P-51 flaps and hydraulics

NovemberEcho

Dergs favorite member
So this is rather random, but I’m in a debate on a modeling forum. It is generally accepted in the modeling community to drop the flaps for accuracy on warbirds because they drop after engine shutdown and hydraulic pressure dissipates. I say this is horsepoop and the only reason you see them down in pics is because pilots lower them because it makes it easier to climb on/off the wing and that they’re designed not to drop just because of lack of hydraulics otherwise a lot of planes wouldn’t be making it home that otherwise could.

So basically, would a lack of hydraulic pressure caused by either engine shutdown or damage cause the flaps to drop?

bonus question, are the Mustangs flaps driven by a pushrod or jackscrew?
 

Minuteman

“Dongola”
In this video it looks like the pilot selected the flaps down shortly before engine shutdown. A diagram I found in an online POH shows there is a "controllable check valve" for the gear doors, which may allow the doors to fall.


Conversely, here's a shutdown where the flaps and doors remain up, so its possible.


So I imagine this is one of those things where a) flaps-down it may make it easier to climb down, and b) there is Old-Pilat-Tale voodoo among owners of these Fabergé eggs about "leaving the system pressurized is hard on the seals" or something, mixed with some old-ass valves that have been left pressurized too often and leakdown on some aircraft.

1605907465945.png
 

trafficinsight

Well-Known Member
So this is rather random, but I’m in a debate on a modeling forum. It is generally accepted in the modeling community to drop the flaps for accuracy on warbirds because they drop after engine shutdown and hydraulic pressure dissipates. I say this is horsepoop and the only reason you see them down in pics is because pilots lower them because it makes it easier to climb on/off the wing and that they’re designed not to drop just because of lack of hydraulics otherwise a lot of planes wouldn’t be making it home that otherwise could.

So basically, would a lack of hydraulic pressure caused by either engine shutdown or damage cause the flaps to drop?

bonus question, are the Mustangs flaps driven by a pushrod or jackscrew?
Flaps are driven by a hydraulic cylinder.

I know the inner landing gear doors will slowly droop over time as hydraulic pressure bleeds off, so really any position is correct as far as they go. I dont know about the flaps other than most ones I've seen parked have the flaps down.

If I had to guess, It could be that they'd drop over time but you'd want them down for preflight anyway and its an engine driven pump. No way to put them down later if you don't.
 

Pilot Fighter

Well-Known Member
Flaps are driven by a hydraulic cylinder.

I know the inner landing gear doors will slowly droop over time as hydraulic pressure bleeds off, so really any position is correct as far as they go. I dont know about the flaps other than most ones I've seen parked have the flaps down.

If I had to guess, It could be that they'd drop over time but you'd want them down for preflight anyway and its an engine driven pump. No way to put them down later if you don't.
Did any WWII fighters have hydraulic flaps?
 
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Pilot Fighter

Well-Known Member
The P-40, P-47, and P-51 all had hydraulic flaps. I dunno about others.
You're right. I think a version of the P-38 had hydraulic ailerons. As a kid I read everything I could find about WWII aircraft. I remembered a reference to a P-38 version being the first to use hydraulics for control surfaces. I guess they were thinking ailerons, rudder, and elevator or my memory failed me after 50 years.
 
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inigo88

Composite-lover
The P-40, P-47, and P-51 all had hydraulic flaps. I dunno about others.
I think they were mostly hydraulic, with a notable exception being the Grumman F4F Wildcat... which was just generally weird.

It has manual hand-cranked landing gear (28 cranks!!!), and vacuum powered flaps.

This Kermit Weeks takeoff Gopro video really captures the absurdity of it all. Hardly an eyes outside after takeoff kind of airplane. :)


Kermit’s Comments
The Wildcat has an interesting flap system that is activated by the vacuum system. Instead of using positive pressure to operate the flap cylinder, this aircraft uses negative pressure or suction. A large tank in the rear of the aircraft stores the “negative” air pressure. At higher speeds, the negative pressure does not create enough force to overcome the air loads to lower the flaps. It will not hurt the system, as in some aircraft, to put the flaps down at high speed. They just simply won’t come down! Pilots used this to their advantage in dogfights and could select the flap handle down before engaging another aircraft. If, during the dogfight, the airplane happened to slow down below a certain speed, the flaps would creep out. This gave the Wildcat a tighter turning radius, which was a major advantage in a dogfight, as they could potentially turn inside the opponent and get on his tail.
Source: 1943 Grumman Wildcat | Fantasy of Flight
 

trafficinsight

Well-Known Member
You're right. I think a version of the P-38 had hydraulic ailerons. As a kid I read everything I could find about WWII aircraft. I remembered a reference to a P-38 version being the first to use hydraulics for control surfaces. I guess they were thinking ailerons, rudder, and elevator or my memory failed me after 50 years.
P-38s had hydraulic aileron boost, I guess it was kind of a truck without it.
 

CFI A&P

Exploring the world one toilet at a time.
So this is rather random, but I’m in a debate on a modeling forum. It is generally accepted in the modeling community to drop the flaps for accuracy on warbirds because they drop after engine shutdown and hydraulic pressure dissipates. I say this is horsepoop and the only reason you see them down in pics is because pilots lower them because it makes it easier to climb on/off the wing and that they’re designed not to drop just because of lack of hydraulics otherwise a lot of planes wouldn’t be making it home that otherwise could.

So basically, would a lack of hydraulic pressure caused by either engine shutdown or damage cause the flaps to drop?

bonus question, are the Mustangs flaps driven by a pushrod or jackscrew?
They've been huffing too much model glue and confusing the aircraft systems. WWII airplanes parked with flaps down to ease ingress/ egress. Otherwise the pilot or maintenance personnel would need to either triple jump or have a ladder nearby to get on and off the aircraft.

Some airplanes have hydraulics that droop, such as gear doors but the fighters intentionally lowered the flaps before shut down.
 
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