XP-39 II - The Groundhog Day Thread

[Ivan1GFP]

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.

Hello Shortround6,

Please remember that the Soviets also pulled the wing guns out of the P-40 Tomahawks and left only the two cowl .50 cal MG. The Tomahawk didn't have a particularly bad roll rate. They just didn't seem to like wing armament and didn't mind lightly armed fighters.

- Ivan.

 

[wuzak]

IMO, without adding some rear ballast, YES!

Adding rear ballast after removing forward ballast?

 

[XBe02Drvr]

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.

Step away for a couple hours and miss all kinds of interesting stuff! I did a little research on tail heavy 1900 crashes and couldn't find one by United Express. The two shining examples I found were Ryan Air in Homer AK, and US Air Express in Charlotte NC, both classic out-of-CG-aft loss of control situations. In an earlier thread I detailed my own experience with a badly out of limits aft Be1900, which I won't reiterate here. Suffice it to say, the 1900 had an exceptionally wide CG range, but due to it's long cabin and relatively narrow chord, was often flown at or very near both forward and aft CG limits at various times, giving crews plenty of experience over the entire CG range. Our adventure happened very shortly after the Homer AK crash, so when we got into the air and realized we had a real squirrel on our hands, we made a point of flying real gingerly and quickly but smoothly correcting every bob and weave.
It was my leg to fly, and I've got to hand it to my captain, he let me fly it, but kept a close watch on my progress. He pulled out my W&B and checked my figures again and confirmed we were near the aft limit, but not over it, then said it wasn't unusual to act a bit squirrelly at the aft limit. More than squirrelly, it wanted to go everywhere but straight and level. More like herding it than driving it, and very tiring. When we got to BTV we agreed we should land fast with only one notch of flaps, since full flaps had been the undoing of the Ryan aircraft. At the gates a couple hefty baggage smashers hopped up into the aft compartment to throw down the bags, and the old girl promptly sat down on her tail.
Turns out we were hauling 400 pounds of undocumented Piedmont Comat hiding under the passenger baggage. If the Ryan crew and passengers hadn't paid with their lives for the lesson, it could have been us.

 

[DerAdlerIstGelandet]

Step away for a couple hours and miss all kinds of interesting stuff! I did a little research on tail heavy 1900 crashes and couldn't find one by United Express. The two shining examples I found were Ryan Air in Homer AK, and US Air Express in Charlotte NC, both classic out-of-CG-aft loss of control situations. In an earlier thread I detailed my own experience with a badly out of limits aft Be1900, which I won't reiterate here. Suffice it to say, the 1900 had an exceptionally wide CG range, but due to it's long cabin and relatively narrow chord, was often flown at or very near both forward and aft CG limits at various times, giving crews plenty of experience over the entire CG range. Our adventure happened very shortly after the Homer AK crash, so when we got into the air and realized we had a real squirrel on our hands, we made a point of flying real gingerly and quickly but smoothly correcting every bob and weave.
It was my leg to fly, and I've got to hand it to my captain, he let me fly it, but kept a close watch on my progress. He pulled out my W&B and checked my figures again and confirmed we were near the aft limit, but not over it, then said it wasn't unusual to act a bit squirrelly at the aft limit. More than squirrelly, it wanted to go everywhere but straight and level. More like herding it than driving it, and very tiring. When we got to BTV we agreed we should land fast with only one notch of flaps, since full flaps had been the undoing of the Ryan aircraft. At the gates a couple hefty baggage smashers hopped up into the aft compartment to throw down the bags, and the old girl promptly sat down on her tail.
Turns out we were hauling 400 pounds of undocumented Piedmont Comat hiding under the passenger baggage. If the Ryan crew and passengers hadn't paid with their lives for the lesson, it could have been us.

You are correct, I meant US Air Express not United. My apologies. There was more to it than the out of CG. Improper maintenance ultimately doomed it. Always set your control cable tensions correctly. She went right into a hangar though and killed all 20 on board, and one on the ground.

The 1900 as you know very well was notoriously tail heavy. If we did not use a tail stand when doing maintenance on them, they would tip right over.

 

[XBe02Drvr]

The 1900 as you know very well was notoriously tail heavy. If we did not use a tail stand when doing maintenance on them, they would tip right over.

And yet with two big guys in the cockpit, with their overnight bags in nose baggage, full fuel and no pax, she was at the forward CG limit.

 

[SaparotRob]

You guys should keep going with this. Eighty years later you'll finally get the P-39 right! Anybody have Mr. Bell's phone number?

 

[P-39 Expert]

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.

The engine bay was 90.25" long in both the P-39 and P-63. The aux. stage did not extend past the aft end of the engine bay, there was a bulkhead there.

 

[P-39 Expert]

This manual page shows how little spare room there was between the ASB and oil tank on the P-63.
View attachment 597432

That's because the ASB was in the engine bay, the oil tank was not, and they were separated by a bulkhead. Engine bay was the same length as on the P-39, 90.25".

 

[P-39 Expert]

Question - what IFF set are talking about here? The only Allied electronic IFF bare units used (before the very end of the war) topped out at around 36 lbs plus a few pounds for wiring and such (most of the installation weights I have seen were around 40 lbs total). Where does the 120-130 lbs value come from?

America's Hundred Thousand lists the weight of the IFF radio on the P-39D-2 as 130lbs, and on the P-400 as 110lbs.

 

[P-39 Expert]

How many times do I need to say this: Bell stated that the nose armor was not needed for ballast on the P-39M. Quoted from Vees for Victory, which most of us recognize as the best reference for the Allison V-1710.

How does that work in your CG calculations?

 

[MiTasol]

That's because the ASB was in the engine bay, the oil tank was not, and they were separated by a bulkhead. Engine bay was the same length as on the P-39, 90.25".

What bulkhead?
Show it to me in any photo or manual page or show it with an arrow in the manual diagrams I provided in post 293. Those diagrams from the manuals do not lie.

Note this is the P-63 I worked on taken immediately after we unloaded it from the truck so I do know what is in that engine bay and I do know how long it is. It is one of my few photos that survived the 2013 floods.

Yes there is 90.25 inches in the forward fuselage but that freaking big hole in the rear fuselage between the forward fuselage and the oil tank is not for storing fairy floss. It is part of the total engine bay

 

[MiTasol]

America's Hundred Thousand lists the weight of the IFF radio on the P-39D-2 as 130lbs, and on the P-400 as 110lbs.

That is interesting. Both were fitted with the SCR-595 if my memory is correct (the Brits called it the R3003 or some such thing) and here is the manual section showing the weight of the radio (32.5lb including mounting) and all its additional components. Maybe they were including the tray in the rear fuselage that the mounting sits on and its support beams but I still cannot see 110 lbs let alone 130.

 

[MiTasol]

The engine bay was 90.25" long in both the P-39 and P-63. The aux. stage did not extend past the aft end of the engine bay, there was a bulkhead there.

What freaking bulkhead - show me some proof.

 

[wuzak]

America's Hundred Thousand lists the weight of the IFF radio on the P-39D-2 as 130lbs, and on the P-400 as 110lbs.

Table 26 has a line item "Miscellaneous Equipment (Radio)" 129.2lb for the P-39D-2 and 109.9lb for the P-400. It doesn't mention IFF.

Under the electrical system section in the table, "Communication" lists 62.0lb, 80.2lb and 35.0lb for the P-39D, P-39D-2 and P-400 respectively.

In the systems description section for communication, navigation and identification systems three radio systems are described as being typical equipment:

SCR-535A radio recognition set with detonator and crash inertia switch (that's be the IFF).

SCR-522A command radio set

SCR-274N radio.

 

[wuzak]

Here is a picture of a P-39 engine bay, looking rearwards

P-39 engine bay by Errol Cavit, on Flickr

 

[MiTasol]

Here is a picture of a P-39 engine bay, looking rearwards

View attachment 597503P-39 engine bay by Errol Cavit, on Flickr

Yep, nice rebuild - the structure at the upper rear is to support the coolant tank

 

[MiTasol]

From P-63 manual - notice the lack of coolant tank support structure.

 

[MiTasol]

This photo from the manual shows the removal of the rear fuselage attach bolts (fuselage splice bolts).
The hole the guys arm is going through is the hole the oil tank fits in.
You will note the total absence of any bulkhead at the end of the engine bay. That is to allow part of the ASB to be installed into the rear fuselage.

 

[FLYBOYJ]

Adding rear ballast after removing forward ballast?

Actually my bad, no ballast!

 

[FLYBOYJ]

How many times do I need to say this: Bell stated that the nose armor was not needed for ballast on the P-39M. Quoted from Vees for Victory, which most of us recognize as the best reference for the Allison V-1710.

How does that work in your CG calculations?

Go back and READ our calculations (Post 368). In our example using numbers from the P-39Q loading chart, IT PUTS THE AIRCRAFT OUT OF CG!! It's probably the same for the P-39M as well.