Gear doors

[jetmex]

Hey everyone, I need a little help. I just finished up a KMP F7F Tigercat but the gear door setup was pretty cheesy. I've never set up gear doors before, so I was looking for some advice. I'd like a mechanical setup if possible, there are already too many servos in the airplane and I don't want to add any more weight.

Any help would be much appreciated!

[Galvin]

The gear door setup I used on the tail wheel of the Staggerwing worked wonderfully and would be easily adaptable to the Tigercat, an airplane which also uses fore and aft retracts on both mains and nose gear.

It involved the use of short threaded rods with ball joint ends that pulled the doors open when the gear went down and closed them when it went up. One end was attached to the gear leg and the other to the door. It worked exactly like the mechanical setups on many full sized aircraft and had the advantage of not requiring another servo nor having to be sequenced into the operation of the other gear components.

I don't know what type of retracts you are using but on mine I was able to drill a hole through the big square plastic socket the gear leg was plugged into (and just ahead of the leg, not through it). Through this hole I placed a 3/16" brass tube with about 1/4" sticking out on each side and flattened both ends.

One might also make up a removable setup using small clevis and fork fittings rather than the flattened tubing depending on how anally retentive one was. The flattened tube ends worked pretty well though.

The tubing was not fixed fixed firmly in the hole but was just loose enough to rotate freely. The flattening of the ends was done carefully both to insure that the "flats" lined up on each side and were close enough to the hole ends to insure as little axial "end play" as possible. It helps to slightly chamfer the ends of the hole the tube runs through in order to prevent binding. Alternatively, a washer could be slipped over each end prior to flattening them.

I drilled a hole in each of the flattened ends of the tube just big enough accept the screw that also goes through the ball joint on the end of the rod. The other end of the rod with its ball joint was then similarly attached to a small control horn attached to the door. It may be necessary to make the horn on the door rotatable to account for changes in geometry as the gear goes up and down but I got away with a fixed horn. The horn on the door is mounted wherever needed to clear the wheel and wherever necessary for the rods to clear everything inside as well. If the horn is mounted close to the hinge line then the doors may move too much. Mine were mounted close to the mating edges of the door as far from the hinge as possible and worked well.

I used threaded rod between the ball ends. I made them overly long to begin with, gradually shortening the rod as I determined the exact length needed to make the setup work. All of this came through experimentation. Once I got the rod length in the ball park, small adjustments were then made through threading and unthreading the ball ends on the rod until I got the doors to close with minimal gap.

Once the correct length of rod for the geometry desired is determined, setting up the other legs and doors should become a lot easier. After their operation is satisfactory it is then possible to unscrew one end of the ball joints and slip a piece of plastic or aluminum tubing of correct length over the threaded portion of the rod if desired for cosmetic reasons. Re-tweaking the doors to fit is obviously in order if you do this.

One big thing is to make sure that the attach points on the doors are pretty much in line vertically with the hole in which the tube rotates when the gear is down. If there is no place to drill a hole in the upper end of the gear leg socket for the tube with the flattened ends, then a fitting must be fabricated.

It is important to keep the centerline of the hole through which the tube with the flattened ends runs fairly close to the centerline of where the gear leg rotates. If it is too far out of line (fore or aft) with the axis of gear rotation, the doors will open too far during operation of the gear and something is liable to break. The distance of the flattened tube's axis down the gear leg from the axis of rotation is also critical; too close and the doors won't open enough and too far they will open too much.

On the nose gear doors, watch that the door operating rods do not interfere with the operation of the nose gear steering at any point in the retract/extend cycle.

Experiment and be patient. Those tail wheel doors on my Staggerwing worked very well but were really the end result of a whole afternoon and evening of false starts.

Hope you can make some sense of this.

[jetmex]

Hey Dave, thanks for the info, I appreciate it. Sorry I've been so long getting back to you, but there's been a wedding, various travels in support of school functions and just general crap at work to deal with....

Anyway, the Tigercat has flown, still without the gear doors. And without gear, for that matter, because the damn things refuse to work at the field. I can get them to cycle all day long on the bench, but they'll only partially retract when we go to fly. My friend (who owns the airplane) is trying to work that out with KMP now....

[baird]

this does not impediment the aircraft performance, and is easier for airlines to maintain, as it remains open. This also lets the wheel sink back in if necessary, rather than breaking off.

[eristo]

Hi friends, I had also the same question to ask, thanks for the above information you all have shared here.