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)

[33k in the air]

I dont think there can be any doubt now that you are just trolling. The maximum speed of the P-39N was about 375MPH, its cruising speed at 15,000ft cruising altitude was 250MPH, Your consumption in cruise at 62gph is only marginally less than your combat 25 gals in 20 mins that is 75gph. most engines used circa 3 times the fuel on max power that they used in cruise.

Fuel usage for military power and war emergency power usage is listed in the upper left corner of the charts in the Flight Operating Instruction Manuals.

I don't know how the P-39 Expert missed that, but evidently he did.

For the P-39N-0/1, military power consumed 147 gallons per hour. (It doesn't list a value for war emergency power fuel consumption.) Allotting 15 minutes for military power equals 36.75 gallons consumed.

 

[FLYBOYJ]

Your calculations are incorrect.

(1) The P-39N-0/1 consumed 147 gallons per hour at full military power. The usual rule for combat fuel allowance was 15 minutes at full military power and 5 minutes at war emergency power. The latter isn't listed on the charts, so let's just go with 15 minutes of full military power.

147 / 60 * 15 = 36.75 gallons

The combat fuel allowance is closer to 37 gallons, not the 25 gallons you stated.

(2) At a take-off weight of 8,000 lbs, the P-39N-0/1 consumed a total of 39.7 gallons of fuel to climb to 25,000 according to the climb data section of the Take-off, Climb, and Landing chart.

The fuel consumed in climbing to 25,000 feet is closer to 40 gallons, not the 20 gallons you stated.

(3) Reserve fuel allowance normally used in radius calculations is thirty minutes, not twenty. Using Column V (max. range) for 7,500 to 7,100 lbs weight, fuel consumption per hour is about 35 gallons per hour. Call it 17 gallons for half an hour.

The fuel reserve is 17 gallons, not the 10 gallons you stated.

(4) The combat radius is determined by how far the aircraft can fly back on internal fuel after deducting allowances for warm-up, initial take-off, combat, and reserve (assuming the cruise out can be done mostly on external fuel in drop tanks).

At 120 gallons full internal: 120 - 37 - 17 = 66 gallons left to cruise back to base.

(Note that the internal fuel remaining would actually be slightly less than 66 gallons, since warm-up, take-off, and initial climb are done on internal fuel before switching over to external fuel. But let's use 66 to be generous.)

Using your own figures of 62 gallons per hour at a TAS of 276 MPH for the cruise back yields the following:
66 / 62 * 276 = 293.8 miles


See the relevant charts on pages 26-28 of the PDF found here:
Pilot's Flight Operating Instructions -- P-39N-0 and P-39N-1 Airplanes

I've come up with similar before. Our friend is confusing allowances for reserves and seems to use parts of the charts that will give the aircraft the most range, ignoring the 30 minutes reserve for emergency. Great post!

 

[33k in the air]

I've come up with similar before. Our friend is confusing allowances for reserves and seems to use parts of the charts that will give the aircraft the most range, ignoring the 30 minutes reserve for emergency. Great post!

He put in the reserve allowance, but underestimated it by using 20 minutes instead of 30 minutes. He also apparently has the aircraft carrying the drop tank out and back, which is odd.

The P-39 manual I referenced only has IAS figures for the cruising values, so there's the question of what exactly those convert to in TAS. Later USAAF manuals included a TAS column.

It also doesn't say anything about a 110-gallon drop tank; the only size explicitly mentioned is 75 gallons. From the manuals of other aircraft, the drag caused by a 75-gallon drop tank was evidently the same as for a 500-lb bomb.

 

[Shortround6]

The reason the P-39 burned so little fuel at 25,000 ft is because it made so little power.
It isn't going to come close to making even military power. Figure roughly a 2.5 percent loss of power for every 1,000 ft higher than 15,000ft. Adjust for exact FTH.
The plane didn't have much reserve power above max cruise. It's ability to actually fight ( manuever ) at those altitudes and not just fly straight and level was limited.
If the fight descends lower the P-39 gets more power but the fuel burn goes way up.

 

[FLYBOYJ]

He put in the reserve allowance, but underestimated it by using 20 minutes instead of 30 minutes. He also apparently has the aircraft carrying the drop tank out and back, which is odd.

The P-39 manual I referenced only has IAS figures for the cruising values, so there's the question of what exactly those convert to in TAS. Later USAAF manuals included a TAS column.

I found that depending on the manual there is a IAS to TAS chart based on a what I believe is a lapse rate calculation (true airspeed is an additional roughly 2% higher than indicated airspeed for each 1,000 feet above sea level) which is the most inaccurate way to calculate this.

 

[pbehn]

According to Parks instructions. a good squadron takes up to 4 minutes longer than a test pilot to reach 25,000ft. two squadrons take 10-12% longer than that and three squadrons 15-18% longer. There maybe good reasons why escort squadrons did not have the same experience but I doubt it is hugely different.

When in contact with a bomber formation, the speed is the ground speed of the bombers, escorts can cruise at a speed to conserve fuel or to be fast enough not to be bounced, fuel used is how much is consumed between RV points. Using an individual aircraft performance to compare operational combat radii doesnt work.

 

[GrauGeist]

I really don't understand what there is to debate.

Perhaps if you paid attention to the mountains of information being provided (often given in intricate detail), you would better understand the Bell product that's the subject of "debate".
In regards to a "debate", there really isn't one - rather it's a case of you stating flawed data and others providing correct data.
Then you completely ignore the accurate data and regurgitate the same flawed data or alter it to back an altered claim, which is once again corrected by accurate data (often times from military sources).
Then you proceed to ignore the peer reviewed, manufacturer's, military's and actual pilot's information and proceed to post flawed, skewed, cherry-picked or imaginary information/data over and over and over and over and over and over and over and over and over and over and over...

It's not even close to a debate, it's called an endless loop.

 

[Milosh]

According to Parks instructions. a good squadron takes up to 4 minutes longer than a test pilot to reach 25,000ft. two squadrons take 10-12% longer than that and three squadrons 15-18% longer.

Wasn't that the problem with Bader's Big Wing during the BoB?

 

[Thumpalumpacus]

According to Parks instructions. a good squadron takes up to 4 minutes longer than a test pilot to reach 25,000ft. two squadrons take 10-12% longer than that and three squadrons 15-18% longer. There maybe good reasons why escort squadrons did not have the same experience but I doubt it is hugely different.

When in contact with a bomber formation, the speed is the ground speed of the bombers, escorts can cruise at a speed to conserve fuel or to be fast enough not to be bounced, fuel used is how much is consumed between RV points. Using an individual aircraft performance to compare operational combat radii doesnt work.

[Emphasis added -- Thump]

Forming up takes both time and fuel. I don't understand why our friend here doesn't seem to take that into account.

 

[Milosh]

Formation flying also consumes more fuel as the throttle is always being moved.

 

[33k in the air]

At 120 gallons full internal: 120 - 37 - 17 = 66 gallons left to cruise back to base.

(Note that the internal fuel remaining would actually be slightly less than 66 gallons, since warm-up, take-off, and initial climb are done on internal fuel before switching over to external fuel. But let's use 66 to be generous.)

Using your own figures of 62 gallons per hour at a TAS of 276 MPH for the cruise back yields the following:
66 / 62 * 276 = 293.8 miles

Based on the P-39N-0/1 original internal fuel load of 87 gallons:

87 - 37 - 17 = 33 gallons left for the cruise back to base.

Using the same 62 gallons per hour at a TAS of 276 MPH that P-39 Expert gave:
33 / 62 * 276 = 146.9 miles.

So increasing the internal fuel from 87 to 120 gallons (+33 gallons, which is 198 lbs of weight) results in the radius being doubled.

 

[pbehn]

Wasn't that the problem with Bader's Big Wing during the BoB?

Exactly although the situation is different, his big wing weren't all at the same airfield and took an age to form up. To turn the big wing and keep formation meant very, very slow turns or those on the inside had to almost stall while those on the outside couldn't keep up.

 

[pbehn]

Based on the P-39N-0/1 original internal fuel load of 87 gallons:

87 - 37 - 17 = 33 gallons left for the cruise back to base.

Using the same 62 gallons per hour at a TAS of 276 MPH that P-39 Expert gave:
33 / 62 * 276 = 146.9 miles.

So increasing the internal fuel from 87 to 120 gallons (+33 gallons, which is 198 lbs of weight) results in the radius being doubled.

Groundhoggery is almost as difficult to fathom as Turingery and Banburismus.

 

[pbehn]

Formation flying also consumes more fuel as the throttle is always being moved.

And you have to make a decision if people cant keep up. Oh and you only find out if your plane isnt performing as per the manual when your engine starts sputtering.

 

[Thumpalumpacus]

Formation flying also consumes more fuel as the throttle is always being moved.

That's my non-piloting understanding as well; it's the same reason my truck gets better mileage using cruise-control than it does when I'm throttling through traffic, right? And in my truck, cruise control is near-useless in traffic because I must consistently adjust throttle to "fly" with traffic. It's great on open highway when everyone is doing the same speed. It follows that I get better mileage driving an empty road than stop-and-go through a freeway jam.

How much more-so it must be when working in three dimensions rather than two. Now I have to throttle up or back for altitude as well as flight-speed. And we've yet to talk about the time it takes to formate and fly out, which is another drain on fuel.

It's why you'll never see mpg-performance (measured on an isolation track here in America) matched by real-world conditions.

 

[Milosh]

Book learning (theory) is useless without the practical experience to go with it.

 

[FLYBOYJ]

That's my non-piloting understanding as well; it's the same reason my truck gets better mileage using cruise-control than it does when I'm throttling through traffic, right?

Exactly! I've flown formation in GA aircraft, a T-34 with a Bonanza and Debonair and in an L29 with 3 other aircraft. I found the jet to be the hardest as it took a few moments for the jet to spool up and when once you got momentum it was tricky to slow down (speed brakes). To throw some gasoline into the fire, we haven't even brought up this fact during our range/ fuel calculations, let alone not flying in a straight formation, constantly making shallow turns within the flight while not only making yourself a harder target but scanning for enemy aircraft. Sorry chaps, I've might have added another week to Groundhog Day!

 

[GrauGeist]

I know I've mentioned this before, but a friend of ours was a B-24 pilot and he spoke of the complexities of large bomber formation missions.
It was not as simple as just taking off and heading to the target.
A typical mission start would be to take off and climb to a certain altitude where his group would form up on their Judas Goat.
Meanwhile hundreds of other B-24s were doing this at the same time, circling for a considerable amount of time waiting for everyone to break out of the overcast, get their bearings and assemble.
The climb to altitude and the assembly loiter consumed quite a bit of fuel before they even got underway.

The.escort groups would be going through similar motions: take off, climb to altitude and loiter for assembly - this burns up alot of fuel before the mission even gets underway.

 

[GregP]

From the Manual for P-39N-1-BE. Gross weight at startup: 7,967 lbs. So, use the figures for 8,000 lbs.

1. Total Fuel on Board with 75 gal drop tank: 167 gal.

2. Combat climb to 15,.000 feet: Book says 35.2 gal. used and it takes 6.9 minutes. That leaves 126.8 gal. Books says climb speed is 150 mph. Assume about a 15° climb. So, Y = 15,000 feet and angle = 15°. So, the horizontal distance is about 10.6 miles travelled during the 6.9 minutes of climb.

3. Assume we allow for 10 minutes of combat. Combat would be at 44.5 MAP and 147 gal per hour per 8,000 pound chart, top left. 10 minutes at 147 gph burns 24.5 gallons, leaving 102.3 gallons of fuel. I can't say if the combat takes you closer to home or farther away from home. Assume it does neither.

4. So, I have 102.3 US gallons of fuel. Assume I decide to cruise at 15,000 feet at 2,400 rpm and 208 mph. I burn 70 gph of fuel, so I can only cruise for 102.3/70 = 1.46 hours. 1.46 hours at 208 mph = 303.97 miles. Call it 304 miles. OK, I went 10.6 miles during climb and I can cruise for 304 miles before I am a glider pilot.

5. Then: 304 miles + 10.6 miles = 314.6 miles. Assume I can turn around instantly, so my range is 314.6/2 = 157.3 miles before I run out of fuel.

That doesn't allow for much use attacking a target even 200 miles away in the Pacific Ocean. Or, suppose you only have 5 minutes of combat. That extends the range from 157.3 miles to 175 miles. Not overly useful from a tactical point of view. A range increase from 157 miles to 175 miles offers very little gain for a fighter mission. You could increase your range at 15,000 feet all the way to 193.6 miles, but then you have no fuel for combat if you want to get home. Not too useful if YOU are flying.

Of course, if you are a suicide pilot, you COULD ram something 387 miles away just as you turned into a glider pilot, assuming no combat time at all until you hit what you are ramming.

All of the above assumes I don't have to drop the 75-gallon tank because the combat happens before I used up all the drop tank fuel. If I DO have to drop it, the range decreases proportionately for fuel unavailable above.

I was likely being kind when I called the P-39N a loggerheaded, swag-bellied, hedge pig ... uuhhh ... groundhog.

 

[Vincenzo]

GregP also your calculation are fault, when you stay at 102.3 gals, 75 are in the drop tank but you don't fight with the drop tank in, so you actually radius is with 27.3 gals so counting the 10 miles of climb a bit more of 90 miles total