Double Trouble!!

[earl1937]

Earl were they counter rotating engines like the P38?

The early tests in the F-82, it would not get off the ground! They had arranged the counter rotating props where the ascending blades on both engines arrived at the same time, in parall to the center section. I don't know the in and outs of the problem, other than what I discovered researching that question for you: The XP-82 was to be powered by two Packard-built Rolls-Royce V-1650 Merlin engines. Initially, the left engine was a V-1650-23 with a gear reduction box to allow the left propeller to turn opposite to the right propeller, which was driven by the more conventional V-1650-25. In this arrangement both propellers would turn upward as they approached the center wing, which in theory would have allowed better single-engine control. This proved not to be the case when the aircraft refused to become airborne during its first flight attempt. After a month of work North American engineers finally discovered that rotating the propellers to meet in the center on their upward turn created sufficient drag to cancel out all lift from the center wing section, one quarter of the aircraft's total wing surface area. The engines and propellers were then exchanged, with their rotation meeting on the downward turn, and the problem was fully solved. The first XP-82 prototype (44-83886) was completed on 25 May 1945, and made the type's first successful flight on 26 June 1945. This aircraft was accepted by the Army Air Forces on 30 August 1945, whose officials were so impressed by the aircraft, while still in development, that they ordered the first production P-82Bs in March 1945, fully three months before its first flight.
Prototype XP-82s, P-82Bs and P-82Es retained both fully equipped cockpits so that pilots could fly the aircraft from either position, alternating control on long flights, while later night fighter versions kept the cockpit on the left side only, placing the radar operator in the right position.

[earl1937]

Earl,
The basis for the design is a lengthened H fuselage, but the reality of building a different aircraft with new points of fatigue necessitated that they redesign the entirety of the fuselage.  I was heavily involved in the former CAF P-82 acquisition and restoration before the Air Force stole the aircraft and took it to Dayton.  The differences are one of the major reasons that these aircraft are so rare.  Almost nothing that goes on a Mustang fits on a Twin Mustang short of GFE and cockpit items.

My memory is getting bad! Just discovered that it was in fact, a 57 inch "Plug" which they placed behind the cockpits, not 47 inches as I had thought. Gosh, time flies and memory fades. Did they recover the F-82 that you were working on from Davis-Monthan? The last time I had a chance to visit that sad place, there were, as I remember 5 or 6 in different conditions, or it might have been just several different parts that I was looking at.(In 1966)
Don't mean to sound melodramatic, but wouldn't you like to have the money and resources to just go though that place and say: "Rebuilt this one, that one". (Talking about all the different aircraft left there).

[earl1937]

Earl,
The basis for the design is a lengthened H fuselage, but the reality of building a different aircraft with new points of fatigue necessitated that they redesign the entirety of the fuselage.  I was heavily involved in the former CAF P-82 acquisition and restoration before the Air Force stole the aircraft and took it to Dayton.  The differences are one of the major reasons that these aircraft are so rare.  Almost nothing that goes on a Mustang fits on a Twin Mustang short of GFE and cockpit items.
  Not sure where you got your info, but: ailerons, flaps, rudders, main landing gear and tail wheel, engines and props were the same as the H. As I understood it, when they "stretched" the fuseledge, they added additional stringers and gussets to strenghen the added 57 inches to fuseledge. The tooling jigs for all those items where already in place, why would they spend research money, when they had something which could be modified to use on the new F-82? The outer wing panels were straight off the P-51H assembley line, the only exception and new development was the center wing section, which had to have the main wing spar designed and produced.

[colmbo]

In this arrangement both propellers would turn upward as they approached the center wing, which in theory would have allowed better single-engine control.

And that doesn't make any sense.  If I understand this correctly the right engine would be clockwise, the left engine counter-clockwise.  This puts the descending (higher thrust) blade to the outside which degrades single-engine performance by creating more yaw.  (This is the way the P-38 was configured however)

[Megalodon]

The wings had the guns and landing gear recesses removed

 

[earl1937]

And that doesn't make any sense.  If I understand this correctly the right engine would be clockwise, the left engine counter-clockwise.  This puts the descending (higher thrust) blade to the outside which degrades single-engine performance by creating more yaw.  (This is the way the P-38 was configured however)

Yep, made no sense to me either! With the acending blade on the outside, though, because of the off centered thrust, the "P" factor and torque was a problem in single engine operations, but you had 2 rudders instead of one to over come those two problems.

[Randy1]

Can you imagine the "Oh crap! feeling they figured that one out.  Thanks Earl. 

[Bodhi]

Mr. Earl,
We acquired "our" P-82 from the CAF in the early 2000 years.  It was an aircraft they had been given by the Air Force in free and clear title back in the 60's or 70's.  They operated it all they way up until the '80's or '90's when a hard landing forced them to put her up.  The lack of available spares and prevailing costs to fix such a unique aircraft were part of the reason it never flew again.  We had intended it to be fully restored but were having a hard time finding a counter rotating prop and were in the process of having a few made when the Air Force decided they wanted it.  So, they used their powers and the US Government to take it back even though the title had been legally transferred.  In the end, it turned into a very costly mess.

Sadly, there is very little if anything left at Davis-Monthan from that era that the smelters did not get.  To have had that money in the '60's and '70's would have been a dream....

As for the info I have, all of it was gleaned from being involved in the project and the research it took to allow restoration to begin.  Could I be wrong, sure, so I apologize if something is in error as it has been several years since we dealt with the P-82.  

Basically, the specifics that I remember were that the wings are redesigns because of the placement of the gear changes the structure significantly.  It is not a simple skin over as the gear wells have large castings and formers that are heavy.  They did not leave them in place as it would not be a tremendous waste in weight.  As I am sure you are well aware, the gear well on a single engine Mustang is a very weak structural link when it opens and actually requires a mechanical up lock on the door to prevent the door from creeping down in flight and thus being ripped off causing subsequent structural failure of the wing.  This area changed significantly from an H series Mustang.  The "plug" in the fuselage in the XP-82 changed from just an add on to actually widening and changing fuselage formers.  The doghouse radiator assemblies are different, but I do not remember if the radiators are or not, but I would lean them towards being different.  The vertical fins are larger and that I believe that changed the rudders.  I can not remember on the flaps or ailerons, it has been too long.  I remember the canopies being different as well as I believe the forward structure for the windscreen.  I also remember the landing gear being stronger to make up for the significant weight increase from the single engine mustangs.

As for changing existing assembly techniques, that's too hard to guess with the military industrial complex.  They have never been good about money, but I do believe they thought there was commonality between airframes up until the XP-82 brought on the changes needed in production birds.  

Anyways, that's all I have to offer to this discussion.  It is amazing to hear of your experiences and stories from that era.  I feel truly blessed to be able to be a part of your discussions on any subject from that era.  Thank you again for your service and thank you for providing all the information that you offer up so freely.

<S>

[Megalodon]

Actually, they took 2 "H" model fuseledge and added 47 inches behind the two cockpits, added, if memory serves 11 inches in length and 4 inches in height to the dorsal fin, for better single engine handling. All 6 of the M2's were "center" wing mounted, therefore they were much more accurate and you could load the 'trees" with 25, 5 inch rockets.

Did they ever use the drop tanks or bombs or the eight 50 gun pak  14 50's ?



The P/F 82E was also the return of the Allison engine and re-designation from P to F.

 

[Brooke]

As for whether the F-82 is substantially two P-51H's or substantially different -- I completely defer to you other guys who know a lot more about it than I do.

[Brooke]

Folks have talked about a couple of the factors in choosing direction of rotation for counter-rotating props on twin-engine planes.

Another one is that, if the inside blades are travelling up and outside blades travelling down, it contributes to the inboard portion of the wing stalling before the outboard portion when you stall the plane under high power.  Designers prefer the outboard portion to stall last so that pilots have a better chance of maintaining roll control during the stall, which is a reason that wings generally are designed with some washout (decreased angle of incidence on the outboard portions of the wing).

In Bodie's book on the P-38, it talks (I think, but could be wrong -- it has been a while since I read it) about how the first prototypes had props with inside blades travelling down, then had some issue, and reversed it in later ones.  I can't remember why.

[earl1937]

Folks have talked about a couple of the factors in choosing direction of rotation for counter-rotating props on twin-engine planes.

Another one is that, if the inside blades are travelling up and outside blades travelling down, it contributes to the inboard portion of the wing stalling before the outboard portion when you stall the plane under high power.  Designers prefer the outboard portion to stall last so that pilots have a better chance of maintaining roll control during the stall, which is a reason that wings generally are designed with some washout (decreased angle of incidence on the outboard portions of the wing).

In Bodie's book on the P-38, it talks (I think, but could be wrong -- it has been a while since I read it) about how the first prototypes had props with inside blades travelling down, then had some issue, and reversed it in later ones.  I can't remember why.

I know that both the 82 and the 38 had "teething" problems with their counter-rotating props, the 82 wouldn't even get off the ground! It took them, from what I have read a month to figure out which way to turn the the props to stop creating so much drag that the thrust couldn't overcome the problem. The 38 would not have had as much problem because it started in a more or less level attitude because of the tricycle gear and there fore asymmetrical thrust, "P" factor and torque, would not have been as acute a problem as the 82 from a nose up attitude on takeoff roll. I never flew a 38, but sat in one, took the check list and was dumb founded by the starting sequence on getting the power plants fired up.

[GScholz]

The way I understand it is that the prop wash creates an airflow that cork-screws around the fuselage in the direction of the prop turning. If both prop blades turns upward in front of the center wing section they will increase the effective angle off attack of that wing in relation to the local airflow, thus stalling it. This is also why one wing stalls before the other on a single engined prop plane with power on. It will be the wing on the side where the prop turns upward. At least that's how I understand it.

[GScholz]

[Brooke]

If both prop blades turns upward in front of the center wing section they will increase the effective angle off attack of that wing in relation to the local airflow

Yep.  Hence maybe the desire for blades to travel up on the inboard (like the P-38) so that inboard stalls first (although, again, other factors might have been more important -- I need to look at Bodie's book).

On the F-82, they seem to travel down inboard, so I guess other factors were more important (like Earl was talking about)?