XP-39 II - The Groundhog Day Thread

P-39 Expert said:

I stand corrected, the oil tank did protrude slightly into the engine bay.

But I am correct in that the engine bay was not extended on the P-63, it was exactly the same size as the P-39, including the XP-39E. Please look at the drawings in my post #236 of the P-39 and the P-63. Those numbers above the fuselage are inches from the tip of the nose. On the P-39 the engine bay starts at station 138.25 and ends at station 228.5 meaning the length of the engine bay was 90.25". On the P-63 drawing the engine bay starts at station 141.25 and ends at station 231.50 for a difference of 90.25". The engine bay was 90.25" exactly for both the P-39 and P-63. The tail cone was lengthened but not the engine bay. The aux. stage supercharger would fit in the P-39 just like it would in the P-63.

Maybe the P-39 oil tank would need to be adjusted to stand vertically instead of at that angle, but the engine bay was not lengthened, it was the exact same size. Exactly.

Click to expand...

There are none so blind as those that will not see.

Ask yourself these questions

Why did they move the oil tank aft and cut a far larger hole in the rear fuselage front bulkhead on the P-63? That much bigger hole requires heavier material and significant additional structure to replace the original bulkhead and that means that bulkhead is heavier and that in turn means the aft CG is made very slightly worse.

Why was it necessary that they create this large volume of space behind the engine, in the rear fuselage, that can only be accessed through the engine bay in the forward fuselage. Apart from inspection panels there is no other access unless you remove the oil tank. That makes this totally dead space unless it is filled with something that extends aft from the forward fuselage engine bay.

Was it to fix the aft CG problem? No because it will actually make the aft CG problem worse.

Was it to install the coolant tank? Obviously not as that was moved to behind the pilot.

Was it to look pretty? Obviously not.

Was it because it seemed like a good idea at the time? Obviously not.

Was it to install helium to fix the aft CG problem? Obviously not.

Was it to fill with horse feathers? Obviously not.

So what does that leave? To extend the engine bay in order to make room for the ASB.

MiTasol said:

on the P-39 it moves considerably aft as the fuselage ammunition is used. On the P-39 it also moves slightly aft as the fuel and wing ammunition are expended.

Click to expand...

Ouch, my ears! LOL!

Hi MiTasol -

Based on the loading chart for the P-39Q and the numbers calculated by Ivan earlier, I came up with a few items...

At 7075 pounds, wheels up, the CG arm is 134.22. Within the envelope but very aft.

First, the wheels up and wheels down moment numbers on the chart are wrong, off by a smidge.

But if we use the CG range of 130.1072 - 136.5584 you'll find that if you use half fuel and expend all ammo, you're right at the aft CG (I came up with 136.58) Funny that when I did the same calculation with 1/4 tanks I came up with similar CG numbers.

If anyone wants to see my math, I can scan my calculations.

I agree Flyboy J that the CG does not change much with fuel consumption, unlike many other fighters, though I would have expected a smidge of a change. Maybe if you do the numbers on the earlier models with the greater fuel tankage it may be more significant, though I doubt it. Likewise wing ammo is not going to move the CG much but the nose ammo certainly does affect the CG and if you remove the gearbox armour, as is consistently suggested, you will find the aircraft is past the aft limit. The Q model had lighter front armour but other changes added weight forward, like the bigger heavier 50 cal blast tubes and the wing gun installation, to allow this.

Maybe it is worth doing those calcs?

MiTasol said:

I agree Flyboy J that the CG does not change much with fuel consumption, unlike many other fighters, though I would have expected a smidge of a change. Maybe if you do the numbers on the earlier models with the greater fuel tankage it may be more significant, though I doubt it. Likewise wing ammo is not going to move the CG much but the nose ammo certainly does affect the CG and if you remove the gearbox armour you will find the aircraft is past the aft limit.

Maybe it is worth doing that calc?

Click to expand...

I'm going off for dinner, I'll do the math in a bit!

P-39 Expert said:

According to you all the Russian P-39s should not have been able to fly.

Click to expand...

There is a difference between exceeding he CG limits to the front and exceeding them to rear. Correction by real pilots welcome.

If the CG moves forward the plane becomes harder to control as the control surfaces become less effective. Primarily in pitch???
More control input (control surface deflection) is needed to get the same result. You also need more up trim or elevator deflection to fly level.

If the CG moves rearward the plane becomes harder to control as the control surfaces become more effective (in pitch?) and at times the plane may decide to change it's AoA (pitch) just about on it's own.

Think of a paper airplane with a paperclip on it's nose, now move the paperclip to the tail and see what happens. A bit extreme but a pilot should be able to handle a plane with the CG a few inches forward of the limit even if the combination losses speed and agility. According to one site with the CG too far forward makes a plane hard to flare for landing.

This manual page shows how little spare room there was between the ASB and oil tank on the P-63.

Shortround6 said:

There is a difference between exceeding he CG limits to the front and exceeding them to rear. Correction by real pilots welcome.

If the CG moves forward the plane becomes harder to control as the control surfaces become less effective. Primarily in pitch???
More control input (control surface deflection) is needed to get the same result. You also need more up trim or elevator deflection to fly level.

If the CG moves rearward the plane becomes harder to control as the control surfaces become more effective (in pitch?) and at times the plane may decide to change it's AoA (pitch) just about on it's own.

Think of a paper airplane with a paperclip on it's nose, now move the paperclip to the tail and see what happens. A bit extreme but a pilot should be able to handle a plane with the CG a few inches forward of the limit even if the combination losses speed and agility. According to one site with the CG too far forward makes a plane hard to flare for landing.

Click to expand...

Correct. An out of CG tail heavy aircraft is worse than an out of CG nose heavy aircraft (none is desirable). Nose heavy will have stability issues and be difficult to raise the nose. Especially during takeoff and landing. It can be difficult to flare and land. Tail heavy, on the other hand, make it difficult to recover from a stall (See United 1900 Crash). Also tail heavy can cause very light control surfaces making It easier to over stress the aircraft.

MiTasol said:

I agree Flyboy J that the CG does not change much with fuel consumption, unlike many other fighters, though I would have expected a smidge of a change. Maybe if you do the numbers on the earlier models with the greater fuel tankage it may be more significant, though I doubt it. Likewise wing ammo is not going to move the CG much but the nose ammo certainly does affect the CG and if you remove the gearbox armour, as is consistently suggested, you will find the aircraft is past the aft limit. The Q model had lighter front armour but other changes added weight forward, like the bigger heavier 50 cal blast tubes and the wing gun installation, to allow this.

Maybe it is worth doing those calcs?

Click to expand...

OK - dinner is down along with a couple of beers!

So I recalculated and removed the gear box armor (70.7 pounds) and "Armor Plate FUME" (27 pounds) which appears to be over the nose 50s. I came up with a weight of 6841.3 with all ammo expended, half fuel left and a moment of 975294.7 which equates to a CG arm of 142.55, way out of the the earlier calculated envelope.

"Better start removing some weight from the tail"!

Shortround6 said:

There is a difference between exceeding he CG limits to the front and exceeding them to rear. Correction by real pilots welcome.

If the CG moves forward the plane becomes harder to control as the control surfaces become less effective. Primarily in pitch???
More control input (control surface deflection) is needed to get the same result. You also need more up trim or elevator deflection to fly level.

If the CG moves rearward the plane becomes harder to control as the control surfaces become more effective (in pitch?) and at times the plane may decide to change it's AoA (pitch) just about on it's own.

Think of a paper airplane with a paperclip on it's nose, now move the paperclip to the tail and see what happens. A bit extreme but a pilot should be able to handle a plane with the CG a few inches forward of the limit even if the combination losses speed and agility. According to one site with the CG too far forward makes a plane hard to flare for landing.

Click to expand...

A good summary. Also think about how the centre of gravity sits in relation to a theoretical pivot point about which the aircraft rotates in all directions. Having a CG above the pivot is inherently unstable. Having it below is inherently stable and the stability increases as the CG moves lower which is why the vertical component of CG matters. In a low wing aircraft the CG moves up as wing fuel is burned off.

Now think of the forces that need to be applied by the elevators as the CG moves forward and aft from this pivot point. If the tail drops with the CG forward of the pivot, the force required to bring it back to horizontal is much less than if the CG moves the same distance behind the pivot point. Simple basic maths.

Now add to this the effect of airflow on the elevators. High set elevators are out of the wing wash (the turbulent air behind a wing) so are fully effective for higher angles of attack. Very low elevators are always out of the wing wash so again remain effective. Mid set elevators, like on the P-39, will be in unstable air when the tail is low and that reduces the effectiveness of the elevators.

Now add aft cg requiring higher elevator effectiveness with the elevators being in the wing wash and you have a major reduction in nose down pitch control. This situation demonstrates why there are defined front and rear limits to CG travel, and why aft CG is more critical than forward CG on the vast majority of aircraft.

It is not exclusive to the P-39 but does demonstrate why CG is important on all aircraft (except balloons).

FLYBOYJ said:

"Better start removing some weight from the tail"!

Click to expand...

God I'm getting old! I'm doing weight and balance calculations just for fun!

You are not old, you just have no life. (You We have plenty of company.)

ThomasP said:

You are not old, you just have no life. (You We have plenty of company.)

Click to expand...

Shortround6 said:

There is a difference between exceeding he CG limits to the front and exceeding them to rear. Correction by real pilots welcome.

Exceeding out of the front is NO WHERE near the penalty / danger that exceeding it out of the aft limit.

If the CG moves forward the plane becomes harder to control as the control surfaces become less effective. Primarily in pitch???
More control input (control surface deflection) is needed to get the same result. You also need more up trim or elevator deflection to fly level.

True, see F15 examples below.

If the CG moves rearward the plane becomes harder to control as the control surfaces become more effective (in pitch?) and at times the plane may decide to change it's AoA (pitch) just about on it's own.

True, see below regards F16.

Think of a paper airplane with a paperclip on it's nose, now move the paperclip to the tail and see what happens. A bit extreme but a pilot should be able to handle a plane with the CG a few inches forward of the limit even if the combination losses speed and agility. According to one site with the CG too far forward makes a plane hard to flare for landing.

Click to expand...

Yes, and in transport aircraft this may be noticed by where your trim is during the approach. This can be dangerous in transport category aircraft when the pilot leaves the autopilot on until just before landing and can be surprised when turning it off and feeling how heavy / sluggish the nose is. He / she may not flare enough to prevent a hard or worse landing.

Shortround6 pardon me for being lazy and cribbing your entire post. You are correct in all your comments / assumptions above. Highlights are for emphasis.

While I have not flown an airplane out of CG I have flown them with forward and aft balances (inside the allowable window). In an Eagle (or any plane) with a nose heavy CG (forward) you will burn airspeed faster doing a hard / break turn (max performance). An identical plane with its CG sitting on the aft limit will burn less energy / airspeed and generate nose movement faster due to the horizontal tail being more effective doing the same max performance turn. Nose heavy aircraft (forward CG) will feel sluggish compared to it's twin with it's CG on the aft limit. I have flown the F15 this way and it's preferred. The F15B was especially "squirrelly" on it's aft stop (longer canopy affected airflow over the tail to a greater extent. I would actually prefer the B as I felt I could get a little more performance out of it.

The F16 is actually tail heavy (but inside it's CG window I assume as it flies like this consistently). The result is that in a hard / max performance turn it will hold energy / airspeed longer. The reality is its horizontal tail moves less to get the corresponding movement out of the nose, and therefor creates less drag. The plane WILL NOT FLY WITHOUT AN OPERABLE FBW SYSTEM. The aircraft is dynamically unstable. The F15 is dynamically stable (very much so). We lost an F16 when it had a midair over the Pacific with a tanker. Its radome was knocked off, and that contains it's AOA sensors. Without those it doesn't fly regardless of how well everything else is working.

P39E I haven't flown the Cobra, or any other WW2 aircraft, nor can I do the W&B calculations like FlyboyJ, Ivan or DerAdler, or read diagrams like MiTasol. However, I can tell you that flying a nose heavy plane is at it's forward limits is much safer than flying one at or aft of it's aft limit. The P39 looks like it operated near or at it's aft limits in normal ops, but low on fuel / oil / ammo and doing hard pulls looks like a recipe for disaster. My second or third flight in the Eagle we went to the airspace and did tail slides. Straight up and until you ran out of airspeed, then flop over, recover and do it again. The bet was to see who could tail slide the furtherest. F16 guys will NOT do that, ever even with FBW. The don't have enough control to easily get out of it. Needless to say when I fought them I knew that, however the point to glean is an aircraft with too heavy of a tail can be at a minimum a handful, or worse.

Cheers,
Biff

No worries Biff, crib away.

FLYBOYJ said:

So based on Ivan's calculations (Thanks again Ivan, I know your math is better than mine!!) we have the "station" C/G range of 130.1072 - 136.5584. The MAC range is 23 - 31%.

The weight and balance chart shown in the P-39N manual has 3 loading configurations (Max Fuel, Bomber and Normal load) at 29.%, 28.8% and 28.9% MAC. From what I see if you load your aircraft in any of those configurations you're good to go.

Now in the P-39Q flight manual I've seen, the same representation is made in inches from a datum point (which seems to be the tip of the nose). C/G data is omitted. ???

View attachment 597400


View attachment 597401

So if we use this information and go back to the original chart we can start removing items and see where they align within the W&B envelope. Mind you this is only for the horizontal C/G, I haven't even touch on the Vertical C/G.

View attachment 597403

View attachment 597405

Click to expand...

Hello FLYBOYJ,

Thanks again, but be very wary of what exactly I was stating and how I got those numbers.
The Percent MAC calculation is most likely correct for every production Airacobra version.
The allowable CoG range of 23-31% is probably the same for all versions.
I don't believe their wings or tail changed from the aerodynamic standpoint though late versions had some structural improvements.

The Weight and Balance Table for P-39Q was used as the basis for calculating the CoG at Basic Weight and Pilot by using the Moments for Loaded Weight and Deducting the Moment for each disposable load using a spreadsheet. This was done for the Wheels-Up condition because in my opinion, it is the more important. The Wheels-Down would tend to move CoG forward and down and improve stability.
Note that FOR THIS SPECIFIC MODEL of P-39Q, with a 200 pound Pilot and no disposable loads (Basic Weight), it is still within the allowable CoG range.

I am pretty certain you already know this but others here may not:
Although this is presumably accurate for the P-39Q, I am quite convinced that earlier models of Airacobra had enough equipment differences to put the CoG with Basic Weight and Pilot at least 2-3 inches further AFT which would make them fall outside the allowable CoG range under certain load conditions.

As for CoG being moved a bit forward, The test P-39D-1 was loaded to have CoG at 25.1% MAC in NACA Memorandum Report L602. Even then, the Elevators were sufficiently powerful to lift the nose of the aircraft at 60 MPH while taxying before the aircraft had reached flying speed.

The Soviets probably also figured this part out. I saw it mentioned in a forum discussion but have not been able to confirm elsewhere that the Soviets also tended to load more ammunition for the .50 cals in the nose than was common in US service. The number stated was 270 rounds per gun.
The aircraft manual confirms that this number is possible.
Some US tests show 250 rounds per gun being loaded for flight testing.

As I mentioned in prior posts, for Spin tests, the Soviets removed the Oil Tank armour for 4 of the 5 test aircraft though I do not know if they did this for their service aircraft.

Seems like they really wanted the CoG forward which may be why they didn't run into as many problems.

- Ivan.

So after all that, the conclusion must be that removing the nose armour would be a bad thing for CoG and stability?

One wonders if the Russians pulled the wing .30 cal guns to improve climb OR to improve roll response. The P-39D was not a particularly good roller, at least until the IAS was near 340mph. And it wasn't that the P-39 got "better" it was that most of the other aircraft got worse quicker and the gap was much reduced.

Ivan1GFP said:

Hello FLYBOYJ,

Thanks again, but be very wary of what exactly I was stating and how I got those numbers.
The Percent MAC calculation is most likely correct for every production Airacobra version.
The allowable CoG range of 23-31% is probably the same for all versions.
I don't believe their wings or tail changed from the aerodynamic standpoint though late versions had some structural improvements.

The Weight and Balance Table for P-39Q was used as the basis for calculating the CoG at Basic Weight and Pilot by using the Moments for Loaded Weight and Deducting the Moment for each disposable load using a spreadsheet. This was done for the Wheels-Up condition because in my opinion, it is the more important. The Wheels-Down would tend to move CoG forward and down and improve stability.
Note that FOR THIS SPECIFIC MODEL of P-39Q, with a 200 pound Pilot and no disposable loads (Basic Weight), it is still within the allowable CoG range.

I am pretty certain you already know this but others here may not:
Although this is presumably accurate for the P-39Q, I am quite convinced that earlier models of Airacobra had enough equipment differences to put the CoG with Basic Weight and Pilot at least 2-3 inches further AFT which would make them fall outside the allowable CoG range under certain load conditions.

As for CoG being moved a bit forward, The test P-39D-1 was loaded to have CoG at 25.1% MAC in NACA Memorandum Report L602. Even then, the Elevators were sufficiently powerful to lift the nose of the aircraft at 60 MPH while taxying before the aircraft had reached flying speed.

The Soviets probably also figured this part out. I saw it mentioned in a forum discussion but have not been able to confirm elsewhere that the Soviets also tended to load more ammunition for the .50 cals in the nose than was common in US service. The number stated was 270 rounds per gun.
The aircraft manual confirms that this number is possible.
Some US tests show 250 rounds per gun being loaded for flight testing.

As I mentioned in prior posts, for Spin tests, the Soviets removed the Oil Tank armour for 4 of the 5 test aircraft though I do not know if they did this for their service aircraft.

Seems like they really wanted the CoG forward which may be why they didn't run into as many problems.

- Ivan.

Click to expand...

Hi Ivan - and thanks for this very interesting exercise! No, totally agree that these numbers aren't exact or finite for every P-39 but I think the point was well made that before you start talking about moving things about an airframe you really need to understand weight and balance, especially on an aircraft like this. There is no doubt the Soviets made their aircraft lighter and probably nose heavier, but the process was not as simplistic as originally presented. I suspect the aft CG measurement of 136.5584 might be off but not that much. I also agree with your comments about earlier P-39s operating outside CG limits as well when put in certain operating situations, corrected on later aircraft (P-39Q). Regardless I think in US and western service, the P-39 carried an aft CG that could be potentially dangerous given certain conditions. I flew an aircraft that was tail heavy, and although it was a GA aircraft, I didn't like the way it felt close to stall speeds and when flaring to land.

"Seems like they really wanted the CoG forward which may be why they didn't run into as many problems." Agree 100%

Now shall we calculate the vertical CG?

wuzak said:

So after all that, the conclusion must be that removing the nose armour would be a bad thing for CoG and stability?

Click to expand...

YES! And just to arbitrarily pick a piece of equipment of the same weight and saying that would counter the weight in the nose doesn't cut it unless you understand W&B calculations. The initial comment was made to remove the armor around the gearbox. I calculated the removal of that armor and some additional armor I believe located by the nose guns. I think if I left that armor in, you can still find a potential out of limit CG when all the ammo was depleted and fuel fell below a certain level. Even though our calculations were an "accurate wag" it still shows that the P-39 stock operated at an aft CG during most if not all operations.

CORRECTED! Removed statement about ballast!