To Trim Or Not to Trim that is the Question.

[guttboy]

I dont know but maybe its me but when I manually trim the p38 I can get speeds that are faster in level flight than without.  It also allows me to control the plane without compression at higher airspeeds.....Tried will back me up on this as well as several guys in my squadron....cant tell you why it does it but it does.

Any thoughts?

[Naudet]

All the planes i fly i always set negative elevator trim before combat, some fully negative others maybe 25% and i do not suffer any lack of manouverability doing so. [/quote)

I switched to manual trim 6 month ago, and most to the very same reason as Cavalier described here.

I generally prefer a nose down attitude in my FW190, cause i also hate it tracking an enemy by pushing the stick forward. I also have the feeling it is easier to track even a level flying target when you have to apply a little up elevator.

And in the low and high speed regions manual trims gives better control. After i got used to the "out of trim" movements of my plane, i can counter them easily by applying a bit stick force.

But i still use auto-lvl/-angle/-speed alot, cause the manual trim reaction is a bit slow. So if i want to trim i.e. for 350 mph i will bring the plane to that speed and hit auto-lvl, than release it and i have the bird trimmed. After that i will mostly correct rudder trim to keep the ball centered, and when the speed changes much elevator trim.

[Apar]

I can see why you want to do that Naudet and Cav, but doesn't 'incorrect' trim setting and the inherent control surface deflections to maintain required flight path induce extra drag?

It seems to me that induced drag by trim tab deflection is lower than induced drag by control surface deflection for the same required effect.

Is the induced drag by trim tab deflection modelled at all in AH?

Apar

[Naudet]

As far as i know, NO, cause AH trims seems to move the control surfaces itself and not only trim tab.

Just look at your right aileron and than move the aileron trim. The aileron moves.

And if this is so, it should play no role if you move the control surfaces to a certain postion bye trim or bye stickforce.
imho when i need back pressure to fly level the elevator is at the same position as if i fly with autolvl.

[Apar]

That makes sense.

Now when trim tab induced drag is modelled, would that then lead to even more drag than applying required control surface deflection manually? If it is modelled it seems to me that it would lead to more drag unless:
The required control surface deflection by trim tab deflection is lower than the required manual control surface deflection.



Apar

[Apar]

I use the combat trim but not for the high speed and low speed reagons because combat trim is not optimized for those regions. I switch combat trim of and use manual trim once I reach those speeds.

I also use auto -speed/-lvl/-angle allot.

But I don't overtrim elevator (I'm lazy). I rather push the stick forward to get a better tracking view than constantly applying back pressure in flight.

Apar

[Mino]

Andy;

Sorry work has been a night mare.

Let me do some more testing, I might be jumping off to a assumption.

But....

I seemed to get a better turn rate trimming "up" near stall speed.

[Ghosth]

Actually Apar, CT is a list of trim positions for given speeds. What throws CT off are things like missing control surfaces, flaps, & gear that are down.

[Apar]

VV Ghosth. I do understand that CT uses a set tabel for a range of speeds. I also do understand that trim tab deflection causes the control surfaces to a certain position.

I still would like to know whether using trim induces more or less drag than not using it and just deflect the control surfaces with the stick.

E.g.:
I'm in level flight but 'out of trim' and have to apply stick pressure to maintain level flight. My plane encounters a certain amount of drag while in flight.

I could also trim the plane (manually or auto) to maintain level flight without any required stick movement. My plane again encounters a certain amount of drag while in flight.

Is the drag higher in the first case or in the second?

[bozon]

Apar,
it's supposed to be the same.
(or totaly negligable difference, to be exact).

Bozon

[easymo]

Thanks guys. This is the funniest thing I have ever read on this BBS.

[Apar]

What is funny about it???  

[Andy Bush]

>>Is the drag higher in the first case or in the second?<<

I've wondered the same thing. I asked a few aero engineers and the consensus was that there may be a difference in total tailplane drag depending on the type of trim system used...but that it was not worth worrying about.

Regardless of whether trim is being used or not, the horizontal tail surfaces are going to have to produce a given amount of lift. That lifting force will have a drag component, most of which will be "induced drag".  Apparently, the effect of a displaced trim tab (in any type of drag) is negligible.

For a trim system that uses a trimmable horizontal stabilizer, I imagine the same is true.

[Apar]

Thanks Andy,