Trim Wheel Travel

[alaskabearhawk]

A question for those that are flying a 4-place BH. If you have a trim wheel like Bearhawk Aircraft sells with the small sprocket, how much do you move you trim wheel during a flight? When I say moving, how much of the 360 degree arc of the trim wheel do you use? I had the spec, but it got lost in a pile of email. If I remember right, someone once posted they only use about 1/8th travel out of the trim wheel arc during a normal flight. For example, flying a Cessna product like a C172 you turn the wheel quite a bit to get the indicator to move a little.

[Zzz]

Having worked the trim in Blackrock's 4-place, it is VERY high geared compared to a Cessna trim wheel. Very little rotation produces a large input, but sorry, I don't know the rotational angle to travel spec.

[jaredyates]

I'm using about 1/4 travel with a fairly forward cg and 360 engine. Aft cg would theoretically reduce travel, while a larger engine and its associated higher top airspeed would probably require slightly more. I'm not especially happy with the current setup, and would like to have a system that is less sensitive.

[alaskabearhawk]

My son who is a Boeing engineer is here for Christmas and we are working on options. That's why I wanted to know. He has some creative ideas that are simple and can use the plans configuration. Thanks...

[marcusofcotton]

It will good to see what you guys come up with! One thought I had was to only put a trim tab on the left as Maule does, realizing that may result in addressing the elevator control horn area as well.

Mark J

[N942VT]

1/4 travel is about right, from take off to landing, but most of the time it stays even more local than that. After 250 hours I've gotten used to it. No problem.
Dave

[alaskabearhawk]

Hey Mark...I really didn't want to change the trim system, just the drive (rotation) ratio of the trim wheel. I suspect with the servo trim, having only a trim tab on one elevator would create an uneven torquing of the elevator spar and may even affect the elevator control to some extent.

[marcusofcotton]

Paul, those were concerns I had as well. Considering Maule, Champion, most Cessnas, and many others are set up with only one side, I'm thinking it is worth further exploration. With my lighter engine prop combo and being on the lower end of the power spectrum, the reduction of weight back there would be helpful as well.

Time to get back out there to work on it,
Mark J

[alaskabearhawk]

They are set up on one side, but they are fixed in travel. You're right...won't find out unless we get something in the air.

[Zzz]

What about just a mod to the wheel gearing to pull less chain per degree of rotation? Essentially lower the gear ratio so that you still work with the same trim control system, just more finely adjust it with ham fists.

[Dan R.]

For Bearhawks built from the older plans like mine that uses the longer elevator trim tabs. Use a Cessna trim jack screw that is in the stab of a Cessna 150-172. It has a 10-1 reduction ratio. There are Bearhawk flying with this mod and the trim is about like a C-172. I have drawing. The jack screw is mount vertical in the fuselage just ahead and below of the L.E of the fin . Dan R.

[Erbman]

This issue has been known for a long time, which is why I created a screw thread trim actuator as shown on the Bearhawk Reference CD. It seems like I once calculated that the reduction ratio was about 13:1. It's worked well for me. With 300 hours on Three Sigma now I'm quite used to it.

[Battson]

I think at landing we find we need about 0.75" - 1" of deflection at the trim tab, nose-up, relative to it's neutral position in-line with the elevator. That's with CG near the front of the envelope.

[alaskabearhawk]

I forgot to look at you CD Russ. I just remembered the trim wheel being the same, so I assumed the trim system was per the "normal" system. I did consider the jackscrew Dan, but I didn't want all that chain. My son offered a few suggestions using a differential belcrank, but I think I have a good solution that is different than all these and much simpler. I'll post details when it's done.