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Mustang Gear Doors and Flaps, the eternal debate

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  • Member since
    December 2002
  • From: Northern California
Mustang Gear Doors and Flaps, the eternal debate
Posted by jeaton01 on Tuesday, December 22, 2020 9:11 PM

Frank's comment was bugging me, there has been so much discussion about gear doors and flaps on Mustangs, so I spent much of the afternoon studying the Erection and Maintenance manual I have which is a WW II document.  It covers the P-51A, P-51B-5 and P-51C-1 Mustangs.  The long and short is that all these airplanes had identical landing gear and flap systems.  The was a mechanical uplock on the inner gear doors, but it was mechanically linked to the main strut actuator and only engaged when the main gear strut was in the retracted position.  It was held in the unlocked position when the gear was down.  The main gear strut down lock was the only part of the system that remained locked in position when hydraulic power was lost.  It had an internal spring which held it to the locked position, and it required hydraulic pressure to cause it to disengage and allow the strut to retract.  The inner gear doors and the flaps had no other means of staying up than the remaining hydraulic pressure after shutdown.  There was an accumulator in the system but it would not hold pressure indefinitely.  The bleed down rate would depend on the internal leakage of the system, which is usually better on new components than old ones.  I once flew an L-17 in the FT Eustis military flying club that had so much internal leakage that if you tried to raise the flaps before the gear was fully retracted everything just came to a stop until I stopped one or the other operation .

But you don't have to take my word for it, I'm posting the relevant pages to my website tonight, here's the link:


To see build logs for my models:


  • Member since
    April 2010
Posted by Theuns on Tuesday, December 22, 2020 10:24 PM

What John is saying is correct. I work on NA T-6 Harvards and whan on the ground they all droop their flaps over time, atleast the ones I see. Unlike the P51 they dont have clamshell geardoors but the down lock on the T5 is somewhat more elabotare and very fiddly to work on believe me, I had to replace on a few years ago and it is NOT a fun job.


On the T6, the main gear , as it pivots down has a metal lockbar ontop of the gearleg. This engages a triangular lockblock (also springloaded) in the top section of the outer centre wingsection, you may have seen the small pannel that allows access and inspection of this lock setup. It actually works in exactly the same way a door in your house does, but there is an overly complex system of small pushrods and belcranks that will pull the lockpin aft as the gear "up" lever is selected before the hydrolics are activated. I am sure on the P51 is had to work basically the same way jusy not through links and rods.


The reason you will see the clamshelldoors at different angles is because the way hydrolics work. Each door has its own ram that wil retract or lower it when the selector pushes oil in either end of the piston. Fluid dynamics will tell you that it will always take the path of least resistance and therefore one piston may be "favored" over the other untill pressure in the system is equalised.

That is why, upon gear retraction on older craft like T6, P51, P40, c47 ect you see one gearleg almost always retracts before the other one does, it is easier for the oil to flow to one piston and fill it first before the other one untll the piston in the ram is under so much pressure that it then diverts oil to the non -up piston and retracts that one.


As for the flaps  on the P51 being in the same "amount" of down, I can only asume that the flaps must have some form of mechanical link between them to stop assymetrical deployment in flight and as the pressure bleeds off both droop at the same rate.

Granted some may say the pilots select down flap  and leave then there but this, to me atleast is a strange thng to do. I know  on the Spitfire and Hurricaine the flaps were retracted as soon at they were on the ground because it desturbed theairflow throught radiators. On the P51 this is not an issue but flaps in the fully down position are prone to be damaged with debris from the wheels being kicked up so my locic tell me they would have retracted them also  on the way back to the apron....I stand to be corrected on this issue though



  • Member since
    December 2002
  • From: Northern California
Posted by jeaton01 on Tuesday, December 22, 2020 10:48 PM

Theunis, both flaps on the P-51 are connected via a torque tube to a single hydrulic actuator that moves them. 


To see build logs for my models:


  • Member since
    July 2004
  • From: Sonora Desert
Posted by stikpusher on Tuesday, December 22, 2020 11:07 PM

If you’ve ever seen footage of a P-51 starting it’s engine, you can see the clamshell doors snap closed as the system gains hydraulic pressure.


F is for FIRE, That burns down the whole town!

U is for URANIUM... BOMBS!

N is for NO SURVIVORS...

       - Plankton



  • Member since
    April 2010
Posted by Theuns on Tuesday, December 22, 2020 11:37 PM

Ta John, I expected as such. same as on the T6, exept the T6has a slider pushrod that actuates pushrods to the flaps

  • Member since
    April 2020
Posted by Eaglecash867 on Wednesday, December 23, 2020 7:53 AM

It has been several months since I worked on a P-51, but IIRC, the inboard side of the flaps have a step on them.  The P-51s wing is pretty high off the ground, so the step on the flap makes it a lot less of a high jump to get on the wing.  Of course, when working on it, we always covered the wing and used a step ladder instead of the flap step.  Couldn't say how any of the mechanical components work, as that's not my field (mine is avionics), but the flaps have always been down, and I think its because of the flap step.

"You can have my illegal fireworks when you pry them from my cold, dead fingers...which are...over there somewhere."

  • Member since
    June 2017
  • From: Winter Park, FL
Posted by fotofrank on Wednesday, December 23, 2020 8:34 AM

Thanks, John. I wish I could remember where I read what I posted about the landing gear doors and flaps on the Allison Mustangs. It's just that all the photos I've see of Allison Mustangs show the flaps and gear doors up so that's the way I built my kits.

Ooops. Except this one:

OK. On the bench: Way too much to build in one lifetime...

  • Member since
    March 2003
  • From: Towson MD
Posted by gregbale on Wednesday, December 23, 2020 8:47 AM

I recall reading years ago -- probably in one of Merle Olmstead's accounts -- that it was common practice among ETO Mustang crews to hand-close and spring-lock the inner doors, purely to keep them out of the way when performing maintenance functions underneath the aircraft.

There certainly are quite a few photos showing this configuration, and that's usually how I've modeled mine over the years without photo sources to the contrary.


 George Lewis:

"Every time you correct me on my grammar I love you a little fewer."

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