Need help with a steerable retract please.

[Allan Bennett]

I've used electric retracts a few times, but this is the first time I've tried to set up a set with a steerable nosewheel. Without the linkage to the servo connected it goes up and down fine, though flopping around a bit while doing so. But when I connect the servo linkage it turns through about 30 degrees while starting to retract, but the clevis then jams against the retract mounting plate before it gets about 1/4 way up.

It seems to me that one improvement I should make is a different clevis, that will clamp the floppy bracket on the retract so that it can't flop around. But it also seems that the floppy bracket shouldn't be able to slide so far down the arm on which it is mounted.

So what do you guys do for a reliable steerable retract please? Or what have I done wrong in this installation?

[Mike T]

The pivot point for the clevis pin needs to be in line with the retract trunnion. From what you describe, it appears to be a little behind, hence the pivot point moves forward when the retract folds back, skewing the steering arm. You will probably have to offset the steering arm and adjust the vertical alignment of the pivot to ensure it is centred.

[Bob Cotsford]

The clevis needs to also be in line with the trunnion pivot in the horizontal plane, you may need to fit a guide to stop the pushrod falling or rising out of line. I usually control the noswheel to rudder mix with the retract switch so that as the wheel goes up the servo centres itself.

[Allan Bennett]

OK, I'm pretty sure you're right about the position of the clevis -- in fact it seems to be below and behind the pivot point of the retract. The below bit I can probably fix by putting some kind of collets onto the 'drop arm', maybe a single turn of copper wire soldered on above and below the floppy bit, to positively locate it. The behind bit will need the drop arm bending a bit, I think. That being the case, why on earth was the unit not built that way in the first place? Oh, and what you can't see in the photo is the hole in the floppy bit that the clevis connects into is actually a slot about 1/4 long, so there's play there too.

Sounds like a pushrod guide will also help, as belt and braces. I like the idea of mixing the steering servo with the retract switch -- I'll do that.

[PeterF]

Also, are you testing it with the plane upside down, also test it the right way up so gravity acts on the steering linkage in the right direction.

[Geoff Gardiner]

This is my setup on the Hawk that I am currently building:

[Allan Bennett]

Thanks Geoff. From your video it seems like restraining the servo pushrod so that the clevis remains at the pivot point is the real key to correct motion, all the other slacknesses being irrelevant so long as the clevis holds its place.

I'll remember to check it right way up too, PeterF. Thanks.

[Bob Cotsford]

Don't stop the control arm slider moving up and down the 'drop arm', that's what allows for servo movement when the leg is retracted ie the slider needs to be free to move with the servo in the retracted position. As Geoff shows, use a guide to locate the pushrod.

[Martin McIntosh]

All of those HK nose retracts twist and were obviously designed by a none modeller who has never tried to use them.

Many earlier servo driven types used a nylon line pull-pull and had a centring spring.

I have done my Hawk quite differently and it is none too satisfactory but I shall attempt to fly it before any changes are made.

[Allan Bennett]

I had complicated things by cutting out a large piece of the bulkhead 'to give the servo link plenty of space'. When I removed the retract from the model I realised that, even in the 'up' position, the slot in the floppy bit was way below the trunnion pivot, which was easily sorted by turning the steering arm about 15 degrees with respect to the nosewheel. Then a simple piece of obechi (arrowed) to prevent the servo linkage dropping below that position, and it now works a treat.

Thanks Chaps

[Allan Bennett]

I've had to add another strip of wood above the servo pushrod to stop it dropping when the model is turned right way up. That was allowing the nosewheel to turn through several degrees when the gear is down. So the pushrod is now running in a slot to restrain its up and down movement.