The International Cessna^® 170 Association

jrenwick wrote:

N9149A wrote:

doug8082a wrote:. The Hatz is designed to have a trim tab, but I plan on installing a Cub trim system.

Doug I don't know if I'd do that. The Cub trim system (jack screw) is considered a week link at least for aerobatics and is one of the first things removed when a Cub is built for serious aerobatics. Thought you might want to know that.

I agree. When I changed my A65 engine out for a C90, I found there's never enough nose-down trim with the stock system.

As you get into more powerful engines and other mods, it gets worse. Super Cubs have even bigger engines, plus flaps. A mechanic I know well believes this was the cause of a fatal PA18 accident locally: Cub driver executes a go-around, and stalls when the flaps are raised. Trim was found fully nose-up in the wreckage. Mechanic thinks the pilot might have saved the situation by reducing power and putting the flaps down again, long enough to retrim. As it was, the elevators apparently didn't have enough oomph to overcome the nose-up trim.

If the design calls for a tab, I'd want to stick with it!

Best Regards,

John

(Thought I'd move this to its own thread...)

Thanks guys... good to know . This is what makes this forum such a great resource.

There are a couple reasons I'm thinking of changing the trim system:

1. Installing the Rotec radial will lower the thrustline a few inches so I thought being able to alter the angle of incidence of the stabilizer would make sense. There have been a few Hatz's built with the Cub trim system. However, there have also been several Hatz's with radials installed that did not go that route and according to some in the Hatz world the change in thrustline really doesn't affect it.... Soooooo , maybe it's not such an issue after all?

2. I'm not that crazy about the current trim system design for the Hatz which involves essentially a push/pull cable arrangement that doesn't appear to have much ability for "fine tuning" the trim like you can with a trim crank or wheel arrangment.

3. This one is strickly cosmetic, but I don't care for the appearance of the trim cable kinda hangin' out there at the tail.

Anyway, I have no problem staying with a trim tab arrangement, I'd just like to have one that I can "fine tune" a bit better and might have a cleaner appearance. As for the thrustline issue, if it IS something I need to address then there is an alternative there as well that has been done on other Hatz's... I was thinking that incorporating the two (ala the Cub) was a workable solution, but now it doesn't sound like such a good idea.

I'm open to suggestions. Ideas anyone?

Just jumpin' in here on PA-18's...something I know nothing about .... (shut up flyguy!)....

...Mechanic thinks the pilot might have saved the situation by reducing power and putting the flaps down again, long enough to retrim. As it was, the elevators apparently didn't have enough oomph to overcome the nose-up trim. ...

Perhaps that was just an unfortunate choice of words...
If the elevators are insufficient ... more efficient trim would not correct insufficient elevators. Insufficient trim would be even less effective on insufficient elevators....(the ones with not enough "oomph".)
What is probably meant ... is that the application of flaps moved the center of lift aft ... ... and the elevators were not sufficient to overcome that shift when combined with the further shift aft of C/L that accompanies an application of power with flaps deployed. (In which case simply placing the flaps in the prior trimmed configuration might have returned the C/L to the former location and possibly have saved the situation... ...depending further, of course, upon whether or not the flap retraction simply stalled the aircraft, which does not relate to the elevator/trim effectiveness...or otherwise.)

That may be a better way to say it, George. As you probably know, Cubs are trimmed by raising and lowering the leading edge of the horizontal stabilizer. When it's fully lowered (full nose-up trim), it takes quite a few turns of the trim crank to neutralize all that trim. Until you've cranked the trim out, you can push the elevator all the way down, and there's still that horizontal stabilizer trying to pull the nose up. With a trim tab system, if you have enough strength to push the elevator all the way down against the dynamic pressure of the trim tab, then you've got full elevator effectiveness to recover from the stall.

Does that make sense?

John

I had the same problem John describes in a Pacer. I was trimed for a slow speed on landing and had to do a go around. When applied full power I put the yoke full forward against the stop and it would not fly level untill I re-trimmed. I soon learned to appreciate the trim system on the 170 even more.

OK, so if the jackscrew is a weak link and is removed if a cub is going to be used for aerobatics, then what do they usually replace it with? A trim tab? Other?

That is what I've seen Doug. But I'm no expert on the subject. I'm just pointing out something the casual observer might not hear. If I where you I wouldn't use a system others would rather get rid of.

Of course I'm talking people who are a little more serious about aerobatics than others. I hope my jack screw will survive an occasional loop. They are perfectly safe for their normal use just a weak link which might be plenty strong for what you intend.

I'm thinking I'd be looking at a Pitts or Christen Eagle or Citabria for my trim system however.

Thanks Bruce. Since the construction of the fuse and tail is still about a year out for me, now is the time to do the research. I'll look into the trim systems used planes designed for aerobatics and see what shakes out. I imagine they are all a trim tab arrangement.

Just to keep things in perspective, I'm not plannig o turning my Hatz into a full time aerobatic platform, but I would like to be able to do the occasional loop, roll, etc. and not have to worry about my tail having problems.

Doug,

You also want to consider the effect of a moving tail trim system on stability of the aircraft as well as drag effects...a whole tail that is trimmed usually has more drag than a trim tab driving an elevator (caveat that with usually). I just know that my design profs always said there were trade off's when changinh things in design, just make sure you know all the trade offs

we can talk offline if you like, I paid all that money for an aero degree, it's fun to use the knowledge every once in awhile.

v/r,
Jud

Thanks Jud. I'll drop you a line.

Doug >>>>

doug8082a wrote:OK, so if the jackscrew is a weak link and is removed if a cub is going to be used for aerobatics, then what do they usually replace it with? A trim tab? Other?

A BIGGER jackscrew.