PDA

View Full Version : Trim misconceptions



AerialTarget
06-28-2005, 04:07 PM
Trimming an airplane correctly doesn't affect speed in real life?



trimming produces less drag i think.

No, it doesn't. It is a misconception, caused by flight simulators' faulty trim modelling, that trim affects speed or maximum elevator authority.

Trim in real life only affects the center position of the stick when you release the controls. If you put a control lock on your yoke in flight (something I do not recommend doing unless you want to die), you can spin the trim wheel all day and the elevator will not budge an inch. The same goes if you hold the yoke in an iron grip, as long as you are strong enough to hold it there.

I stated that trim does not affect speed. For all practical purposes, this is true. However, the tiny trim tab, only a few square inches in surface, does move when you move the trim wheel, regardless of elevator position. And it moves into the airflow. There will be a tiny, unmeasurable change in drag when you trim. However, when your aircraft is correctly trimmed, the trim tab actually causes more drag. In short, there is no measurable difference in speed with trim changes, but if you could measure it, you would find that being correctly trimmed actually makes you slower by a minute figure, perhaps a few tenths of a mile per hour.

http://users.adelphia.net/~j.r.engdahl/josh/Trim.gif

If any real life pilots would like to add or correct anything, then please do so.

LStarosta
06-28-2005, 04:47 PM
Those are some funky ailerons you got there.

3.JG51_BigBear
06-28-2005, 05:58 PM
I don't see how being properly trimmed could make you slower. Trimming a rudder for instance would cause the plane to track straight through the air rather than crab. I would think the increased efficienty of flying straight would outweigh drag caused by a trim tab.

LStarosta
06-28-2005, 06:33 PM
I think he's talking about trimming the aircraft as opposed to applying contant pressure on the controls.

3.JG51_BigBear
06-28-2005, 06:55 PM
I see, thanks.

TX-EcoDragon
06-28-2005, 07:06 PM
He's talking about something that can entertain you for a brief period (this is another one of those engineer vs. pilot sort of issues), but in reality isn't exactly correct since the fact of the matter is that if too far out of trim you will ultimately be less efficient. Sure, if you removed all trim tabs or at least faired them with the control surface and then just flew around with whatever pressures you had to hold to fly smoothly and deliberately without trouble then yeah, you'd be better off without trim. Despite this, real world aircraft make that a challenge in particular those with heavier control pressures or very wide performance envelopes. I don't understand why this keeps coming up. . . since while he's got a point, from a practical perspective it's still wrong. Use the rudder, use the rudder tab but center that ball and there will be a performance increase. . . I for one haven't noticed a difference between holding the surface against the pressure with a neutral tab, and trimming it, other than that when trimmed the surface will not be subjected to the inevitable motions, and drift that a pilot will impose on the aircraft as they hold the pressure. Over time or at either end of the performance envelope this will have a greater impact on aerodynamic efficiency.

altstiff
06-28-2005, 09:33 PM
Problem with this game is the trim is much too slow to ctach up. It is much easier to trim a real airplane than one of these sim ones.

My 2 cents....

LStarosta
06-28-2005, 10:28 PM
Originally posted by altstiff:
Problem with this game is the trim is much too slow to ctach up. It is much easier to trim a real airplane than one of these sim ones.

My 2 cents....

Agreed 100%. Especially if you dont have a trimwheel.

Quaxx2
06-29-2005, 12:06 AM
I fly Cessnas(172's & 172RG's) in the real world and trimming the plane correctly can add up to 5 knots to my ground speed! Just my 2 cents worth!

AerialTarget
06-29-2005, 12:23 AM
Ecodragon, I do not disagree with anything you said. I am not trying to discourage the use of trim. Far from it! I recommend using it both in the game and in real life, for the same reasons you have mentioned here and elsewhere. The reason I made this thread is because many people simply do not understand what trim does, and propagate the incorrect information!

Nearly everyone I have talked to online, and also many people at this forum, seem to think it makes the elevator move regardless of where the stick is, gives you a greater limit to your elevator deflection, and that it somehow substantially decreases drag by moving the trim tab out of the airflow (quite the opposite of real life).

You are, of course, correct; human inputs are imperfect and therefore it is less efficient to attempt to hold controls in a certain position than to trim. However, trimming in itself does not affect the airflow at all, except for a very minute, immeasurable decrease in speed due to the trim tab sticking out into the airflow.

All trim does in real life is moves the position that the stick will move to and remain at if you let go of it. I am not addressing you, Ecodragon, or any other of the many people here who know what trim does. I am addressing the large number of people who have been misled by widespread, unavoidably incorrect trim modelling in flight simulators.

tatali0n.
06-29-2005, 05:44 AM
Originally posted by AerialTarget:All trim does in real life is moves the position that the stick will move to and remain at if you let go of it. I am not addressing you, Ecodragon, or any other of the many people here who know what trim does. I am addressing the large number of people who have been mislead by widespread, unavoidably incorrect trim modelling in flight simulators.

Elevator trim changes the presures on the stick (as you say, the position it returns to if you let go of it) which in turn changes the "neutral" position of the aircraft's nose in relation to the horizon which, in turn, influences the speed that the aircraft "wants" to fly at.

It's therefore incorrect to say that trimming an aircraft doesn't affect speed in real life. Elevator trim does just this. What it doesn't do is influence the speed in any way beyond which just putting and holding the nose into that position manually would not do, regardless of trim, it simply makes it much easier to hold the correct attitude in relation to the horizon and thus the required speed.

I would imagine that changes in drag are inconsequential regardless of whether the forces on the stick are influenced by trim tabs on the control surfaces or spring-loading on the stick itself (or other direct, mechanical as opposed to aerodynamic means).

That said, my real life experience of these things comes from flying modern gliders, not vintage WWII warbirds, nor anything else with an engine, for that matter http://forums.ubi.com/groupee_common/emoticons/icon_wink.gif

AerialTarget
06-29-2005, 06:06 AM
Originally posted by tatali0n.:
It's therefore incorrect to say that trimming an aircraft doesn't affect speed in real life. Elevator trim does just this. What it doesn't do is influence the speed in any way beyond which just putting and holding the nose into that position manually would not do, regardless of trim, it simply makes it much easier to hold the correct attitude in relation to the horizon and thus the required speed.

Yes; I meant, "Trimming an aircraft doesn't affect speed in real life, assuming you've got a firm grip on the stick." Again, I'm not trying to downplay the importance of trim. I am trying to dispel a widespread myth about what trim does. Talk to a few - say twenty - people playing the game online about trim. You'd be surprised at the number of those who don't have a clue what trim really does.