P40 Vs all other fighters in Europe
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FLYBOYJ
"THE GREAT GAZOO"
A lot! Most performance charts start at a baseline, sea level 59F 29.92 air pressure. High temperatures and humidity will make the aircraft perform at sea level as if it was at a higher altitude. Here is a simple calculator. At 100F and 95% humidity your plane will perform as if it was at almost 3,000' MSL
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Simon Thomas
Senior Airman
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If you were at sea level, those conditions are equivalent to 3365 ft.
Clayton Magnet
Staff Sergeant
- 903
- Feb 16, 2013
Assuming a sea level field elevation and an altimeter setting of 29.92, the density altitude at 100 degrees F would be just under 3000'
Edit: Sorry, I missed the other replies. Disregard
Ok thanks! Very helpful and interesting. So that means in the SW Pacific the critical altitude for a P-39D drops from around 13,000 to about 10,000 ft. Of course, early P-40s also have the same problem. F4F has much better altitude performance than either army type, still producing ~ 1,000 hp at 20,000 ft (maybe 17,000 in these weather conditions, or does that still apply once you are up in the colder air?). This maybe helps explain why F4F was still so important despite seeming to have much lower top speed. It's still in the game for another 10,000 ft.
With a bit smaller wings, P-39 may not be as stable as some other types in the (effectively) thinner / hot air?
With a bit smaller wings, P-39 may not be as stable as some other types in the (effectively) thinner / hot air?
Thumpalumpacus
Major
The warmer the air, the less the density. The less the density, the less lift can be developed at a given speed. This is a variable that will affect all planes operating in the same airspace in a roughly equal manner. This is also affected by altitude. A warm day in Denver requires a longer takeoff roll than a cold day in Boston.
I get that, my observation was, maybe aircraft with a little bit higher wing-loading and a shorter wingspan might have a bit more trouble in thinner air. Not saying that's it so, just speculating.
FLYBOYJ
"THE GREAT GAZOO"
A little be more complicated than that - you have to consider the wing design, camber, etc. I'm Showing the P-39 had a NACA 0015 wing at the root, a NACA 23009 at the tip, someone like Bill or GregP could probably do the math and tell you how that wing will perform in thinner air
FLYBOYJ
"THE GREAT GAZOO"
Living and flying around the Denver metro area, I could attest to that. The airport where I fly out of is about 5600' MSL, I've seen days when the density altitude was close to 10,000'
FLYBOYJ
"THE GREAT GAZOO"
It's calculated. You start with the chart previously posted and then you use performance charts specific to the aircraft
Instruments on airplanes are funny things. In pilot training we had to do a TOLD (Take Off and Landing Data) problem every flight in the T-38 to figure out all your speeds (max abort, se t/o) plus some others I've no doubt dumped from memory. However, while sitting on the ramp at Roswell (home of the little green men and a instrument approach called the widow maker) my IP told me to dial 2992 into the altimeter (kollsman window) and it will show you what altitude the aircraft "thinks" it's at or, more accurately the pressure altitude of the airfield. Get the temp, do your TOLD, go fly. It's also useful if you think the ATIS (Airport Terminal Information Service) is inaccurate (usually updated once an hour unless conditions change enough). The altimeter can tell you more than just your altitude. In the OV-10 we had a thermometer that stuck out of the canopy, and we would do our TOLD problem just prior to take off. Not sure why, we took off below engine out take off speed(V2) (plane didn't have enough power to get to V2 on the ground), and in the event of an engine out missed it probably didn't have enough power to go around. If you touched down you would not get airborne again on one engine. That airplane (OV-10A) was a performance dud.
I started flying the Eagle in late 91. We didn't use TOLD data. The IPs told us if your nose wheel was off the runway go, if it wasn't you could abort (with one or less bags of gas). If you had 3 bags of gas it was full A/B and use around 190 as your single engine take off speed (SETOS). Fast forward about 10 years and we have two accidents and lack of TOLD data was supposedly a contributing factor. First accident was an ex F-18 guy at New Orleans who brought the gear up and settled back on the runway. That only happens if you lift off slow. Duh. Second was an accident at Nellis where the A/B nozzle gauges were backwards. So the left shows blown out, and per the checklist you pull the left out of burner and continue the take off if able. He tried, and went off the end of the runway hauling ass, punched out and got banged up pretty badly. Come to find out his nozzles where cross wired, and he pulled the good engine out of AB and tried to continue the take off in a heavy jet, high pressure altitude, with neither in AB. TOLD may have helped him make a timely decision to reject.
I got off the beaten path a bit, but just some context on why pressure altitude is important knowledge to have.
Cheers,
Biff
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FLYBOYJ
"THE GREAT GAZOO"
Hey Biff - something funny. Up until a few years ago the old T-41s operated at the Academy had no TOLD data above 10,000'. IPs used to love to fly to Leadville on their "proficiency" flights. A sharp IG inspector caught this.
It's one of those things that after near 40 years of reading about airplanes practically every day, I never really understood this until today. It's quite interesting and I suspect it does help explain why some types did better in tropical environments and some in cold.
When I first read that description of takeoff etc., I misread and thought you were describing the T-38 as a 'performance dud'.
Does the TOLD etc. have the same relevance once you are up high and it's colder ?
When I first read that description of takeoff etc., I misread and thought you were describing the T-38 as a 'performance dud'.
Does the TOLD etc. have the same relevance once you are up high and it's colder ?
Clayton Magnet
Staff Sergeant
- 903
- Feb 16, 2013
EVERY aircraft does better in cold, assuming not too cold. Cold dry air is best for performance, no matter what aircraft you are flying
I get that, but if you have a higher wing-loading and maybe a bit of a propensity to spin, might this not be a bit more of a problem in tropical conditions?
I.e. I am thinking a Hurricane might actually be better than say, an I-16 in say, Egypt in August, while the reverse may be true in December in Leningrad.
Shortround6
Major General
As a very rough estimate the plane that has a higher loading (higher wing loading and higher power loading) will have more "trouble" than the plane with a lower loading.
You are dealing with a "double whammy". The engine is going to make less power at the same manifold pressure and the wing is going to generate less lift for the same airspeed.
I am not a pilot so I may be way off on this.
Say plane A takes off at 80mph and plane B takes off at 100mph on a standard day (59 degrees/standard pressure) and with the temperature, humidity and barometric of the time of day at take off we calculate that we are going to need 10% more speed (lift?) to get the plane up to take-off speed. Plane A now needs 88mph and plane B needs 110mph, small advantage to Plane A BUT both planes are going to accelerate slower due to lower engine power (and the props may have have less bite) so plane B is going to need a longer distance to take off.
The Climb performance is going to be off as plane B needs more power just to stay in the air (Higher stalling speed).
You not only need the calculator for the air conditions you need the manuals or charts for each each plane because they are all not at the same, Hot and High will be higher for each plane but not always the same.
You are dealing with a "double whammy". The engine is going to make less power at the same manifold pressure and the wing is going to generate less lift for the same airspeed.
I am not a pilot so I may be way off on this.
Say plane A takes off at 80mph and plane B takes off at 100mph on a standard day (59 degrees/standard pressure) and with the temperature, humidity and barometric of the time of day at take off we calculate that we are going to need 10% more speed (lift?) to get the plane up to take-off speed. Plane A now needs 88mph and plane B needs 110mph, small advantage to Plane A BUT both planes are going to accelerate slower due to lower engine power (and the props may have have less bite) so plane B is going to need a longer distance to take off.
The Climb performance is going to be off as plane B needs more power just to stay in the air (Higher stalling speed).
You not only need the calculator for the air conditions you need the manuals or charts for each each plane because they are all not at the same, Hot and High will be higher for each plane but not always the same.
Shortround6
Major General
once you are dealing with high speeds you have more variables to come out and play with
Drag goes up with the cube of speed.
Your Lift goes up with the square of the speed.
The engine power (as opposed to propeller efficiency) is a combination of air density (not much change in speed) and the RAM effect of the intake and the pressure ratio of the supercharger on WW II airplanes.
Some of this is somewhat academic.
If your plane will only fly at 330mph true instead of 345mph true at 10,000ft due to hot temperatures you aren't going to know it. You are going to be looking at IAS for airspeed and the airspeed indicator is reading the existing air pressure (density). If the enemy airplane is also by 10-15mph from it's "book" figures it is going to take a long tail chase to to figure out the speed difference.
The speed/climb differences at 80-150mph when taking-off and landing are what can kill you every time you take-off and land.
I have no idea why they figured out the chart this way instead of using 59 Degrees (standard day) but the Army Charts are measured from O degrees C and 32 degrees F. (maybe they figure you know the difference between your boots sliding on ice or splashing through a puddle?). But at just about 92 degrees for the P-38 your take off distance will be 20% higher than the chart shows. North Africa can be worse.
Note that they are only figuring a 10% increase in time to climb compared to the 20% increase in take-off run.
Edit: Screwed up the math, 32 degrees F from 92 degrees is 60 degrees so we need a 30% increase in take-off run and not 20%.
Change in temperature of 60 degrees F is worth about 2,000lbs of take off weight to the P-38 on a clean, dry runway.
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Spitfires were extensively tested by the RAE in regards to the who what and when and their effects speed, aerials, guns, blisters, ejection ports, rear view mirrors, exhaust types, panel fit and over paint condition and fit and finish where all tested and they had no trouble getting a 20-30mph increase in speed from Mk1's through to MkIX's by incorporating all the above.
