RV-6 HS Incidence

[sjrossi]

Hi Guys,

I'm finishing an RV6 that another builder started. I decided to check the incidence of the HS and it is half a degree high in front. He had 1/4" spacer under the front spar. I can change it to zero easy enough, but will have to rework the fairing slightly. I have read other posts about their plane needing some nose down trim in cruise, and it seems that having the HS incidence a little high would provide that. Also, would it add some drag?

Question is, am I wasting my time sweating half a degree here? In the absence of any new information I'll just change it to zero incidence and rework the fairing.

Thanks for any input....

[Kyle Boatright]

Depends. My airplane, with an O-320 and a wood prop has an empty CG of around 71" at a weight of 1032 lbs. Knowing that I had a light prop, and reading comments by others at the time (circa 2000), I intentionally set my incidence 1/2 degree lower than recommended. With no bags on board, the elevator is in trail in cruise, and with bags aboard, the leading edges of the counterweights are slightly above the stabilizer tips.

I'm going to call my setup perfect. ;-)

Now, if your airplane is going to have a forward CG, it might be worthwhile changing your HS incidence, with the resulting changes to your empennage fairing.

[scsmith]

Hi,

It is true that having the leading edge up a bit, like a half a degree or so, will improve cruise performance slightly. You will find you don't have the slight elevator deflection at cruise speeds that most RV's (most airplanes) do. I don't know if you could ever measure a speed benefit from this, but at least psychologically, it will seem faster.

However, there is a price to be paid for this, which may or may not be objectionable depending on your flying experience and expectations. You will likely end up with very little stick force gradient at slow speeds. Possibly even a force reversal at very slow speeds. At the very least, the stick forces are going to be very light.

A force reversal is fairly unpleasant to fly -- it means that as if you trim at a slow speed, say, approach speed, if you go slower, it takes some forward stick pressure to keep from slowing more, and aft pressure to keep from speeding up more. This is backward from what you would expect -- you would like to feel more back pressure is needed to go slower, and forward pressure to speed up.

As long as you don't get an actual force reversal, and you don't mind flying an airplane with extremely light control forces, it may be just fine for you. Most glider pilots don't find the very light pitch forces objectionable at all.

[lr172]

Be sure it is wrong before you change anything. In my case, in order to make my edge distances and due to some twist in a wing, I could not get the main wings set with the fuselage level. You end up setting the main wings to the correct level position and then set the HS incidence while ignoring the Fuselage level. This is documented in Van's material on the subject.

My point is, be sure you are checking the HS incidence directly against the wing incidence and don't trust that they are both referenced to the fuselage.

Larry

[MikeyDale]

I think if you aren't going to paint before you fly, fly it first and see if it needs adjusting.

[WingsOnWheels]

After reading through all the posts from other RV-6 builders I opted to add a 0.063 spacer under my stab. That seemed to be the most common shim size builders ended up using. That is in addition to the 1/8" spacer called out in the plans, for a total of 3/16". This is all from memory, so you may want to double-check those numbers.

[skylor]

Originally Posted by sjrossi

Hi Guys,

I'm finishing an RV6 that another builder started. I decided to check the incidence of the HS and it is half a degree high in front. He had 1/4" spacer under the front spar. I can change it to zero easy enough, but will have to rework the fairing slightly. I have read other posts about their plane needing some nose down trim in cruise, and it seems that having the HS incidence a little high would provide that. Also, would it add some drag?

Question is, am I wasting my time sweating half a degree here? In the absence of any new information I'll just change it to zero incidence and rework the fairing.

Thanks for any input....

Keep in mind that if the VS forward spar bracket has already been drilled to the HS, then changing HS stab incidence will change the alignment of the VS hinge brackets...

Skylor