Elevator, rudder & aileron trim

[IO540]

The only thing I came up with is IAS error

Wouldn't seem likely as the change in the aircraft pitch between say flying 1-up with 20USG in the tanks, and flying at MTOW (approx 1070kg to 1400kg) is not readily visible, at say 140kt IAS.

Also, one would tend to notice it in the GS, over time.

The change in IAS for a given fuel flow is at most 3kt i.e.2%.

no speed gain should tempt anyone to violate an aeroplanes aft limit.

Obviously, but you are entitled to load the plane anywhere inside the envelope.

If you feel otherwise, you should write to the FAA and to EASA and ask them to revoke the Type Certificate

[Big Pistons Forever]

Originally Posted by IO540

I often wonder why the type I fly (a TB20) does almost the same speed (for a given fuel flow) across a wide range of total weights, and the only explanation I can come up with is that the elevator AoA (and drag) is indeed changing, and improves with larger total weights.

A Cessna 340 I occasionally fly, also seems to fly at about the same TAS regardless of the aircraft total weight. Your explanation seems reasonable. Also small changes in wing AoA may affect drag and because the airflow patterns over the whole aircraft may be driven in a slightly more favorable direction at higher aircraft weights therefore slightly reducing drag and offsetting the higher weight.

You have me intrigued now. Does anyone with a theoretical aerodynamics background care to comment ?

[24Carrot]

OK, one more uninformed speculation!

We think of these aircraft as rigid, but if you stick an extra 330kg in the middle of a 10m beam, (aka wing), you must get some deflection, and perhaps some twist.

I am sure the designers made sure that nothing bad happened as a result, but perhaps they managed to be cleverer than that?

PS the AoA change with those weights would be something like:
(1 - 1070/1400) x 3 degrees, maybe 0.7 degrees.
So the IAS error would indeed be small, and I think it is in the wrong direction anyway.

[maxred]

Yes, because at any given power setting the aircraft will be slower at high than at low weight. In steady level flight, thrust equals drag, and lift equals weight.

If therefore weight is added to an unaccelerated aeroplane, it would begin to descend. To compensate, you would have to increase lift, but with power held constant, the only recourse is to raise the nose, and therefore the AOA.

However, this also would increase induced drag, which slows you down.

In reality however, this speed reduction is quite small, and is generally because the addition of weight, generally moves the cg aft.

It may be in the two cases above 340, and TB20, that the cg does not move back far enough with increased weight, to alter the speed.

I am not an expert in theoretical aerodynamics however

[IO540]

Certainly, on the TB20, burning fuel has a negligible effect on the CofG. On the computer graph you can just about see a small shift.

The reality is that I can depart on a ~950nm flight with 86USG and land with say 22USG, and the IAS is the same for a given flow rate.