Stab incidence + or -?

[No Step]

This has been bugging me for a long time. A wing with the L.E. up 1 degree common to the fuselage datum is known as +1 degree incidence.

But I'm confused about the stab. Now LE up 1 degree on the stab, you'd think would be +1 degree but that would be mathematically a +2 decalage which it isn't,.. it'd be 0 decalage... So, L.E. up on the stab is actually called -1 degree since it's making the airplane pitch down? ....is that right?

Fred
https://home.comcast.net/~guilfoyle72/

[slipstick]

No. For some reason you've assumed that decalage is wing incidence + (plus) stab incidence. It ain't, it's the angle between wing and stab which is wing incidence - (minus) stab incidence. 1 - 1 = 0....problem over.

Steve

[No Step]

Yeah, I know but, L.E. up >is< considered positive stab incidence then right?

[slipstick]

You obviously don't want to believe me so let's wait for someone else to come along with an opinion ;).

Steve

[Montag DP]

Yeah, I know but, L.E. up >is< considered positive stab incidence then right?Yes.

[No Step]

Thanks montag,

slipstick - sorry you feel that way. I'm trying to get a clear picture of the correct terminology. You have misunderstood me.

[Go4it]

Example:
Upper wing +4°
Lower wing +1°
Subtract .... 4 - 1 = 3° decalage

[vintage1]

Leaving aside what purists call decalage - the angle between two wings of a biplane, and talking about common usage, the difference in rigging angle between main wing and tail, for stability the CG will always be at a place where the tail is negative with respect to the wing.

Where you place wing incidence is simply a question of whether the model will fly tail up or tail down.

Full size practice is to make the fuselage as parallel to the airstream as possible at the most efficient cruising speed.

This leads to a slightly nose down attitude at high speed and a definite tail down attitude when landing and at other low airspeeds, like climbing.


When setting up a model, the issues play out this way.

1/. Set main wing incidence to determine the models flying attitude. In general a clark Y type section should be with the flat bottom parallel or about +1 degree wrt to the datum.

2/. Set tailplane for expected stablity in pitch. For a free flighter this might be 2-4 degrees -ve of the wing, for RC 1-2 is right. With a flat based Clark Y and a flat plate tail, set the tail parallel to the flat bottom.

3/. Set CG position by calculation of tail volumes etc, or just by doing glide tests..move CG till glide is stable on zero trim.

4/. Add down and sidethrust to get more or less straight climb under power.

[No Step]

Thank you Vintage1, well said.

[kcaldwel]

Leaving aside what purists call decalage - the angle between two wings of a biplane, and talking about common usage, the difference in rigging angle between main wing and tail, for stability the CG will always be at a place where the tail is negative with respect to the wing.

Although I agree with the rest Of Vintage's post I think, I do not believe the statement "for stability the CG will always be at a place where the tail is negative with respect to the wing" is always correct.

First, all this discussion so far has been about measuring incidence from the chord line of the airfoil. With cambered airfoils, the zero lift line can easily be 2 or 3 degrees from the chord line. This alone can make it so there would be positive incidence in the tail with respect to the wing, if you are measuring from chord lines.

If the tail has an inverted cambered airfoil like many full size aircraft, the tail could easily have positive incidence with respect to the wing.

Also, trim with a given stability margin can easily require positive incidence on the stabilizer if the stabilizer or tail volume is unusually large (old FF models, tandem wing designs). Wing wash effects can also require positive stabilizer incidence settings.

So I don't agree with the blanket statement that the tail has to have negative incidence with respect to the wing for stability, particularly if you are measuring from the chord line of the airfoils, and not the zero lift lines.

Stability is set by the distance the CG is ahead of the aircraft neutral point. Trim speed is adjusted with incidence and/or elevator trim.

Kevin

[Rodney]

What Vintage says is correct for normal aircraft setup, i.e. the tail always is set to lift DOWN to counteract the pitching component generated by the wing. Now true, some few designs do not do this but they are rare.

[kcaldwel]

The tailplane does not have to lift down. Most properly trimmed sailplanes and pattern aircraft have very little stabilizer load, that may be + or - depending on the flight condition. Tandem wing and canard aircraft all have positive load on both surfaces, as do large tail volume airplanes such as some FF models.

I believe all pattern aircraft are set up 0-0, so I don't think it is that rare.

For trimmed flight, the sum of all moments around the CG (thrust, drag and lift of wing and tail, Cm of wing and tail, drag and lift of fuse, etc.) must sum to zero. Whether the tail lifts up or down depends on may things.

Kevin