P-51A and the Allison Engine

[Lightning]

Hi all,

I was reading my latest issue of "Air Classics" magazine when I came across an article on a newly restored, very rare P-51A--the one with the Allison engine.

Pilot <s>Steve</s> John (corrected by edit) Hinton has been putting the required "break-in" hours on the airplane prior to certification for cross-country flight. He made an interesting comment concerning its low-level performance. He stated that the "A" is 20 mph faster down low than the Merlin-powered P-51D.

The Merlin is, without doubt, one of the greatest piston engines of all time, but I think that sometimes the Allison V-1710 tends to be given short shrift. Some of the war's best low-level performers were powered by the V-1710. It was easy to "streamline," and it was very reliable--having a longer "mean time between failure" (MTBF) than that of the Merlin.

The P-51A's stablemate, the A-36 "Apache" (originally "Invader") was very successful in the "Air Commando" ground-attack role. It performed very well under harsh conditions of climate and maintenance.

The need in Europe was for a high-altitude, long range escort fighter. The Merlin-powered Mustang was obviously better suited to this task than the Allison-powered P-51. Had the greater demand been for a low-altitude fighter that had good ground-attack capability, I firmly believe that the Allison Mustang would have been preferred.

Regards,
Lightning

[Kutscha]

Lightning, do you know of the Spitfire Performance site?

Lots of info there.

such as, http://www.spitfireperformance.com/p-38/p-38.html

The Invader was an un-official name. It came about with units in the MTO wanting it to be named such.

[GregP]

Good post Lightning. The ALlisons were VERY good engines that were well-made, reliable with a high TBO, and were easier to tune and keep tuned than the Merlins.

If effort had been expended on a suitable turbo or supercharger for the Allison, it might very well have been a better chouice than the Merlin. That being said, history is history, and the Allison never got the "suitable high-altitude boost" it needed.

I like the Allison a lot and let's remember that when new-maufacture Yak-3s were made in recent years, they chose the Allison to power it. That might well be to keep the original cowling lines, but I think availability, reliability, and TBO also played a part, too, and the Merlin is beaten by the Allison on all threee accounts.

[Groggy]

quote:Originally posted by GregP

Good post Lightning. The ALlisons were VERY good engines that were well-made, reliable with a high TBO, and were easier to tune and keep tuned than the Merlins.

If effort had been expended on a suitable turbo or supercharger for the Allison, it might very well have been a better chouice than the Merlin. That being said, history is history, and the Allison never got the "suitable high-altitude boost" it needed.

I like the Allison a lot and let's remember that when new-maufacture Yak-3s were made in recent years, they chose the Allison to power it. That might well be to keep the original cowling lines, but I think availability, reliability, and TBO also played a part, too, and the Merlin is beaten by the Allison on all threee accounts.

Hi GregP

What it the TBO for the Allison, Yes I agree the Allison engines were very good and well laid out designs.

[montanamotor]

One Reason, why an Allison-powered Mustang may have been faster at low levels than a Merlin-powered one, may have been caused by the differences in the engine's compressor-layout.

Mechanical compressors tend to drain LOTS of horsepower from any given engine. On average, a typical WWII-plane's compressor needed up to 20 percent of the engine's total power output for spinning at full throttle, rated altitude. If an engine was said to put out, let's say, 1500 HP at rated altitude, in fact it was putting out a full 1800 HP at the crankshaft, instead; yet, 300 HP of it would be de-routed to spin the charger, instead of the prop. And only those 1500 HP finally reaching the propeller-spine would be measured on the dyno. The rest would be attributed to as "internal loss".

The Allison - without use of it's turbosupercharger, as it was designed for in the XP 39 (and which was SUCCESSLULLY retained in the P 38) still featured a simple single-stage/single-speed mechanical supercharger.

One must know that, TURBO-supercharging was ONLY used in WWII-aircraft-engines to compensate for the loss of air-density at altitude. Even in turbo-supercharged planes, SEALEVEL-POWER in the engines was ALLWAYS and WITH NO EXCEPTION generated solely by use of a fixed-gear, single-stage mechanical supercharger, that was to supercharge air into the engine at a fixed given rate, to make it put out it's basic maximum power at sealevel. At sea level, the turbo would always remain idle, whatever power-setting the pilot would choose.

Only as a plane gained altitude, the TURBOsupercharger slowly would be engaged more and more by closing the exhaust-bypass (manually or, controlled by an air-density-meter). The closing butterfly-valve would route more and more exhaust to the turbosupercharger's turbine, to spin the turbosupercharger's compressor faster an faster, to continously supply the mechanical fixed-gear-single-stage-supercharger with sea-level-air pressure during climb (up to 8000 meters / 27000 ft).

When Allison dispensed with the turbosupercharger in all applications but the P 38, the engine retained the single mechanical supercharger stage. But this was geared up to spin faster, so as to retrieve at least some of the loss of altitude-performance it had suffered by the dispensation of the turbo.

Besides: Dispensing with the turbosupercharger is the reason, why some people believe, P 40 and P 39 would have had no supercharging at all, by the way. But that's simply not true (see above). After deleting the turbo, Allison's supercharging was only limited to the fixed-gear single-stage-charger.

BUT: Compared to the highly evolved mechanical supercharger used by the MERLIN (up to two-stage/three-speed intercooler-aftercooler in later variants), the Allison's simpler single-stage/single-speed - later single stage/two-speed - no intercooler, no ADI/MW 50-supercharger of course never could achieve similar high altitude performance; it could not come even close to the Merlin high up.

But there is a good thing in everything.

This Allison's evident high-altitude-deficiency really had advantages, too! Because: Less stages and slower revving compresser-blades would consume much less "parasitic" power from the engine's crank in a low-level-rated Allison, than the high-level-rated compressor did from the Merlin's crank. Thus, more of the produced HP would reach the propeller in an Allison (At low level!) than in a Merlin, to be transformed into thrust by the prop.

Additionally, No intercooler/aftercooler availlable meant less drag and wheight-penalty for the airframe, also - which would also contribute to an aircraft's advantageous performance-envelope at low levels.

Note: You can optimize an engine for high altitude, of for low altitude - but not for both. You can try to find a compromise, of course. But even a compromize won't be as good as an optimized set of machinery would. And optimized for low-level-performance the Allison in the P 51 A was.

I would say: Yes - it's definitely credible, that an Allison-powered Mustang was faster at low levels than a Merlin-powered-one. It's totally logical to me.

Cheers!

Montanamotor

P.S. Aaah - I forgot to mention something else, which is very imporant in this topic: Remember the prop! What goes for the engines in specializing for high-altitude vs. low-altitude-work, goes identically for propellers, too! You either have a good low-level-prop, or you have a good high-level-prop - but you can't have both.

For example: Somewhere I read that, by upgrading the Boeing B 17 from C to D-version, along with it's revised tail and interior came new, wider-chord-props, which greatly improved high-level-performance - maximum ceiling as well as speed - at the expense of low level-efficiency. The same must have gone for the Merlin-reengineered Mustang, too.

As propeller-blades are facing exactly the same aerodynamic influences and, conditions as a plane's wings do, it looks quite logical, that a wide, thick wing for high altitude-efficiency in thin air simply can't be equally effective under twice or even three-fold the air-pressure at the deck. Vice-versa, this goes identically for a narrower, thinner-airfoil-wing suited for thick "bottom air", which was forced to work at altitudes above the the top of Mount Everest.

Cheers again!

[Mark J]

The P-39 and P-40 were equal to any European fighter up until about 1942 as long as their altitude stayed below 15000ft. They were both capable low level fighters.

The Damiler Benz and Junkers inline engines had a mechanical/fluid driven supercharger, automatic in operation that worked something like the torque converter in an automatic car.
Montanamotor, are you saying these German engines were spinning up their supercharger vanes at sea level?
I thought they didn't 'cut in' until a certain, pre selected altitude was reached.

cheers

[Lightning]

Hi Kutscha:

Quoting you:
quote:The Invader was an un-official name. It came about with units in the MTO wanting it to be named such.

This is one of those subjects on which one can never find a definitive answer. Where the names came from, and even which came first, seems to be a matter of which reference you happen to be readig at the time. Your explanation is as good as any.

The Apache was the original name given the P-51 by the USAAF or the RAF (take your pick--I have read it both ways). This, of course, was changed to Mustang, which obviously stuck. The original dive bomber versions were originally ordered as P-51s, but before delivery, the designation was changed to A-36. So, upon delivery, were they A-36 Mustangs or A-36 Apaches?

At about the same time, the name "Invader" appeared. Which name--Invader or Apache--was first used for the A-36 production aircraft is not all that clear.

I even read one source that said the name Invader was originally used to confuse foreign intelligence gathering efforts into American warplane production.

I have a copy of the official publication "Air News Yearbook" from 1944 (i.e later in the war) that shows an A-36 in flight. The caption on the photograph describes the aircraft as an A-36 "Invader," so the name was not short lived.

While we're on the subject, it is not 100% certain that the British came up with the name "Mustang" as is genrally accepted. On occasion, the question has arisen as to whether the British would have chosen the name of an American wild horse. It doesn't seem all that hard to believe to me, but some have questioned it.

Regards,
Lightning

[montanamotor]

Hi, Mark J - you are right.

MECHANICAL compressors always spin in a WWII's engine - at full throttle, that is. This is essential to boost the manifold-pressure, to achieve the rated power output at ANY altitude, even at sea level. without a compressor, a naturally aspirated DB 601/605 would have been restricted to perhaps 700 HP max. power output at sea level. Not enought BMEP - or torque, to achieve higher power-outputs, in that case. Only supercharging allows cramming so much more air into the cylinders ABOVE the natural air pressure, as to produce sufficient torque to deliver the respective, known rated take-off-power.

The DB's hydraulic coupling, on the other hand, especially allowed the DB-engines to let the compressor drain only so much pover as to drive the compressor-wheel as was definitely neccessary at any moment in flight.

This meant that, if you were going to go in a low-level-cruise setting with the enging putting out LESS than about those 700 HP which it was able to put out naturally aspirated, anyway, the hydraulic clutch only THEN may have completely disengaged the compressor from the engine, to cut power-drain to zero, and hence, reduce fuel consumption significantly.

Only if qou were again requiring MORE than those 700 oblique HP, the hydraulic coupling would again engage and spin the compressor up to the maximum rpm for the given required manifold pressure. Only when the air got too thin for further sufficient compression at high altitudes - which occurred at about 7000 meters for the DB 605 - from there up the power output of the DB would drop again, in direct reference to the drop of the ambient air pressure.

The DB-powered planes therefore had a single-stage-unlimited speed-compressor, capable to adapt itself continously to any variation in altitude.

Other planes relied on hydraulically, or even manually selected gearboxes for their engines' compressors. The first speed would always be selected for full rated power on the ground again.

For cruising at low altitude, the first speed would be disengaged, to leave the engine naturally aspired. First speed would be reselected - either triggered by an air-density-meter and hydraulically switched, or simply via a clutch and hand-lever (The single-stage, two-speed-compressor of the Curtiss Wright R 1820 in the Brewster Buffalo featured this rudimentary, yet reliable system, for example!) - at full throttle in climb up to a level, where the manifold pressure would start to drop again.

Then - manually - the pilot had to pull back the throttle, grabbed the compressor-gearselector-stick with the motorcycle-like clutch-hand-lever on top, pulled the clutch, switched the compressor's gearbox into second gear, released the handclutch, pushed back forward the throttle-lever - and off he went again.

And if you now thought, that this was a complicated way of flying - then imagine what was your duty, if you were flying in a F4F Wildcat wit a two-stage, two-speed PW 1830, instead...

Yes, hydraulically operated, automatically switched speeds for compressors eased a pilot's life a LOT, of course. But then it could happen that, the gearchange in the compressor-gearbox - which always had to be executed with an automatically trottled back engine, too, like the way you are easing the pedal, whenever you change a gear in your car - might occur automatically in the most unpleasant of instances: Like, climbing up the side of a mountain with a full load of bombs and drop tanks in extreme low-level-flight, or if you were just about to open fire from a very steep angle of attack, after hou had sneacked under the belly of an enemy bomber from six o'clock low without being detected... Those instances were numerous, and they DID occur in WW II.

Therefore: As good as the manually or automatically triggered and selected multi-gear-compressors may have been in WW II-fighter-planes - the best and smoothest compressor-driving-systems for any pilot to fly back in that era were either the self-adjusting DB 601/605 hydramatic clutches, or the combined compressor-turbosupercharging-systems as featured in the P 38 and P 47.

Hey, and did you ever hear about the P 43? Neat plane. Based on the Seversky P 35. Small as a Wildcat - but: Turbosupercharged! The Chinese Airforce in cooperation with Chennault's Flying Tigers used several of them as high altitude photo reconnaisance-planes against the Japanese. They didn't even suffer one single loss from enemy action throughout the war. Not bad, eh?

Cheers!

Montanamotor

[Lightning]

Hi montanamotor,

Quoting you:
quote: One Reason, why an Allison-powered Mustang may have been faster at low levels than a Merlin-powered one, may have been caused by the differences in the engine's compressor-layout.
No doubt this played a part, but the major reasons for the P-51A's low-level performance were aerodynamic in nature. Remember, the P-51A had ony 1200 hp take-off power and used a three-bladed propeller. Compare this to the P-51D's 1490 hp driving four blades. As time went on, the "D" became heavier, but also more powerful. When you compare the two planes side-by-side, the "A" presents a much slimmer configuration because of the Allison installation.
quote:. . . as a plane gained altitude, the TURBOsupercharger slowly would be engaged more and more by closing the exhaust-bypass (manually or, controlled by an air-density-meter). The closing butterfly-valve would route more and more exhaust to the turbosupercharger's turbine, to spin the turbosupercharger's compressor faster an faster. . .
I believe this was called a "waste gate" on the P-38's turbosuperchargers.
quote:P.S. Aaah - I forgot to mention something else, which is very imporant in this topic: Remember the prop! What goes for the engines in specializing for high-altitude vs. low-altitude-work, goes identically for propellers, too! You either have a good low-level-prop, or you have a good high-level-prop - but you can't have both.But, again, neither the "A" nor the "D" had specialized propellers optimized for use at any specific altitude, and the "D" had significantly more power and a four bladed prop.
quote:For example: Somewhere I read that, by upgrading the Boeing B 17 from C to D-version, along with it's revised tail and interior came new, wider-chord-props, which greatly improved high-level-performance - maximum ceiling as well as speed - at the expense of low level-efficiency. The same must have gone for the Merlin-reengineered Mustang, too.
This was evident in the P-47D and the never-produced P-38K with their "paddle-blade" propellers. The Mustangs in question never had such propellers. Again, it was 1200 hp with three blades vs 1490 hp with four.
quote:As propeller-blades are facing exactly the same aerodynamic influences and, conditions as a plane's wings do, it looks quite logical, that a wide, thick wing for high altitude-efficiency in thin air simply can't be equally effective under twice or even three-fold the air-pressure at the deck. Vice-versa, this goes identically for a narrower, thinner-airfoil-wing suited for thick "bottom air", which was forced to work at altitudes above the the top of Mount Everest.
Its not just a matter of wide and thin. Speed, airfoil section, and angle-of-attack (pitch) are major cosiderations--on propellers as well as wings.

As a final comment, I believe it is very interesting to this discussion that the fastest of all Mustangs--the XP-51J--reverted back to the Allison engine. Using the Allison V-1710-119 with a take-off power rating of 1500 hp, it attained 491 mph at 27,400 feet. Compare this with 487 mph at 25,000 feet for the P-51H. The "J", however, never made it into production.

Regards,
Lightning

[montanamotor]

Good points, Lightning. I will definitely take them in account.

The Allison V-1710-119 finally had the auxilliary-compressor, that altered the complete package into a two-stage/two-speed, intercooler/ADI equipped super-powerplant. The Allison -119 was the pinnacle of V 12-piston-engine-development - for me, that is. Beauty lies in the eyes of the beholder, folks!

But for me, the Allison V 1710 definitely IS a beautiful engine, indeed.

Cheers!

Montanamotor

[GregP]

Hi Montanamotor. I agree with you. The Allison IS a very good looking engine that had a high state of development before production was shut down.

Any rebuilt Allison will give the new owner 2000+ hours of operation before needing a a horrendously expensive rebuild ... but, if you can afford an Allison, you can afford the rebuild, too ... no problem.

Anyone who contemplates making a new Dardo should consider the engine carefully. The Allison may not fit into a stock cowl, but it WILL give a lot of service reliably. The Continental may or may not, and Dardos are probably not very good gliders and don't have a 40-knot touchdown speed ... the probability of survival is inversely proportional to the angle of arrival (and speed of same). :)

All the best luck with the Dardo ... when it is completed, PLEASE post a video! Good luck with tailwheel tires!

[BlackWolf3945]

quote:Originally posted by Lightning

I was reading my latest issue of "Air Classics" magazine when I came across an article on a newly restored, very rare P-51A--the one with the Allison engine.
Is this the specific airframe from the article?

http://img.photobucket.com/albums/v428/achtungindianer/GP/IMG_1616.jpg

If so, then allow me to make a correction. It's not a restored aircraft, it's a newly-constructed replica built from NAA plans by a fella named Gerry Beck. More info about this particular aircraft HERE (http://www.warbirds-eaa.org/news/2006%20-%2005_22%20-%20Ultimate%20Homebuilt%20Warbird%20Coming%20to%20 Oshkosh.html).


Fade to Black...

[montanamotor]

COOL! [8)][:0][:p]

Well - maybe we should ask HIM about directional stability or, instability with turtle-deck Mustangs. He should definitely be the one to know about it first-hand...

Cheers!

Montanamotor

[Lightning]

Hi BlackWolf3945,

Quoting you:
quote:Is this the specific airframe from the article?
The aircraft in the article is restored P-51A-10NA, USAAF s/n 43-6251. It carries civil registration NX4235Y.

It is painted olive drab/green with light-colored diagonal stripes and has the name "Miss Virginia" painted on the nose. This is quite interesting because the P-38G flown by Rex Barber when he shot down Admiral Isoruku Yamamoto on April 18, 1943 was also named "Miss Virginia."

Regards,
Lightning

[pmjwright]

quote:No doubt this played a part, but the major reasons for the P-51A's low-level performance were aerodynamic in nature. Remember, the P-51A had ony 1200 hp take-off power and used a three-bladed propeller. Compare this to the P-51D's 1490 hp driving four blades. As time went on, the "D" became heavier, but also more powerful. When you compare the two planes side-by-side, the "A" presents a much slimmer configuration because of the Allison installation.

I agree, and (side note since you're using the "D" for comparison), the A's razorback fuselage also contributed slightly to aerodynamics. Since majority of flying in Pacific and CBI was at altitudes &lt;15000, it is no surprise that many CBI pilots preferred the A over the C or even the D.

[Kutscha]

Basic weight of the XP-51J was 6030lb while for the P-51H it was 7148lb. Max gross weight was 9140lb and 11050lb respectively.

Both used an Aeroproducts an A-542 prop, -B1 or -B2 on the J and H. The -B was 1" larger in diameter and had 0.2* more pitch (both high and low) than the -A prop.

The XP-51J had no guns fitted (no gun ports) so there would be better airflow over the wings.

[Lightning]

Hi Kutscha,

quote:Basic weight of the XP-51J was 6030lb while for the P-51H it was 7148lb. Max gross weight was 9140lb and 11050lb respectively....The XP-51J had no guns fitted (no gun ports) so there would be better airflow over the wings.

But the P-51H had 2250 war emergency power.

Regards,
Lightning

[Kutscha]

The V-1650-9 only put out 1630bhp at 23,000ft, wet, high blower. At 10,100ft it was 1930bhp, wet, low blower.

Mustang: The Story of the P-51 Fighter
Gruenhagen
Arco Publishing
ISBN 0-668-03912-4

[Lightning]

Hi Kutscha,

Quoting you:
quote:The V-1650-9 only put out 1630bhp at 23,000ft, wet, high blower. At 10,100ft it was 1930bhp, wet, low blower.

I have consulted quite a few web sites as well as my own personal reference books on this subject. Many (too numerous to cite individually here) give, between them, a range of WEP for the P-51H of 2218 hp, 2220 hp, 2250 hp, and 2270 hp. Most cite 2270 hp. I think you are quoting "military power."

Regards,
Lightning

[Kutscha]

No Lightning, it says war emergency (wet) power for the numbers I posted. The numbers are for 80" Hg. The numbers you state are for 90" Hg 'wet'. Afaik 90" Hg was not cleared for service.

http://www.spitfireperformance.com/mustang/p-51h-powercurve.jpg

More a/c data can be found, http://www.spitfireperformance.com/

[ickysdad]

According to Mustang Designer & America's 100,000 the P-51H WEP was 2250 HP but on 90" . According to Mustang Designer it was cleared for service though.

[Lightning]

Hi Kutscha,

I've looked at several more sources regarding power ratings of the Merlin V-1650-9, and they all give over 2200 hp for WEP. One gives the power output as being 1735 hp @ 3000 rpm and only 66" boost (i.e. not WEP). Another gives normal power as 1380 hp @ 3000 rpm but 2280 hp @ 3000 rpm at WEP.

All-in-all, almost every reference source I have consulted gives WEP for the P-51H and the V-1650-9 engine as well over 2000 hp.

Regards,
Lightning

[Kutscha]

I hope those references Lightning are not general a/c books and web sites.

icky's references I will believe.

[SkyChimp]

Regarding the V-1650-9: the differences in ratings had to do mainly with the fuel grade.

74" was a dry WEP rating on 100/130 grade fuel (AWF-28).
80" was a wet WEP rating on 100/130 grade fuel.
90" was a wet WEP rating on 115/145 grade fuel (AWF-33).

All of these ratings were approved for use in the P-51H at one time or another. 115/145 grade fuel was mainly a post WWII fuel, though limited quantities were available during the war for operational trials.

[Lightning]

Hi SkyChimp,

Quoting you:
quote:Regarding the V-1650-9: the differences in ratings had to do mainly with the fuel grade.

74" was a dry WEP rating on 100/130 grade fuel (AWF-28).
80" was a wet WEP rating on 100/130 grade fuel.
90" was a wet WEP rating on 115/145 grade fuel (AWF-33).

All of these ratings were approved for use in the P-51H at one time or another. 115/145 grade fuel was mainly a post WWII fuel, though limited quantities were available during the war for operational trials.

Do you happen to know what the WEP rating in hp was for each of the three cases you listed? I think that would add to this discussion.

Regards,
Lightning