Wing incedence question
 

What effect to the increase or decrease of incidence cause?
Thanks for the help.

RE: Wing incedence question
 

An increase in incidence will require a little additional nose down trim and the airplane will fly a little more nose down when trimmed for level flight. In inverted flight it will require a more nose up attitude. Other than these changes in deck angle it won’t have any significant effect. Decrease incidence will have the opposite effect on deck angle.

RE: Wing incedence question
 

http://www.fsea.org/pdf/MA2%20Studen...plementpdf.PDF

Down load this and the answer is on page 3. This is a good beginners guide.

RE: Wing incedence question
 

The discussion of incidence in Ben’s referenced article, dealing with trimming of rubber powered free flight models, is only marginally relevant to radio controlled aircraft with adjustable trim and elevator control. First there is the definition of incidence. The article defines it as the “angle between the wing chord plane and the tail chord plane”. Normal convention defines it as the angle between the wing chord and the aircraft (fuselage) reference line. The horizontal stabilizer is normally set to zero incidence with reference to this same line but can have either plus or minus incidence also.

The article deals with balancing a model by moving the wing fore and aft on the fuselage, which changes the tail moment, and adding ballast to the nose or tail which changes the cg. It only implies that there must be some difference between the wing chord angle and the tail chord angle for the airplane to be both stable and balanced (which for the type of model it's dealing with is basically true). As significant as this is for trimming a free flight model, I don’t believe it answers the original question.

On a radio controlled aircraft with adjustable trim and elevator control, if incidence is increased and the cg and tail moment remain the same, The trim must be adjusted a little nose down, which in effect restores the original angular relationship between the wing chord and the tail chord. The airplane will then fly a little nose down in level flight and when inverted will fly a little more nose up. Since the thrust line remains the same the effect is like adding a little downthrust, but for relatively small angles that effect is negligible.

The overall effect of changing the incidence is a change in the deck angle period.

RE: Wing incedence question
 

Thank you much for the help information!
I am working on a FF conversion to rc and am trying to increase the stability.
Again thank you for the knowlagable info!

RE: Wing incedence question
 

Most free flights are very stable laterally, and marginally stable in pitch. To convert to R/C remove most of the dihedral to reduce lateral stability and balance the aircraft with the cg just a little more forward to increase pitch stability. If the converted model is to be three control without ailerons, maintain on the order of 4-6 degrees of dihedral. If ailerons are going to be used and depending on your experience level, the wing can be flat, or just enough dihedral so that the wings doesn’t appear to droop. Incidence alone won’t effect stability. For a given configuration, pitch stability is only affected by cg position, forward cg is more stable, rearward cg less stable. Dihedral only effects lateral stability. It doesn’t effect pitch. Free flight models also generally have a small fin/rudder area to avoid spiral instability. For R/C you might want to increase it a little bit.

The pictures are of two free flight models converted to R/C. Both are three control aircraft with no ailerons. The red one had too much dihedral at first that resulted in jerky turns. I cut the wing and reworked it to the configuration shown here, and it has become one of my favorites. I have flown it with several two strokes and a diesel and now am flying it with a little four stroke engine.

RE: Wing incedence question
 

Hi ya'll,
Wing incidence> this should be a permanent setting, not to be modified.
Reason> the airfoil should be set initially at its best L/D point. ( Best
ift for drag produced) Thus most efficent.
Then horizontal tail lift is adjusted to produce desired line of flight.
These fundimentals have been violated in many designs for what ever
reasoning.
In that case first move should be to reset wing at proper L/D angle.
Then tail angle will also probably demand attention
How's zat?

Hal deBolt

RE: Wing incedence question
 

I agree with Hal as far as he goes. He is describing an airplane designed for most efficient cruise flight and full scale aircraft usually have the incidence angle set to the maximum L/D for that reason.

However if the airplane being designed is a free flight model where maximum endurance is the goal, the best L/D incidence will result in the fuselage being aligned with the direction of flight when gliding at the flattest angle, but being a little nose high at the minimum sink rate speed. A little more incidence than best L/D will more nearly align the fuselage with the direction of flight in that condition, and give less overall drag and slightly more endurance.

Aircraft designed for aerobatic flight where inverted flight is a consideration require a different approach. Here the airfoil is symmetrical about the chord line and both wing and horizontal stabilizer are set at zero incidence. With this setup, the aircraft will fly basically the same upright and inverted. Of course, in level flight, the attitude will be nose up in both cases, but it will be the same.

Sport models that are used for general all around flying with some limited aerobatics often use a slightly cambered airfoil (sometimes erroneously referred to as “semi-symmetrical”). These are set at somewhat less than max L/D in order to minimize the nose high attitude in inverted flight.

In most R/C flying neither endurance, range, nor fuel efficiency are major design considerations. Rather incidence is usually set to provide attitudes appropriate to the maneuvers desired.

RE: Wing incedence question
 

Lou, no offense as I'm sure your conversions fly quite well but I'm going to offer the opposite line of resoning and say that the article IS pertinent to RC with only the obvious differences (no sliding wings and such items) needing to be thought about how they apply to the RC world.

You say that the rearward CG is not desireable but the first thing RC gliders do these days for serious competition models is dive test them and move the CG back until they are barely pitch stable at all. Much as the free flight models do as per my own experience and as described in the article. I've also trimmed more than one old timer to fly on the edge of the pitch envelope in this manner with success. Just reduce the elevator throw to compensate. I personally found no real stability advantage to keeping the balance forward. It just limits the model's ability to float in the glide.

The article didn't seem to have mentioned it but the angle between the wing chord line and the tail chord line is known as the decalage angle. We often forget about this angle because we use the angles to the arbitrary fuselage reference line. But the air and the model do not care about any fuselage line. Rather it is always the angles between the thrust, wing and tail that count. And when the engine quites it's just the decalage angle between the wing and tail that counts. The fuselage line is just a handy reference line to use to compare them all.

I also have found that lowered dihedral it isn't truly required as you can compensate for the strong yaw to roll couple by reducing the area and throw of the rudder until the model flies as you wish. And in some cases where the original vertical tail volume coefficient was marginally too large reducing the dihedral COULD result in a model that is spirally unstable. The Comet Clipper is one such model. This is one design that is known to be spirally on the edge and does not do well with high climb angles as a result. Hence the release of the Clipper II with polyhedral, but no other changes, to adapt to the stronger engines. I've personally seen a few examples of Mk1 Clippers auger in thanks to what strongly appeared to be this spiral instability. In one case the owner took another's advice and added a deep engine cowl and the forward area fixed the "problem". Further reducing the dihedral in the Clipper's case would be a serious mistake. But on the whole I will agree that many old time designs can safely have their dihedral angles reduced at the expense of loosing some of their authenticity. If the "look" is important then I maintain that limiting the rudder area and throw is a viable alternative. Another is to prevent the models of this type from flying too fast. The extra speed will certainly boost the rudder's effect to a nervous level. But old time models don't really look good flying like that anyway.

Unless you're using the model's shape as a pattern for a more modern approach to sport flying then all your comments make perfect sense. Just watch out for the spiral stability issues. A reduced area vertical fin is called for in those instances.

It all just goes to show that there's more than one way to make our models fly well.

DriverM, You say you're trying to INCREASE the stability of the original free flight design. I'll offer the suggestion that the model IS stable enough as is or it would not have been able to fly successfully as a FREE flight model in the first place. Depending on the design you've chosen increasing the incidence and moving the CG forward could actually make the pitch stability MORE nervous and result in requireing a lot more downthrust to avoid a strong nose up and down pitch reaction to power shifts. Moving the CG forward on very high wing designs like a Playboy with it's pylon is a prime example of this. We went through this with a guy in the Vintage forum a few months back that did it that way and when the model was trimmed to glide he could not hold the nose down under full power. Moving the CG back close to it's original FF position made the model much more tolerable to fly. These designs had very high tail volume cefficients in the first place so the "odd" balance point locations are actually not as bad as you would think. Just start with a smaller elevator throw than normal and I think you'll be pleasently surprised.

RE: Wing incedence question
 

Bmathews, no offense taken. Based on your many posts, I have great respect for your aeronautical knowledge. I don’t think we are very far apart, but are looking at the question with different goals in mind.

A free flight model, whether powered or glider has unique requirements that often result in unique solutions. The flight profile involves a drastic launch (power or tow) followed by a minimum sink rate glide. The transition from extreme power to gentle glide without benefit of any control from the pilot has resulted in all sort of aerodynamic gyrations in an attempt to achieve satisfactory flight. High dihedral angles, polyhedral, limited pitch stability, pylon mounted wings, “lifting” tails, ad infinitum, are all means of achieving some control during the high power climb and the transition to glide. An aircraft with significant pitch stability tends to be “loopy” with sharp pitch changes with power. This and the fact that with the cg near the neutral point, there is a slight improvement in sink rate, dictate that sort of balance for endurance type models. However it was not uncommon for these aircraft to make contest winning flights in still air but to become erratic and unpredictable if there were significant turbulence.

When I first started in radio control, we simply added a rudder escapement to an existing free flight model and didn’t touch it until the top of the climb and the engine had quit. It was then used it to keep the model in the area until it glided to the ground, eliminating the necessity of following it cross country (at least when the radio worked). If the goal is to reproduce this kind of old time flight profile, your comments are right on the mark.

The fact is that if we had had a way to control the elevator and rudder most of the aerodynamic gimmicks would not have been required. As we begun to fly R/C (rudder only, escapement) the airplanes begun to change dramatically. The pylon disappeared, dihedral was flattened, cg was moved forward, and most of all, the flight profile was changed to a gentle climb with a long engine run, and a glide to a landing somewhere in the vicinity of the launch area. The much better flight characteristics resulting from these changes are just as valid today as they were then.

If one wants to authentically reproduce an old timer free flight airplane and add radio control, the pre- WW-2 models designed before limited engine runs were introduced are the best choice. My ‘Miss America” is an exact replica of the 1936 nationals winner, except it is scaled down to fit an OS 10 engine. It has several hundred flights and even won a SAM radio assisted free flight contest (because I was the only contestant to enter that category).

I built the Berkley Brigadier as per the plans except I left off the polyhedral and moved the cg slightly forward. After several dozen flights, I cut the wing and reduced the dihedral to that shown in the photo, as turn entries were pretty jerky. I take off, throttle back and watch it slowly climb and circle above with the sunlight shining through the doped silk and think about all the models I’ve chased over hill and dale in years past. Sometimes I make a few swoops and dives that I could only imagine doing in the old days. If the goal of building an antique free flight and adapting to R/C is to capture some of the nostalgia of bygone days, without all the headaches we went through, I think my approach works quite well.

Back to the original question concerning incidence, granted the wing incidence and resulting decalage set the trim point (for a given cg location). However incidence has no direct effect on stability. It gives an indication of stability because a forward cg (stable) requires more decalage to balance at the trim speed than a more rearward cg (less stable). The reason I said it is not relevant to R/C models is that as soon as the pilot changes the trim, he in effect changes the decalage. Regardless at what angle the horizontal stabilizer is physically mounted, the actual decalage is changed by the trim. If the incidence of the wing is changed, and everything else stays the same, the pilot must adjust the trim to balance the aircraft and the effective decalage is right back where it was originally. The only difference will be the pitch angle of the fuselage.

(Now regarding driverm391's comment regarding stability, sometimes what a person means by stability is not necessarily the same as what an engineer means. To many, an aircraft is considered stable if it is easily controlled and doesn't require a lot of control input. It may or may not be stable in an engineering sense.)

RE: Wing incedence question
 

Quite right, I see our two options as trying to butter opposite sides of the same slice of toast... :D

I brought up the examples of the Clipper and Playboy to show what CAN happen if you don't approach these mods in an educated manner. As you say there were many designs that were quite extreme in their layouts where one element supported another element in a delicate balance that was all aimed at one flight goal. To change elements of that balance without a full appreciation of the factors can often make for a very dissappointing outcome. Of course avoiding the more extreme designs in favour of the semi scale types like the Brigadier can greatly help avoid such calamities.

What you describe about your Brigadier makes perfect sense for the thankfully common old timer cabin semi scale type models if you're not bound by the need to stick with the SAM rules about modifying the model design. There were many that follow that pattern and will respond wonderfully to what you described. I just wanted to bring up that it IS possible to live with the original design and original trim if the SAM restrictions require it and still end up with a fine flying model.

I've got plans here for the Modelcraft Pacific Ace that I plan on electrifying as a sport model one day. I'll be cutting the dihedral in half for that one just as you suggested for appearance reasons mostly but this one is for ME rather than any contest work so I don't mind "improving" it.

And in the end anything that brings another Old Time design into the world is a great way to beautify the skies....