I don't understand how some planes ended up being so fast

grampi · Nov 2, 2017

There were certain fighters that were extremely fast, but for the life of me, I can't figure out why. I can understand planes like the P-51, Spitfire, and Me 109 being fast, as they were relatively small and aerodynamically clean aircraft. But how in the world does a P-47 have any business having a top speed of 500+ MPH, or a Corsair being able to do in the high 400s? Both of these planes were big and heavy, and neither look to have anywhere near the clean aerodynamics of the 51, Spit, or 109. So how were they able to achieve such high top speeds with the planes?

tomo pauk · Nov 2, 2017

Horsepower at altitude matters. P-47 that went close to 500 mph have had 2800 HP at 30000 ft. Weight plays almost no role in speed.

pbehn · Nov 2, 2017

2000+ horsepower helps. In fact the corsair had some aspects that were aerodynamically "cleaner than the Spit and the 109. The gull wing joins the fuselage at 90 deg which is a better set up. The big air cooled radials had a higher frontal area but didn't have the radiators.

Shortround6 · Nov 2, 2017

Speed without altitude given is almost meaningless.

Drag depends on the density of air while engine power, given good superchargers, does not.

Air at 20,000ft is 53.27% as dense as the air at sea level (on a 59 degree F day standard pressure) while the air at 30,000ft is 37.40% as dense as sea level air.

An Allison P-51 using "cruise power" (2600rpm and wide open throttle) or about 1000hp at a bit over 12,000ft could keep up with if not slightly beat a P-47D using WEP with water injection. over double the power (2300hp) but at 31,000ft the P-47 could hit 435mph while the Allison Mustang would be down to under 340mph at that altitude due the engine haven fallen to under 600hp while the P-47 still had 2200hp. Amazing what over 3 times the power can do

A Corsair, without water injection would be down to about 360-365mph at 31,000ft going by charts. Engine power might be around 1100HP?

Koopernic · Nov 3, 2017

grampi said:
There were certain fighters that were extremely fast, but for the life of me, I can't figure out why. I can understand planes like the P-51, Spitfire, and Me 109 being fast, as they were relatively small and aerodynamically clean aircraft. But how in the world does a P-47 have any business having a top speed of 500+ MPH, or a Corsair being able to do in the high 400s? Both of these planes were big and heavy, and neither look to have anywhere near the clean aerodynamics of the 51, Spit, or 109. So how were they able to achieve such high top speeds with the planes?

The equation for air resistance is
F=1/2 Cd A ρ V²

F is the force in Newton's (about 100 grams)
Cd is the coefficient of drag
A is Frontal Area
ρ is the air density.
V² is velocity squared.

The P47 because of its turbosupercharger did not loose as much power at altitude as other designs and was fast there. It was not particularly fast at sea level, even the P47M on 150PN fuel. For instance at 25,000ft the air is only 0.4 times as dense as at sea level yet the P-47 has lost no power. It's said the American turbo engines were all flat rated from seal level to 25,000ft. At sea level the P51 and Tempest did well because of laminar flow wings and because their non turbo charged engines didn't have cost in terms of turbo weight.

The Corsair didn't have a turbo but it had a two stage mechanical supercharger with an inter cooler so it's high altitude power as quite good. This type of arrangement has significant exhaust thrust because of the high pressure.

Also the blunt nose is not as unaerodynamic as it seems. It's more important to have a smoothly tapering fuselage so that the air remains attached (thereby not becoming turbulent)

The air flowing into the cowling also came out and there were probably some attempts to eject the now heated air at higher velocity to recover some thrust.

The 1930s Gee Bee racers were particularly fast despite their stocky bodies. Things climbed faster than a Griffon Spitfire.

I would say that the packaging that BMW did on their radial the 801 was particularly effective. They closed the nose of and ensured adequate airflow by using a geared fan. Don't know if any American engines did this, perhaps only the R4360. Might have helped the R3350 during its struggles.

fastmongrel · Nov 3, 2017

Power.

You could get a shopping mall up to 500mph if you had enough power

pbehn · Nov 3, 2017

fastmongrel said:
Power.

You could get a shopping mall up to 500mph if you had enough power

That was the principle behind the F-4.

DogMD11 · Nov 3, 2017

The XP-47J had a fan-cooled R-2800 and the XP-72 and XF-12 Rainbow had fan-cooled R-4360's

Koopernic · Nov 4, 2017

DogMD11 said:
The XP-47J had a fan-cooled R-2800 and the XP-72 and XF-12 Rainbow had fan-cooled R-4360's

At around 500mph the XP-47J was an extremely impressive aircraft. I'm curious as to whether it could have gotten into service baring the fact it was decided to focus on the R-4360 engined XP-72 instead.

Shortround6 · Nov 4, 2017

Koopernic said:
At around 500mph the XP-47J was an extremely impressive aircraft. I'm curious as to whether it could have gotten into service baring the fact it was decided to focus on the R-4360 engined XP-72 instead.

According to one source:
"In addition, 70% of the P-47 production line needed to be re-tooled in order to produce the P-47J."

I don't know how true this was but they claim it used a "lightened wing". The "empty weight" is claimed to be 300-500lbs lighter than various D models.

drgondog · Nov 5, 2017

tomo pauk said:
Horsepower at altitude matters. P-47 that went close to 500 mph have had 2800 HP at 30000 ft. Weight plays almost no role in speed.

Tomo - you are correct that Induced Drag, which is a function of CL is much less important at top speeds, but very important at Cruising speed. Near optimal cruising speed CDp=CDi

drgondog · Nov 5, 2017

Koopernic said:
The equation for air resistance is
F=1/2 Cd A ρ V²

The P47 because of its turbosupercharger did not loose as much power at altitude as other designs and was fast there. It was not particularly fast at sea level, even the P47M on 150PN fuel. For instance at 25,000ft the air is only 0.4 times as dense as at sea level yet the P-47 has lost no power. It's said the American turbo engines were all flat rated from seal level to 25,000ft. At sea level the P51 and Tempest did well because of laminar flow wings and because their non turbo charged engines didn't have cost in terms of turbo weight.

The Corsair didn't have a turbo but it had a two stage mechanical supercharger with an inter cooler so it's high altitude power as quite good. This type of arrangement has significant exhaust thrust because of the high pressure.

Also the blunt nose is not as unaerodynamic as it seems. It's more important to have a smoothly tapering fuselage so that the air remains attached (thereby not becoming turbulent)

The air flowing into the cowling also came out and there were probably some attempts to eject the now heated air at higher velocity to recover some thrust.

The 1930s Gee Bee racers were particularly fast despite their stocky bodies. Things climbed faster than a Griffon Spitfire.

I would say that the packaging that BMW did on their radial the 801 was particularly effective. They closed the nose of and ensured adequate airflow by using a geared fan. Don't know if any American engines did this, perhaps only the R4360. Might have helped the R3350 during its struggles.

"F is the force in Newton's (about 100 grams)
Cd is the coefficient of drag
A is Frontal Area
ρ is the air density.
V² is velocity squared."

F=1/2 Cd*S*rho*V^^2 is correct for Aero when S=Wing Area and Cd = (CDp1+CDp2+CDp3)*CDc +Cdi
CDp1 is a function of Reynolds Number and the Parasite Drag build up of airframe components derived at a Specific RN during wind tunnel testing
CDp2 is composed of additional items such as guns, radio masts, leaks, surface roughness
CDp2 is increment of CDp due to angle of attack (profile drag)
CDp3 is increment of CDp due to cooling drag and items immersed in slipstream during climb - also important to derive Power Required calculations.
CDc mulitiplier for Compressibility - increasingly important at M>0.3
CDc is a compressibility factor which includes drag rise of compressible flow
CDi is the Induced Drag which is function of CL^^2, divided by AR*Pi*e
Actually the "e" factor is the reason that the P-47 elliptical wing offset Mustang advantage in WL and AR.

The major reason the Mustang drag was so low was a.) the wing and b.) the extremely well designed 2nd order curve layout of the airframe. The latter was only slightly lower than the P-47 and FW 190.

Frontal area per se, was not a linear comparative factor

Todd Secrest · Nov 6, 2017

grampi said:
There were certain fighters that were extremely fast, but for the life of me, I can't figure out why. I can understand planes like the P-51, Spitfire, and Me 109 being fast, as they were relatively small and aerodynamically clean aircraft. But how in the world does a P-47 have any business having a top speed of 500+ MPH, or a Corsair being able to do in the high 400s? Both of these planes were big and heavy, and neither look to have anywhere near the clean aerodynamics of the 51, Spit, or 109. So how were they able to achieve such high top speeds with the planes?

I would assume the P-47 and Corsair had bigger propellers, then the P-51, Spitfire, Me-109.

mad_max · Nov 8, 2017

Todd Secrest said:
I would assume the P-47 and Corsair had bigger propellers, then the P-51, Spitfire, Me-109.

Size of the propeller was proportional to the power the engine produced, prop gearing and problem of the prop tips going supersonic. All the test results I've studied pointed to props that provided good climb and good speed. Some tests showed certain props had slightly better speed, but a lower climb rate, while others had slightly better climb with a slower speed. You want the best of both worlds, but compromising has to be done so you have a respectable climb and speed.

Propellers of all Nations at War were constantly changing as new data came forward and tested. All of them slowly changed to a wider chord on their props with prop gearing to keep the tips under mach 1.

Take this for what it's worth. I'm not an engineer, just an aircraft junkie.

Todd Secrest · Nov 9, 2017

mad_max said:
Size of the propeller was proportional to the power the engine produced, prop gearing and problem of the prop tips going supersonic. All the test results I've studied pointed to props that provided good climb and good speed. Some tests showed certain props had slightly better speed, but a lower climb rate, while others had slightly better climb with a slower speed. You want the best of both worlds, but compromising has to be done so you have a respectable climb and speed.
Propellers of all Nations at War were constantly changing as new data came forward and tested. All of them slowly changed to a wider chord on their props with prop gearing to keep the tips under mach 1.
Take this for what it's worth. I'm not an engineer, just an aircraft junkie.

Did not the F4U Corsair and F6F use the same engine? (answer is yes)
And the Corsair still had a higher top speed and I'm assume because it had a bigger propeller (13ft 4in)?
The Corsair was designed with the gull wings, to accommodate the big propeller (and to shorten the landing gear).
The F6F Hellcats propeller was 13ft 1in, Could 3 inches make much of a difference?

tomo pauk · Nov 9, 2017

mad_max said:
...
Take this for what it's worth. I'm not an engineer, just an aircraft junkie.

I'm thinking about stealing that line for the signature; credited, of course

Todd Secrest said:
Did not the F4U Corsair and F6F use the same engine? (answer is yes)
And the Corsair still had a higher top speed and I'm assume because it had a bigger propeller (13ft 4in)?
The Corsair was designed with the gull wings, to accommodate the big propeller (and to shorten the landing gear).
The F6F Hellcats propeller was 13ft 1in, Could 3 inches make much of a difference?

The F6F have had bigger Cd0 (C d zero, coefficient of drag at high speed), IIRC by some 10%, even though it used a thinner wing. Wing was also bigger on the Hellcat.

pbehn · Nov 9, 2017

Top speed is not always the ultimate measure of high performance, it is just one of them. The most successful F1 cars are rarely the fastest through the speed trap.

fastmongrel · Nov 9, 2017

pbehn said:
Top speed is not always the ultimate measure of high performance, it is just one of them. The most successful F1 cars are rarely the fastest through the speed trap.

How often did a WW2 plane operate at maximum speed.

pbehn · Nov 9, 2017

fastmongrel said:
How often did a WW2 plane operate at maximum speed.

For carrier based planes there are many aspects of performance that are important, by 1945 enough was known about aircraft performance for me to say that the Bearcat was as fast as the designers wanted bearing in mind what they wanted in other areas like rate of climb, turn and take off/landing.

tomo pauk · Nov 9, 2017

fastmongrel said:
How often did a WW2 plane operate at maximum speed.

Time-wise rarely.
But always when it was really important & possible.

I don't understand how some planes ended up being so fast

Senior Airman

Creator of Interesting Threads

Colonel

Major General

Banned

1st Sergeant

Colonel

Recruit

Banned

Major General

Major

Major

Airman

Airman 1st Class

Airman

Creator of Interesting Threads

Colonel

1st Sergeant

Colonel

Creator of Interesting Threads

Users who are viewing this thread

Similar threads