XP-39 II - The Groundhog Day Thread

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P-39 Expert said:
I understand the critical engine.
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No you don't.

P-39 Expert said:
If both props are turning right as viewed from behind then the right engine would be the critical engine. If the left engine is lost then the right engine torque (and the drag of the dead left engine) tends to pull the plane into a left bank.
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WRONG! You've been corrected on this at least twice, and yet you persist in this erroneous explanation. Critical engine is the engine most dangerous to lose, in this case, left engine. I earned my multi engine rating and multi instructor rating in planes of this sort, then instructed in them. The differences between critical and non-critical engines are due to P factor, and torque has next to nothing to do with it. The real killer here is asymmetric thrust and the yaw and roll it induces, and that happens with EITHER engine out, it's just a little bit worse with the critical engine out.
Don't come to me with your shiny new Whizbang 260 looking for a multi engine rating! From what I've seen of your willingness to absorb new information if it conflicts with your preconceived notions, I wouldn't get in an airplane with you and put both our lives on the line for all the tea in China. Engine-out training is risky enough with a student who's got his/her head screwed on straight. You never know what creative new stunt they'll pull, out of confusion, misunderstanding, or nervousness. In your case it could easily be a suicide mission.
 
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I thought the 'critical' engine on the P38 was the one with the generator on it because they only put one generator one one engine for most of the P38's life and if you lost that engine you would lose electrical power soon after and that would cause issues that I can't remember resulting in loss of the aircraft.
 
That is a classification based on equipment piled on the engine. If we're talking performance and depending if "the glass is half full or half empty" The P 38 either has two critical engines or no critical engine. This is been debated for years.
 
Different definition of "critical" than the classic twin engine one. Since the P38 didn't have a classic critical engine (or two of them!), then this definition could suffice.
IIRC, that single generator issue only applied to the earliest models, including the non turbo versions built for the Brits.
The Doyne Conversion Apache I got my multi in had a single generator on the left engine and a single hydraulic pump on the right, and since both gear and flaps were electrically controlled and hydraulically actuated, you couldn't afford to lose either engine. I spent lots of time trying to keep the dang thing straight on one engine while wailing away on the the manual hydraulic pump. Executing a single engine go around was an athletic workout, as aileron and rudder forces were HEAVY and the hydraulic wobble pump was very stiff to operate and required you to bend down almost to the floor while you struggled to gain 100 feet/minute until you got the plane cleaned up. Only VFR in flat country, as the climb gradient was pretty marginal. If I had to do that night IFR at a "hole in the hills" airport in mountain country, it would be doomed to failure. Just belly it in, wheels up. A year later, the old girl got a second generator (actually, converted to alternators), and an electrically driven aux hydraulic pump. Hallelujah!
 
I always thought the critical engine was the one with the worse performance in a twin with same-turning engines. It is invariably the one that rotates outward at the top of the prop arc. Since the P-38 had BOTH engines turning outward at the top of the prop arc, then either one should perform substantially the same within normal engine variation ... ergo, there doesn't seem to BE a critical engine.

But, as you say, there seem to be room for debate. Actually, there always seems to be some debate, whether there is room for it or not, doesn't there? So, there we have the answer: We have: 1) no critical engine, 2) two critical engines, or 3) one critical engine (only one generator), depending on who is writing the text.

Another victory for clarity ...

Cheers.
 
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Hello P-39 Expert,

P-39 Expert said:
Flat trajectory out to 400yds from "Cobra" by Birch Matthews.
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I have that book as well. It has a lot of nice photographs and references but the author is a "fan boy" for the Airacobra.
His opinions which are asserted as fact often do not stand up to scrutiny so I would be very wary of quoting him.
Regardless of who made that statement of a "Flat trajectory out to 400 yards", you should have the sense to know it can't possibly be true. NOTHING flies flat (mostly true). It is just a matter of HOW curved the trajectory is.

P-39 Expert said:
P-39, P-59, P-63.
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If the 37 mm were such a great weapon, why was Bell the only company that chose to use it?
Everyone else seemed to go with multiple .50 cal or 20 mm instead even when the aircraft clearly could carry the 37 mm if it needed to.

P-39 Expert said:
Should have been easy to reduce the weight also. I have no idea why it wasn't implemented.
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Perhaps because increasing the ammunition capacity wasn't that easy.
As for reducing weight, the operators of the aircraft were of the opinion that more armour and not less was needed.


Hello Mad Dog,

From what I have read about the King Cobra, it seemed to have very good maneuverability as compared to the types already in service. The problem though was that it was also significantly slower than types already in service and those in power seemed to prefer speed over other qualities.

- Ivan.
 
GregP said:
Here's a guy mostly at or below 250 mph who isn't having any trouble.

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Of course he's not having trouble, they don't let 21 year old kids fly 75-80 year old warbirds worth the big bucks, that means the guy is very experienced and knows his way around one. There's no doubt the young kids flying the 39s and 400s back in the war had a lot less experience, and were possibly more rattled if things went sideways..........
 
glennasher said:
......There's no doubt the young kids flying the 39s and 400s back in the war had a lot less experience, and were possibly more rattled if things went sideways..........
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There was a book by Edward Parks called Nanette about his experience of flying the P-39 in New Guinea. It's a very good read. In it, he recounts an event where the electric propeller pitch control failed on a P-39 on a training flight, and the rookie pilot was so flustered he did nothing but carried on flying straight and level until the plane stalled and crashed.
 
What would have an experienced pilot done?
 
SaparotRob said:
What would have an experienced pilot done?
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Not having flown a P-39, I'm not sure of the proceedure to remedy that problem, but I can safely say that if my plane were experiencing issues, I'd call it out and either turn back or, if I was past the PONR, look for a suitable place to put it down (within friendly territory, if possible).
As a last resort, steady her out and hit the silk.
 
On the 37mm,
from a ballistics perspective (and going back to high school physics) we can have a practical range where the guns seems to shoot flat, It is pointed up slightly and so does not vary from the line of sight by very much out to the preset distance. EVERY projectile falls at the same speed, the old high school 32 ft per second per second. the projectile falls 8 feet in the first second of flight, (starts at zero fps in the fall and ends at 32 fps ) it falls 48 feet in the 2nd second of flight. (starts at 32fps and ends at 64fps). The 37mm, 20mm, .50 cal and .30 cal will all fall at the same speed.
However what really matters to our pilots is the time of flight to the target. This governs both how far the bullet falls and how far the target moves between the bullet leaving the barrel and reaching the target area. With the 20mm and .50 cal both having about 40% higher velocity they will cover around 40% more distance in the same period of time. This is where the flatter shooting comes from. It also makes deflection shooting very difficult with plane that has different guns, forget looping trajectories. if the .50 cal bullets are hitting the 37mm shells are behind the aircraft. If the 37 does hit it means the .50s are flying in front of the nose.

This is a bit of simplification as the 37mm should keep it's velocity better than the others but since the difference is so great the crossover point (where the 20mm and .50 cal bullets do travel slower than the 37mm) may be well beyond any practical range, the .50 and 20mm were well matched out to over 600yds, The closer the firing plane gets to the 6 o'clock position the less difference in velocity makes.

edit, flunk high school math, Projectile falls 16 ft in the first second not 8ft, the 48 feet in the 2nd second of flight would be correct.
 
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SaparotRob said:
What would have an experienced pilot done?
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1. Maintain Aircraft Control
2. Analyze the Situation
3. Take Appropriate Action
4. Land as soon as conditions permit

1 means fly the plane. If you are about to stall get the nose down and add power (if that's an option). 2. Using aircraft systems knowledge assess. 3 means coming up with a plan. 4 means get it to a safe conclusion.

Freezing up indicates task saturation / sensory overload.

Cheers,
Biff
 
GregP said:
Here's a guy mostly at or below 250 mph who isn't having any trouble.

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Hello GregP,

That Guy probably isn't flying a P-39 with a CoG at or beyond its aft limit either.
The fellow trying to recover from a tumble has an aircraft that doesn't necessarily want to fly nose first.
He also may have some remaining rotational momentum from the tumbling he has just done and wants to give the aeroplane a chance to point itself nose down without any significant yaw or pitch and gain some speed so he can safely pull some G to get out of the vertical dive.

Those late model and war weary P-39 that served state-side as trainers probably never carried full armament and ammunition and were most likely ballasted in a manner so that they simply could not get CoG out of range by expending stores. This would be the same situation as modern airshow aircraft.

SaparotRob said:
What would have an experienced pilot done?
Click to expand...

Hello SaparotRob,

My guess is that an experienced pilot probably would have put the propeller into manual control and tried to adjust the RPM to something reasonable for the power setting.

- Ivan.
 
SaparotRob said:
What would have an experienced pilot done?
Click to expand...
When the P-39 prop pitch control went, the blades went into fine pitch which meant they produced less forward thrust whilst being easier for the engine to spin. If you did nothing, the aircraft speed gradually drops whilst the engine quickly over-revs. Whilst the engine screams, the aircraft loses speed to the point where there is not enough lift generated by the wings, at which point you will stall and usually drop into a spin. Without engine thrust to help you pull out of the spin, you might not recover control before you hit the ground. The textbook answer is to feather the prop (if you still can), switch off the engine and adopt a shallow-angled glide whilst looking ahead for a clear spot to land. If you can't feather the prop then it becomes a big airbrake, meaning you have to dive at a steeper angle to maintain flying speed, which reduces the amount of time you have to either find a landing spot or bail out. Even if you can feather the prop and stretch your glide, if there's no spot to land then you have to take to your parachute. The worst thing you can do is to do nothing, which is what the rookie did in the book.
 
Those were great responses guys. Very understandable.
I especially liked Ivan1GFP's answer. "..an airplane that doesn't necessarily want to fly nose first" put it all together for me. I believe I understand what the plane wants to do and why it's so difficult correct it. Put that whole center of gravity thing into perspective.
The explanations of what the pilot has to respond to made it real to me.
 
You can make a VERY safe bet that the P-39 in the clip is not at aft CG! Just seemed like too good a clip not to post is all, considering how dangerous it is purported to be under 250 mph.

Of course, it also doesn't have all the armor or armament since it's a civilian bird, now back in the U.S.A. Just to give you an idea what that means, the Planes of Fame operates a P-51A with Allison power. The wartime specs for it say the empty weight is around 6,433 lbs (2.918 kg). But, ours doesn't have armament, armor, or any wiring not needed for aircraft operation. It also has a modern radio, which weighs about nothing compared with a WWII unit.

The end result of it is that the empty weight is WAY less than stock. That P-39 you see in the clip isn't anywhere NEAR combat weight, and is quite spritely in acceleration.
 
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