Question for the A&P guys

[Tim Wescott]

I've seen plenty of footage of WW-II planes retracting their wheels one at a a time.

I've always assumed that this was inherent in the mechanism, i.e., they're hydraulic powered and the least sticky one goes up first.

My flying buddy mentioned that he thought they're intentionally sequenced -- i.e., there's one that always goes up first, then once it's up, it flips a switch to let the other one go up.

So -- how's it done?  Or is it done each way on different planes, with another two dozen variations besides?

[Jim Roselle]

Although I'm an A&P I haven't worked on full scale aircraft in over twenty years, so here's my somewhat unqualified answer. 

If memory serves the gear retract one at time so as to require less hydraulic pressure than if both gear swung at the same time. Therefore you can use a smaller pump, less fluid, and less weight.
 I may be completely wrong on this.

Jim

[Tim Wescott]

Thanks Jim.  It makes a lot of sense -- I've worked on big electrical systems that start up stage-by-stage for much the same reason, to spare the input cables the load of everything trying to come on at once.  Do you know if they do it automagically, or is there an actual valve in one wheel well or another that does that part of the sequencing.

[Douglas Ames]

Modern Aircraft have efficient hydraulic pumps and an accumulator to handle an increase in fluid demand when the gear is sequenced.
There is a drop in pressure that occurs when a large actuator(s) is filled.
The accumulator helps by preventing the pump from cavitating and reducing the pressure drop in the system.
I'm going to guess that the early Fighters might not have had these, and their pumps were marginal considering the only hydraulic system was the gear. (The constant speed prop was actuated by engine oil press.)
I say this because one of the mods done on "Glacier Girl" (P-38) was to replace the hydraulic pumps with a more modern design.
Even on Commercial Airliners, the gears don't retract at exactly the same time.

[dave siegler]

Looks like a single line system.  A pressure regulator to supply constant pressure and one leg comes up first.

[Jim Dincau]

P-51 was both at once



Spitfire was a hand pumped hydraulic

F4 wildcat was a hand cranked chain drive

[bill rutherford]

I have worked on a lot of airplanes including world war two stuff and I have never seen any sequence valves to make one gear go up faster than another. what would be the point? Internal friction , would be the only reason that I could think of. Not saying that it never happened in a design, but then again what would be the point. Did a retract check on a Cessna 210 today and the left gear was always up first and down first. Both cylinders feed of a tee fitting so get equal pressure.  Bill

[dave siegler]

I stand corrected on the mustang.  Single line both up together.


Fg1 corsair
[youtube=425,350]https://www.youtube.com/watch?v=Ppq5uhX3WKs[/youtube]

P40 just watch the first 8 seconds

[youtube=425,350]https://www.youtube.com/watch?v=SM1mFnILZDI[/youtube]

Bf 109  just watch the first 8 seconds
[youtube=425,350]https://www.youtube.com/watch?v=QTxGDI2-wDs[/youtube]

hellcat
[youtube=425,350]https://www.youtube.com/watch?v=bBsg3WZeheY[/youtube]

Wildcat.


Probably correct,  system friction is the difference. 

[De Hill]

A hydraulic sequence valve is used to make things happen in a certain sequence. Example: Landing gear goes up, and then the landing gear doors close.

[Russ Popel]

I could go on for pages about warbird  hydraulic systems but basically they follow path of least resistance or friction as some have called it. If the resistance is very similar, then the gear will go together, if an iactuator is "sticky" or a gear pin etc is tighter, then you will see gear that seem to be independent . Some have accumulators,some have hydraulic timers that keep system "live" for a few minutes then go into bypass mode,some have electrical actuated bypass systems (t-28) which are triggered by the gear up,flaps up,speed brake up,canopy closed micro switches. The t-6 has hydraulic gear and flaps as does the p-51.The b-25 has hydraulic gear,flaps ,Bombay doors,cowl flaps,and brakes. The pby Catalina had hydraulic windshield wipers and some autopilots used hydraulics as well. That's enough,now back to building.

[Chris Cox]

Russ - back to building?  Seems more likely you will be going snorkelling today... You should be back home sanding!

[Alan Resinger]

Russ,
Get back to the "Hobbit Workshop" and get some sanding done.  I finished all the base color yesterday and will start putting trim on in the next day or so.  I may be spraying clear coat within a week or so.  Joan and I got all the paint masks cut last weekend before Chris left for Winterpeg.
Missed you last Saturday for lunch.  How's the new ship coming.
Alan

[john e. holliday]

From what I seen the Mustang is one of the few that the gear retracts at the same time.   The Corsair and P-40 I seen in a video showed the Right gear halfway up before the left even moved.  Watched several videos in which this happens. 

[CircuitFlyer]

The Saunders ST-27 (a turboprop converted DeHavilland Heron) was all pneumatically operated gear, brakes & flaps.  The pilots didn't like it much when one flap would operate slower than the other.  It was a bit of a wild ride for a second or two after selecting the flaps.

[jim gevay]

As others have said I think it's just the way some systems are designed by that particular manufacturer. For lack of a better term it's the path of least resistance on whether one gear leg go's up before the other.
On P-40's and such, one always go's up before the other, and on others like the Mustang, both are nearly together. Now obviously on Mustangs they have sequencing valves for the gear doors. On retraction the inboard or clamshell doors open, then the gear go's up and then the clamshell doors close. Just the opposite on extension.

I'm not sure why you showed the video on the Wildcat as it has a fully mechanical gear system, only the shock strut is hydraulic. The gear is hand cranked up through sprockets and chain, kind of like a bicycle chain,
with about 30 turns from a handle inside the cockpit.

[phil c]

This is a pretty nifty scale P-51A Mustang.   I wonder what the line pull would be on a 7/8ths scale plane?

Phil C

[Michael Schmitt]

Years ago when we flew 727's I noticed the Electric driven pumps would raise both Inner Flaps  before both Outboard flaps. But put the airplane outside and start an Engine (now using the engine driven Hydraulic Pumps) the Inboard and Outboard Flaps would drive up and down together. The Engine pumps provide more flow. Having said that I've done many gear swings on airliners and its a rare thing to see them go up exactly together. As long as the gear is up and locked within the Aircraft maintenance Manual time limit its good to go. There are a lot of moving parts in aircraft systems--the distance each gear is from the pumps--the amount of tubing in the system, are any of the actuators Bypassing (bad internal seal in an actuating cylinder) Etc. Etc.