Thanks again for your post. I just re-read it and wanted to ask/comment on it. Let me see if I have it right: Decalage is the difference in angle between the wing and horizontal stabilizer. Wing incidence is the difference in angle between the FSR and the wing. Question (I hope not a dumb one): If I know the decalage angle, is there any reason for me to know the wing incidence?

Thanks.

Marl

If you presume the horizontal is parallel to the FRL, then decelage tells you the incidence.
As the horizontal in most cases is parallel, it's a good assumption, but don't hang your hat on it.

It's not a dumb question at all.

What you have to realize is that there is only two relationships that reallly count. One is the decalage angle and the other is the thrust line angle to the wing. Yeah, we just added another scoop to the mix...

The FRL or center line or whatever you want to call it is strictly an imaginary reference line it means absolutely NOTHING other than as an arbitrary reference line that the designer uses to set the other angles and parts to. Because of that there is no way to determine what it truly is.

However, let's consider that the human animal is basically lazy. Being that way I know that when I design my models I'll set the formers to sit at 90 degrees to the FRL for the most part unless it's a special case like a cabin former that sets the landing gear angle or some other special and obvious case. SOMETIMES, and don't rely on this, the upper edge of the main side panel will be drawn parallel to the FRL but there's absolutely no guarantee of that but if that line is 90 degrees to the majority of the formers you can pretty much rely on that being the case. If it's an aerobatic design or a super sport model intende as being close to aerobatic then you can expect the wing's airfoil angle to be very close to parallel to the FRL and the stab along with it. Often such models don't show more than a degree or so of decalage at most and are often 0-0. Such models typically also have the FRL passing through the center of the rear of the engine or motor although the front may be depressed slightly with downthrust. More oddly shaped designs won't follow any of these suggestions with any degree of reliability.

The bottom line is that if you need an FRL for some reason just draw your own in. It'll be YOUR reference and that's all you need.

To illustrate how confusing it can be I've seen old time Vintage RC models where the stab is set to 0 or even a couple of degree positive, the wing is set at a large positive incidence and the engine is set with only a little downthrust. But in reality the model set like this will fly very tail high so the wing sits at it's preferred angle of attack, the tail will be negative at that point and the engine will be working with a bunch of downthrust. The FRL in flight will be at a very tail high angle. In effect the plan angles the wing upwards to promote a larger downthrust angle. A VERY arbitrary reference line that has nothing to do with the reality of flying.

Whoops! Wrong acronym. I meant FRL. I'm pretty sure I understand it now. Thanks for your posts. New question: Is the thrust line angle the angular difference between the engine/motor shaft and the wing? Thanks.

Marl

Ah, now you got me and things are going to get complex. I have to admit that I've never seen a plan where the designer shows the downthrust angle with reference to the wing's airfoil center line. It's always given as an angle to the ... er..... FRL ..... er.... which of the smilies is the "embarrased" one ?

That makes things a bit complex doesn't it? Even though the angle that COUNTS is the one between the wing and the thrust line no one gives it that way. I practice the angle between the wing and the thrustline can be between 0 for a hot pattern model or up to 8 or 9 degrees for a rudder only old time vintage model. Most trainer models like a Goldberg Eagle will be around 4 or so and something like a Great Planes Sportster would be perhaps 2 to 3 tops.

Let me add a bit to the discussion.

At McDonnell Douglas we called the FRL a Water Line (WL). The nomenclature was stolden from the boat builders. That is a line that you wanted the hull of the boat to be level when at rest. In airplanes it is a line that we might want to always be level when the airplane is flying at 1g level flight.

To make something like a transport in full scale a designer and aero engineer would huddle over his drawing board. The designer says the cargo floor must be level in flight and have a certain volume. He draws a horizontal line and calls it zero WL. Things above it are positive and things drawn lower are negative. It's just bookkeeping as far as the designer goes.

The designer draws a fuselage around the zero WL. The wheels would be negative and if it is a high wing the wing will have a positive WL. The aero engineer says OK, now based on our calculations for the lowest drag when attached to a fuselage and to get the needed lift at the speed we are flying the wing should be at an angle of attack of 4 degrees. The designer draws in the wing at 4 degrees with respect to the WL and calls it the WRA (Wing Reference Angle). So if the fuselage is flying level the wing will be at 4 degrees angle of attack.

Based on some more calculations (after all he has gone through 4 years of college and 10 years experience and is supposed to know this stuff) the aero guy says for stability and trim he has determined that the tail needs to be at an Incidence Angle (IA) with respect to the wing of 6 degrees. He is thinking of the wing and tail connected with an invisible fuselage but with the whole assembly flying level.

The designer says, hummm, a 6 degree Incidence Angle and I have the wing at 4 degrees already and I don't want to change that then the tail must be at a -2 degrees. The Incidence Angle is a total angle between the two surfaces.

The model is made for a wind tunnel tests, the test ran and the data tells the aero guy that he was close (I am an aero guy and writing this so of course he will be close) but the wing needs a little more lift and the tail is a little off. The designer makes the WRA a degree more, the TRA a degree more in the negative direction. Always keeping the balance of wing and tail forces so that the fuselage will fly level. So we have the WRA = 5, TRA = -3 and Incidence Angle = to the difference between them = 8 degrees.

Again it is bookkeeping. The need to discuss the same thing no matter who is in the discussion. The designer thinks in terms of WRA and TRA and the aero type in terms of angle of attack and Incidence Angle. They are all the same thing just looking from a different point.

The ideal design would have a varying wing and tail angle so that they could be individually adjusted in flight for the lowest drag while always keeping the fuselage cargo hold at zero WL.

Looking at our RC models we can note some trends. Always keep in mind that the designer wants the fuselage to look right in the air. When he draws that look on the paper and draws a horizontal line through it that is called the WL. Then the angles are measured with respect to that WL.

Pattern airplane WRA = 0 TRA = 0 Incidence = 0
Sport airplane WRA = 2 TRA = 0 Incidence = 2
Trainer airplane WRA = 3 TRA = -2 Incidence = 5

If you know the WL and any two of the above the third one can be determined.

Keep in mind that the best way to design an airplane if you don't have experience or knowledge is to copy a successful design. It is the equivalent of the professionals performing wind tunnel and flight tests. Just someone else has done it for you! Don't start from scratch each time. The pros don't.

After you have spent a lifetime of messing with models it becomes automatic but the brain is still going through the process. The infamous TLAR (that looks about right) is actually a very complex design process that the brain carries out. The only difference between an engineer and a normal guy using TLAR is that the engineer knows why, the TLAR works by feedback from previous models and what works. It is nicer to know why in my experience.

Did this help any?

Merry Christmas to all including RAPPTOR!

Ben

We dood the same WL thing at Lughead, but set the 0-0 WL some distance below the lowest part of the plane, usually the extended landing gear, so all measurements on the plane would be positive.
Knowing where 0-0 WL is then, the hardware can be checked in the hangar by locating the hangar floor in terms of WL.
Similarly for the fuselage stations, the 0-0 FS is set some distance ahead of the nose cone, so all dimensions on the plane will be positive when measuring the hardware as built.

Great Discussion. I understand the definitions of incidence, but don't completely understand the effect of the "tail" incidence. First, can someone define "Angle-of-Attack" stability or "Balance"?

Next,
Why -2 degrees? I assume for a cargo plane the cg is in front of the wing center of lift, so the tail has to push down when in level flight to keep the plane from rotating. Is that all there is to it?

Does the downthrust of the engine effect where the WL or FRL are in level flight?

Carl

All of that makes perfect sense to me now. When the plane is originally designed, the FRL is somewhat arbitrarily chosen, but has to do with setting up some sort of measuring convention so that the numbers will be convenient to talk about (e.g., all positive). The FRL is indeed a useful and convenient line from which to make measurements. It can also be chosen based upon the attitude of the flooring during level flight. Right?

Thanks to all and a Merry Christmas!

Marl

.
Yes, it's not much more complicated than that. The tail's down lifting opposes the wing's natural nose-down moment, which is a result of the wing's camber.
The actual value for the tail's flying angle relative to the wing depends on a whole herd of things: speed, weight, moment arms, configuration, but the tail is built-on usually to have the least drag at the most economical cruise speed.
In Ben's example of the FRL's position, on the Tristar the floor was set to zero, when the plane was cruising at .92 Mach. The "fuel crunch" of 1972 forced the planes to fly at no more than .85 Mach, which raised the floor angle, and made pushing the food carts a chore for the gals.
The Tristar horizontal is all-flying, so there's no problem "streamlining" it as there is with a fixed horizontal and elevator, with a conventional layout.

You are right of course, we did too, but I wanted to get the 0 WL on the airplane for the example.

Can you imagine to possibilities for messing up with 00 through the centerline of the airplane!

Getting back to models, if you look at some of the "classic" Pattern designs, like the Joe Bridi Super Kaos, the wing had some positive angle when referenced to the fuselage datum, and the engine had some down angle. The stabilizer was at zero. These angles gave the model the desired effect of needing virtually no stabilizer trim in upright level flight, and a small amount of down elevator when inverted, at the speeds flown in the then-current AMA Pattern competition.

Naturally, the design was intended to fit Joe Bridi's style of flying the AMA Pattern at the time, so what fit his style might not have fit other flyers. Even so, the airplane had become known over time as an excellent flying airplane, and variants are still being produced today, over thirty years later!

On the Super Kaos, the fuselage top would be leveled and used as the angular reference, and the wing's, stabilizer's, and engine's angles would all be measured in relation to that. Level the fuselage and then use an incidence meter (angle meter?) to set the wing, stab, and engine.

As has also been stated, the fuselage reference lines (FRL---I've learned to call them datum lines) are arbitrarily-set by the aircraft's designer to have a convenient reference to make all other relative measurements. On the Van's Aircraft RV-9 I'm building, the longitudinal FRL is the top of the top fuselage longeron, and the vertical FRS is defined just forward of the propeller spinner. To make it easier on the home aircraft builder, the wing's and stabilizer's angles are measured in inches from the datum line at the leading edge and trailing edge of the wing and stab.

Bill the problem with using something actually on the airplane fuse is that you end up with negative WLs or FRS or whatever we call it. It is awfully easy to drop the negative sign when measuring below it. If you start really low everything is positive so you elinimate the possibility for 1000s of mistakes, especially for larger airplanes with multiple designers.

When thinking of different force setups don't forget Hal deBolts setup. The wing was at a +1 deg (trying to remember the exact number gave me a headache) with respect to the WL, the tail was at +2 deg or so. So the incidence angle was -1. The Kaos would be +1 (assuming Incidence = wing angle - tail angle). Two different ways of looking for a solution of a pattern ship.

I have come to the conclusion that the human pilot can fly a large range of model airplanes well and if the pilot is good enough he might win with almost anything (within reason of course).

Start of editoral

I get a kick out of someone that comes out with a "new design" but if we overlay the top view and the side view over several dozen other airplanes the only differ in the smallest nuances. But since the latest champion "designed" them it is looked on as the tool of choice for the pattern wars.

End of editoral

BAX.. said it right.. this was a super model.. did all things good... i would say this would be a good all around base line ..RD

Thanks for the lower case RAPPTOR. I did a lookup on your name and did see that you have a lot of experience in model Jets. You probably have had experience with the all flying tail. That is an example of a case where incidence is basically not defined until you are in level flight. If someone would come up with a very cheap, simple, ball bearing all flying tail assembly that would fit in a model it would be useful in a host of airplanes. A person would need to be close to some estimate for the first flight but there after it would be what ever the model needed.

We would be able to buy them now except that a conventional horizontal tail only needs a few hinges to make it work and the servo loads are very light by comparison.

One of the highlights of my last year was seeing Terry Nitch's F-100 at the Scale Masters. It was an example of the all flying tail use in modeling at it's best. Made me realize how feeble my efforts were!

Merry Christmas.

Ben,

I agree with your statement about having a line outside of the aircraft, especially with a host of other people involved in the process. That would make things easier in a team setting.

In the case of the RV-9, Van's has one person who did the actual design. Since they then produce kits, the references used make it very easy for the builder to get things aligned. All that's needed for alignment are a good level, an accurate ruler, and a plumb bob. The components are put on a stand/jig and levelled and plumbed as needed...shim as necessary and set the clamps. For example, when I set up the wings for drilling the rivet holes, I set it up on a stand that had the chordline vertical with the wing spar horizontal. A plumb bob was hung from the main spar and the distance from the plumb line to the rear spar was measured. As long as the root and tip had less than 3/32" of twist, the wing was considered good (over a 12' span in the wing panel). Of course the object of the homebuilder is to get things as close to perfect as possible.

With respect to models, something similar is happening. The designer has to make it easy for the modeler to true-up the airframe. Having a straight-line reference on the model makes it easy. A fuselage longeron, top deck, crutch, and so on as the reference make it easy to set the model and get it levelled in the building cradle. An "incidence meter" can then be used to get the wing and stabilizer (engine thrustline, too) to the angles detailed in the instrucions/plans. If the datum line was a foot below the model, it would be much more difficult for the modeler to set up the alignment.

Ultimately, the actual location of the reference is irrelevant, as long as it makes it easy for the builder/assembler to get the job done with the least possibility of error. As a long-time modeler, I know I make enough of them on my own!

I know the feeling well, given a chance I'll make the mistake. I really like the modern lazer cut interlocking kits. They can almost make them fool (and me) proof.

Certainly Van's experience would let him know how to present his design to the average builder (who as we all know is a genius!) and eliminate most of the mistakes that would happen due to a slip of the minus or plus sign. I don't know if I would want to take on that chore as a kit designer especially when someone is going to strap himself in it and go flying.

[X(] WOW ,MY 15 MINUTES OF FAME!! ---IMHO ,,all flying stabs should not be used on models.. only scale ,if needed.. they are very hard to set up. they are less effective,and can be a danger, if not done correctly.. RD

But, If (the big if) the mechanical part of the flying stab could be made with nice bearing and a proper pivot point that didn't have slop, etc. then they would be better because the problem of incidence setup goes away. What ever the angular relationship you end up with is would be good for that airplane. I think the all flying tail would be lower drag (both creating the same lift and pitching moment) than a flapped (which is what a horizontal/elevator is) surface making the same lift, but, I don't have the numbers in front of me at present.

Actually they are just as effective as a regular stab. Think of the system with a 10 percent chord elevator. You get a certain pitching moment with elevator deflection and is good enough for a lot of airplanes. As the percentage of chord of the elevator is increased the pitching moment at the same deflection angle is increased until you get the percentage chord to be 100 percent. The pitching moment with percent chord always increases although the increase might not be linear.

But the problem is indeed with the mechanical aspects of the system. When I think of the workmanship I have seen at the average flying field being aimed at a flying stab installation it makes me very nervous. At least with an elevator you can look and seep pretty quickly if the thing has been knocked out of alignment and especially so if there is some aero balance in the design. Not so with the all flying stab.

I have a couple all-flying stab planes...
They're most handy on planes where the flight incidence might be unknown, and yet give the least amount of drag when flying.

Unique designs, what purpose (other than because you could) did each design have. The second one is certainly very unique. Where do you find the time to make all of those designs?

.
Second question first... retire 10 years early...
The first was an attempt for a long distance electric for the Baker Marathon. Intended to use 3 sets of 10x3000 Nimhs in parallel for the maximum flight time.
Gave up that idea after burning up two expensive motors and speed controls. It now flies quite nicely with a Speed 700 and a single battery pack.
100 inch span. The flying vee-tail was an interesting exercise in allowing for tail wheel steering...
The second is a first cut at an SAE lifter for the 2003 72 inch wingspan rule. The pylon-mounted wing got most of the airfoil as lifting, not enclosed in the fuselage.
Flies fine.. but the flaps were overkill.
I'm looking for a practical use for both of these.

I said, "Actually they are just as effective as a regular stab. Think of the system with a 10 percent chord elevator. You get a certain pitching moment with elevator deflection and is good enough for a lot of airplanes. As the percentage of chord of the elevator is increased the pitching moment at the same deflection angle is increased until you get the percentage chord to be 100 percent. The pitching moment with percent chord always increases although the increase might not be linear. "

I missed something in there. Why I just remembered it now, I don't know!!

If you look at a regular wing section going up through the stall and post stall you get a given CLalpha curve shape. If you put a flap on the section it creates an incremental CL increase at each angle of attack. There will then be all flying horizontal tail designs with small elevators deflecting in the same direction as the flying tail that make the combination design more effective than just the all flying tail. But these are hard to make and probably not necessary.

The question is then - how does a fixed horizontal tail with respect to the fuselage equipped with a moveable elevator compare to the all flying tail. There will be some airplane angles of attack where the horizontal is at the section Clmax and when the elevator is deflected in the right direction that will have a higher Clmax than an all moving horizontal tail can achieve. But for the majority of the possible tail loads (those that don't approach the Clmax of the section) that either system can achieve there isn't any difference between the two in effectiveness. Indeed at the condition where the elevator approaches 100 percent chord it must approach (probably in a funny nonlinear manner) the all flyling horizontal tail in effectiveness.

Hey Ben,

Your just the man I need to talk to. My new CG Chipmunk was built , 0 tail, 1/4 wing , and 0 thrust. The darn thing climbs like a homesick angel. The down trim required was very visible. Before I tried to raise the trailing edge of the wing, I decided it would be easier to crank a little down into the engine, about 1 3/4 degrees. It takes a little less trim, but nowhere near none. So now I am going to fix the saddle and move the wing to zero. How could 1/4 dgree up in the wing have such a huge effect ? And once the wing is zero , should I put the engine back to zero? Or is this a personal preference thing ? There are no numbers mentioned in the instructions or the plans. I thought a wing had to have a positive AOA ?

Realizing the question was directed at Ben, I’m going to jump in here and give my two cents.

Any airplane will climb when power is added, if the cg is ahead of the neutral point (it has positive pitch stability). If the climb is excessive it always indicates that the cg is too far forward. The fix is always to move the cg rearward. Changing the wing incidence will not affect the tendency to climb with added power if the cg is not changed. It will only affect the trim position in level flight.

There is only one power setting at which the trim will be zero. Faster flight will require down trim while slower flight will require up trim. Typically I trim for about half throttle which is the basic power setting about which I fly, then use elevator to adjust for power changes. But that’s just a personal preference. For an airplane to fly “groovy” (that is to say just go where it’s pointed) at various power settings requires balance at or very close to the neutral point.