Can anyone tell me what is meant by Incidence and it's importance? Also what wing wash-out is and it's affects? Thanks in advance for your help.

Incidence is the angle of the centerline of the main wing and/or horizontal stabilizer to the thrust line. It can be used to control ballooning at higher speeds, or other unwanted flight characteristics. Look at a profile pic of a Fokker Dr.1, D8, or D7. See how the leading edge of the H. stab is raised relative to the thrust line? That's positive incidence. In this case, it forces the leading edge of the wing down in level flight, keeping the tail flying when the wing stalls, and allowing a more gentle, slower stall and an easier recovery.

Washout is when the wing tip leading edge is lowered in relation to the root L.E. (twist). This allows the wing to stall at the root first, which is much better than the tip stalling first. Washin is the opposite, and I don't know any plane that would benefit from it.

Dr.1

Normally, we measure incidence relative to a fuselage reference line, usually down the center of the fuselage, as opposed to the thrust line. Positive incidence is always leading edge up.

Some people get confused and think that positive incidence in the stab means the plane will nose up. This is not correct. Positive incidence in the stab will cause a nose down pitch.

Incidence is used by the designer to correct aerodynamic problems he sees in the plane. A lifting or flat bottom airfoil will cause a climb at full power so he adds in positive stab incidence to cause the tail to raise as speed increases, countering the climb tendency. We in RC tend to over power planes so many times we need to make adjustments ourselves. One example is to lower the wing incidence on a trainer to reduce the climbing tendency with a .46 size engine. (Most trainers were designed for partial power with a plain bearing .40.) Raising the trailing edge with a couple of "popsicle" sticks under the TE normally does this.

I largely agree with Dr1 Driver --- except --- the thrust line is not the reference line. The reference is an arbitrary longitudinal line (sometimes the mid-line of the fuse) & all other things are referenced to it. For convenience, the extended plane of the horiz stab is often used as the reference line.

The thrust line can be positive or negative relative to the reference line, as can the chord line of the main plane (wing) & the horiz stab, but that is unusual.

Sounds confusing? It can be -- which is why the horiz stab is a good thing to use as a reference.

BTW it is the chord line of the wing, not the centerline.

Taking the plane of the stab as the reference, if the wing chord-line is tilted up at the leading edge, then the wing has positive incidence. Similarly, if the chord-line is tilted down at the leading edge, the wing has negative incidence.

As noted, the engine can be tilted up or down (and often is) relative to the incidence reference. To further confuse you, it can also be angled right or left & offset right or left relative to the axial centerline of the fuse.

I'll buy the reference line idea. However, since 99% of my planes have 0-0 thrust, the thrust line is usually the datum line.

Dr.1

That may be the case with your own particular models, but it is most certainly not the case with the majority of others. Virtually no standard trainers, for example, have the thrust line coincident with the reference line -- they invariably have down thrust.

I know that, brat. Most sport models, however, are also built 0-0.

Dr.1

agree that many sport models are like that.

True. Trainers need the downthrust because of the high lift wings they're fitted with. Some balloon horribly when full throttle is applied. In their case, negative incidence may create trimming problems, so downthrust is used, since it's proportional with throttle. It supposedly self-corrects the ballooning.

Dr.1

Pitch-up results from the aerodynamic center being offset from the thrust line. The wings produce drag that is offset from the thrust line & hence there is a turning (pitching) moment upwards (in the case of high-wing aircraft) toward the center of lift.

As speed increases, lift (& hence, drag) increases, thus changing (increasing) the pitching moment. Since speed is dependent on power, down thrust works (more or less) to cancel the pitching moment. The effect of downthrust is not directly proportional to throttle setting -- it is speed dependent, but it works fairly well across a usefull range of airspeeds.

Most sport models I've dealt with actually have right thrust, hence they aren't 0-0 on thrust, but that's a minor point.

Incidence was well described, i.e. it is the relative angles of the engine, wings and stab. I'd like to try to expand on it's importance.

Planes are a balancing act between many forces. These forces change as conditions such as thrust, airspeed, altitude, etc. change. The designer of a plane will attempt to set the engine thrust, wing incidence, stab incidence and CG such that the plane will fly at a constant altitude at a given airspeed and thrust setting at zero trim, typically this would be your cruise speed and is not necessarily wide open throttle. If you think about how planes fly, the effect of thrust, and why most planes have stabs, it should be obvious that the angles between these components is critical. Their relation to CG may be a bit more difficult to grasp, but take the easy one, the stab. If your CG is rearward, the stab will need positive incidence to hold up the heavy tail. Move the CG forward, the incidence on the stab needs to be lowered, etc. Beyond "level flight", the incidence of these components (and CG) can have a large effect on aerobatic models. Issues such as knife edge tracking, upline/downlines, etc. all relate to incidence. Small changes in incidence can have large effects in how the plane flys. By small, on precision aerobatic models one can easily see the effect of 1/8 degree or less change in incidence. While sport planes and the average sport pilot are probably less sensitive, it should go without saying that incidence, thrust, CG location are very important and you should take care to ensure they are set "properly" on your plane.

"Properly" - this is subjective. There is a wide range of combinations that will work for a given plane, but the plane will act differently. CG is the obvious one as it is common knowledge that you can move this around some, but wing, stab and engine incidence can also be "adjusted." While typically a bit more difficult to fiddle with (unless you have wing/stab adjustors), plane performance can be greatly enhance, or diminished, by changing the incidence of the wing, stab, engine.

To get a feel of how this all works, download an advanced trimming chart that offers solutions that include incidence changes. There is one at the NSRCA web site --> http://www.nsrca.org/trimA.htm

THANKS EVERYONE for your input. It is as clear as mud now... he he he Just kidding, I definately have a better understanding than I did before, I guess some experimenting will help to. Thanks again for all your information...

If you are going to experiment, you must determine how your model is currently configured before you start changing things.

I suggest that, for convenience, you take the line of the horizontal stab as your reference. Measure the existing wing incidence & engine thrust lines relative to that, then change only one thing at a time while you are experimenting.

Make your changes small & easily reversible -- for example, change wing incidence by shimming the leading or trailing edge, at the wing saddle, with popsicle sticks to see what happens -- then it is easy to go back to the original config. Same thing for thrust lines --shim the engine mount with thin, easily removable shims.

The changes will be interactive. If you change the wing incidence you will get a particular result -- same for the engine thrust. If you change both, the result will be different from either of the others -- the process is itterative.

If you find a set-up that works better than what you currently have, you can make it permanent when all of your testing is completed.

Good luck & have fun.

Quick question: How do you measure incidence if you don't have an incidence meter? I wanted to experiment with my 4 Star 40 (I built this from a kit, first build) as it seems to want to balloon on me a lot, especially with the first flight or so. I am not sure why this happens. It seems like I have to always give a lot of down trim to this plane every time I take it out for the first time of the day. Then as time goes on during the day, I can come back on the trim some. It is also somewhat sensitive to wind which most people tell me that low-wing planes are usually not affected much by wind. Could this be an incidence problem? I am pretty sure the CG is correct but I might check it again tonight when I get home from work. My father-in-law (who got me started in this hobby and also has a 4 Star 40) told me some time ago that the style control rods (plastic rod inside a plastic tube) the 4* has tend to warp in different temps and need to be trimmed every time you fly. Is this correct? Thanks in advance...

Quick question: How do you measure incidence if you don't have an incidence meter?
Quick answer: you don't.

It seems like I have to always give a lot of down trim to this plane every time I take it out for the first time of the day. Then as time goes on during the day, I can come back on the trim some.
This may be due to the nylon tube in a tube rods. Real Golden Rods don't have much expansion/contraction. I've measured less than 1/32" total from 200 degrees to 32 degrees. Cheaper nylon rods will contract and expand more, giving the trim problems you describe. Also, the outer rods MUST be well supported, no farther apart than every 3".

It is also somewhat sensitive to wind which most people tell me that low-wing planes are usually not affected much by wind.
Not true. The placement of the wing doesn't affect wind sensitivity. The wing loading/weight and airfoil affect that the most.

Dr.1

When planes "balloon", they are setting the AOA of the wing too high. This can be caused from anything from over speed to not enough down thrust to CG and incidence issues. You must first attempt to determine the root cause of the problem, and it may be more than just one item. Download the trim chart I posted or another similar chart. I'd first check for thrust issues, trim for level flight at 3/4 throttle, then chop the throttle. Does the plane descend in a nice arc or does it sharply head down. If it is the latter, you don't have enough down thrust, etc. The try dives at idle to check for incidence, etc. as indicated on the trim chart. As BritBrat mentioned, everything is interdependent, i.e. a change in one thing effects everything else, so you end up chasing configurations around. To really trim a plane out well, it often many small adjustments and many flights, but it is worth it in the end.

The part about the trim changing during the day is curious. It may be possible you are having expansion/contraction issues with the control rods, or maybe a fuel tank issue is shifting your CG a lot. If you are using the plastic pushrod (Nyrod, etc.), they suck. They will change a lot with temp and cause you to retrim all the time. Replace them with pull-pull cables or steel rods, etc. and the majority of the retrim issue will probably go away.

Here where I fly in Nebraska, we consider wind anything over 30MPH. Anything less is a gentle breeze. So I have some experience with wind. What effects model performance in wind more than anything is wing loading. A lightly loaded model has a lot of wing for it's mass, and hence is pushed around easier in the wind. The low mass (read low inertia) for the area of the plane is easily upset by wind gusts. For that reason, sometimes it isn't always best to build as light as possible in areas that have more wind. The extra mass (assuming you don't go overboard) actually helps penetration in gust conditions. 4*'s are lightly loaded planes. I'm 95% sure what your perceive as a wind handling issue is due to the light wing loading on that plane.

Cheers

If you are using the plastic pushrod (Nyrod, etc.), they suck. They will change a lot with temp and cause you to retrim all the time.

Wrong, John. Perhaps you missed my post where I stated that I have actually measured Golden Rods (Sullivan brand name) between 200 and 32 degrees. The total difference in length of a 18" section was less than 1/32". That will not make a noticable trim change on most sport planes. I can't peak for Nyrods. When I use plastic tube-in-a-tube, I always use Golden Rods.

Properly installed, Golden Rods are a reliable control linkage.

Dr.1

One little mistake that can cause your problem, is having your ailerons deflected, rather than in-trail. If they are both deflected downward, that will indeed have the same effect as incorrect incidence (too much positive incidence).

If your ailerons are perfectly in-trail, try reflexing the ailerons upward a little & see if the problem goes away.

The trim changes could be caused by push-rod expansion & contraction, but it shouldn't be especially noticeable -- unless you have the throws maxed.

That's a VERY good suggestion, brat. Many times ailerons are trailing in flap or spoiler position, causing trim problems.

Dr.1

WOW, you guys are great!!! All that information makes much sence to me as they all sound like possible answers. It may be the aileron deflection problem as britbrat mentioned, sounds like a real possibility. My kit did come with a guage for testing this but at the time when I built this plane, I was only 2 months into the hobby and didn't know all that much about building but I am pretty sure it was close to being flat onto the gauge. Something I will have to check again. I am not sure if the pushrods are nylon or plastic inside of the tubes, but as I mentioned in my earlier posts, my father-in-law had somewhat of the same problem with having to constantly readjust the trims everytime he flew for that day. All my other planes have either metal or wood/metal and I rearly have to adjust trim, just take off and fly. Now, being that the outer tubes have been glued into the formers during building, how can I replace these with soemthing more reliable/stable? Do I just remove the inner pushrod and replace with metal pushrods inside the tubes? Or is there a better method/pushrod to use? Thanks again guys, I love these boards, it really helps to have people who know so much, makes a world of a difference.

There is a type of linkage made with 2-56 metal rods inside plastic tubes. You can buy a set of these and simply replace the inner nylon rod with the metal one. Unless there's a really tight curve on the existing rod, it will work.

Dr.1

My Avistar has that kind of setup with the metal rods inside of the tubes, seems to work. There aren't any significant bends in my 4* but I hope there is enough clearance in the outer tube for the metal pushrods to move freely. I guess I will just have to try and see. Thanks for your help again... One question about the incidence issue. From what I am understanding from what everyone said, the affect on my plane, say if I would shim the leading edge of the wing down, this should make the plane's nose want to point down. Maybe possibly reducing the ballooning, that is if it isn't because of my ailerons?

You are more-or-less correct, but it is easier to shim the trailing edge up, rather than try to lower the leading edge.

Try reflexing the ailerons first. If that doesn't work, the problem may not be an incidence issue.

Get back to us with the results from the aileron changes, before you try anything else.

Ok, I am hoping to get out tomorrow to fly some, HOPEFULLY, if weather permits... I will let you know my results... Thanks.

"From what I am understanding from what everyone said, the affect on my plane, say if I would shim the leading edge of the wing down, this should make the plane's nose want to point down. Maybe possibly reducing the ballooning, that is if it isn't because of my ailerons?"

Actually, you haven't given enough information yet to determine what needs to be changed. Specifically, when does the plane balloon. At cruise speed? Only at full power? Only at low power? More toward the end of the flight, etc.

Here is what I suggest you do.

Check a low power dive. The purpose of this test is to remove engine thrust from the equation. We are then just left with CG and incidence. Get up high, throttle back to idle and put the plane into a straight down dive. Once in the dive, be sure you send elevator back to neutral (release the stick, etc.) Watch the plane closely. Does it continue straight down? Does it try to tuck and exit inverted, or does it try to pull out to upright?

If the plane continues straight down in the dive test, there is a very good chance your CG and incidence of the wing/stab are OK. If the plane pulls out this may be somewhat normal for a 4*, but it can also mean that you are nose heavy and/or wing/stab are set to low. If the model tucks, you may be tail heavy and/or wing/stab are set to high.

Next, Fly the plane at your cruise speed. Trim for level flight at this speed. This is probably somewhere around 2/3 throttle. Pull the throttle quickly but smoothly back to idle and watch the plane. As it slows down it should gently lose altitude at a very slight nose down attitude. Does the plane climb at all, even a little bit? Does the plane quickly descend? If the plane climbs, you probably have too much down thrust. If the plane descends rapidly, you probably don't have enough down thrust.

Can you do these tests and report back?

Cheers.