Oops! I think you're barking up the wrong tree here. For parameters such as top speed, best climb, and to a lesser extent, ceiling, best measured performance occurs with the least airframe drag, which is with CG at the aft limit. This is usually not the most desirable from a handling standpoint, but this is where the horizontal stabilizer is required to generate the least amount of negative lift with its attendant drag.
XP-39 II - The Groundhog Day Thread
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Hello P-39 Expert,
The point I was trying to raise was that WHERE that weight difference was might make a significant difference in performance.
I believe GregP's explanation is correct but I am still thinking there was something else that wasn't so obvious that might explain the difference in performance.
Under some load conditions, it WAS easy and pleasant to fly. Under some load conditions it was NOT. For a P-39 Expert, you really don't seem to have been reading too many pilot reports. Don't just read the ones you like. Read the others as well and figure out why those pilots came up with something different when they are flying "the same" airplane.
Not hardly. The requirements were just a bit different. Without the Turbo, the P-39 was no longer an interceptor.
Actually I was discussing the possible differences in flight performance because of a difference in weight distribution.
- Ivan.
The point I was trying to raise was that WHERE that weight difference was might make a significant difference in performance.
I believe GregP's explanation is correct but I am still thinking there was something else that wasn't so obvious that might explain the difference in performance.
Under some load conditions, it WAS easy and pleasant to fly. Under some load conditions it was NOT. For a P-39 Expert, you really don't seem to have been reading too many pilot reports. Don't just read the ones you like. Read the others as well and figure out why those pilots came up with something different when they are flying "the same" airplane.
Not hardly. The requirements were just a bit different. Without the Turbo, the P-39 was no longer an interceptor.
Actually I was discussing the possible differences in flight performance because of a difference in weight distribution.
- Ivan.
Hello XBe02Drvr,
I agree you are correct on this. I was thinking more along the lines that without an autopilot, it would be more difficult to hold a marginally stable aircraft at a constant attitude for an optimal climb.
I have attached NACA L-602.
Please read Stability and Control results starting at page 4 and see if you come up with the same conclusions I did.
Seems to me that these people were not convinced that 31% MAC was the correct Aft limit.
Hello P-39 Expert,
With the Gear Box Armour in place, the P-39Q at Basic Weight + 200 pound Pilot has a CoG at 30.1623% MAC which is still within the limits of 23-31% MAC.
WITHOUT the Gear Box Armour of 70.74 Pounds at Station 21, The P-39Q at Basic Weight + 200 Pound Pilot has a CoG at 31.6876% MAC which is outside the allowable limits.
- Ivan.
What format is that attachment in? My Android phone says CAN NOT OPEN FILE. No explanation.
However, judging by the W&B worksheets posted by Joe upthread, it appears your conclusions are supported FOR A P39Q. BUT, as pointed out by some of the wiser heads aboard this vessel, extrapolating from one model to another on the assumption that all things remain equal puts one on shaky ground. Devil is in the details, and I haven't seen any credible W&B data yet for the D.
In my experience with aircraft at or near the aft CG limit, at high speed, steady state conditions such as a climb or max speed run, the squirrelliness manifests itself primarily in maneuvering rather than "hunting" in trimmed out steady state flight. I have been slightly out of limits aft on a few occasions, mostly as a result of using standard weights for passengers and baggage. (We used to double the bag count for soldiers going in and out of Ft. Drum, and that probably wasn't adequate still.) You suspect you're slightly out when the nose starts to "hunt" up and down in level flight and you're constantly adjusting pitch trim. On a hot day at 6000 feet between ART and SYR with a full boat and an aft CG, every little thermal bump makes your bird waddle and wag its tail and soon you're driving a barfwagen.
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I take that back. I got to digging through the stuff jmcalli2 linked for us back in post 839 (thanks jm!), and found a NACA test report on a P39D s/n 28378, done in early 1943, (no NACA Report # given) which had some scary details mentioned in matter-of-fact language.
1. CG @ 30.2% MAC (0.8% MAC fwd of AFT limit!) is referred to as "Normal". This leaves 0.8% for fuel burn and ammo expenditure. (20MM on test a/c, w/4 .30s in wing LE, 2 .50s in nose)
2. Stall is described as abrupt without buffet warning, accompanied by wing drop, roll oscillations, and a tendency of the stick to thrash laterally if not firmly held. This seems to happen in all speed ranges and configurations. Specifically stated that this makes for an unsatisfactory gun platform for deflection shooting in a turning fight.
3. If ailerons are not exactly centered at the stall, the stall tends to roll sharply away from direction of aileron deflection.
4. Stick force gradient @ "Normal" CG of 30.2% is described as 1.8 lbs/G! A 14.4 lb. pull (which you can EASILY accomplish between fingertips and thumb of your right hand, especially when adrenelated) will put you at 8Gs! Scary!
This is well below USAAF minimum acceptable value.
5. Ailerons effectiveness is described as unsatisfactory.
6. Have I scared anybody yet? This is all outlined in straightforward unemotional test pilot language which the uninitiated could easily read without the context to bring out the meaning. The exclamation points, bold print, and caps are mine.
7. I haven't even got into the graphs yet as they aren't easily readable on my Android phone.
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P-39 Expert
Non-Expert
Can you give a specific location in the report?
Peter Gunn
Master Sergeant
SO! Just as I suspected, you would find a tiny loophole to escape an affair of honor! So be it! But fair warning sir fair warning... I will NOT be so generous in any future "disagreements", you are a cad sir, A CAD! But I will magnanimously forego satisfaction at this time.
Send your vaunted Airacobras SUH, there is a flight of Fairey Battles that will be more than a match for your tail heavy - ground loving - spin happy - engine behinders.
Peter Gunn
Master Sergeant
The ultimate insult, to accuse an organic all-natural guy of being a Computer Aided Design! I'm adding my Kingcobra squadron, all flown by former Soviet aces to the strike force. Say your prayers, laddie!
Peter Gunn
Master Sergeant
KINGcobras you say? You've heard the expression "twice nothing..."
Bring it big boy, we're ready...
Funny looking Battles! Might as well be a Sever-sky!
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Peter Gunn
Master Sergeant
We upted the ante for your KINGcobras...
Peter Gunn
Master Sergeant
OK, I'm obviously not an expert on this, but to my untrained eye, for a plane that tips the scales at roughly 3.5 - 4.0 TONS, the removal of a mere 70.74 pounds puts the CoG outside of the allowable limits by 1.5253%. To me that means this thing is CLOSE to being out of whack even on a good day with everything in place. Going back several dozen pages, I believe the case was made that the Gear Box Armour was probably added to keep the CoG issue from cropping up.
It seems to me that that theory holds quite a bit of water.
*EDIT*
Also regarding something I saw in post #852 "Interceptor vs. fighter"
In 1939 there was a difference. Actually, there probably still is.
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Hello XBe02Drvr,
The NACA Memorandum Report L-602 which I attached is in PDF format. It IS a fairly lousy quality scan but readable.
The aircraft tested is the same one 41-28378 (P-39D-1) as the report you are reading and the conclusions are the same so it is probably the same report. It is also why I asked you to start reading at Page 4 for the Stability and Controls section. It kind of struck me as a bit scary as well.
If you have the same report, please observe the CoG range for testing.
The most forward CoG was 25% MAC
at Take-Off Weight 7800 pounds. Actual Flying Weight for testing estimated at 7620 pounds
The most rearward CoG was 30.8% MAC
at Take-Off Weight 7600 pounds. Actual Flying Weight for testing estimated at 7420 pounds
Note that the weights at the test for Aft CoG were still much higher than the numbers we were getting for the Basic Weight + Pilot for the P-39Q. For the P-39Q, that Basic Weight + Pilot is as low as the aircraft can get and the P-39D-1 wasn't even close to its lowest possible weight.
That is why I believe the P-39D and others of the era had a CoG that was several inches further Aft than the later versions.
As for how much, it isn't possible to determine with the data presented in this report because what the loads were to get to 7600 or 7420 pounds were not specified.
Hello P-39 Expert,
I already stated Page 4 in the message YOU QUOTED! Did you even read the message before you quoted it???
Hello Peter Gunn,
That is the thing about the P-39. When the CoG is at the forward end of the allowable range, it handles very well. Burning fuel doesn't really affect it to a great degree because the fuel tanks are very near where the CoG should be. The problem is that when the CoG migrates aft handling gets much less predictable.
The L-602 report also brought up another point: The allowable aft CoG limit was stated to be 31% MAC, but in testing at even 30.8% MAC, they were getting some pretty nasty handling and their Take-Off weight to get to this situation was 7600 pounds which is only 50 pounds below the "Normal" Loaded Weight for the aircraft.
The problem as I stated earlier is that the report doesn't say how they got to 7600 pounds.
The problem as you may be noticing is that SOMEONE, presumably the manufacturer made the determination that 31% MAC was safe. The NACA Test Pilots didn't seem to agree in their report. Handling also normally doesn't go instantly from great to lousy at a specific number. There are generally plenty of degrees in between.
I believe the best illustration of the difference is the Lockheed F-104 Starfighter.
Interceptor, Yes. Fighter..... Not Really, regardless of how it was employed.
- Ivan.
P-39 Expert
Non-Expert
You're doing your best to scare everyone over basically nothing.
The stick only oscillated laterally in one condition, when high accelerations were reached from recovery from rolls.
"In any condition, at any time after the stall occurred, recovery could be effected promptly by applying down elevator." This was stated more than once.
The report was generally complimentary and all requirements were met.
P-39 Expert
Non-Expert
Translation?
SaparotRob
Unter Gemeine Geschwader Murmeltier XIII
I think GregP said what we were all thinking.
Can someone tell me what we are supposed to be afraid of?
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