I notice that Flair Legionaire has positive tail incidence. What are the effects of the various incidence angles on a biplane? Specifically:

1. Was is the effect of a positive, neutral, and negative incidence on the elevator stab?

2. How critical is wing incidence? I'm just planning on using scale incidence for a scale airfoil.

2. Is there a relationship between the tail incidence and the wing incidence?

Hi Abufletcher, to answer your questions, my opinions only -

1. Changing the incidence on the stab is just like elevator movement or elevator trim. On full size aircraft it's used to adjust the trim of the plane so the pilot isn't pulling up or pushing down on the stick to allow for changing speed/load/balance situations. If you raise the front of the stab it's like lowering the rear of the stab, like lowering the elevator, tail comes up - plane goes down.

2. Wing incidence is very critical, usually adjusted at first by trial & error or a guess following experience with similar models. Just build as per plan would be my advice, go with the forward limit for the centre of gravity for the first flights.

3. Yes. And with the engine thrust line. If the wing, the tail and the engine are all in one line (like an Extra 300 and most aerobatic models) and the wing section symetrical or almost symetrical there's a good chance all three will be set at zero degrees - the model is like an arrow & will fly fairly straight regardless of speed or whether its upside down or not. When you start raising or lowering engines, wings and tails things go out of whack so the angles are then changed to make aircraft fly correctly. Raising the wing incidence is like lowering the tail incidence, the plane will tend to loop. If the wing incidence is raised more than a little the tail incidence must be raised as well to keep the plane flying straight.

Hope this helps - John.

John, thanks for the clarifications. At the moment I have 0 degrees on the stab and engine and the wing incidence will be whatever it is on the scale technical drawings that I'm using as "plans" for my scratch-built Nieuport 11. As I understand things (which is not saying much ) undercambered wings used a higher incidence angle than later symetrical airfoils.

Since the stab is connected with three screws I can be adjusting the tail incidence after the maiden flight, I guess. So ideally, the tail plane should be set to that in "straight and level" flight with the trim set to around neutral the elevator surface should be level with the horizontal stab. Is this right?

Don, back in WWI some planes used the wing and stab incidence to effectively give down thrust to the engine. As an example the SE5a has +5 degrees on the wings AND the stab. What this does is give the engine -5 degrees, effectly, when the wings are flying level. Just thought you might like to know.

OK. Time for the next silly question. Why would one want to use the wing and tail incidence to create effective engine downthrust?

And to return to the original question, would it be better off to start off with a bit of positive incidence on the tail (and if so how much) and zero up or down thrust on the engine? I always though of adding engine thrust as something you did to solve serious flight problems, for example, a strong tendency to climb even with the elevator trim set all the way up.

Also if positive tail incidence means that the plane would fly more in a nose down orientation then does that mean you'd have to add less weight to the nose? As you can see I'm totally muddled!

Here's another silly question. Why would you want to use the wing & tail incidence to create effectively longer undercarriage legs? To raise the wing angle to help take off the Armstrong Whitworth Whitley bomber of WW2 used a heap of positive incidence on the wing instead of longer legs which would have the same effect. Of course they had to add a heap of positive on the tail too so it wouldn't loop so the poor things spent their lives flying around in a silly nose down attitude.

If I had to guess I'd say the Nieuport would have a couple of degrees positive on both wings, the tail at zero & the engine about 5 deg down. Post another thread titles Nieuport 11 rigging angles & see how you go. Engine thrust adjustments are used to cure climbing or diving problems at high power settings. Changes to tail incidence angles to cure tail heavy planes is a recipie for disaster. - John.

Several manufactures use incidence in the wing to fly nose down so you can see where you are going.
Darryl Usher

One thing to consider is that it was probably easier to add positive incidence to the wings and stab and leave the engine at neutral. This would have made making the airframe and cowls easier not to mention the radiators..etc..

BTW, what is the standard method of adding a couple of degrees of downthrust to a model engine? Do people angle down the entire mount (with, for example, a couple of a washers on the top mounting bolts) or do you tip th engine forward by add a couple of washers to the rear two engine bolts?

I'm tempted to leave the engine at neutral unless I find there is a problem I can't solve with the tail incidence. On the wing incidence, I'm just going to stick with the scale incidence for this scale airfoil -- and assume the designers had it right to start with (and to heck with any scale effects).

After reading the above posts, I just have to make a couple of comments: I have designed many airplanes in my day, of all sorts and configurations. There are three things that you must do if you want your first flight to be uneventful.

1. You must have enough power to fly the airplane.

2. The C.G.(center of gravity) must be in the exact location for the airfoil being used. In most cases this is between 25 and 30% of the wing chord. I start all my planes out at 27.5% with a full tank of fuel. For bi-planes with straight staggered wings, locate the 27.5 point on each wing, and draw a line between them. Where that line crosses the thrust line is a good starting spot. For wings with sweep back, locating the exact balance point requires additional calculations.

3. You must KNOW your incidence angles. With the stab set at zero, for flat bottom and undercambered wings, I use 1.5 to 2 degrees positive incidence.

DON'T ASSUME ANYTHING. Get an incidence meter and read your incidence angles. Adjust as necessary. Put the plane on a balance stand so you know where you are. I balance so that the stab is at zero with a full tank of fuel. I determine the power requirement from experience.

Don't fly any plane until you KNOW the incidence angles, the correct CG, and are satisfied that the engine will develop enough power to fly the airplane.

Attached are a couple of photos of my new J-3 on it's balance stand. In this case, I had to add 12 oz. to the nose to get it right.

Wow, Jim! Thanks for the advice. Once the models start getting THAT big, making a balance stand becomes a serious matter! I'll definitely pick up an incidence meter this summer.

Sticking with strict scale construction, the Nieuport 11 looks to have zero tail incidence and about 2-3 degrees of upper wing incidence. The question is to what degree can a (very) scale model simply use the incidence angles of the real aircraft. The Flair Legionaire has a couple of degrees of positive incidence on the stab (created by an angle cut to the rear longerons). Not sure on the wing yet.

I'm going to have to do some research to figure out the CG point on the Nieuport 11's swept-back and slightly staggered wings. If anyone with a Proctor N.11 could give me a starting point that would be great. I prefer my models to be a bit nose heavy for that first flight. From experiements with RealFlight it's obvious that a plane can be seriously nose-heavy and still be manageable but just a bit tail-heavy can spell disaster on a maiden flight.

BTW, I thought we were supposed to balance the aircraft "dry" (with an empty tank).

Continuing the discussion: The incidence of the wings are always referenced to the stab.

With the stab at zero, set the wings at 2 deg. max. positive. That gives two degrees differential, no matter how you look at it.

Now take a WWI fighter: Some of them have the stab set as much as 5 degrees positive, relative to the thrust line. In that case, the wings must be set at 7 deg. max., relative to the thrust line. However, the differential between the wing and stab is still 2 degrees. Get it?

This has the effect of building in 5 degrees of downthrust to the engine, (7 - 2).

The reason that I balance my planes with a full tank of fuel, is that is the way it will be flown. I never take off with an empty tank. So, I know then that when it does take off, the balance is as good as I can make it, and as it burns off fuel, the CG will move backward. However, it's very rare that I ever have to change the original CG, once determined.

As you build more models and gain experience, you will do things "your way"; things that work for you. That's all I am doing here - what I say is not gospel - it's just the way that I do it, and it works for me.

By the way - do purchase a good incidence meter. It's one of the best investments you can make. It takes all the guess work out of setting up a model correctly.

So let me see if I'm understanding this. Positive incidence on the wing/stab combo create effective engine downthrust. Is this right? So if the stab is at zero and the wing at +2 then I have the equivalent of -2 downthrust? Sounds like a winning combo.

But again I have a question (I always do!) -- Can a model with a scale airfoil use a scale amount of incidence?

Jim Messer is right.

I have the Hanger Nine Cub flying with an Astro-Flight Geared 40 electric motor. I have the elevator at zero degrees incidence and it flies better than a friend of mines that has the stock positive incidence. Like Jim all of my designs have the engine set at zero degrees, stab set at zero degrees and the wing at 2 or maybe 3 degrees incidence. Even if the scale 3-view shows positive incidence in the stab I always put the stab at Zero incidence.

So if you Hanger Nine Cub flies tail high add 1/4" balsa underneath the trailing edge of the stab and you will notice the plane flies level...

Good luck

abu: re. your last post: No - you don't have it right. The thrust line is referenced to the stab only: With the stab at zero, the amount of downthrust is also zero. With the stab set at 5 deg. positive, and the thrust line at zero, then the amount of effective downthrust built in is 5 degrees.

With the stab at zero, the wings should be set at 2 degrees positive. Built in downthrust is zero.

With the stab at 5 deg. positive, and the wings at 7 deg. positive, there is still a 2 deg. differential between the wing and stab, and the effective downthrust is 5 degrees.

I wish I could draw you a diagram, but don't know how to do it and post it here. So words will have to suffice.

Humm.. I see you are back from the UK Don and asking questions again ..hope you enjoyed your visit.

Fortunately for scale buffs there is no need to have everything zero'd out ... I cant think of anything worse looking than something like a Fokker DVII with its tail at zero [:'(] I totally agree with Jim Messer that on flat or undercampered wings, they have to have a slight positive incidence with respect to the tail incidence for the aircraft to fly . I also believe that it is the incidence of the main wings relative to the fuz datum line and not the incidence of the tail that makes the aircraft fly with its "Tail in the air / or tail down" If the tail incidence is out .. as Boomerang1 said .. you dial in trim for level flight .. a "draggy" solution but it will still fly fine... and the fuz will still be at the angle dictated by the main wings. I work in reverse from Jim as I dont have an incidence meter. Going by the 3 view, I set the main wings to the angle on the drawings .. then set the tail keeping in mind that the main wing needs some positive... I also put a couple of degrees downthrust on the engine. Next is the maiden flight and I trim the model out at cruising speed, land and check the elevator/tailplane (stab) are in line ..no up or down trim.. if trim is applied .. I reset the tailplane until it is in line with elevator. Once happy I then fly again under different throttle settings and adjust downthrust as required ... ok ..not scientific, more trial and error, but .. It works for me and I never could understand all them forumlae to do it technically. All my models fly fine though they certainly are not the most efficient flyers .. but then..were old canvas jobs efficient to begin with

Just to add a bit, If you go back far enough, Bleriot etc, they had barely enough power to get off the ground so they had very high incidence angles, sort of like flying with the flaps down, which is, with flaps down a very high incidence angle wing. If the wing was parallel with the fuse, it would look like it was flying uphill and causing a lot of drag. When you think about it there is a relationship between power and incidence as time progressed. WW1 planes100 HP etc, had a lot less incidence than the older 35-40 HP Bleriots. WW2 1500 HP planes had a lot less incidence than the earlier ones. So it's a matter of keeping the fuse level as flying speed changes. I imagine the jets have very little inc. angle. Just my thoughts too

Yup Flypaper 2 .. I would also thingk that weight and drag would influence the incidence too .. thats why aerodynamics get too complicated for me. Perhaps that helps answer one of your questions Don. "Can a model with a scale airfoil use a scale amount of incidence" yes ..if everything else is scale ..which is unlikely. So what to do... me ...Id pick the most characteristic incidence on the aircraft and adjust the other factors so it flew in the correct attitude. By that..I mean if the main wing incidence was the most prominant feature ..example the Bleriot .. I would use its scale incidence and try to trim the model by adjusting the stab and downthrust etc. With my Fokker DVII .. I set the stab and altered the top wing incidence as it wasnt so noticible .. only my 2 cents worth and as stated I know little about aerodynamics so could be wrong

For all the reasons people have mentioned I think it makes the most sense for me to use essentially scale incidence on the main wing (which would be the most obvious feature) and initiall set zero on the tail and zero on the engine. Then after the first initial inflight trim, I'll do as Donnie suggests and, if necessary add in a bit of positive stab (and possibly a degree or so of downthrust).

Donnie, do you add downthrust by adding a shim to the whole engine mount (if you've used an engine mount vs. wood beams)?

One last tip on incidence "by trial and error" for you Don.. if you have the rear stab set with not enough positive incidence .. this has the same effect as having a bit of up elevator .. on a first flight..this can be a disaster as the model may try to jump off the ground before its reached flying speed. I always apply a little down trim to the elevator for first flight in anticipation of this happening.

As for downthrust and sidethrust ..I usually build that into the firewall or shim the rear of the motor mount . as a rule of thumb, I always build in 2deg down and 2 deg side thrust on day one for bipes. Doing this at the construction phase ensures the centre of the prop is in the centre of the cowl (the mount is off set to achieve this). Later, fine adjustment is done with shims between the engine and its bearers. Before you ask ..how much side thrust .. a model has the correct side thrust if you pull it vertical and it goes up in a straight line without falling to right or lift. Its worth doing a search for an "aerobatic trimming chart" and trimming a model (your space walker for example) properly following the chart... you will learn a lot about how different settings interact with each other and will be surprised how much better your model can behave

Donnie, there's a lot to take in there -- which only goes to show how much I don't know! Very good advice about dialing in a bit of down trim for that first flight. No doubt I'll eventually be back with more "first flight" questions.

I'm curious about those "2 degrees" of down and side thrust. You've said above that you don't have an incidence meter. How does one "eyeball" 2 degrees? Do you just know from experience? Also you forgot to anticipate the question I was actually going to ask () which is "side thrust to WHICH side" -- I know this should be obvious but it's not.

I'd feel a little nervious trying to build thrust into the firewall. What if I mess up the angles? It's just about all I can manage to just get it on straight. Good point also about how the offset of the engine will affect the thrust washer's placement in the cowl. I hadn't thought of that at all. Looks like I might have to redo the firewall I've made already (but not installed).