Groundhog Thread Part Deux - P-39 Fantasy and Fetish - The Never Ending Story (Mods take no responsibility for head against wall injuries sustained)
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Peter Gunn
Master Sergeant
I believe you are correct, also
drgondog
once told me that every 40-50 minutes the Mustangs would rev up to even higher RPM's for several minutes to keep the plugs from fouling.
Yeah - I prefer the fluffy pictures of the P39 in winter camo......
HiI believe you are correct, also
The plugs were one of several problems that arose with the introduction of the P-51B, it appears to have been 'solved' by using British RC5/5 plugs instead of the original ones. There is a good summary of the various problems, and 'solutions', in 'The Mighty Eighth War Manual' by Freeman, pages 195-196, extracts below:
Mike
P-39 Expert
Non-Expert
Not a "stock" P-39N for the Wright Field test? Of course it was, it was taken straight from the production line. We've been over the test weights many times before, the quoted figure represents the average weight of the plane during a flight accounting for fuel burn. Virtually all the Wright Field tests used this method to compute weights for tests for all AAF fighter types. Their test weights were lower than published gross weights.
You're comparing takeoff power at SL and WEP at 9500' and it certainly was available for squadron use.
In comparison with a Spitfire V the normal P-39 held 20% more internal fuel. There is no way on this earth that a Spitfire V had a combat radius of 550mi. Escort fighter bases in England were only 520mi from Berlin. Most escort range charts credit the Spitfire V with a combat radius of 175mi. Barely across the English Channel.
Somebody certainly is "cherry picking" and it's not me.
Where do you get your fuel consumption figures from for cruising at 25,000ft with a 110 gal tank? A 75 gal tank knocks 40 MPH off all cruising figures as per the tests. Could a P-39 actually get to and cruise at 25,000ft with all the fuel you want on it?
drgondog
Major
Roger Freeman did well on the narrative regarding the P-51B in general, but incorrect on several points:
1. Malcolm Hoods were being installed on Mustang III (P-51B-15-NA 'FZ100') which arrived in October but transferred to 9th and 8th AF in December in a Reverse Lend Lease from RAF. Substantial part of kits for Malcolm Hood model for P-51B went to BAD2 Warton, and when more kits were available, were released to Service Groups located around the fighter bases. Priority was to squadron/group leaders but many had filtered to the 'general pool' by May 1944.
2. The 358th FG was Assigned to and began operations with the 8th AF December/January but swapped for 357th FG.
3. Arnold prioritized P-51B/C for 8th AF in August 1943, but Leigh-Mallory leading TAC which included 9th AF took 'exception' to 354th (and succeeding 357th and 363rd and 339th) being assigned in 'any role' not under his command. Took Spaatz/Arnold/Portal to sort out the TDY assignments of 354 and 363 to 8th AF until Invasion preparation at end of May. Also to effect the trade of the P-7D equipped 358FG for the P-51B equipped 357FG ad re-assign the 339FG in return for the P-38 equipped 367FG.
4. The 0.50 cal four gun mount scheme/design for P-51B/C was Exactly the same as the A-36/P-51A.. The jamming problems and solution were as described by Freeman.
5. The engine mount bolt failure was due solely to QA from subcontractor - failure to properly heat treat the finished bolt to high tensile (90,000+ psi) strength. The Brits quickly secured a supply meeting NAA specs, NAA solved the problem and shipped to ETO a backload reserve as the production line was inspected and bolts replaced at the factor. The grounding order was lifted i three days.
6. The notion that a 'few' P-51Bs were sent to 20th and 55th (also 364th) P-38 FGs for conversion in July implies that a 'flood' of P-51Ds were available and that the P-38FGs ere getting 'hand me downs'. No, by Mid July only 300+ P-51D-5-NA had been delivered to ETO, compared to 1200+ operational P-51B/C in ETO. Brand new P-51Bs and Cs were still arriving in August-September 1944. His comment regarding 'small portion' being installed on P-1B/C was technically correct for the number of B/Cs produced - but all Malcolm kits went to ETO based Mustangs - zero for MTO/CBI/CONUS.
P-39 Expert
Non-Expert
Switch to drop tank is a lot earlier, right after gear and flaps up and climb speed has been established. Most of the fuel used on the ground and during takeoff will drain back into the main internal tanks. Most of the 20gal takeoff/climb allowance is used climbing toward the target and is therefore used in the range calculations.
Of course drop tank does not count for combat, the reserve for combat has already been deducted from the fuel total. Calculation is based on using the drop tank during climb and the outbound leg of the mission. The range calculation assumes that the tank remains in place on the return leg while internal fuel is being burned since cruising speed was not increased when the tank was dropped. Pilots were encouraged to bring the tank back if possible if no combat had been encountered. Otherwise pilots would have simply dropped the tank when the fuel was exhausted and the tank could not have been reused. Naturally if combat is encountered before the fuel was exhausted in the drop tank then the tank would be jettisoned and combat radius would be decreased. This is true of any plane carrying drop tanks, not just the P-39. Remember the P-39N cruising speed at 25000' without the drop tank was 350mph at the same power setting (2600rpm normal power) instead of 276mph with the tank. If combat is encountered the drop tank is dropped and the combat reserve (24gal) and the landing reserve (10gal) would be deducted from the 120gal internal fuel leaving 86gal to get home. 86gal divided by 62gph = 1.4hr x 350mph = 485mi, well over the combat radius quoted in the calculation.
Regarding my estimate for fuel burn at combat power vs normal power, you are correct that going from 2600rpm to 3000rpm is about a 15% increase. That's the increase in fuel burn that I used for my estimate. Throttle setting was the same (full), mixture setting was the same (auto rich), plane was well above critical altitude. Only difference was rpm. You are entitled to your estimate, but that doesn't make it correct. Let's say that your estimate is correct at 31gal. vs my estimate of 24gal. Those 7 gallons represent 7 minutes less flying time (7 divided by 62gph) and 31mi at cruising speed (7 min x 276mph). That reduced combat radius by 16 whole miles. Hardly relevant at all.
Compared to a P-38G the P-39N climbed a good bit better up to 25000' and was about the same speed up to 20000'. P-39N was well matched with contemporary 109s and 190s they would have encountered in early 1943. And nothing climbed with a Spitfire IX. Nothing.
Shortround6
Major General
Nope,
At least at a speed that would be considered useful.
The escort fighters cruised at whatever speed, (within reason) that would get the job done.
about 315mph true was what the nominal (chart) radius figures was based on. Does not include the S turns/weaving.
Some missions they may have cruised slower.
However the escorts did have to cruise at a high enough speed so they could quickly respond to the German attackers.
Cruise too slow and the escorts took too long to get up to a speed useful for combat.
Here is where the P-39 is really going to suck.
There are a couple of charts at the back of AHT that compare the acceleration (calculated) of the 8 major American fighters. The calculations are for 250mph starting speed and for sea level. One chart/table is for early versions using military power and the other is for late versions using combat power. The P-39 is in 4th place on both tables and in the early table is not behind the leaders by very much, only about 10% behind the champ (P-38F) of the early planes.
However that is comparing a P-38F with two 1150 hp engines to a P-39D with one 1150 hp engine.
At 25,000ft the P-38 still has at least 1000hp per engine while the P-39 is done near 700HP?
The later chart show things much wider apart.
While the P-38L is now about 15% better than the 1420hp P-39Q-1 it's two 1600hp engines let it accelerate about 80% faster than earlier P-38F.
this is at sea level.
At 25,000ft the P-38L still has two 1600hp engines. The P-39 has an under 800hp engine, in fact nearer to 750hp. What is it's acceleration now?
The P-47s have 1900 hp engines at 25,000ft in 1943, they get better later.
The Merlins in the P-51Bs have 1410hp available in the mid 20,000ft range. Granted they are not available in in 1943.
The P-39N could eventually get up to a useful speed but by then the attackers would be gone.
It is not fast enough even at top speed (at 25,000ft) and once it does anything except fly straight and level either speed falls off or the plane looses altitude.
The expert has made much of the P-39s climb rate. However max climb rate is done at 216mph at 25,000ft.
Test of a P-47D in Aug 1943 show 1900hp available for climb at 25,000ft. Best climb speed is 246mph.
Once both planes stop climbing the P-47 is flying about 30mph faster and despite it's weight and drag (which is not double that of the P-39) it's 2 1/2 times greater power is going to allow it better acceleration. The P-47 is about 30-40mph faster at top speed.
IF both planes are doing 370mph at 25,000ft the P-39 is pretty much maxed out (assuming the test results reflect real world conditions) while the P-47 has the extra power needed to go 30-40mph faster in reserve to help it climb slowly or to compensate for a slight bank/gentile turn.
Shortround6
Major General
This works ONLY if your escort fighters arrive over the bomber stream and in enemy territory strung out in a line in the order they took off. If you actually want to form up into flights or squadrons it does not work. The AAC planners did not count the climb for range calculations, you have been told this many times.
Which really sucks if you are the fighter pilot/s you just had their radius decreased in mid flight. That translates into POW.
It's wrong unless you can repeal the laws of physics.
As you pile on internal and external fuel and go higher, this mythical beast becomes more economical and faster, even my dear old grandmother would raise an eye brow at the proposition, it sounds like perpetual motion.
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At 25,000ft there is no difference between the different types of power, because the engine could not develop enough power to trouble the bearings or any other component. This can be seen in the tests.
FLYBOYJ
"THE GREAT GAZOO"
Hmmm - "combat power vs normal power"
Word search no such thing...
"Military Power/ Takeoff Power" found in manual.
Word search no such thing...
"Military Power/ Takeoff Power" found in manual.
drgondog
Major
No. Test weights stated are Always GW before takeoff.
Conditions including taped gun ports, removal of sway braces, external tank/no tank. The Test weights Were mostly conducted Under combat weights - and as such Optimistic'. Major Price was the Wright Field Bell project officer and notorious for producing 'optimistic' reports. I draw your attention to comparisons of RAF test results to Wright and Bell tests.
P-39 Performance Tests
Looking at Op manual for P-39Q and 75 gal external tank, a couple of entries stand out, beginning on pg 24:
GW for stated FERRY combat load (full internal load of fuel plus 75 gal tank but ZERO 50 cal ammo) = 8,100 (of which the wt of tank/fuel & sway brace =800). Not stated but with full ammo, 330 pounds (1000 rnd), The P-39Q could only take off with 75gal external if 55 less gallons of fuel (or ammo) carried - in which case - leave the 75 gal tank at home.
All Ranges stated are Ferry type profiles, with Maximum range =1000 miles for P-39Q-1 at 8100 GW.
Data above 15000 feet are left 'blank', save Condition II -->20K cruise settings of 30"MP, 2600rpm and IAS of 179mph/TAS and 76 gpm for Cruise.
The Optimal Cruise for distance is at 14,000 feet at 32"MP, 2280RPM and 74mpg (Clean)
There are no service ceilings published for 75 gal tank, nor is any data provided above 20K. Any proof that escort of any type was possible at 25K??
Note also that the fuel allowance for warm up, takeoff and climb to 20K consumes 42 gal.
In the escort role, there is no provision for forming up, nor consideration that when the P-39 actually reached 20K, that the TAS per the tables with 75 gal tank is 179IAS/270mph TAS was adequate to support US Heavies at 20K. - Well above FTH and Extremely sluggish to throttle up airspeed to deflect attacks. About the same speed as a fully loaded B-24 at 20,000 feet (cruise =170 to 180mph IAS).
The worst part of this ad-nauseum rendition of the 'super escort' P-39 (any production version) is that you never think to compare Wright Field/Bell test data with RAF test data - folks that REALLY needed a suitable fighter
FLYBOYJ
"THE GREAT GAZOO"
For the sake of discussion here are the charts. Colors were added by me.
LET THE GROUNDHOGGERY BEGIN!
LET THE GROUNDHOGGERY BEGIN!
FLYBOYJ
"THE GREAT GAZOO"
Here are the charts for the P-39Q
Peter Gunn
Master Sergeant
Are you sure it isn't GROUNDHUGGERY? We are talking P-39 here...
Don't give me a P-39
The one with the engine behind
Peter Gunn
Master Sergeant
There are a few problems with this statement. First of all, we KNOW that the evil Brits were conspiring to kill the P-39 what with their demands to add weight to it for no apparent reason (just for starters), so data from RAF not trustworthy.
Second, it's doubtful that the evil Brit RAF were using anything other than just "Joe Pilots" to run their P-39 program to heighten the sabotage of the Bell super plane, so again, data not trustworthy.
My God man, have you learned NOTHING in this thread?
Peter Gunn
Master Sergeant
Yeeeaaaaah...
I still don't think you get the whole "range" and "mission planning" thing let alone the mission profile for deep penetration escort.
In flight test reports here P-39 Performance Tests there is Normal rated power, Military power and War emergency power and the critical altitudes these were at, for level speeds and climb, this sort of stuff isnt my "bag" which is why I was asking where he got his info. I am pleased to see from other posts the laws of physics also apply to the P-39.
I would also say I have noticed in that test, they are very specific about the propeller used, and in the list of variants (below) there seems to have been many different ones, different numbers, different diameters even a 4 blade type. Bell P-39 Airacobra - U.S.A.A.F. Resource Center - A Warbirds Resource Group Site In the tests mentioned they quote different design numbers, diameter and even the blade angle range in one case. There is absolutely no way a test pilot knows what the implication of this is unless he is told by someone who does.
Bell P-39 Airacobra - U.S.A.A.F. Resource Center - A Warbirds Resource Group Site
This page deals with various technical aspects of the Bell P-39 Airacobra.
