XP-39 II - The Groundhog Day Thread

[Shortround6]

from an old thread with some edits, edits are in italics.

1, no test report the XP-39 did fly 390mph.
2. reports of continued oil overheating and/or high temperatures causing at least one flight to be cut short.
3, At least one report of high coolant temperatures.
4, reports or accounts of modifications done to oil and radiator ducts. Which did not solve the problem
5, accounts of how the AAC wanted a new drive shaft installed in the plane because of vibration problems or worries if the engine misfired. Until new drive shaft was fitted engine rpm was restricted to either 2600or 2700rpm. The New drive shaft was not fitted until after the plane went to Langley. No full power flights?
6 time available.
a, first flight April 6th, 15/20 minutes.
b 2nd flight April 7th cut short due to oil temp
c. 3rd and 4th flights on April 22 total 47 minutes.
d 5th flight April 23, Nose wheel fails after manual lowering of landing gear, 1 hour 40 minutes total flying time in 5 flights.
E. Taking out the 1st flight and 3rd & 4th flights we have either 33 or 38 minutes to split between flights 2 and 5. While I have not seen how long flight 5 was accounts say they spent at least some time trying to get the landing gear down.
F. for the accounts that claim the XP-39 did 390mph on it's first flight, consider that the flight, depending on who wrote the account was either 15 or 20 minutes. In that short space of time our intrepid test pilot (James Taylor) would have had to take off, get the landing gear up, climb to 20,000ft, perform the speed run, descend (dive?) back down to ground level, lower flaps and landing gear, line up on the run way and land the plane. Taylor would have had to do this in plane that had never been flown before although taxi tests up to 80mph had been performed several months earlier.
G. Birch Mathew's book 'Cobra' says
"Taylor climbed into the diminutive interceptor, warmed the engine and lifted of uneventfully for a 20 minute flight on 6 April 1939. The only difficulty he experienced was an elevated oil temperature. The next day Taylor demonstrated the airplane for General Hap Arnold. He was forced to cut the flight short because once again oil temperatures shot up causing oil pressure to plummet."
No mention of a high speed run in this account.
e. accounts do not say what flying or tests were done in May of 1939. could have been some.
f. Plane is delivered to Langley on June 6th. After arrangements made at the end of April.
7, Langley claimed that the oil cooling problem was resolved or minimized during initial flight tests by using a higher drag duct than originally fitted.

So, if the XP-39 did do 390mph or anywhere near to it it had to be done in May, (repeating the above none of the April flights were long enough to get to 20,000ft, do a high speed run and get back down on the ground again.) with a high drag oil cooler duct, a radiator that had problems (like at 350mph the radiator was supposed to have 16,900 cubic ft of air going through but only 10,250 cu ft was needed to cool the engine) and the engine was limited to 90% or less of rated rpm, unless the pilot disobeyed instructions.

 

[GregP]

I still have a book from 1946 that says 390 mph. "U.S.Army Aircraft 1908 - 1946 SC - AEF - AAS - AAC - AAF," by James C. Fahey, published and distributed by SHIPS and AIRCRAFT, P.O. Box 48, Falls Church, Virginia. As you pointed out, Shortround, we do not have flight test data to back it up. But, and here's the rub .... all of the OTHER data I have checked against available period flight test reports matches said tests. That makes me wonder where they got the top speed. We all know it had to come from somewhere; it wasn't just "made up" as fake news.

I like your summary above. I believe that many of us doubt the 390 mph datum is accurate. But, not finding the test data doesn't mean it wasn't generated accurately in a flight test report that has been lost, or at least undiscovered, up to now.

But, let's remember that the P-39Q had a top speed of 385 mph at 8,300 pounds with 1,200 HP. 6.92 pounds per HP. The XP-39 weighed in WITH a turbocharger, at 6,204 pounds and had 1,150 HP. 5.39 pounds per HP. So, the XP-39 is down 50 HP and lighter by 2,100 pounds, which might be a good recipe for another 5 mph due to the power-to-weight ratio alone (22% better). We might not want to believe, but it is entirely possible it DID go 390 mph in some test we have not yet found.

I don't think so and you don't think so, but I suppose we COULD be wrong ... who knows for sure? And that missing test may yet surface at some point. I'll say that I'm with you in thinking it did NOT go 390 mph ... but ... hey, maybe.

 

[Mad Dog]

......So, if the XP-39 did do 390mph or anywhere near to it it had to be done in May, (repeating the above none of the April flights were long enough to get to 20,000ft, do a high speed run and get back down on the ground again.) with a high drag oil cooler duct, a radiator that had problems (like at 350mph the radiator was supposed to have 16,900 cubic ft of air going through but only 10,250 cu ft was needed to cool the engine) and the engine was limited to 90% or less of rated rpm, unless the pilot disobeyed instructions.

Yes, but you're forgetting the XP-39 had no guns, no armour and virtually no equipment for those first few flights. It weighed less than 4000Lbs empty, which is one reason why it climbed to 20,000ft in five minutes. So if Bell had decided to make a carefully planned publicity speed run, twenty minutes might give time for the extra-lightweight prototype to climb to 20,000ft and make the run, and 47 minutes would be plenty. And the XP-39B without the turbo and weighing over 300Lbs more still managed 375mph in the thicker air at 15,000ft. So, no, you have not conclusively proven the XP-39 didn't fly at 390mph.

 

[Shortround6]

This rather depends on almost suicidal test pilots.
What other aircraft in all of WW II established record setting climbs to altitude and did high speed runs in the first two hours of testing?

Let alone the first 20 minutes?

There is no record of the plane making that fantastic climb to 20,000ft. like on what day?

There was no 47 minute flight, two flights in one day equeled 47 minutes.
The plane was plagued with overheating issues at this time, mostly oil, and they spent 15 days trying to sort that out between flights 2 and 3? and yet we are supposed to believe that they hammered the crap out of the engine on the first flight to set all these marvelous performance records?

Bell was lying like 2020 politician (pick one from either/any side) back in 1939. The XP-39 was supposed to weigh 5550lbs gross, it didn't, it was weighed at Wright field at 6,104lbs. Unless they added over 500lbs to the useful load that means the Bell was off by 500lbs on the empty weight Bell had admitted before hand that the gross weight had climbed to 5,855lbs before the plane was completed.

NACA figured that the XP-39 was almost a flying parachute when they got it. The oil cooling problem had NOT been fixed and was causing even more drag than the original set up, the radiator was a disaster, the intercooler was only good for about 25% down to 12% of the cooling needed, and nobody has mentioned the fact that the engine was not allowed to run at full rpm/power on the days in question.

See Bell's lies to the British about performance and the special aircraft needed to even get to 3% under the contract performance to keep the contract going.

 

[overbeck]

And the XP-39B without the turbo and weighing over 300Lbs more still managed 375mph in the thicker air at 15,000ft. So, no, you have not conclusively proven the XP-39 didn't fly at 390mph.

If anything this argues against the 390 mph figure. Removing the turbo also deleted the large ducts for intercooler and turbine oil cooler air, which was the main reason for the change. An extra 300 lbs would have a very small effect on top speed. Induced drag doesn't change that much at moderate angles of attack.

 

[Mad Dog]

...What other aircraft in all of WW II established record setting climbs to altitude and did high speed runs in the first two hours of testing?....

Which other manufacturer was as desperate as Bell to get an order to avoid going bust? Which then extends to how desperate a test pilot might be to keep his job that he'd agree to make such a flight. And then you have bravado and just plain guts. During the second flight of the Hawker Typhoon prototype, chief test pilot Philip Lucas noticed the fuselage had started to suffer a structural failure, with a tear large enough to push his hand through opening in one side. Lucas stubbornly refused to bailout, the accepted procedure in such a situation, and saved the prototype. Whilst he was awarded the George Medal for his bravery, he admitted in later years that one possible cause of the failure was some flight manouveres "not in the official test plan".

....See Bell's lies to the British about performance and the special aircraft needed to even get to 3% under the contract performance to keep the contract going.

The British would have rejected the whole order if Bell hadn't have been able to meet the "within 3% of 400mph" requirement. As I already mentioned, the British Direct Purchase Commission to the US weren't exactly the best and brightest, so I'm not surprised they signed some bad contracts. As it was, they were under pressure to buy just about anything. The P-40 snuck under the barrier because they arrived reasonably early and were judged suitable for the Middle East. By the time the P-400s arrived in the UK, we had loads of Spitfire Vs and the Hawker Typhoon was supposed to soon be sweeping the Luftwaffe from the sky. The Bf109F meant nothing without good altitude performance was going to be used as a fighter in the UK. The Desert requirement was already being met by the mediocre but trusted Tomahawk, and a "much-improved" Kittyhawk was being promised by an equally dishonest Curtis, leaving no role for the troublesome P-400s. Not surprisingly, the RAF jumped at the chance to offload the P-400s to the Russians.

 

[GregP]

Good thing you stayed with a boring thread title that doesn't generate comments, huh, Shortround?

 

[wuzak]

The British would have rejected the whole order if Bell hadn't have been able to meet the "within 3% of 400mph" requirement. As I already mentioned, the British Direct Purchase Commission to the US weren't exactly the best and brightest, so I'm not surprised they signed some bad contracts. As it was, they were under pressure to buy just about anything.

Most of the British order ended up in Russia. And they weren't close to the performance estimates (guarantees), which Bell based on an unarmed and unarmoured prototype.

Why do you think the BPC were "not the best and brightest"? Because they bought P-39s?

The P-40 snuck under the barrier because they arrived reasonably early and were judged suitable for the Middle East.

The P-40 was ordered because the RAF needed fighters and the P-40 was the only US fighter that had any chance of being available in a short period.

What made the P-40 suitable for the Middle East but not for northern Europe?

 

[Mad Dog]

......Why do you think the BPC were "not the best and brightest"? Because they bought P-39s?....

Because they didn't say "No, none of this will do." They should have brought a pallet of RR Merlin engines with them and said, "Here, build me something round this, we'd rather wait for something better than order just anything." But they ordered the available P-40. If they had of insisted on the Merlin as the powerplant, we might have had a Merlin-engined P-38 in 1941, which would have been invaluable in every theatre and much more useful than the eventual P-40F/Kittyhawk MkII, and the Mustang MkI would have been a world-beater from the start.

......The P-40 was ordered because the RAF needed fighters and the P-40 was the only US fighter that had any chance of being available in a short period....

Agreed, but it doesn't say much for the state if the US industry when you consider the US was technically ahead in stressed-metal construction, wing design and turbocharging, that the best they could provide was the Tomahawk. The Mustang was definitely a better effort, but, IIRC, a lot of the design work on the P-51 was actually done after they spent a lot of time studying a captured ME109..... After all, the American bombers were very good, probably the best available anywhere until the Mosquito arrived.

.....What made the P-40 suitable for the Middle East but not for northern Europe?

The P-40 had two big advantages over the Hurricane (and Spitfire). Firstly, it had much better range, which was vital for overseas operations. Strangely, the RAF didn't use drop-tanks with the Tomahawks. The other advantage was the Allison V-1710 stood up to Desert conditions better than the RR Merlin. Whilst some of this was down to the down-draught carb inlet, the Allison came with superior air filters, probably because the USAAC expected their aircraft to operate throughout the USA's many and varied environments. And in the Desert theatre in mid-1940, the RAF expected to only be fighting the Italians, and their fighters were inferior to the P-40B/C and had equally poor altitude performance. But in the ETO the fighting was getting to higher and higher altitudes, and the RAF was worried about high-altitude bombers like the Ju86 (which actually proved to be a bit of a flop).

 

[Shortround6]

a lot of the design work on the P-51 was actually done after they spent a lot of time studying a captured ME109.....

I really love these theories that don't have any real resemblance to the actual time line.

Sept 9th 1940. There is no engine installed at this time and the exhaust stacks are dummies.

Just when and where did the NA engineers study this captured 109?
The French had a captured one in the spring of 1940 that wound up in Britain, British were doing trials with it in Sept of 1940
It is 5400 miles from Los Angeles to London. Not very convenient for a team to go study the 109.
The British had approved the preliminary design on May 4th 1940. The NA guys must have been very fast workers indeed to incorporate anything from the study of the French 109 which started in March.

 

[tomo pauk]

Because they didn't say "No, none of this will do." They should have brought a pallet of RR Merlin engines with them and said, "Here, build me something round this, we'd rather wait for something better than order just anything." But they ordered the available P-40. If they had of insisted on the Merlin as the powerplant, we might have had a Merlin-engined P-38 in 1941, which would have been invaluable in every theatre and much more useful than the eventual P-40F/Kittyhawk MkII, and the Mustang MkI would have been a world-beater from the start.

There was no Merlins enough for everyone in 1940-41, especailly once the 2- and 4-engined A/C were being powered with them. Ford, and then Packard deal were made for a reason.

The Mustang was definitely a better effort, but, IIRC, a lot of the design work on the P-51 was actually done after they spent a lot of time studying a captured ME109..... After all, the American bombers were very good, probably the best available anywhere until the Mosquito arrived.

In other words, Americans studied the draggiest 'new' fighter of 1940 and came out with the streamlined wonder? If someone believes this, I have Brooklyn bridge on sale, real cheap.

Chart of XP-39 doing 340 mh at 20000 ft, as rolled out at Bell: picture

 

[GregP]

Hi Tomo,

I love the chart but am a bit curious about it. I have some observations.

1) It shows the "original airplane" doing 340 mph;
2) The "modified airplane with supercharged engine" doing just over 390 mph;
3) And the "modified airplane with altitude blower engine doing just over 400 mph.

But ...

The "original airplane" DID have a supercharged engine; the 1,150 HP Allisons V-1710-17 (E2). It had a B-5 two-stage turbosupercharger. They never flew an Allison in an airplane that wasn't at least supercharged. So, exactly what is the first curve referring to? Some other powerplant that wasn't supercharged? I can understand the curve with and without the altitude turbosupercharger, but the third curve (the so-called "original airplane") makes me scratch my head and wonder what they were thinking about.

Did the source for this set of curves explain the three curves enough to answer what the first curve was supposed to depict? Just curious, no other agenda here.

 

[tomo pauk]

Hi Tomo,

I love the chart but am a bit curious about it. I have some observations.

1) It shows the "original airplane" doing 340 mph;
2) The "modified airplane with supercharged engine" doing just over 390 mph;
3) And the "modified airplane with altitude blower engine doing just over 400 mph.

But ...

The "original airplane" DID have a supercharged engine; the 1,150 HP Allisons V-1710-17 (E2). It had a B-5 two-stage turbosupercharger. They never flew an Allison in an airplane that wasn't at least supercharged. So, exactly what is the first curve referring to? Some other powerplant that wasn't supercharged? I can understand the curve with and without the altitude turbosupercharger, but the third curve (the so-called "original airplane") makes me scratch my head and wonder what they were thinking about.
Did the source for this set of curves explain the three curves enough to answer what the first curve was supposed to depict? Just curious, no other agenda here.

'Supercharged engine' = in this case, has turbo to help out the engine-stage supercharger. (for total of 2 stages of supercharging)
'Altitude blower engine' = engine has 'just' the engine-stage supercharger as the only stage of supercharging; the impeller turns faster than on engines that were supposed to have also turbocharger. Eg. 8.77 times the crankshaft speed for the known V-1710-33 engine, vs. 6.44 times the crankshaft speed for the early turbocharged V-1710s.
B-5, or any other turbo of ww2, was 1-stage supercharger - the 2-stage turbochargers were mainly on drawing boards, and on prototype shops/test benches.

 

[GregP]

Thanks, Tomo!

Here's the original XP-39 in 2 configurations, both as the XP-39 with turbocharger (right) and as the "modified" XP-39B.

In the top row, I have it mis-labeled. The pic on the right is the XP-39 and the pic on the left is the same airplane without the turbocharger, but still with the original canopy. All protrusions have been removed, including the exhaust stacks.

In the second row, this is the same airplane, but you can see the canopy has been lowered and streamlined a bit. In the second row on the right, I'm not too sure, but it looks like the same airplane with dummy exhaust stack (possibly no engine yet). The stacks look kind of "flat" to me.

 

[Ivan1GFP]

Hello All.

There is no doubt that Larry Bell was a bit loose with the facts when advertising the new and wonderful P-39 to the British but there are a few other factors to consider that don't seem to get brought up in these discussions.

The test conditions under which Bell conducted tests and the British did were a bit different.
First of all, for British service, a snow screen was added to the supercharger intake.
This was an obstruction and caused a noticeable loss in performance.
For later US tests, the backfire screens were removed from the Allison engines. Were they still in place during the British tests?
Many US aircraft at the time used the practice of declaring a partial fuel and ammunition load as "standard" for normal loaded weight.
As an example, even though the P-39D could carry 120 Gallons of Fuel, 100 Gallons was considered the standard load.
Although the .50 cal guns in the nose could carry 270 rounds per gun, seldom did anyone load more than 200..... Except the Soviets.
The wing guns could carry up to 1000 rounds per gun but "standard" was only 300.

In reading through descriptions of the aircraft in the British tests, it seems that they did not get the instructions and loaded their aircraft to capacity.
Also, the substitution of a 20 mm cannon for the 37 mm sounds like a serious weight reduction but it really was not. The aircraft with the 20 mm carried two pieces of "cheek armour" that were not found in the 37 mm aircraft. This was probably also to help maintain proper CG but with everything else and with other equipment substitutions, the British aircraft were heavier.

Another odd thing is that for climb and maximum speed tests, the throttle settings were quite a bit lower than what the engine was capable of.

No doubt there was some additional Larry Bell "Magic" with testing aircraft with no armament or armour or very early versions but there were definitely a few more factors involved.

- Ivan.

 

[GregP]

Hi Tomo,

Just so we're communicating with clarity, the turbo charged version of the XP-39 was a 2-stage system, with the lower-altitude stage being the engine-driven supercharger and the higher-altitude stage being the turbocharger. The turbocharger by itself was one-stage but, when combined with the engine-driven supercharger, resulted in a 2-stage system. Your explanation above makes me think we are essentially saying the same things, but in different words.

Classic mis-communication, probably on my part.

Again, thanks for the charts!

 

[Shortround6]

To get the "production" Airacobra I to within 4% of the contract speed (it actually did a somewhat better, about 1% low) Bell resorted to a number of 'tricks' like 20 coats of primer sanded between coats. And the finish camouflage coat was lightly sanded to eliminate the seams left by the camouflage templates.
They also used a wood filler (plastic wood?) around the canopy to smooth the canopy to framework.
Certain panels were made of heavier gauge sheet metal to prevent distortion at high speed.
Improved fasteners/linkage for the landing gear doors to improve fit when closed.
and others, like no radio mast and fairings over the

One of my favorites
the test plane actually had slightly smaller elevators and rudders and slightly larger horizontal stabilizers and vertical fin. Not adopted for any other aircraft.

I don't know if some of these changes did anything or not but some certainly did and the the trick with the paint was totally unusable on a production aircraft.

The difference in speed between S/N AH 579 (standard ) and S/N AH 571 (modified) was 20mph in top speed.

Now take a 371mph aircraft that was only a few days old at the factory ramp and fly it in service (or extended tests at a different location) for several weeks and see how the fit/finish deteriorates.

However, I can think of no other fighter in WW II that was modified to such an extent for an acceptance test, there may have been some?

 

[FLYBOYJ]

To get the "production" Airacobra I to within 4% of the contract speed (it actually did a somewhat better, about 1% low) Bell resorted to a number of 'tricks' like 20 coats of primer sanded between coats. And the finish camouflage coat was lightly sanded to eliminate the seams left by the camouflage templates.
They also used a wood filler (plastic wood?) around the canopy to smooth the canopy to framework.
Certain panels were made of heavier gauge sheet metal to prevent distortion at high speed.
Improved fasteners/linkage for the landing gear doors to improve fit when closed.
and others, like no radio mast and fairings over the

One of my favorites
the test plane actually had slightly smaller elevators and rudders and slightly larger horizontal stabilizers and vertical fin. Not adopted for any other aircraft.

I don't know if some of these changes did anything or not but some certainly did and the the trick with the paint was totally unusable on a production aircraft.

The difference in speed between S/N AH 579 (standard ) and S/N AH 571 (modified) was 20mph in top speed.

Now take a 371mph aircraft that was only a few days old at the factory ramp and fly it in service (or extended tests at a different location) for several weeks and see how the fit/finish deteriorates.

However, I can think of no other fighter in WW II that was modified to such an extent for an acceptance test, there may have been some?

Sounds like Reno the day before qualifying!

 

[ThomasP]

"...flying parachute..."

Good one.

 

[tomo pauk]

Hi Tomo,

Just so we're communicating with clarity, the turbo charged version of the XP-39 was a 2-stage system,

Yes.

with the lower-altitude stage being the engine-driven supercharger and the higher-altitude stage being the turbocharger.

No.
There was no such thing as 'low altitude stage' nor 'higher-altitude stage'. Both stages were operating when engine was operating - just like on the P-47 and similar US made aircraft with turboed engines. The control system operating the waste gates was controling the speed and thus pressure from turbocharger (lest it would've overboosted the engine up to the point of destroying it) under the rated altitude
Note that both stages were also always operating (when engine was running, obviously) on 2-stage Merlins, Griffons, Jumo 213E or 2-stage V-1710s, even though here both stages were run by engine.
It took both stages operating to achieve pressure ratios of beyond 5:1 (pressure ratio from 1st stage + pressure ratio of 2nd stage), needed for engine operating at thin air at 20000 ft and above.

The turbocharger by itself was one-stage but, when combined with the engine-driven supercharger, resulted in a 2-stage system.

Yes, again just like on P-43s, or P-47s, or combat-worthy P-38s, or most of B-17s and B-24s we know.