Groundhog Thread Part Deux - P-39 Fantasy and Fetish - The Never Ending Story (Mods take no responsibility for head against wall injuries sustained) (1 Viewer)

[Shortround6]

A P-40F at 25,000ft had almost 10% more power when climbing than the P-39N and that is the P-40F running at 2850rpm and the P39N using 3000rpm.

Nobody was suggesting using the the P-40F as an escort fighter.

See this report for P-40 numbers.

http://www.wwiiaircraftperformance.org/P-40/P-40F_41-13635_FS-M-19-1578-A.pdf

 

[FLYBOYJ]

A P-39 that just dropped it's tank was 10mph slower at all engine settings than a P-39 that never had one fitted, stated in the frequently quoted tests)

And that probably due to the installation of the braces to hold the tank

 

[pbehn]

And that probably due to the installation of the braces to hold the tank

It is specifically tested, without shackles, with shackles and with shackles and tank full (tank is 75gal type) 10MPH is a ball park figure but all figures are around that, and probably within experimental error. Tests at 13,000ft.

	Belly Tank Brackets & Shackles Removed MPH​	Belly Tank Removed Shackles & Brackets in Place MPH​	Belly Tank (Full) Shackles & Brackets in Place MPH​	RPM​	Chart BHP​	Throttle Position​	Mixture setting​
	358​	-​	-​	3000​	1170​	W.O.​	A.R.​
	336.5​	328​	-​	2600​	974​	W.O.​	A.R.​
	-​	349​	-​	3000​	1165​	W.O.​	A.R.​
	-​	-​	311.5​	3000​	1145​	W.O.​	A.R.​
	-​	-​	294.5​	2600​	967​	W.O.​	A.R.​
	321.5​	312.5​	282​	2280​	850​	Part​	A.L.​
	301​	292​	264​	2200​	700​	Part​	A.L.​
	276​	267​	241​	2100​	550​	Part​	A.L.​
	257​	247​	221​	1900​	450​	Part​	A.L.​
	222​	212​	184​	1700​	330​	Part​	A.L.​

 

[GrauGeist]

Even the Allison powered Mustang was at a disadvantage at higher altitudes (also why it wasn't used as escort) much like the Fw190, who like the Merlin powered Mustangs, required an entirely different engine in order to be effective at typical altitudes used by bombers.

 

[FLYBOYJ]

Some observations:

I've been going through the P-39N and Q manuals and been finding some "hokey" things in both (Imagine that). In the performance charts I can easily see how someone with little or no flight or aviation experience can be misled with some of the data, especially if you have conflicting information.

First, flight planning 101 - you use BOTH climb and cruise charts, end of story! The comment on the Flight Operation Instruction Sheet about using 20 gallons for fuel for taxi, warm-up and climb to 5000' is actually wrong and should be removed as if conflicts with the climb data sheet on P 26. Now if you look at the flight tests on www.wwiiaircraftperformance.org it is showing similar climb times that is shown in the flight manual at combat setting, the big difference is during the flight test very aggressive Vx speeds were used. Again the climb times to 5000' in the flight manual are very similar to the flight tests, but using 20 gallons as a "one size fits all" does not really work although it is noted in the Flight Operation Instruction Sheet that "20 USG not available for flight." That fuel got you to 5000' and in some cases that "20 gallons" could be as high as 26.8 depending on climb and weight.

From 5000' to what ever altitude you intend to cruise at you have to chose a climb setting and speed (Vx or Vy) and you are going to burn fuel getting there unless you have some Pixie Dust, Unicorn Poop, Groundhog mystical incantations! That has to be deducted once you get to your cruise altitude. After that you can start allocating for higher power settings that will be used in combat (columns II to V of the Flight Operation Instruction Sheet). I don't think you can ever accurately predict what going to happen in combat so this is the most difficult thing to allocate for. In another thread Bill described a mission that his dad participated in and how the flight plan continually changed. So you get out a pencil (when you can) and start some in flight planning!

After this you can probably chose an economical altitude and speed to return home but note in our infamous Flight Operation Instruction Sheet it states clearly "NO RESERVE FUEL ALLOWANCE" so you're going to have to calculate at least 30 minutes (I would put 45 in there if I was flying over water returning to a land base).

Again - it was clearly pointed out by several others some of the errors in these flight manuals but it's funny how through-out this long and tedious ordeal GregP , ShortRound and I have come up with similar calculations. I would guess that our combined aviation experience is well over 100 years!!!!!

OK I'm done

GregP, ShortRound and myself paying respects

 

[pbehn]

Methinks you are cherry-picking again, trying desperately to use the absolute lightest P-39 you can find, with the highest internal fuel tankage you can find, and then use the highest speed and climb numbers you can find along with the longest range you can find.

I think those speed and climb numbers were obtained by using the same plane but with different props.

 

[33k in the air]

The comment on the Flight Operation Instruction Sheet about using 20 gallons for fuel for taxi, warm-up and climb to 5000' is actually wrong and should be removed as if conflicts with the climb data sheet on P 26.

I've seen other manuals for other aircraft make the same sort of understatement. It isn't limited to the P-39. I presume the lower value is just a rough figure used to ease the math for simple missions.

 

[FLYBOYJ]

I've seen other manuals for other aircraft make the same sort of understatement. It isn't limited to the P-39. I presume the lower value is just a rough figure used to ease the math for simple missions.

Exactly! But in the case of our iron bird could mean the difference of making it home or becoming part of the jungle.

 

[FLYBOYJ]

I've seen other manuals for other aircraft make the same sort of understatement. It isn't limited to the P-39. I presume the lower value is just a rough figure used to ease the math for simple missions.

Just looked in my P-38 manual, it allocates 60 gallons for this.

 

[P-39 Expert]

The "expert" doesn't seem to grasp that the drop tank is immaterial.

Hang a 110 gallon tank on the P-39 and you might wind up in place you can't get home from.

You are down 10-15 gallons of internal fuel after warm up, take off and climb to safe height to switch tanks.

Now what is critical is how much fuel is remaining in the the internal tanks after 15-20 minutes of combat and allowing for a 30 minute reserve.

That is the fuel available for egress from the combat area.

what was in the drop tank is smoke and mirrors.

Give him the 120 gallon fuel capacity, he is down to 105-110 when the tank (of whatever size ) is punched off. The P-39 with a tank attached is just a target for the Luftwaffe fighters.
30 minute reserve to get home is around 15 gallons. Down to 90-95 gallons.

Now the much debated combat allowance.
If the P-39s can defend the bombers by flying straight and level for 20 minutes they just might squeak out just over 30 gallons of fuel used in "combat".

If they have to do anything else, like turn, then the bets are off and the P-39s will not be able to stay at 25,000ft. The lower it goes the more fuel per minute it burns.

Unfortunately some of the P-39 charts are sheer nonsense. Both the take-off an climb charts and the range charts.

However the P-39 and other aircraft "fight operation instruction chart/s" do have a phrase/sentence that the "expert" has completely over looked.

In the box near the top that says "instructions for using chart" when you get to the notes, note "A" says...

"AVOID CONTINOUS CRUISING IN COLUMN 1 EXCEPT IN EMERGENCY"

The "expert's" entire flight plan except for landing is based around continuous cruise in Column 1 conditions.

Okay, let's use your numbers. I don't agree with them but let's use them. 110gal internal when drop tank is jettisoned. Less your combat reserve 35gal and your 30 minute landing reserve 15gal leaves 60gallons to get home. Burning 62gph = .97hrs x 350mphTAS = 340mi. From the P-39N manual.

Now if things get tight climb to 30000' and cruise at 305mphTAS burning 48gph. Same 60gal divided by 48gph = 1.25hr x 305mphTAS = 381mi. These are your numbers straight from the P-39N manual.

And regarding cruising at 2600rpm (normal or max. continuous power) the engine chart says "UNLIMITED".

 

[P-39 Expert]

I checked my P-39N-0 and N-1 manual. It says 87 gallons internal, too. Then I checked my P-39K manual. It says 104 gallons internal + 16 gallons internal overload, for 120 gallons.

Let's see. They built 9,558 to 10,092 P-39 Airacobras. The P-39N accounted for 2,095 of them. The P-39Q accounted for 4,905 of them. That's 7,000 P-39s in the Q and N models alone, or 73% of them if you use the 9,558 build total.

They built 210 P-39K models. That's 2.2%. Not exactly the "mass-production model," is it?

The vast majority of P-39s that got sent anywhere had 87 gallons of internal fuel and about 2% of them held 120 gallons internal fuel if you look only at the K models. I decline to look any farther into the other 25% of P-39's. Feel free. But, and here's the thing to notice, the 87-gallon models were built AFTER the P-39K's were built.

Maybe they found out that the 120 gallons of internal fuel were just too much for the airplane to fighter well with? Whatever the reason, the later P-39s did not hold 120 gallons.

Methinks you are cherry-picking again, trying desperately to use the absolute lightest P-39 you can find, with the highest internal fuel tankage you can find, and then use the highest speed and climb numbers you can find along with the longest range you can find.

The things is, these things above did not happen on the same model P-39. You need to choose a single model, accept the weight, accept the fuel specified, and accept the numbers that aren't in column 1 of the cruise charts. You also need to accept that the P-39 didn't fight very much at 25,000 feet. It was in its element at 12,000 feet and below. Maybe 15,000 at the highest. It was mainly seen within 175 miles from the departure point.

But, I'm pretty sure you will ignore this and continue to claim 120 gallons of internal fuel. After all, it's in the simulator, right?

See Shortround's post #3079. If the AAF HAD decided to use the P-39 for escort work then they would have certainly used all available internal tankage. And probably 30gal fuel for the wing guns.

 

[P-39 Expert]

A P-40F at 25,000ft had almost 10% more power when climbing than the P-39N and that is the P-40F running at 2850rpm and the P39N using 3000rpm.

Nobody was suggesting using the the P-40F as an escort fighter.

See this report for P-40 numbers.

http://www.wwiiaircraftperformance.org/P-40/P-40F_41-13635_FS-M-19-1578-A.pdf

Yes and a top speed of only 369.5mph and climb of only 2185fom at only 18000'. Escort was at 25000'+.

 

[P-39 Expert]

It is specifically tested, without shackles, with shackles and with shackles and tank full (tank is 75gal type) 10MPH is a ball park figure but all figures are around that, and probably within experimental error. Tests at 13,000ft.

	Belly Tank Brackets & Shackles Removed MPH​	Belly Tank Removed Shackles & Brackets in Place MPH​	Belly Tank (Full) Shackles & Brackets in Place MPH​	RPM​	Chart BHP​	Throttle Position​	Mixture setting​
	358​	-​	-​	3000​	1170​	W.O.​	A.R.​
	336.5​	328​	-​	2600​	974​	W.O.​	A.R.​
	-​	349​	-​	3000​	1165​	W.O.​	A.R.​
	-​	-​	311.5​	3000​	1145​	W.O.​	A.R.​
	-​	-​	294.5​	2600​	967​	W.O.​	A.R.​
	321.5​	312.5​	282​	2280​	850​	Part​	A.L.​
	301​	292​	264​	2200​	700​	Part​	A.L.​
	276​	267​	241​	2100​	550​	Part​	A.L.​
	257​	247​	221​	1900​	450​	Part​	A.L.​
	222​	212​	184​	1700​	330​	Part​	A.L.​

This is for a 1941 P-39D loaded to 7800lbs. Escort would have been in 1943 with a P-39N.

 

[33k in the air]

Just looked in my P-38 manual, it allocates 60 gallons for this.

Yes, but what fuel consumption is given in the Climb, Take-Off, and Landing chart for a climb to the same altitude at that weight as to the allowance stated on the cruise/range charts? As I recall the allowance listed there is always less than what the climb chart shows.

 

[pbehn]

This is for a 1941 P-39D loaded to 7800lbs. Escort would have been in 1943 with a P-39N.

Did shackles stop creating drag around 1942? Your P-39 with full tanks and external shackles cruises at 25,000 ft just a few miles an hour slower than its absolute top speed and using only 62 g/hr. It is utter nonsense, as is the idea of climbing higher to save fuel.

 

[Peter Gunn]

Pick any of the four (4) responses by @P-39 Expert above and:

 

[Glider]

The "D"s all had 120 gallons, the K & L & M all had 120 gallons.

The Q-1 had 87 gallons.
The Q -5 had 110 gallons (?)
The Q-10/-15/-20/-30 all had 120 gallons.

120 gallons was quite doable. (leave out the wing guns for weight compensation )

However the whole thing is based off the "N" and one extraordinarily good test and a one or two strange numbers in the flight charts.

A P-39 with a drop tank, no matter what model, is not fast enough at altitude to do escort work. The P-38 and P-47 could cruise at over 300mph at 25,000ft with drop tank/s attached and not use column 1 on the charts.
A P-39 without drop tank, no matter what model, is 20-40mph slower than the P-38s and P-47s of mid 1943 and slower than the 109s. at the altitude the bombers are flying at.
A P-39, no matter what model, at 20,000ft and above has the worst power to weight ratio of the available allied fighters and of the defending fighters.

the P-39 is a low drag airplane and can reach a pretty good speed if given enough time. But it can't bleed off speed in maneuvers and get it back quick.
P-39 as a bomber escort for B-17s would be of less use than the Bf 110 was to the Germans in 1940.

Heck, stick a 25imp gallon tank in the back of a Spitfire IX and a 90 gallon tank under it and you would have a better escort fighter than a P-39.

They did wind up sticking a 41 gallon and a 33 gallon tank in the back of Spitfire IX so it was possible. Maybe not a good idea but possible.

The attached may be of interest. It includes the long range of the Spit IX when fitted with internal extra tanks and the drop tanks. Its also noting at the bottom that these are still air figures but as a rule of thumb an average of 75% of the still air range is considered to be the operational range.

No one is pretending that the Spit was equal to the P51, the modified versions had quite a respectable range and the Tempest were pretty good. Both of these I believe comfortably exceed the P39.

 

[FLYBOYJ]

Yes, but what fuel consumption is given in the Climb, Take-Off, and Landing chart for a climb to the same altitude at that weight as to the allowance stated on the cruise/range charts? As I recall the allowance listed there is always less than what the climb chart shows.

On the climb chart for a P-38H Tank Supports Only "Allow 50 (60 for 2 165 gallon tanks) gallons for warm up, take off and an initial climb plus allowance for winds, reserve & combat as required."

The climb data chart starts fuel consumption at 10,000' Depending on weight and Vx/ Vy my manual shows between 60 and 70 gallons

 

[P-39 Expert]

Some observations:

I've been going through the P-39N and Q manuals and been finding some "hokey" things in both (Imagine that). In the performance charts I can easily see how someone with little or no flight or aviation experience can be misled with some of the data, especially if you have conflicting information.

First, flight planning 101 - you use BOTH climb and cruise charts, end of story! The comment on the Flight Operation Instruction Sheet about using 20 gallons for fuel for taxi, warm-up and climb to 5000' is actually wrong and should be removed as if conflicts with the climb data sheet on P 26. Now if you look at the flight tests on www.wwiiaircraftperformance.org it is showing similar climb times that is shown in the flight manual at combat setting, the big difference is during the flight test very aggressive Vx speeds were used. Again the climb times to 5000' in the flight manual are very similar to the flight tests, but using 20 gallons as a "one size fits all" does not really work although it is noted in the Flight Operation Instruction Sheet that "20 USG not available for flight." That fuel got you to 5000' and in some cases that "20 gallons" could be as high as 26.8 depending on climb and weight.

The 20gal on the Flight Operations Instruction Chart is correct. It is an allowance. And it assumes climb to higher altitude starts at 5000' having traveled zero miles. This is being conservative since at least as soon as gear/flaps are up and climb speed is attained after form up the pilot will vector toward target. Then another 4.5min climbing to 5000' toward the target means an extra 13mi not included in calculations.

From 5000' to what ever altitude you intend to cruise at you have to chose a climb setting and speed (Vx or Vy) and you are going to burn fuel getting there unless you have some Pixie Dust, Unicorn Poop, Groundhog mystical incantations! That has to be deducted once you get to your cruise altitude. After that you can start allocating for higher power settings that will be used in combat (columns II to V of the flight manual). I don't think you can ever accurately predict what going to happen in combat so this is the most difficult thing to allocate for. In another thread Bill described a mission that his dad participated in and how the flight plan continually changed. So you get out a pencil (when you can) and start some in flight planning!

Yes fuel will be burned climbing from 5000' to 25000' but 73mi range will be gained by climbing at 170mphIAS (220mphTAS average) for 20minutes in the direction of the target. One more time, this has all been factored into the range calculations for every altitude and every power setting. Take any amount of fuel available at any altitude at any power setting (RPM/manifold pressure) and do the math. Available fuel divided by gallons per hour x TAS is always more than the range figure quoted. The very smart AAF officers who put together these charts accounted for the fuel burned/miles gained by climbing from 5000' to whatever altitude is chosen. They did this to lessen pilot workload and get all the range information on one chart. You don't need the Takeoff, Climb and Landing Chart for any range calculations.

View attachment 633243


After this you can probably chose an economical altitude and speed to return home but note in our infamous Flight Operation Instruction Sheet it states clearly "NO RESERVE FUEL ALLOWANCE" so you're going to have to calculate at least 30 minutes (I would put 45 in there if I was flying over water returning to a land base).

But I thought that previously you couldn't find any mention of any reserve for landing anywhere. Now we need 30minutes (you would use 45) to find our home field. That hasn't moved. I have always heard that the AAF used 20min.

Again - it was clearly pointed out by several others some of the errors in these flight manuals but it's funny how through-out this long and tedious ordeal GregP , ShortRound and I have come up with similar calculations. I would guess that our combined aviation experience is well over 100 years!!!!!

OK I'm done

Or maybe you have been in your pixie dust, unicorn poop or mystical incantations again. I just use the appropriate flight manual.

View attachment 633242
GregP, ShortRound and myself paying respects

 

[P-39 Expert]

On the climb chart for a P-38H Tank Supports Only "Allow 50 (60 for 2 165 gallon tanks) gallons for warm up, take off and an initial climb plus allowance for winds, reserve & combat as required."

The climb data chart starts fuel consumption at 10,000' Depending on weight and Vx/ Vy my manual shows between 60 and 70 gallons

Yes, the P-38 was a twin meaning that two engines must be started (separately) and two sets of propeller and mag checks must be undertaken (separately).