Angle of incidence

[Danny]

Confused!

Please help me understand incidence. I know what it is, but simply put, what is it for and how do you find how much angle is needed for a particular design?

Are there any calculations to find the angle for either the mainplane or stab (just interested in orthodox layouts for now)? If there are calculations available, can you build the design with the calculated angle and have it work on the first try? In other words, do you have to keep adjusting the angle to find what works best?

Also, what happens when you either have too much or too little angle for either the wing or stab? What will the flight characteristics be like?

I've been reading a aerodynamics book suited for model aircraft, and it is very sketchy on the subject. It basically just impplies that it's up to the designer to find the correct angles. So, I'm kinda lost, and would like to find the correct angles without going through too much aggrevation.

Thanks,

Dan

[Ollie]

The definition of an angle of incidence is the angle between a flying surface and an arbitrary reference line. The purpose of incidence is to make it easier to set up the relationships among the thrust, wing chord and tail chord lines. The angle between to wing chord line and the tail chord line is called the longitudinal dihedral (LD) or decalage. The LD has great aerodynamic significance. The angles of incidence of the wing and tail have little aerodynamic significance until they are added to get the LD. LD is not to be confused with L/D.

To understand what is going on, it is instructive to consider a glider whose wing chord line pivots and is controlled by a servo (pitcheron) and whose horizontal tail chord line is also controlled by a servo (all moving tail). Either the wing servo or the tail servo or both can control the LD. For a given center of gravity, there is an LD angle that results in a certain trimmed flight speed and pitch atitude. If the LD angle is increased the plane will take on a more nose up pitch attitude and if the LD angle is decreased the plane will take on a more nose down pitch attitude. It matters little whether the LD angle is changed by the wing servo or the tail servo or both. If the CG is moved forward it will take a larger LD angle to reestablish the original pitch attitude and airspeed. If the CG is moved aft, it will take a smaller LD angle to reestablish the original pitch attitude and trimmed flight speed. Notice that the word incidence isn't necessary to describe what happens.

With powered models the angular relationships are more complex and the thrust force and direction relative to the wing and tail have to be taken into account.

It is possible to calculate all the angular relationships that are involved but it is quite a complex calculation that involves the zero lift angle of attack of the wing airfoil, the aspect ratio of the wing (because it affects the induced angle of attack), the aspect ratio of the horizontal tail, the tail moment arm length, the wing chord, the wing area, the tail area, the CG location, how much the wake of the fuselage and wing slow the airflow over the tail and the desired trimmed airspeed. That's just for a glider. It is even more complex for a powered aircraft. BTW, that's why they put trim tabs on full scale aircraft.

As a practical matter is is best to find the LD of a new design by a process of flight testing and adjustment starting from angular relationships known to be safe for aircraft of similar configurations.

The angle of the fuselage to the direction of flight affects the drag of the fuselage but has little effect on the pitch trim unless the projected area of the fuselage is large and the angle of the fuselage to the direction of flight is quite large.

[Danny]

Thanks Ollie. That was a lot of help, but a few more questions arise. Does the LD affect the stability at all, or just the balance (trim) of the model. Would I need a bigger or small stab. area if adjusting the LD affected the stability?

Do you have any references to those complicated equations? I could probably work through them if I knew where to find info. about the chosen airfoil.

Also, how could you adjust the angle of incidence, especially on the stab., when it is glued into place? How large of change, in degrees, would affect performance? Would .5 of a degree matter much?

I'm just worried that I won't be "in the ballpark" when setting the angles for a first flight of a new design, causing a disaster. Could the model be uncontrollable once lifting off? Any rules of thumb on this?

Anybody else care to shed some more light?

Thanks again,
Dan

[Sail 'n Soar]

Dan,

What Ollie says is true, but by the fact that you are asking the questions, I'm not sure you want to get into the math - it's more than a simple equation. But if you are a glutton for punishment, I recommend "Theory or Wing Sections," by Abbott and Von Doenhoff. You can use either the tables or equations from Chapter 1. As an alternative, check out the site:

http://aero.stanford.edu/wingcalc.html

This will give you the angles of attack relative to the zero lift angle to reach a certain CL at the wing root and wing average CL. Set the wing AoA relative to the fuselage centerline at some mean angle between the angles associated with your minimum and maximum operating CL's.

An even easier approach, assuming you are planning a model you expect to fly upright most to all of the time, is to set the wing angle of incidence at ~2 - 3 degrees measured between the wing cord and fuselage centerline, align the tail plane with the fuselage centerline as a starting point, and trim with the elevator.

As long as the angles are not too far off the norm, your CG not too far back, and your tail volume adequate, model aerodynamics are relatively forgiving.

Gerry

[Ollie]

Stability is a matter of having the CG ahead of the neutral point (NP) which is the aerodynamic center of the whole aircraft. The bigger the tail area in relationship to the wing area and the longer the tail moment arm relative to the wing chord, the farther aft the NP will be and the farther aft the CG can be while still staying ahead of the NP for stability. Martin Simons' "Model Aircraft Aerodynamics" tells how to assure this.

As far as the angles between the wing chord, the tail chord and the thrust line are concerned, copy the angular setup of a model of similar configuration, airfoil and purpose. Usually a few clicks of elevator trim are all that is necessary to achieve level flight. Fine tuning the angles can proceed from there. Prototypes of new designs should be assembled so that all the angles are adjustable. Use rubber bands or mounting bolts on the wing or tail so that LD angle can be adjusted with shims. Make provisions on the engine mounting so that washers or shims can be employed to offset the thrust line.

To some extent, the fine tuning of the angular relationships is a matter of personal taste and flying style rather than a hard number to be calculated. How would you know what number to put in the calculation for the trimmed airspeed that woud satisfy you? How would you know how much the wake of the fuselage and wing slows the flow over the tail?

[Ollie]

Here is an outline of the calculations necessary to find the longitudinal dihedral (LD) for a particular trimmed flight condition.

The basic relationships for a glider are:
1. The vector sum of lift and drag equals weight.
2. The sum of the pitching moment of the airfoil, the tail moment and the lift plus drag moments about the CG equals zero.

The basic relationships for a powered aircraft are:
1. Lift equals weight.
2. Drag equals thrust.
3. The sum of all the moments about the center of gravity is zero.

These equations must be solved simultaneously. Pick an airspeed and calculate all the forces and moments. Solve for the angles of attack that satisfy the simultaneous equations. Calculate the induced angles of attack, the zero lift angles of attack and subtract them from the total angles of attack to get the geometric angles of attack. Subtract the tail geometric angle of attack from the wing geometric angle of attack to get the longitudinal dihedral.

These calculations are complicated by the down wash angle of the wing wake over the tail which affects the tail angle of attack and the speed of the flow over the tail being less than the airspeed because of wake effects.

The vertical distance between the thrust line and the CG must be known to calculate the moment associated with the thrust force. The pitching moment coefficient of the airfoil must be known. The horizontal distance between 25% of the wing's mean aerodynamic chord and the CG location must be calculated. The horizontal distance between the CG and 25% of the tail's mean aerodynamic chord must be calculated.

The maximum coefficient of lift of the wing must be greater than the coefficient of lift required in the above mentiond equations to insure adequate stall margin.

Now, maybe, you are beginning to appreciate why these calculations are seldom made in practise, why they put trim levers on transmitters and why they put trim tabs on full scale aircraft.

All of the many equations necessary can be found in Model Aircraft Aerodynamics by Martin Simons or any engineering text on basic aerodynamics.

[Sparky Paul]

Flew this one today sucessfully twice.
The extreme incidence angle did have a butterfly or two going before the 1st flight..
Hoping it wouldn't pitch up while accelerating.
It didn't.
Flies well.. the flaps-down position is limited (on purpose) by the fuselage,.
Pitch up with flap isn't uncontrollable, another pre-flight concern.
All-flying horizontals take a lot of the worry out of the unexpected..

[Danny]

Thanks a lot guys.

I'm finally starting to develop a mind set for everything that I've been reading and studying. Things are begining to "click". It's a great feeling.

I'm sure I'll have more questions later to test your knowledge.

Thanks again,
Dan

[MikeSt]

Danny,

No formulas here, but a good *simple* explanation.

http://www.monmouth.com/~jsd/how/htm...eyond-decalage

Mike

[Novy]

Hello everybody,

Please excuse my poor englinsh as I am french speaking,...
I am involved with two different LD issues at the moment.
I am currently building a f5d pylon racer, this plane is all fiberglass molded, it is a very fast airplane.
The incidence of the wing is fixed, but there is some "play" for the stabilizer,... I made some measurements in my workshop before gluing the stabilizer.
I fixed the plane to get 0° of incidence on the wing, and then I measured +1.5° (positiv) on the stabilizer!!! This must be wrong.
I contacted the designer of the plane, he told me to set the stabilizer at -0.5°.
This 0.5° longitudinal dihedral sounds more appropriate but I read (forgot where), that this LD angle could be set at 0° for fast airplanes, what would the flying caracteristics be if I did so?

On the other hand, a friend of mine build exactely the same plane, he did not check anything, he need lots of up trim to fly level, does is stabilizer got some positiv incidence like mine?

Last week-end, I flew a new beginner airplane at our field, I checked for proper CG, throws and all usual stuff.
I was badly impressed with the instability on the pitch angle of this plane. The nose went up at 45° very fast!!! despite a proper CG.
On the next flight, I added some weight in the nose, it didn't change anything but the beginner flew himself like that, the plane went up "like a horse" a few times and then,... the wing snapped suddenly! Big crash,...
My feeling is that this was a LD issue, the angle must have been too much, don't you think? My feeling would have been to put a shim under the trailing edge,...

Does this do any sense to anyone of you.

Thanks in advance for any inputs.

Novy

[Sparky Paul]

The positive incidence on your race plane stabilizer IS wrong. Take it out... set to zero or the -.5° the designer says.
.
The second plane does (did?) appear to have the wing on wrong, or upthrust on the motor.

[Sparky Paul]

Interesting when an old thread gets revived.
I flew the SAE Lifter illustrated earlier, 2 weeks ago. It's a good flier, can carry some weight, needs a job.

[Ollie]

Stability and trim are two different things. Pitch stability is defined as the tendency to return to an air speed and pitch attitude after a disturbance. It depends on the CG being ahead of the neutral point. The distance that the CG is ahead of the neutral point is called the static margin. It is measured as a % of the average wing chord. The larger the static margin the stronger the tendency to return to the original trimmed flight condition after a disturbance.

The trimmed airspeed and pitch attitude of a stable plane are determined by the decalage angle between the wing chord and horizontal tail chord. As the decalage angle is decreased (tail less negative relative to the wing) the trimmed airspeed increases (nose down) and as the decalage angle is decreased the trimmed airspeed decreases (nose up).

Because the CG location affects both stability and trim it should be adjusted to the desired stability first. The decalage affects trim but not stability, therefore, the decalage should be adjusted to obtain the desired airspeed and pitch attitude.

The initial settings of CG location (within its allowable range) and decalage (within its allowable range) are only to have a safe place to begin flight testing. The fine adjustments of CG and decalage are made to suit the purpose of the model and the flying style of the pilot through flight testing.

[Novy]

Well, gentlmen,

Thanks for these incredible fast replies!!! I appreciate very much.

Sparky Paul, I have been diging here and exuming much older posts than this one only one year old!
With your experience (the plane you are showing us looks like "unusual") what difference will I get between a LD of 0,5° and 0° in flight with my race plane? Will I ever notice something? I am gluing the thing this week-end!
The beginner airplane is no more (I surely would have made some measurement on this one!).
Thanks for your input!

Ollie, I appreciate your explanations, but,... when you tell us "The decalage affects trim but not stability" it makes me like confused, you know, unlike you, I am a "practical and less theorical guy" I fit and try!
I flew a beginner plane last year, it was very toutchy on pitch attitude, adding weight in the nose did not cure the bird, but as soon as we added a 0,04" shim under the trailing edge, it became more gentle,... how are we going to explain this,...
Also, if the LD on my race plane is not set properly, I will be able to correct this with the trim but,... is this elevator pointing up or down not going to generate some unwanted drag? resulting in losing some precious seconds,...

Thanks again for your inputs

[Ollie]

If a plane is so out of trim that it stalls it may seem like it is unstable when it is trimmed to fly too slow or too fast to be controllable. This is not instability in the narrow sense of the definition of stability.
A stable plane can be trimmed but an unstabnle plane can not be trimmed. If enough weight is added to the nose the controls will become sluggish even if they have excessive deflection.

Control response depends on three things: airspeed, control deflection and CG location. Control response increases as the square of the airspeed. Control response decreases as the CG is moved forward and control response decreases as control deflection decreases. For control deflections up to about 3 to 5 degrees there is very little additional drag because the flow is not separated if the hinge line is sealed and the hinge line gaps are closed. With the control surface deflected more than 5 degrees or so the flow seperates at the hinge line and produces extra drag. For minimum drag the CG and decalage have to be set so that there is very little download on the tail. When there is down load on the tail, the induced drag of the tail increases as the square of the download. This happens when the CG is too far forward., The CG should be moved aft in small increments until the plane goes where it is pointed without returning to its original flight path. This is neutral stability. Then a very small weight is added to the nose to give the plane some stability without overlaoading the stab at high speed. After finding the CG for neutral stability, the CG shoulld be moved forward a distance equal to about 2% of the average wing chord. Then the CG should remain fixed and the decalage or elevator trim adjusted for a trimmed flight speed that is appropriate to the purpose of the plane. After the CG and decalage are set, the control throws may be adjusted so that they suit the flying style of the pilot. The throws should be no more than enough to control the plane in any normal flight mode from takeoff to high speed and back to landing. The only prerequisite to this adjustment dicipline is to start with the CG in a stable location and with the trim somewhere in the safe range of operation. The only reason to mention theory is to explain why this practical adjustment dicipline is required.