XP-39 II - The Groundhog Day Thread

[P-39 Expert]

Yes - 20 degrees C (36F) and when computing performance calculations, it will make a difference.

Yes - 20 degrees C (36F) and when computing performance calculations, it will make a difference.

Okay, how much would that 20 degrees affect the performance of a P-39K or A6M2 at 24000'?

 

[Ivan1GFP]

The chart says that even with a drop tank the "service ceiling" was over 27000'. Wish the report had been more specific.

I think most would be inclined to believe that the P-39K could handle itself at 27000' with the weight reduction. General was pleased with the results.

You are quoting performance under standard conditions. As has been explained to you, there is no evidence these were "standard" conditions and the greatest likelihood is that the conditions were much less than optimal.
As for being "inclined to believe", you are welcome to believe what you want as you tend to anyway, but that does not change the contents of the document which states that 27,000 feet was a service ceiling.
General Harmon should have been pleased with this experiment; it actually worked! The other experiments did not work out nearly as well.

Reducing the weight increased the climb rate and ceiling just like I have been telling you.

I don't believe anyone is disputing that reducing weight will usually increase climb rate and ceiling. The question is really how much effect they will have. We just don't believe your "calculations" have any basis in reality and that there were other more important factors that differed between the P-39C and P-39D tests.

Just to humour you for a second, what do your calculations tell you the increase in climb rate would be at 25,000 feet and please show your work. What do your calculations show the increase in service ceiling would be?

- Ivan.

 

[FLYBOYJ]

Okay, how much would that 20 degrees affect the performance of a P-39K or A6M2 at 24000'?

That would be indicated in performance charts based on the flight testing of both aircraft.

 

[P-39 Expert]

That would be indicated in performance charts based on the flight testing of both aircraft.

I was wanting a number, like 3mph or 50fpm.

 

[P-39 Expert]

As they say, please expand.

So, they throw one really crappy propeller (Rotol) in with the other 4 and it's 330fpm less than the standard prop. Wonder why the HP was so low on that one?

The differences in the other props were in the 200fpm range, barely measurable.

We're talking about the P-39C and D, and two props by the same manufacturer, same diameter, different blade (if it was really different). Why would the AAF specify a different prop if it hurt climb rate that much?

 

[pbehn]

So, they throw one really crappy propeller (Rotol) in with the other 4

Rotol props needed a Spitfire behind them to produce a decent climb performance up to 40,000 feet.

 

[Ivan1GFP]

We're talking about the P-39C and D, and two props by the same manufacturer, same diameter, different blade (if it was really different). Why would the AAF specify a different prop if it hurt climb rate that much?

Very often, two different propellers have characteristics that give them advantages under different conditions.
A propeller's efficiency curve is not constant but varies with speed and RPM. Usually this is simplified to just Advance Ratio (J).
The simplest difference is that one propeller may have an advantage in climb while the other is better for maximum speed or cruising speed.

You seem to be hung up on climb rate. The US Army folks were much more interested in maximum speed over just about everything else when it came to fighters.

- Ivan.

 

[XBe02Drvr]

I'm not buying the less efficient wing.

Whether you buy it or not doesn't change the fact that a wing of that design with that loading is just not going to perform as efficiently in air that thin with so little excess power available as the more lightly loaded, better airfoil, higher L/D Zero wing will.
The only planes that could match or exceed the Zero in combat performance at those altitudes were those of later generations that had significantly more surplus power available at altitude.

 

[Peter Gunn]

So, they throw one really crappy propeller (Rotol) in with the other 4 and it's 330fpm less than the standard prop. Wonder why the HP was so low on that one?

The differences in the other props were in the 200fpm range, barely measurable.

We're talking about the P-39C and D, and two props by the same manufacturer, same diameter, different blade (if it was really different). Why would the AAF specify a different prop if it hurt climb rate that much?

On my office wall behind my desk I have a 3' x 3' picture of an F-86 Sabre. When I have to explain to someone whatever issue is at hand and I get that blank stare, I point to the Sabre, as in the point went over your head like this Sabre.

Perhaps... the Air Corps was oh, I don't know, TESTING different propellers? I mean why else would they use the same plane weighted the same with different props? It does happen.

Also, if Wes talks about wing efficiency I wouldn't really argue with him unless you're a pilot with a sh!t ton of hours or a aerodynamic engineer, just my opinion of course.

 

[P-39 Expert]

You are quoting performance under standard conditions. As has been explained to you, there is no evidence these were "standard" conditions and the greatest likelihood is that the conditions were much less than optimal.
As for being "inclined to believe", you are welcome to believe what you want as you tend to anyway, but that does not change the contents of the document which states that 27,000 feet was a service ceiling. As has been explained to you, the "service ceiling" of a standard P-39K was over 30000' BEFORE any weight reduction. They reduced the weight by 650lbs and increased the combat ceiling to 27000.
General Harmon should have been pleased with this experiment; it actually worked! The other experiments did not work out nearly as well.



I don't believe anyone is disputing that reducing weight will usually increase climb rate and ceiling. The question is really how much effect they will have. We just don't believe your "calculations" have any basis in reality and that there were other more important factors that differed between the P-39C and P-39D tests.

Just to humour you for a second, what do your calculations tell you the increase in climb rate would be at 25,000 feet and please show your work. What do your calculations show the increase in service ceiling would be? This has been explained to you before. The difference in climb rate between the two virtually identical (except weight) planes was 1000fpm at 10000'. The difference in weight of the two planes was 836lbs. Divide 1000' by 836lbs and you get 1.2fpm per pound of weight. That means if you reduce the weight of the P-39D by 300lbs the climb rate improves by 360fpm. If you add 200lbs to the P-39C then you decrease the climb rate by 240fpm. That's at 10000'.

No amount of slightly different propellers (of the same diameter by the same manufacturer), an inch in length, fin fillet, symmetrical airfoil, adjustments to standard atmosphere or ANY OTHER FACTOR is going to make a difference of 1000fpm climb except almost a half ton of weight. The planes were virtually identical except for 836lbs weight.

- Ivan.

You are quoting performance under standard conditions. As has been explained to you, there is no evidence these were "standard" conditions and the greatest likelihood is that the conditions were much less than optimal.
As for being "inclined to believe", you are welcome to believe what you want as you tend to anyway, but that does not change the contents of the document which states that 27,000 feet was a service ceiling.
General Harmon should have been pleased with this experiment; it actually worked! The other experiments did not work out nearly as well.



I don't believe anyone is disputing that reducing weight will usually increase climb rate and ceiling. The question is really how much effect they will have. We just don't believe your "calculations" have any basis in reality and that there were other more important factors that differed between the P-39C and P-39D tests.

Just to humour you for a second, what do your calculations tell you the increase in climb rate would be at 25,000 feet and please show your work. What do your calculations show the increase in service ceiling would be?

- Ivan.

Expand above

 

[fubar57]

Armchair experts have way more knowledge than anyone

 

[P-39 Expert]

Whether you buy it or not doesn't change the fact that a wing of that design with that loading is just not going to perform as efficiently in air that thin with so little excess power available as the more lightly loaded, better airfoil, higher L/D Zero wing will.
The only planes that could match or exceed the Zero in combat performance at those altitudes were those of later generations that had significantly more surplus power available at altitude.

Absolutely nothing wrong with a symmetrical airfoil. The A6M2 will be gasping for air just like the P-39K at 27000'. Sure it has a much lower wing loading, and will be more maneuverable at 27000' just like it is at 10000'. But it won't climb any better and it sure won't dive any better.

 

[pbehn]

Armchair experts have way more knowledge than anyone

Especially in the technical ideas supermarket, if you don't like it for some reason, you don't buy it, and that, is that.

 

[Ivan1GFP]

I'm not buying the less efficient wing.

You can buy the less efficient wing or not, but just consider that if it was such a great idea, then why didn't anyone else use such a simple idea as a symmetrical airfoil?
From what I have read in various accounts, the symmetrical airfoil seemed to be a very quickly passing fad in the 1930's that some people thought might give an advantage in speed.
Actually one other major US type DID use a symmetrical airfoil and its results were at least as bad as those in the Airacobra.
The B-26 Marauder used a symmetrical airfoil and a tiny wing to start and was a relative speed demon for the time. It had to lose that in stages as more practical requirements became more important and in its last model ended up with the entire (now muh larger) wing offset several degrees from the fuselage to provide additional lift at more practical speeds.

When interviewed after the war, Peyton Magruder, the designer admitted that the symmetrical wing was probably not a good idea.

Symmetrical airfoils make sense if the plane is intended to do a lot of inverted flying as might be expected of an aerobatic plane.

This is why I have been commenting from the start that one of the fixes to the Airacobra should have been replacing the outer wing sections with something with a more modern airfoil such as the NACA 23000 series that everyone else was using. Bell clearly knew about it because the wing tips of the Airacobra were NACA 23000 series airfoils.
They were meant to prevent the outboard sections at the ailerons from stalling when the rest of the wing stalled. In this manner, the pilot could retain lateral control. The problem is that if there was a bit of yaw, the entire inboard symmetrical section of one side would stall at the same time and flip the plane.

- Ivan.

 

[P-39 Expert]

On my office wall behind my desk I have a 3' x 3' picture of an F-86 Sabre. When I have to explain to someone whatever issue is at hand and I get that blank stare, I point to the Sabre, as in the point went over your head like this Sabre.

Perhaps... the Air Corps was oh, I don't know, TESTING different propellers? I mean why else would they use the same plane weighted the same with different props? It does happen.

Also, if Wes talks about wing efficiency I wouldn't really argue with him unless you're a pilot with a sh!t ton of hours or a aerodynamic engineer, just my opinion of course.

Yes the AAF tested different propellers on the P-40F. Obviously some were better than others. The P-39 had an Allison engine. The props tested on the Allison P-40E were all remarkably similar in performance, both in speed and climb. One would think that would be the better comparison in this case.

If you all are done patting yourselves on the back, can any of you divide 1000 by 836?

 

[FLYBOYJ]

I was wanting a number, like 3mph or 50fpm.

Again, that would be shown on a performance chart that includes temperature and density altitude into the equation. I believe most if not all US WW2 fighters computed at "Standard Altitude" (59F at sea level 29.92"HG).

 

[DerAdlerIstGelandet]

... because the atmospherics are different in different parts of the world. That is why you see "adjusted to standard atmosphere" of data sheets.

Not just different parts of the world. It can very anywhere. Different times of the day. Different seasons. Different anything really.

 

[XBe02Drvr]

Very often, two different propellers have characteristics that give them advantages under different conditions.

Case in point: a prop with the same blade count but a higher propulsive surface area (a "paddle blade", if you will). In the denser air at lower altitudes the added rotational drag of the blades will keep trying to slow RPM, driving the governor to "fine" the propeller pitch to keep RPM constant, reducing the efficiency of converting HP to thrust. This is not so much an issue with a behemoth like the R2800, as the P47 so impressively proved. But where the paddle comes into it's own is at altitude, where the thinner air reduces drag, allowing more efficient pitch angles, but also demanding more propulsive surface area to convert the limited available HP into thrust.
Any non-turbocharged Allison fighter equipped with a more paddle oriented prop would show a deficit in low altitude performance in trade for improved performance at upper levels compared to the stock setup.

 

[Dash119]

Okay, how are you going to make up the other 700fpm? Phantom CG problems? A tail fin fillet? The extra inch of fuselage length?

Think it was maybe the 836lbs of extra weight?

If you all are done patting yourselves on the back, can any of you divide 1000 by 836?

Am I supposed to divide 1000 by 836, or 700 by 836?

 

[Milosh]

So, they throw one really crappy propeller (Rotol) in with the other 4 and it's 330fpm less than the standard prop. Wonder why the HP was so low on that one?

The differences in the other props were in the 200fpm range, barely measurable.

We're talking about the P-39C and D, and two props by the same manufacturer, same diameter, different blade (if it was really different). Why would the AAF specify a different prop if it hurt climb rate that much?

I suggest you look at the Me109K which was tested with one prop but used a different prop for serial production. Iirc one was best in climb and the other for speed.