rear and geared together (or not?) at the front.

View attachment 202402

Yes it was a single powerplant but in size and volume it was two side by side engines.

True, but I would think that two separate engines could not be mounted as close together as the V-3420 halves were, because engine mounts would have to be fitted between the two engines and access to them would be required - making the installation wider.

 

[wuzak]

That may be a shortened drive shaft set up from a XB-42, I am not certain. But keeping to the Allison style of building in moduals there were different front cases/gear units for different applications.

That is the drive for the P-75. The XB-42 actually used a pair of V-1710s on opposite sides of the fuselage, with their driveshafts angled towards the centre.

 

[FLYBOYJ]

True, but I would think that two separate engines could not be mounted as close together as the V-3420 halves were, because engine mounts would have to be fitted between the two engines and access to them would be required - making the installation wider.

 

[wuzak]

And?

 

[wuzak]

It was brought up in another thread, but probably belongs here.

The question was asked if a Merlin could fit into a P-39 and/or a P-63.

A single stage single speed Merlin, such as a 45 or 50, should fit comfortably well inside a P-39, being about the same width (29.8" vs 29.3"), slightly taller (41.2" vs 35.7", due to the downdraft carby?) but considerably shorter (69.0" vs 85.8") than the V-1710 single stage engine. The weight is about 50lbs more for the Merlin. Using a 45 would be swings and roundabouts - lose at takeoff and low altitude, gain at altitude. Using a 45M or 50M would give a decent power boost at low altitudes, but you would be again stuck with a low altitude aircraft.

A single stage two speed Merlin, such as a XX, would give better altitude performance, and not lose so badly at low altitudes. Weight goes up a further 50lbs or so, length goes up to 71", but still shorter than a V-1710, and height increases to 41". Again, using a downdraft carburettor should reduce the height somewhat.

The third option is the Merlin 60-series. Weight goes up by 200lb, length up to 88.7", and now longer than the Allison. Width is up by just under 1", but height is reduced to 40". A space will be required for the aftercooler radiator.

An alternative is the Griffon. At 30.3" wide and 81.0" long for the two stage versions it is narrower and shorter than the two stage Merlins! However it is substantially taller at 46". And weight is up by around 600lbs compared to the original Allison. You will also need larger radiators and an aftrecooler radiator. It would probably be too powerful for the P-39's structure. But what about the P-63? It was designed around a heavier engine (the two stage Allison at around 1650lbs) and woul dlater be fitted with the even heavier and longer turbocompound V-1710-127 - though it never flew.