Horizontal Stabilator Inclination Angle

[Andreas K]

Hi All,

I have around 200 hrs. in my RV-10 and just recently noticed that when I fly by myself at 10000 ft at 170 Ktas the elevator horn (the part where the ballast mounts) sticks up above the stab fairing cap by an estimated 1/4 inch. I built the Stabilator and shims to plans. I don't fly with ballast. I was wondering if anybody else experiences the same.
Thanks for your input

Andi

[RV8JD]

The RVs appear to be designed this way. From Scott McDaniels ("rvbuilder2002"):

"... and a very slight amount of down elevator was specifically designed in by the designer (Van) and considered desirable. The slight down elevator improves pitch stability at the cost of a very slight amount of drag."

And this post by Scott on the RV-10 specifically: http://www.vansairforce.com/communit...0&postcount=11

Here is a pic of my previous RV-8. Note the slight amount of down elevator, as evidenced by the elevator horn:

[Carl Froehlich]

Andi,

Had the same situation on my RV-10. I added a 0.063 shim under the forward HS spar to address (I did a 0.040 shim on my RV-8A and it did the trick). On the RV-10 however this shim achieved close to nothing. I pulled the shim.

I went back and took careful measurements to find the true ?in trail? position for the elevators. In this position the elevator horns did stick up about a quarter inch.

So recommend you do this same exercise before adding a shim.

Carl

[paul330]

It seems to me that the HS must produce a given amount of downward force to keep the aircraft in balance. This will vary depending on the C of G. So, you can rig the HS so that the elevators are in trail for one - and only one - C of G position. In all other cases, the elevator will be slightly up or down.

The bottom line is that the majority of the drag from the HS comes from the negative lift it produces. The form drag from 1/4' of horn in the airflow is insignificant.

So I wouldn't mess with anything but would leave it as the designer intended .....

[Carl Froehlich]

Paul,

You are correct. At issue here however is excessive HS downward force. On my RV-8A the elevators were always deflected down in cruise, even when solo. More down with a passenger in the back. Here the HS downward force was more than needed and was, in my opinion, unnecessary drag in cruise. Note - the drag I?m referring to is the HS and deflected elevators, not the horns.

The 0.040? shim I added under the forward HS spar mitigated this, and still left me with enough HS down force so that I did not run out of trim during solo landing, resulting in the elevators more in trail during cruise.

On the RV-10 this was not the case. It seems the horns sticking up may not be an indicator of elevators deflected down to counter excessive HS down force. So again recommend careful review of what the horns look like when the elevators are exactly in trail before considering a shim under the HS forward spar.

Carl

[Andreas K]

Thanks for your help. I will measure the true trail position of the elevators and experiment with different CG's before doing anything.

[paul330]

Originally Posted by Carl Froehlich

Paul,

You are correct. At issue here however is excessive HS downward force. On my RV-8A the elevators were always deflected down in cruise, even when solo. More down with a passenger in the back. Here the HS downward force was more than needed and was, in my opinion, unnecessary drag in cruise. Note - the drag I’m referring to is the HS and deflected elevators, not the horns.

The 0.040” shim I added under the forward HS spar mitigated this, and still left me with enough HS down force so that I did not run out of trim during solo landing, resulting in the elevators more in trail during cruise.

On the RV-10 this was not the case. It seems the horns sticking up may not be an indicator of elevators deflected down to counter excessive HS down force. So again recommend careful review of what the horns look like when the elevators are exactly in trail before considering a shim under the HS forward spar.


Carl

Sorry Carl but you can't have "excessive" downward force if the aircraft is in trim. The force is not produced by the elevator alone. The elevator merely changes the effective camber of the HS/elevator combo to vary the amount of negative lift. By re-rigging the HS you simply do this by changing the angle of attack instead - you are still producing exactly the same amount of downward lift for any given weight/balance/airspeed and thus the same drag.

I will give you that the efficiency of the HS "wing" may be slightly different depending on whether you generate lift by increasing camber or increasing AoA but short of putting it in a wind tunnel I don't know how you would quantify that. In any case, I suspect any difference would be negligible.

Running out of trim or up elevator with a forward CG is another issue and what you suggest may well help ........

[BobTurner]

Originally Posted by paul330

Running out of trim or up elevator with a forward CG is another issue and what you suggest may well help ........

Paul, I was with you until that last line. I think reducing the (negative) angle of attack of the stabilizer with a shim will reduce, not help, the downward force that can be generated with full up elevator. My -10, at forward CG and full flaps, barely has sufficient elevator authority in landing. I wouldn't want to reduce it.

[paul330]

You are right Bob - I got my negatives and positives mixed up ......