OK, well not exactly. I know what wing incidence is. What I can't find is an explanation of HOW it is set and WHAT it is measured against. That is, what is used as the baseline to measure the incidence against. Also it does not seem to me that you can measure this even if you want to unless you have one of those expensive machines/tools. Can someone take the time to explan or give me a site with a good basic explanation. I keep reading all this stuff about how important it is. I've built about 30 ARF's and have no idea if the incidence was good or bad on any of them and probably couldn't tell anyway. So someone help me out here.

I think you can measure it relative to whatever datum you like. Measuring it relative to the horizontal stab makes sense. A horizontal line through the fuselage also makes sense. A plan specifying an incidence value should make clear what reference is used.

I don't think you'll find any hard rule concerning what value the incidence should be for any particular model. You can fly the plane and get an idea it is correct or not.

For example, if when trimmed for straight and level flight you find the fuse is drooping at a slight angle in order to achieve the required angle of attack for the speed at which you are trimming, then raising then increasing the wing's incidence will get you the same angle of attack and keep the fuselage level.

Some aerobatic trim chart includes flight tests for incidence:
http://www.nsrca.org/technical/trimming/trimming.htm

In my brief experience differences can be observed if you change the incidence.

I have a stick that will fly around with the fuse drooping at a noticeable angle. It has a slight negative incidence. When I change that (stick some fuel tubing under the leading edge) it flies with the fuse closer to level.

I have an aerobatic plane that pitches strongly towards its belly in knife edge. Decreasing incidence will resolve that (so will moving the CG forward.)

Incidence can be measured against any arbitrary line. That line can be selected due to influences based on what the aircraft mission would be.

For an airplane optimised for cruising, the zero line would be set along the axis of the fuse that results in minimum fuse drag. Then the wing incidence would be set at the angle of attack typical for cruise so that both wing and fuse are in their best position. That's how you would do it for a sailplane.

For aerobatics, the zero line is more arbitrary, but would run somewhere in the region of the line from the engine crank to the stabilizer, or some other way of setting a central line for the fuse. In this case, it is the relationship between the wing, thrust, and tail angles that is most important, so the datum on the fuse is not a critical choice.

Yes, but I have been reading a lot of threads lately on incidence, especially on Bipes. When people keep talking about setting incidence at 0 or +1, etc; they are also talking about setting the elevator incidence. You can't just pick "something" to measure the incidence against. It has to be an exact baseline. good example: My DP Ultimate says 0 wing incidence. But 0 to What? It doesn't say. I think you get my confusion.

PS: If the zero line is "arbitrary" then this entire discussion is useless.

Steve

As I recall from a long time back, the incidence is measured against a fuselage reference line. Most full scales set this line so the fuselage is in an attitude of least drag, or the attitude they want the plane to crusise at.

In RC monoplance we normally use a 0-0-0 set up. This is wing, stab and engine all at zero degrees. Some biplanes and others use positive incidence, but sport and acro planes mostly use 0-0-0.

Robart used to sell a very inexpensive incidence meter. I still use the one I bought 20 years ago.

My procedure is to put a level on the stab (if it's flat) and prop the fuselage so the bubble centers. If it's an airfoil stab I use the incidence meter. With the fuselage set, I carefully put the wing on and re-check the stab. Then I check the wing incidence at both tips, shimming if necessary. In this position, I also check the angle of the firewall and shim the engine as necessary for either 0 deg. or whatever down or right thrust may be required.

I should say that I also join wings with the meter. It doesn't matter what the root is, most of the rolling force comes from out near the tips. When I join 2 panels, I prop 1 side up and get the incidence at 0. I weigh the wing down with magazines. Then I put the second wing panel in place ahd check it at the tip. If I can get both to 0. I glue them together. Some times the root is off by a good amount (meaning there is a warp in the wing.). It's the tips that count.

...In those cases, you would have to ask those who are commenting. I would guess they are setting the incidence relative to the stab. But it could be relative to the thrust line. It could be as simple as using the bottom of the fuselage.

It could just be to a line on the plans though, as it really is arbitrary. The value of "+1" incidence on the wing is completely useless for aerobatics if you don't know the tail incidence or thrust line. It's the relative positions of the 3 that make a difference.

BTW - I think when you are refering to "elevator" incidence - you are really talking about stabilizer incidence

Steve, pick one. Specifically, one you can't change. I'm betting your horizontal stab is fixed, unchangeable. Use that for your zero. Set your wing zero, and you're at a good starting place.

Fly the plane. If it tucks to the belly in knife edge, either (a) cut your stab loose and move it to -1 degree, or (b) shim down the back of your wing, moving it to + 1 degree. Fly again.

I know someone will point out I've omitted a bunch of steps, but you need to get past this. Pick a datum. Start measuring. Fly the plane, define what's wrong with it. Fix something. Repeat.

If you haven't built the plane, get busy!! And try to enjoy it!!

Good luck,
Dave Olson

Ed, your getting close to making sense for me.

John, you are correct I meant stab. And your comment about "it could" seems correct also. But nobody, and I mean nobody, says what to measure it against on an ARF. The manufacturers and cabitzers all talk at great length about setting it at "x" ; but I have yet to have them say "x" to the stab, or "x" to the fuselage bottom etc. So it seems to me this entire incidence thing is probably way overblown. Am I correct.

Well, gitting it right can be important for the behavior of the plane, but the best place to pose this question is the next person you run into who puts forth a value for incidence. Ask them directly what they measure that against. And ask the next person after that - since there's no hard & fast rule you need to find out every time.

A perfect example of what I am talking about is about two threads away in the "Biplane Incidence Question" thread. Lots of +this and -that, but no specifics of + or - against what.

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The "what" is usually the Fuselage Reference Line... a line from nose to tail.
Mostly with the horizontal at zero, or the top fuselage longeron, or the floor,...
When the horizontal is set at 0 for the plane in question. do that.
Measure the wing(s) incidence(s) from that reference.
For models, it is usually the case the horizontal being fixed will be at "zero" when the plane is designed.
Scale models OTOH to be scale will do as is mentioned in the "Biplane" thread have other references.. such as the Stearman with its +3 horizontal. That is referenced to a line running thru the prop towards the rudder, paralleling a main fuselage longeron. Just to give the plane designer someplace to start from.
What is important isn't the absolute reference, but the difference in settings, wing to tail, relative to that reference.
The difference is called "decalage"... or "longitudinal dihedral", which is more explanatory.
Harold DeBolt used to set his wings and tails at a positive angle to a horizontal reference line, with the motor mounted on that line with no downthrust. + wing, + tail. This would result in a slight amount of tail-high appearance in flight.
Worked for him.
I built one of his models and set wing and tail at 0.
Worked for me.

you cant measure your wing incidence by using the horizontal if you've never checked the horiz.against the datum set to zero on the fuse.99% of the time,your fuse datum line must be set to zero before you can check other incidence measurements along with the up and down thrust of the motor.If your lucky sometimes with a removeable hatch,you can use the flat part of the fuse to set you fuse datum line to 0.If not,I draw a line using a dry erase marker down the fuse where the datum should be,I then take a small bubble lever,or incidence meter and tape it lever with the line and then prop up the fuse until everything is lever or zero.Then you can proceed and set your horiz and wing incidence.This has worked perfectly

Tall Paul:

There is no such thing as decalage on a monoplane.

It is, by definition, the incidence difference between (or among) two (or more) lifting surfaces.

Many misuse it, referring to the incidence difference between the lifting surface and the horizontal stabilizer, but that's incorrect.

Most common decalage is having the lower wing around +2 degrees, this makes the stall more gentle by having the lower wing stall before the upper.

By having the lower wing set negative in relation to the upper, the stall is more sudden, with a greater tendency to snap. You usually find this in planes designed specifically for aerobatics.

But remember please, there is no such thing as decalage on a monoplane.

Thanks.

Bill.

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but.... if the wings plug in, and one side is at a different angle than the other?
If they're semi-symmetrical, does it matter?

Another way to look at the issue is to remember that the reference is arbitrary, and what matters is the angle of the wing, horizontal tail, and engine in realation to each other.

So, you could set your airplane on a table, and hook up a meter to your horizontal stab and observe a "+5". Then check your wing. If it also reads "+5", then you've got 0 differnece between the two, and you're good to go if the kit calls for "0". Now check the engine, and see what it's doing compared to the first two. If someone says they use 0, 0 on the wing and stab and -1 on the engine, just make sure the engine shows one degree less than the others, and you're good to go.

Splais.....Large commercial aircraft use the Fuselage Wl to compute wing/stab incidence....the stab is fully trimmable for both "+" and "-" degrees of incidence......its important when building a model to determine what reference one would like to use for setting up wing/tail incidence......they both have an important role on how an aircraft will fly.......I like Nick Zirolis way of using a "Crutch" system to build a model...it gives me a Wl to refernce the wing/tail incidences....for example if I set the crutch @ "0" using an incidence meter then all my wing/tail measurements will reflect a"+" or "-" from that reference point.....different models have incidences that reflect their own individual flying characteristics......The "+" or "-" is used as a description of where the Leading edge of the wing/tail is in relation to the fuselage reference point...... with the smaller models changing the wing/tail incidences are fairly easy to accomplish...the larger giant scale models its best to make wing/tail incidences as described by the plans designer....Bill....

Warbirds,I agree.0 is always the best refrence to measure off of.

"There is no such thing as decalage on a monoplane.
It is, by definition, the incidence difference between (or among) two (or more) lifting surfaces.
Many misuse it, referring to the incidence difference between the lifting surface and the horizontal stabilizer, but that's incorrect. "

Actually it can be shown that for a monoplane as well as a biplane that the tail is indeed a lifting surface. For zero pitch acceleration - wing lift times moment arm about CG is equal to tail lift times moment arm about CG. This is true whether conventional configuration or canard.

That being the case the term incidence is correctly used when comparing the angular difference between the wing and horizontal tail.