A36 Apache

[lyric1]

Most definately 1941 make em feel better propoganda

The book also describes the Devesator as the Navy's "Mighty Torpedo bomber!"

descriptions of the Airacobra and others follow that same path

So your saying our leaders of the day lied to us?

[Chalenge]

That is an interesting pic Dan. High altitude battles with an Allison powered airplane.?????

I think at the time that propaganda drawing was released the war department probably didnt want too many people talking about service ceilings but I also dont think too many airplanes were operating much higher than the Apache could operate. The push for high altitude operations was just beginning at that point. Notice in Guppys drawing that the number of exhaust cylinders are not clear the tail looks nothing like the 'Apache' and you cant even tell how many guns it might have.

[Stoney]

IThe push for high altitude operations was just beginning at that point.

P-38 and P-47 were purposefully designed with turbos instead of a multi-speed superchargers.  The B-17 and B-24 were purposefully designed with turbos instead of multi-speed superchargers.  All of these aircraft were designed before the war started for the U.S., and had turbos for high-altitude design missions.  The only reason the A-36 had a single-speed super'd Allison was its design mission--ground attack.

[Chalenge]

Yes I understand the Americans had designs for high altitude and thanks for pointing that out. When the first prototype NA-73X rolled out they didnt have an engine at all but the designers must have been thinking far ahead because the design details of this project didnt really make for much advantage until the plane was at higher altitudes. Now concerning the A-36 itself I would agree it was intended to fill a role that North American knew the Army had to fill yet but no one else was considering. When I think of the German or Japanese projects that eventually fill the high altitude roles I can only recall late war designs. Nothing about the time of the A-36 comes to mind as being intended for high-altitude but of course I tend to stick to familiar territory and that would be the Mustang projects.

[Stoney]

the design details of this project didnt really make for much advantage until the plane was at higher altitudes.

Considering the plane used the same Allison powerplant as the P-40 series, was heavier, and bigger than the P-40, it showed the aerodynamic efficiencies gained by the design, regardless of altitude.  Those same efficiencies were demonstrated at the higher escort altitudes where the Pony truly developed its reputation. 

[Chalenge]

Im sorry but while what you say sounds good I dont believe it approaches the real reason the P-51 was better than the P-40. Aerodynamic efficiencies alone are in my mind at least a very small part of the picture.

[MiloMorai]

Stoney, the P-51 and P-40 were pretty close to being the same size.

P-40E
# Length: 31.67 ft (9.66 m)
# Wingspan: 37.33 ft (11.38 m)
# Height: 12.33 ft (3.76 m)
# Wing area: 235.94 ft² (21.92 m²)

P-51D
# Length: 32 ft 3 in (9.83 m)
# Wingspan: 37 ft 0 in (11.28 m)
# Height: 13 ft 8 in (4.17 m)
# Wing area: 235 ft² (21.83 m²)

weights

P-40E
# Empty weight: 6,350 lb (2,880 kg)
# Loaded weight: 8,280 lb (3,760 kg)

P-51D
# Empty weight: 7,635 lb (3,465 kg)
# Loaded weight: 9,200 lb (4,175 kg)

Mustang I
basic: 5990lb
Max gross: 8633lb

[Stoney]

Stoney, the P-51 and P-40 were pretty close to being the same size.

Hmm, right you are.  Thanks for the clarification.  Either way, same powerplant in the same sized aircraft, faster at sea level doesn't have anything to do with the supercharger.

[Chalenge]

You are comparing the P-40 to the D model. The 1150 horsepower Allison V-1710-F3R was in the A-36 Apache but I couldnt tell you what the P-40 had so I will take your word it had the same engine.

A-36 empty         -     6087
A-36 normal T.O.  -     8600
A-36 max G.W.       -  10700

Same horsepower as the P-40 similar weight and less area in profile but I dont think the airfoil really enjoyed a lot of advantage at sea level. Its tough to tell really how the weights compare without knowing what the different configurations actually were. I believe the A-36 carried more fuel for one thing.

[Stoney]

You are comparing the P-40 to the D model...

I dont think the airfoil really enjoyed a lot of advantage at sea level.

Yeah, apparently I did.  But, the airfoil would be just as advantageous at sea level as it was at high altitude.  The lower cooling drag and all of the other aerodynamic refinements of the Apache would be as apparent at sea level as they would at altitude.  The two speed supercharger on the Packard Merlin was what gave the B and D models their high altitude performance, not aerodynamics.  If they were looking for aerodynamics for high altitude, the P-51 planform wouldn't have been a 5.5 aspect ratio wing.  To make it more aerodynamically suited for high altitude, they would have made it high-aspect ratio like the Ta-152.

[Chalenge]

I dont think it is possible for that to be true. The engine enhancements you mention are intended to maintain the same atmospheric pressures as low altitude but the airplane performs better at 25k which is critical altitude for the D model. So if the engine output is the same (which only in a perfect world would it be the same) why does the D model perform better at 25k than at sea level?

Being a glider pilot I have a high appreciation of wingspan. 

[Stoney]

I dont think it is possible for that to be true. The engine enhancements you mention are intended to maintain the same atmospheric pressures as low altitude but the airplane performs better at 25k which is critical altitude for the D model. So if the engine output is the same (which only in a perfect world would it be the same) why does the D model perform better at 25k than at sea level?

Being a glider pilot I have a high appreciation of wingspan. 

Dynamic pressure is the term used to describe the relative density of the air at a certain speed.  It is represented by the term "q" in most equations, and is = to:

1/2*p*v^2

Where p = density of air and v = velocity

At altitude, air density is lower so dynamic pressure is lower, so skin friction drag is reduced.  The constant amounts of manifold pressure that are created by the turbo or supercharged induction systems, generally speaking, create constant amounts of thrust.  We also know from the thrust equation that an aircraft that is at equilibrium:

Thrust = Drag

If the engine produces the same amount of thrust up to critical altitude, thrust in the equation is constant.  If the drag is reduced due to lower dynamic pressure, then the velocity of the aircraft will increase, so that the True Air Speed of an aircraft will continue to increase, with the same amount of thrust being applied in the face of decreased friction drag.  So, the aircraft will continue to fly faster, until the constant thrust accelerates the aircraft to a speed where the drag and thrust components reach equilibrium again.  Generally speaking, all aircraft fly faster with increases in altitude, at least until they begin to lose thrust (either less engine power or prop efficiency) or the drag component begins to increase despite lower dynamic pressure (typically because of higher induced drag).

So, generally speaking, lower drag at altitude means faster planes, even if the engine power remains constant. 

[Chalenge]

Yes and following your own argument then an airplane with a smalled profile will be faster still in thinner air than an airplane with a thicker and coarser profile will since it will require less horsepower and result in less drag also.

[Stoney]

Yes and following your own argument then an airplane with a smalled profile will be faster still in thinner air than an airplane with a thicker and coarser profile will since it will require less horsepower and result in less drag also.

Not necessarily.  The Spit 9 versus P-51 is a good example.  Even though they are both powered by the same engine, and the P-51 is larger and heavier (I double checked this time ), the P-51 is still faster.  The Spit airfoil is also thinner at the root than the P-51, but still generates more drag.  My original point is the same though--the aerodynamic design of the P-51 was more efficient at all altitudes, not just at high altitude.

[Bronk]

Quick question.
Wouldn't drag be more noticeable down low where the air is thicker?





Isn't there a larger speed dif at sea lvl than say 25k?