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View Full Version : Something you didnt know about P38s



XyZspineZyX
10-14-2003, 06:33 PM
Why does most P38s props rotate outwards
I have wondered about this , and here is maybe the answer


David Pincus wrote:

> Craig Wall wrote:
> >
> > Terry Schell wrote:
> >
> > > highflyer wrote:
> > > <snip
> > > >
> > > > I do know that, during the war, they decided the P-38 needed to
> > > > have counterrotating props. They installed them on the P-38-J
> > > > which was a quite common P-38 model. In the P-38-J both props
> > > > rotated outboard at the top. Think about that for a while.
> > >
> > > Are you sure they counter-rotated the "wrong" way? That seems strange.
> > > Any ideas on the rationale? Could it be related to the fact that the
> > > rudders on this bird were directly in the propwash, unlike most twins?
> >
> > Yep, they do, and I once got to ask Kelly Johnson that very same
> > question. And he answered me.
> >
> > Craig Wall
>
> The counterrotation props were used on ALL USAAF models of the
> Lightning, including the original pre-production YP-38s. Only the
> British export Model 322 Lightnings had the same rotation on both props.
>
> As Craig has related Mr. Johnson's comments, the ORIGINAL direction of
> rotation of the prop wash on the rudder and horizontal stab was part of
> the problem that caused a number of test a/c to loose their tail
> asssemblies during high speed dives, near-mach tuckunder not
> withstanding. Many fixes were tried, including switching the left and
> right engines. This help reduce the stresses on the tail assembly,
> though it took the underwing dive brakes to finally tame the near-mach
> dive problems. The dive flaps were instituted during the late "K" series
> (-25 series I think) but the countrarotating propellor switch happened
> earlier on in the production run.
>
> David Pincus

I didn't relate any comments at all, yet!

I met Kelly in about '82 or '83 at a seminar in Norman, Oklahoma. I asked
him about the P-38 props turning outboard at the top, giving two critical
engines.

I though I knew the answer: because the spiral flow off the props was
opposite the tip vortices, the ship should be cleaner. And it is. But that
wasn't the reason he gave....

He just said "it made a better gun platform".

That was it. The entire purpose of the airplane was to shoot, and anything
that made it do that job better was the deciding factor in all decisions like
the prop rotation, etc...

He said they actually tried it in every possible combination of prop
directions, and that's the one that worked best and gave the highest gunnery
scores.

That day I learned to *focus*.

Craig Wall





VICTOR MAY HAVE BEEN A WEIRDO,BUT HE WAS A DAMN GOOD FIGHTERPILOT.
<ceter>http://www.boners.com/content/788904.1.jpg </center>

XyZspineZyX
10-14-2003, 06:33 PM
Why does most P38s props rotate outwards
I have wondered about this , and here is maybe the answer


David Pincus wrote:

> Craig Wall wrote:
> >
> > Terry Schell wrote:
> >
> > > highflyer wrote:
> > > <snip
> > > >
> > > > I do know that, during the war, they decided the P-38 needed to
> > > > have counterrotating props. They installed them on the P-38-J
> > > > which was a quite common P-38 model. In the P-38-J both props
> > > > rotated outboard at the top. Think about that for a while.
> > >
> > > Are you sure they counter-rotated the "wrong" way? That seems strange.
> > > Any ideas on the rationale? Could it be related to the fact that the
> > > rudders on this bird were directly in the propwash, unlike most twins?
> >
> > Yep, they do, and I once got to ask Kelly Johnson that very same
> > question. And he answered me.
> >
> > Craig Wall
>
> The counterrotation props were used on ALL USAAF models of the
> Lightning, including the original pre-production YP-38s. Only the
> British export Model 322 Lightnings had the same rotation on both props.
>
> As Craig has related Mr. Johnson's comments, the ORIGINAL direction of
> rotation of the prop wash on the rudder and horizontal stab was part of
> the problem that caused a number of test a/c to loose their tail
> asssemblies during high speed dives, near-mach tuckunder not
> withstanding. Many fixes were tried, including switching the left and
> right engines. This help reduce the stresses on the tail assembly,
> though it took the underwing dive brakes to finally tame the near-mach
> dive problems. The dive flaps were instituted during the late "K" series
> (-25 series I think) but the countrarotating propellor switch happened
> earlier on in the production run.
>
> David Pincus

I didn't relate any comments at all, yet!

I met Kelly in about '82 or '83 at a seminar in Norman, Oklahoma. I asked
him about the P-38 props turning outboard at the top, giving two critical
engines.

I though I knew the answer: because the spiral flow off the props was
opposite the tip vortices, the ship should be cleaner. And it is. But that
wasn't the reason he gave....

He just said "it made a better gun platform".

That was it. The entire purpose of the airplane was to shoot, and anything
that made it do that job better was the deciding factor in all decisions like
the prop rotation, etc...

He said they actually tried it in every possible combination of prop
directions, and that's the one that worked best and gave the highest gunnery
scores.

That day I learned to *focus*.

Craig Wall





VICTOR MAY HAVE BEEN A WEIRDO,BUT HE WAS A DAMN GOOD FIGHTERPILOT.
<ceter>http://www.boners.com/content/788904.1.jpg </center>

XyZspineZyX
10-14-2003, 07:08 PM
Good info and a nice read!

fluke39
10-15-2003, 11:37 AM
i guess this fits with what you stated but i have always read that it was due to excessive vibration caused if both props turned the same way ( why this was i can't recall - maybe due to tail plane and propwash)

<center><img src=http://mysite.freeserve.com/Angel_one_five/flukelogo.jpg>

XyZspineZyX
10-15-2003, 11:52 AM
Very interesting info, thanks.

If you want other good infos about the P-38, go here:

http://www.yarchive.net/mil/p38.html


So, if the FM of the P-38 will be accurate in FB, the Lightning will be an excellent gun plateform, have a good roll rate (with the "J" hydraulic ailerons), good in dives with its airbrakes and fast.

Cheers,

XyZspineZyX
10-15-2003, 01:36 PM
it was a good gun platform with all guns
consentrated in the nose,this gave it a tremendous punch
I know the German pilots avoid head on passes agains it,and thats pretty fun ,when you know all other
allied fighter were warned about doing the same against
the German fighter.(Fw190)

VICTOR MAY HAVE BEEN A WEIRDO,BUT HE WAS A DAMN GOOD FIGHTERPILOT.
<ceter>http://www.boners.com/content/788904.1.jpg </center>

XyZspineZyX
10-16-2003, 12:19 AM
and bump

VICTOR MAY HAVE BEEN A WEIRDO,BUT HE WAS A DAMN GOOD FIGHTERPILOT.
<ceter>http://www.boners.com/content/788904.1.jpg </center>

XyZspineZyX
10-16-2003, 12:57 AM
It was was done to offset the p load factor on the booms, the gun stablity was an extra. I alos remove the the "critical" engine condition of a one engine out situation.

BTW a critical engine is the engine on twin engine plane that if lost will cause the aircraft to suffer severe yaw due to thrust and torque severly affecting its stablity

http://www.imagestation.com/picture/sraid79/p9141f290fa1c1c59a2dc382c77af21f3/fb1a8321.jpg


Lead Whiner for the P-47D-40, M and N and Hvars

XyZspineZyX
10-16-2003, 01:44 AM
Loosing a engine on a plane - during takeoff - that has two counter rotating props is more dangerous than loosing a engine on a plane that has both props going the same way.

http://www.mechmodels.com/images/klv_ubisig1a.jpg


Oh yeah, I'm a P-63 whiner too! /i/smilies/16x16_smiley-very-happy.gif

XyZspineZyX
10-16-2003, 02:37 AM
Have you seen the pilot training video? They demonstrate the loss of an engine on takeoff a few times and say its no problem as long as you know the procedures and feather the dead engine soon. The P-38 was a very good aircraft even on 1 engine. Infact, there was no manuver it could not do on 1 engine. To install confadence in 1 engine flight, they would play "fallow the leader" in flight school. The leader was always the instructor and he would do loops, rolls (in and out of the dead engine) battel rolls, split S and every basic combat manuver known, and the students would fallow. The guy who told me this said he never lost a student, and he would do this on there 2nd time solo in the P-38. That tells me it was a damn solid aircraft with 1 engine if he is having his students do combat manuvers with 1 dead engine on there 2nd flight!!!

Gib

No fancy quote or cool photo.... YET

XyZspineZyX
10-16-2003, 07:55 AM
Fjuff79, thanks for the post. It answered a question I'd had in my geek brain for some time about the P-38 and critical engine(s).

Sniper762x51, you're right that the counter-rotating props add to the aircraft's stability, especially near the stall (assuming both engines are turning at about the same speed there's no net torque to roll the plane off to one side.) But with the engines turning in the directions they did (the upper blade rotating toward the wingtip for both engines), there were two critical engines, as the original post stated. This is due primarily to P-factor.

P-factor refers to the asymmetric thrust produced by the propellor as the aircraft's angle of attack increases. As the AoA goes up, the ascending blade in the propellor disk is at a lower angle of attack (blade pitch minus AoA) than the decending blade (blade pitch plus AoA.) With the decending blade on the outside of both props, losing either would produce an asymmetric thrust on the outside of the prop disk, just where you don't want it for yaw control when single-engine. Rotating the engines in the opposite direction (upper blades rotating inward for both engines) would have been better from a P-factor standpoint. Apparently the airflow twisting inward (in the same direction as the tip vortices the author mentions) had an adverse affect on gun aiming. Most likely the folks making the decision also deemed there to be enough rudder authority with the loss of an engine to deal with the critical engine factor as well.

Blotto

"Only the spirit of attack, born in a brave heart, will bring success to any fighter craft, no matter how technically advanced." - A. Galland

"Look, do you want the jets, or would you rather I slap the props back on?" - W. Messerschmitt

http://home.mindspring.com/~blottogg/sitebuildercontent/sitebuilderpictures/14fsPatch.gif

XyZspineZyX
10-16-2003, 11:20 AM
I like your inputs,
ceep em coming

VICTOR MAY HAVE BEEN A WEIRDO,BUT HE WAS A DAMN GOOD FIGHTERPILOT.
<ceter>http://www.boners.com/content/788904.1.jpg </center>

XyZspineZyX
10-16-2003, 02:27 PM
Out of curious, I had read few performance charts along WWII fighters. Tell me how good rate of roll of P-38F/G/H and P-38J/L against other enemy planes? Men, tell me which well-know emeny planes that P-38 can outroll them.

Regards
SnowLeopard

XyZspineZyX
10-16-2003, 02:48 PM
some say there was lag on the roll
between movin stick and plane movin

XyZspineZyX
10-16-2003, 04:35 PM
Yes there was. But what are you compairing it too? Light single engine fighters nearly half its weight like the 109 with not even fuel tanks in the wings, or other twin engine fighters with the engines on the wings? It was the best rolling twin engine aircraft and 2nd only to the FW-190, and the L could even out roll the FW-190 at high speeds above 300MPH (I think its above 300MPH, but im sure the Luftwhiner twins will quickly correct me). Skychimp and Milo have posted a roll chart here a few times. Im sure it will pop up again rather quickly.

Gib

p1ngu666 wrote:
- some say there was lag on the roll
- between movin stick and plane movin
-
-
-
-



No fancy quote or cool photo.... YET

XyZspineZyX
10-16-2003, 06:16 PM
Gibbage1 wrote:
- Have you seen the pilot training video? They
- demonstrate the loss of an engine on takeoff a few
- times and say its no problem as long as you know the
- procedures and feather the dead engine soon. The
- P-38 was a very good aircraft even on 1 engine.
- Infact, there was no manuver it could not do on 1
- engine. To install confadence in 1 engine flight,
- they would play "fallow the leader" in flight
- school. The leader was always the instructor and he
- would do loops, rolls (in and out of the dead
- engine) battel rolls, split S and every basic combat
- manuver known, and the students would fallow. The
- guy who told me this said he never lost a student,
- and he would do this on there 2nd time solo in the
- P-38. That tells me it was a damn solid aircraft
- with 1 engine if he is having his students do combat
- manuvers with 1 dead engine on there 2nd flight!!!

What I'm saying is, if you lost a engine on takeoff, <U>and were not experienced with the aircraft</U>, she'd go up, then roll over, then down. Surely you've read about that?

Thomas H. Jones, P-38 pilot:

"With 3,000rpm and 55 inches of mercury on take-off, if either engine failed it was impossible to hold torque with the one leg muscle. The drill was to ****** the good throttle, crank in trim, then re-boost and don't forget to feather the dead engine. Otherwise the bird went up - then over - then down!"

This effect will also happen on other twin engine aircraft, but is more pronounced on some. Look around for a Grumman OV-1D Mohawk - they had a similar problem, and probably one of the most dangerous.

In flight, loosing a engine wouldn't be as big a deal, but on take-off with that power... Whoo boy! /i/smilies/16x16_smiley-wink.gif

http://www.mechmodels.com/images/klv_ubisig1a.jpg


Oh yeah, I'm a P-63 whiner too! /i/smilies/16x16_smiley-very-happy.gif

XyZspineZyX
10-16-2003, 06:50 PM
It would have a multitude of benefits, and this is not at all uncommon in twins, the idea being that you can negate the effects of torque and p-factor with both engines running (the reason we need to hold rudder on takeoff and climbout or any high power low speed portion of flight) which means that the aircraft would be less prone to slippng or skidding, (ie the nose would point more accurately into the direction of flight), it would have less pronounced departure characteristics (accelerated stalls would have less pronounced wing drop, spins would not be as aggravated by power,), and it would have more neautral ground handling, and by making the top of each blade rotate toward the center of the aircraft would also mean that in the event of an engine failure that torque would have the natural tendency to roll the aircraft toward the running engine instead of the dead one engine (the correct response that the pilot needs to have, with a dead engine you will need alot of rudder and aileron into the running engine)

Of course this would also make it a good gun platform!

S!
TX-EcoDragon
Black 1
TX Squadron XO
http://www.txsquadron.com

(Former)Reserve Pilot Aircraft #2 of Gruppo 313
Pattuglia Acrobatica Virtuale
http://www.pav-amvi.it

http://www.attitudeaviation.com/

http://www.calaggieflyers.com/



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Message Edited on 10/16/0310:26AM by TX-EcoDragon

XyZspineZyX
10-17-2003, 02:18 AM
p1ngu666 wrote:
- some say there was lag on the roll
- between movin stick and plane movin
-
P-38 control rolls by turn yoke not stick.

Regards
SnowLeopard

XyZspineZyX
10-17-2003, 02:39 AM
The FB 262 is a uber roller in my opinion.... could a big heavy twin jet roll like that?? Does anyone have access to official performance data for the 262, apart from speed and the like. I'm talking roll rates and unusual stuff. landing speeds, runway lengths,

Virtually I'm wondering if anyone has a Me-262 ops manmual they could scan.... lol

FuryFighter