Hand job

[easymo]

 This is a question about rate of speed the trim tabs move at. My understanding is that most of the trim done in WW2 planes, was done by turning a wheel by hand. I have noticed, over the years, that the wheels on most machines are sized to maximize the torque, while minimizing the number of turns required to do the task. I would think the trim wheels would have been designed to go through there full range of motion, with no more than 360 degrees of travel. The tabs on my nik are incredibly slow. Is this accurate?

[funked]

A big problem with a system like that is "backdriving" - where the air forces on the tab will cause the tab to move from the position that the pilot set it at.  

The best way to get around this is to use a big old gear ratio in the linkage.  So you have a high ratio of trim wheel angular travel to trim tab angular travel.  With a high gear ratio, a small amount of friction on the trim wheel will translate into a large force at the trim tab, so it won't backdrive easily.

The high gear ratio also has the effect of reducing the forces that the pilot has to exert on the trim wheel.

So yeah I (who hasn't flown any of these planes) would expect that it would take a pilot a while to crank the trim wheel from maximum to minimum trim - there is a good solid mechanical reason why it would have to be designed that way.

[This message has been edited by funked (edited 11-08-2000).]

[Toad]

I can say with near certainty that...it depends.

 

Depends on which plane you are talking about.

The experience I've had in a few WW2 trainer type aircraft (various PT's, BT's and AT's) the answer would generally be "no, you don't get full range of motion in one full turn (revolution) of the wheel."

However, you rarely, if ever need full range of motion of the trim tab at any one particular time. Trimming is more of a gradual process, as speed and configuration change.

I can only recall one time I have ever had elevator trim near the stop, and that was a "non-standard" takeoff situation.

Easymo, this isn't directed at you or anyone else but there still seems to be a general misunderstanding about why trim is even on the airplane. Despite what we have all seen or learned in various computer flying games, the primary use of trim is to relieve control pressures, not to maneuver the aircraft.

Hope this helps.

[ra]

From WWII training videos I've seen I seem to recall most planes having small trim cranks which had to be turned through many 360 degree cranks.  Go to Zeno's and watch one of the fighter training films.

Some light aircraft I've flown have some different mechanisms.  A Piper Super Cub doesn't have an elevator trim tab, it has a fully adjustable horizontal stabilizer.  You adjust it with a little crank sort of like rolling down a car window.  It can take 10 360 degree cranks to get the desired result.

The elevator trim on a Piper Pawnee moves the entire elevator, and the crank only has about 180 degrees of motion.


ra

[Rickenbacker]

I always thought the funniest thing about the pawnee was the flap handle. Looks and works _exactly_ like the handbrake on a car.

No, I don't fly them, I generally hang more or less straight behind them  .

------------------
        Rickenbacker (Ricken)

                -ISAF-
the Independent Swedish Air Force

[easymo]

 Toad, I think I have a fair understanding of what a trim tab does in a real plane. But that's a real plane. Trim tabs are still, very much, a control surface, as modeled in AH. Since this is a feature in AH, I wanted to find out if the manipulation of trim was fairly realistic. It appears that it is.

 BTW this is NOT a complaint. The FM is to easy already. I don't want trim to be done away with.

[Dinger]

Well, that's the problem isn't it.  Real flight controls aren't springloaded to auto-center.  So our trim situation is very different from that WWII planes.
(although didn't the FW190 have electric vator trim?)