Elevator Trim Deflection value

[vrapp]

Hello,

I have a question about the Elevator trim deflection at offset 2EA0. What I see is the factored value read at 2EA0 is always much lower than what the plane panel says - see the picture. On all planes, trim indicator has zero at the very top, which is not neutral but the lowest deflection. Is it something different? what value corresponds to zero deflection? also, when, for example, in PMDG B737 FMC calculates the trim to be set at 6.25, should it be the value at 2EA0 or what's shown on the panel? I tried the former, and the airplane took off by itself without any extra elevator angle.

So, as the bottom line, if FMC says to have elevator trim angle X at takeoff, at what offset should I expect it?

thanks

[Pete Dowson]

I have a question about the Elevator trim deflection at offset 2EA0. What I see is the factored value read at 2EA0 is always much lower than what the plane panel says - see the picture

The units on the aircraft scale are not radians or degrees of trim angle, which is what 2EA0 represents, with 0 = Neutral. In your picture the actual 2EA0 value is not the factored one but the small value (.076) in radians. I assume the factored one is a conversion to degrees.

I suspect aircraft trim indicator units are rather arbitrary, and used simply in relation to the performance tables which show correct trim values for different circumstances (mainly safe take-off trims for different weights etc).

On all planes, trim indicator has zero at the very top, which is not neutral but the lowest deflection.

The values are arbitrary I think, as I said.

what value corresponds to zero deflection?

You'd have to find out for the specific aircraft -- when 2EA0 reads zero, see what the trim indicator reads.

also, when, for example, in PMDG B737 FMC calculates the trim to be set at 6.25, should it be the value at 2EA0 or what's shown on the panel?

Well, I would have thought that would be obvious. It must relate to the panel. That's what the pilot would see. On a real aircraft you don't actually have offsets like 2EA0 visible through FSUIPC, and the folks who made the panel are trying to simulate the real aircraft.

So, as the bottom line, if FMC says to have elevator trim angle X at takeoff, at what offset should I expect it?

No offsets whatseover. It's purely related to the markings on that particular aircraft's trim indicator.

Pete

[Pete Dowson]

Hi again,

Just as extra information on this matter, I have a PFC 737NG cockpit, and the trim indicator scale in that is marked in units going from 0 (full nose down) to 19 (full nose up).

What is effectively happening on this is that the full trim scale (offset 0BC2) of -16383 to +16383 is being maped, linearly, onto the sale of 0 to 19. If you are trying to drive a separate indication somewhere, I'd use the 0BC2 indicator, a 16-bit integer, rather than the 64-bit floating point value at 2EA0.

Simply scale -16384 -> 16383 to the needed range 0-19. In other words something like:

Aircraft Trim Set = ((FStrim +16384) * 20) / 32768

Here I am assuming that the aircraft scale is actually linearly proportional to the trim position. I cannot imagine that it might be otherwise.

Regards,

Pete