Wing-Ding

I've read a number of threads and still do not completely understand wing incidence. Firstly, how is it defined and secondly, how is it measured? If wing incidence is measured in relation to the fuselage, then where is the reference line on the fuse? In my way of thinking, it can't be arbitrary because you could end up with no difference between the wing and stab (zero decalage?), but could (exaggerating to make a point), be 90 degrees to the fuse (obviously absurd) and still have zero decalage (mono-wing, wrong term perhaps?).

Maybe I'm getting mixed up with angle of attack. Please help me understand this as I'm going nuts (already gone?). Thanks.

Marl

Tall Paul

Incidence -is- arbitrary. A designer selects some point on the plane, or off.. In full scale, a "waterline" designated WL00 is where everything else is mearsured from.
Usually this is well below the fuselage with the landing gear extended.
The incidence (of the wing) is important relative to the incidence of the horizontal. For ease of use, it's common practice to set the horizontal at 0 degrees incidence. The arbitrary fuselage reference line (FRL) then is placed parallel to the fore and aft extended chord of the horizontal.
For optimum aerodynamic performance, generally at a chosen weight condition and airspeed, the wing is set relative to this FRL for most efficient use of the wing.
Models seldom have the speed range between takeoff, landing and cruise/highspeed flight that full-scales experience, so precise determination of the wing angle is usually determined first by what has worked, and second by flight testing, with the elevator trimmed off 0 (faired with horizontal) as the situation requires.
On this factory drawing of the T-28, the FRL goes thru the motor, parallel to the horizontal. The motor has signifioant downthrust, and the wing significant positive incidence.
But essentially, a line is placed for reference. It might be no more than the bottom of the sheet the plan is drawn on, the horizontal drawn parallel to this, and everything else fitted.
For example, on the Lockheed S3A, the WL00 was placed 200 inches below the lowest part of the airplane., if memory serves. Why? Who knows.
As long as all measurements are done properly from that point, it's a moot question.

Wing-Ding

Thanks Tall Paul,

I think I'm starting to get it. Still confused, though. Dumb question: What is "the horizontal"? Horizontal in reference to what? Maybe an imaginary line that runs through the geometric center of each fuselage former? I guess I'm screwed up on the definitions.

I think that what you're say is that when specifying wing incidence, the angle of incidence can be in reference to some arbitrary zero. But suppose I had an R/C model for which I had no plans. Would it be possible to measure wing incidence or stab incidence?

It seems that the designer must have had some relationship between the fuse and the wing in mind when he arbitrarily decided where zero was going to be on the drawings. Seems like the line along the length of the fuse (WW00, FRL, etc.) must be defined in relation to some feature or geometry of the plane. Right- or am I missing something? Thanks again for your help.

Marl

Mike James

Yep...(Agreeing with Paul)

The datum line or "waterline" is just an arbitrary reference. I think in commercial aircraft, this would most often be the "deck angle", (angle of the floor/seats) so the designers can come up with a zero or near-zero deck angle for cruise. The wing, engine, and tail parts might all have different angles, to make the thing work.

My approach to designing models, using a typical planform with a forward wing and standard or "T" tail is:

I try and calculate the lowest drag position (typically my "zero" reference) of the fuselage, and then, using known airfoil data, try and figure the wing's angle of attack at cruise speed, so that in cruise configuration, the fuselage is in it's lowest drag position. Then I mount the wing on the fuselage at that cruise-optimized position. (usually a degree or so positive for symmetrical airfoils, and usually zero or slightly negative incidence for cambered airfoils.)

I believe that stabilizer incidence is arrived at by what the "zero trim" angle for cruise would be. (to reduce drag on the stabilizer) CG variations, different airfoils, and varying airspeeds will change the amount of download on the tail too, of course, but the general idea is the same.

Wing-Ding

Ok, that makes sense. So you're using some sort of minimum drag on the fuse to get your zero and wing relationship. Right?

So back to one of my obtuse questions: Is it possible to measure wing (or stab) incidence on a plane for which you have no plans (and you're not an aeronautical engineer)? Thanks.

Marl

BMatthews

Mike makes some good points about designing for a purpose but many models are set up for general fun flying and fuel efficiency isn't really a major factor for us anyway.

Here's another term for ya... Decalage (deka-laj). The word has a French origin and hence the pronunciation. It refers the the angular difference between the wing and the horizontal stabilizer and takes no notice of any arbitrary reference line. It's also sometimes innacuratley refered to as "longitudinal dihedral" as it's a key factor in pitch stability of the aircraft. For models where you don't have any reference to work from this decalage angle is a good place to work from.

On a model that's already built you can measure the decalage angle using something like a Robart Incidence guage. Ah, another bad use of the term incidence in naming this guage. It actually registers the current angle of the wing compared to level as set up for the test and NOT the incidence angle. The only way it would actually measure the arbitray incidence angle would be if the model was proped up in a jig so that the reference line is level. However if you measure the wing angle and tail angle with the guage without moving the model then the difference between the two is the decalage angle. It's most usually a negative number since the stabilizer always (or usually always) has a negative angle compared to the wing.

The more negative that number the more forward the balance point needs to be. Aerobatic models often have 0 decalage while a trainer may have -3 to -5 degrees. The more angle then generall the more the balance point needs to be forward. High decalage angles with forward balance points tend to produce models that tend to want to climb as power is added or in a dive they tend to want to nose backup. Taken to extremes the model becomes hard to fly with each speed increase causing the model to balloon up or cutting the throttle making the model want to dive. Taken the other way the model is very neutral and requires much more involvement from the pilot. Like in the 3 bears the bowl that is just the right temperature is the one that most people prefer.

Wing-Ding

Thanks Bruce,

I understand the decalage perfectly. I guess I just had trouble with the concept of "arbitrary" fuselage reference line. Maybe it was just a philosophical point, but it must be arbitrary only for the designer. I, for example, cannot decide arbitrarily where that line is. I can guess, but I can't, for example, arbitrarily decide it goes from the cockpit to the landing gear (exaggeration to illustrate a point). Then the wings and stab would be about 90 degrees to the direction of flight.

Marl

Mike James

Here's an idea to make it a little less arbitrary, although it's not especially scientific...

If your airplane has formers, chances are fairly high that those formers are aligned 90 degrees to what the designer thought of as the reference horizontal. The height of this line, would be approximately through the centerline of the engine mount on most airplanes. (again, not too scientific, but probably very close)

Tall Paul

.
Yes. Place the plane on a flat surface. Using a bubble level (easiest) or measuring up from the surface, set the horizontal to level.
Measure the wing leading and trailing edge distances from the surface. That's the wing incidence, relative to the horizontal.

khodges

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I don't think it gets used much in model aircraft, but there is usually some slight difference in the incidence between the left and right wing in some light general aviation aircraft to help counter torque reaction. In other words the right wing has slightly greater incidence than the left, which creates slightly greater lift on the right wing due to a greater angle of attack, which counters the plane's tendency to roll to the right. It is very slight, I think on the order of a degree or two.

Wing-Ding

Thanks everyone for your comments. Tall Paul: The only thing I could see that was parallel to the FRL on the T-28 was the border on the drawing, so I wasn't sure where to start. Based on Mike James' suggestion that the designer may have intended the FRL to be 90 degrees to the formers, it looks like on the T-28 drawing that the FRL is perpendicular to section lines B-B and D-D. Maybe the formers are parallel to those section lines. So once the FRL is drawn perpendicular to the formers, then in the drawing, the whole plane is oriented so the FRL is parallel to the drawing border- "the horizontal".

Thanks for your responses.

Marl

nitrowatt

Matthews decalage example/explanation is a welcome one for me...I've heard of different explanations but his seems easy to understand...I've heard of the "falling leaf" example in the past to illustrate the pitch stability that dacalage angle gives a plane. KHodges brings an interesting point as well...I think that some twin piston planes (maybe the Barrons?) use a differential flap rigging to help with torque effects in landing configuration when torque may pose a problem...on an aborted landing for example. I work on Lears and the flaps are equal...but I think I remember a friend that was rigging a Barrons flaps after installing a motor and the flap rig was a bit different...this was a while ago though...so don't quote me.

RAPPTOR

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PLEASE!!!!!!!!!!! DECALAGE HAS NOTHING TO DO WITH C.G. ,,,C.G --------IS GEOMETRIC..WING,, OR ENTIRE AIRCRAFT.. POOR DESIGNS WILL REQUIRE TRIM OR SOME OTHER "TRICK" TO GET GOOD FLIGHT ...IF YOU "BALANCE" TO GET LEVEL FLIGHT AT A GIVEN AIR SPEED,WHAT HAPPENS WHEN THAT SPEED CHANGES?? SYM. AIRFOIL, SYM. STAB. ZERO EVERYTHING,,BEST ALL AROUND SET UP..FLYS ALL ATTITUDES.. GROOVES.

Ben Lanterman

Rapptor - right decalage has nothing to do with CG - BUT - CG determines the airplane pitch stability because of its relation to the Neutral Point. Decalage determines the angle of attack stability of the stable airplane. You want angle of attack stability in a trainer and a strong pitch stability due to CG location.

Using trim or some other trick to get good flight isn't a sign of a poor design - its a sign of an airplane designed for a particular task. If the task is lowest trim drag at the most efficient crusie speed of the engine at a particular altitude then any trick or trim device you can drag up is fair game and will make it a good design.

When the altitude changes - then the criteria has changed and of course things need to be rebalanced aerodynamically.

For a trainer a symmetrical setup isn't too good since the angle of attack stability will help keep the airplane level.

That leaves up with the symmetrical setup is good just for great pattern flying airplanes - or anything that is going to spend as much time upright as inverted as flying sidewise and in snaps. Any of our hovering firebreathing monsters will work better with symmetrical as will the 7 ounce indoor mini firebreathers.

Ben Lanterman

A little more on stability. There is basically no difference between a stable 0-0-0 pattern ship trimmed to fly at 1g level flight and a stable trainer with 2 degrees of incidence that is flying at 1 g level flight. If both are statically and dynamically stable they will return to 1g level flight when disturbed.

If both are put into a 3g looping maneuver then the elevators are putting each at a new trimmed angle of attack. Both are still stable due to the CG placement and are going to return to the 3g trimmed conditon if a gusts hits them in the middle of the loop.

It is impossible for a 0-0-0 airplane to fly level without some elevator trim input either from the trim lever or just holding a little back stick. With 0-0-0 and no elevator trim input the airplane is at a 0g condition but is still stable. When disturbed it will return to the 0g condition.

BMatthews

If you have a proper decalage angle and the CG to go with it just like your "bad" example quoted here then the model will nose up if the model gets into a dive or nose down if it gets nose high and slows down. Both of which will return the model to it's stable speed ..... Just like a good trainer or general fly around non aerobatic model design SHOULD do.

Yes it does but it also takes a skilled pilot to fly such a model when it is balanced right on the neutral point. Chances are even YOUR model is not on that ragged edge and you are using a slightly forward balance point with some up elevator trim to compensate. If you trim for hands off level and roll it inverted does it still fly hands off? If you need to use a little down elevator to keep it level at that point then you HAVE some decalage even if it's induced by the up elevator trim you have set into the model.

And surprising as it may seem not everyone wants to fly a neutrally stable touchy aerobatic model. Some that are rusty on the thumbs, don't have the reactions any more. or just fly for casual fun to putt around the sky LIKE some pitch stability.

LouW

Much of what has been said here is true but seems unnecessarily complicated. The fact is that cg position determines pitch stability, while decalage (or trim*) determines balance.

*Aerodynamically there is no difference between decalage and trim though they are accomplished by different means.

RAPPTOR

My Feedback: (41)

[] LOUEW ,IS RIGHT ON.. AS ALWAYS,SIMPLE ANSWERS TURN INTO PAPER AND WORD WARS.. "FLYING WINGS" HAVE NO DECALAGE...... "I KNOW ABOUT REFLEX" PLEASE DONT GO THERE.. O-O-O STATIC. WILL RESULT IN A FLYABLE AIRCRAFT..THERE IS ENOUGH DRAG ABOVE HOROZONTAL C.G TO ADD POSS.INCEDENCE. IT WILL HOWEVER, NOT WANT TO COME OUT OF A DIVE WITHOUT, A LOT OF ELEVATOR.INPUT..AS PER BAD DESIGNS,,FLIGHT SPEED SPECIFIC,,WHY NOT JUST FLY WITH A DROUGE SHOOT DEPLOYED?? I STRIVE TO MAKE THINGS WORK WELL THE FIRST TIME, NOT PATCH THEM LATER TO GET RESULTS.. I HAVE FAILD IN THIS QUEST MANY TIMES.. "THEY" SAID A HONEY BEE CAN NOT FLY ALSO..SAVE ALL THE, HO-CUS PO-CUS. FOR JOB INTERVEIWS..

BMatthews

Raptor, that's fine for YOUR style of flying but legions of students have trained on models with what you would consider, by the sounds of what you type, to be "bad" design. And yet those models are accepted in the modelling community as being very successful designs. It's not that either way is wrong. It's just that different models are designed for different flight tasks. You've obviously moved beyond the need for any built in stability and prefer to trim in the minimum required as needed and that's fine. Many, including me, would agree with this method *FOR SOME TYPES OF MODELS*. However not everyone is at that point or perhaps there are reasons for certain models in your stable to use a different force setup. I fly models that are close to neutrally stable and I also like to fly Old Timer free flights with RC in them. The aerobatic models I try to trim to the edge like you. The Old timers wouldn't be as nice to trim and just watch them fly around if I did the same thing.

Basically I'm trying to point out that not everyone flies as you do. What's good for you and others of similar skill level isn't neccesarily good for everyone.

Wing-Ding

Rapptor,

Please turn your shift lock off. It looks as though you're shouting. Thanks.

Marl

Ben Lanterman

LOUEW ,IS RIGHT ON..

He often is.

AS ALWAYS,SIMPLE ANSWERS TURN INTO PAPER AND WORD WARS..

Trying to be accurate isn't bad, just good science.

"FLYING WINGS" HAVE NO DECALAGE...... "I KNOW ABOUT REFLEX" PLEASE DONT GO THERE..

And this is relavent because.......

O-O-O STATIC. WILL RESULT IN A FLYABLE AIRCRAFT..THERE IS ENOUGH DRAG ABOVE HOROZONTAL C.G TO ADD POSS.INCEDENCE. IT WILL HOWEVER, NOT WANT TO COME OUT OF A DIVE WITHOUT, A LOT OF ELEVATOR.INPUT.
.AS PER BAD DESIGNS,,FLIGHT SPEED SPECIFIC,,WHY NOT JUST FLY WITH A DROUGE SHOOT DEPLOYED??

Differential drag above and below the vertical location of the CG (I believe this is what you wanted to say) doesn't make it stable - It gives an incremental pitching moment that will be dependent on velocity squared. Not something that will work at a constant velocity which is what decalage will do. We don't fly with drouge chutes (I believe this is what you wanted to say) because we fly airplanes.

I STRIVE TO MAKE THINGS WORK WELL THE FIRST TIME, NOT PATCH THEM LATER TO GET RESULTS.. I HAVE FAILD IN THIS QUEST MANY TIMES..

Making a trainer that is stable and self righting work well is making things work well the first time. I would be interested to know what you consider to be a "patch" on an airplane.

"THEY" SAID A HONEY BEE CAN NOT FLY ALSO..SAVE ALL THE, HO-CUS PO-CUS. FOR JOB INTERVEIWS..

No aerodynamic eng. that knew what he was talking about ever said this. What we typically used in airplane aerodynamics is just a subset that works OK in the Reynold's number range that our airplanes happen to fly in. The aerodynamics of the Honey Bee as well as certain flies, etc. work on the "clap and flap" (I think I remember hearing it as clap and fling also) mechanism which is a very low Reynold's number phenomena. The flat plate wings hit at the top of the upstroke (the clap) and then sweep down (the flap). In the process a big vortex action is produced (the lift). As the wings make the upstroke the rotate and produce no vortex action until the clap again at the top.

Other insects make use of other flat plate aerodynamic vortex producing actions. The aero engineer worth his salt knows this. Again it is a subset of what the total fluid dynamics study is about.

This isn't hocus pocus, it's just good science. People can understand it and don't need the information dumbed down for them. Knowing how things work isn't a bad thing, rather it is something to strive for.

And yes, lose the caps. If you want to use the caps but sure and use very smooth sentence structure.

RAPPTOR

My Feedback: (41)

[>:]HI ,,SORRY FOR CAPS.. IM NOT YELLING..JUST LIKE THE WAY THEY LOOK..PLEASE DONT BE UPSET .. OK NOW THIS IS FOR THE SCEINCE GUYS..I HAVE SAID MANY TIMES,MATH IS NOT A SCEINCE,!! NASA HAS THE BEST "PAPER BRAINS" IN THE WORLD, CORRECT?? IM NOT A PAPER BRAIN, JUST WORK THINGS OUT TILL THEY WORK.. I HAVE NEVER "BURNT UP ANY ASTRONAUTS",IN A TEST,OR LOST ANY OF MY AIRCRAFT DO TO "BEING HIT BY STYROFOAM"!!!!!!!!!! {SHUTTLE} LOST ONE TO ,{O RING} LEAKAGE.HOW CAN THIS BE????? YOU GUYS ALWAYS NO WHAT TO DO "AFTER THE FACT"!!! IF YOUR SO SURE ,,WHY TEST?? JUST DO IT..ENOUGH SAID.. PUT THE WINGS ON AND FLY IT.. TRIM IT TO DEATH ,IF NEEDED..THATS THE WAY THE PROS DO IT.. I FLY ALL JETS,F-18,F-16 F-22,F-14,F-5,F-20,BD-5,A-10, NO COMPUTERS!! GO FIGURE.. THROW A STONE INTO A PACK OF DOGS,THE ONE THAT YELLS IS THE ONE YOU HIT.. RD

Ben Lanterman

Where do you fly all the jets?

The rest of us really don't like the way all caps look, I believe there is something in the forum rules that asks that you don't use them because it is normally used as if you are shouting! If not then certainly refrain for politeness.

No offense intended but you honestly don't sound like you know what you are talking about. Your comment, " IM NOT A PAPER BRAIN, JUST WORK THINGS OUT TILL THEY WORK.." is usually made by someone that has had no formal engineering education. There is nothing wrong with that, my wife is a remarkable teacher without an engineering education, my dentist gets along just fine without one and my proctologist certainly hasn't had one although he majored in big fingers.

You don't have to be a "PAPER BRAIN" , what ever that is, you just have to be accurate. Look at what you are reading this on, I would believe it was designed by a PAPER BRAIN, wouldn't you. Why do you think airplanes are any different?

I'm sorry - you bring nothing to the table.

RAPPTOR

My Feedback: (41)

i fly my planes in the sky,{paper brain}semi scilled over paid} you would rather complain about the fonts, then get the message!! this makes my point!! i have some formal trainig, i stopped ,before it removed all my common sence..you have formal training i asume!! i see you use it as a trump card.. why would it be then ,i can improve on any thing paer brains design???my ,its all about bottom line??

Tall Paul

Rapptor. Go away.