Hey Guys...I have two identical airframes that fly completely different....One is extremely pitch sensitive (elevator)....same control surface throws, same radio setting values, same cg, ....can someone tell me what to look for?....Which airframe design parameters influence pitch sensitivity?...
Thanks
Ed

Ed,

More to follow from REAL technical experts, but if everything you mentioned is the same, the only thing I can think to look at is the incidence angle between the wing and the horizontal stabilizer. More pitch stability with more negative angle of the stab relative to the wing. Less with 0-0 or positive angle stab to wing. There are a number of sub-variables involved too, but this is the first thing I would look for.

Good luck,
Bedford

Thank/You Bedford!....That's the kind of info. I'm looking for....

Are the hinge gaps sealed? If not that could lead to a difference in controls.

Hi tande
The one that is more pitch sensitive can probably be tamed down a little with a little nose weight. Or a slight movement of the battery or equivalent.

If the planes are truly identical, I'd swap out the RX and elevator servo, plus check the linkage with firm hand pressure as you cycle the elevator from full up to full down.
The elevator joiner on some designs can be weak / flimsy.
I had an ARF Q500 Lanier Predator that was built with such a positive wing incidence that the model was unstable in pitch. I had to jack the trailing edge of the wing up 1/4" to get the plane to properly track.
That "ARF" was more work and frustration than it was worth, but at least it was a good basic design to scratch build copies from.

Yes, i'd guess that you have some difference in control linkages. The other possibility is that the CG is a little different but your chosen CG location method (end of fingers?) isnt sensitive enough to detect it. Incidence angle would not cause this, if all else was identical then diffrent incidence would just mean you had to trim the elevator, it would not effect stability.

Another option is that something is loose or flexing, maybe (esp. ifit's a foam plane) the wing or tail surfaces themselves are twisting, this can cause all sorts of erratic behavior.

Steve

Jet Plane above is right.
............................
Pitch sensitivity is down to an aft CG or a structural problem.
Lack of pitch sensitivity can be caused by large hinge gaps, bad control runs (blowback).

WOW alasdair, your comment about Bedford is kinda blunt. I happen to know the man and he isn't over in Moscow because he's stupid. He's actually fairly sharp when it comes to R/C. I think the OP mentioned the CG has been checked and as combatpigg mentioned wing vs stab incidence angles can and do play a big part in pitch control.

We should be able to help the OP with his problem without resorting to degrading each others input.

Jezmo, if you look at the aerodynamic formula for quantifying pitch stability it's clear that AOAs have no part whatsoever. It's a formula that's been the basis of design for many, many years and proven to be true. There are ways that screwing up the AOAs will screw up your first flight, but if the elevator is trimmed on that flight, you'll land an airplane that is less efficient but still has whatever pitch stability it was designed and built into it.

Pitch stability is a function of 6 things. CG location, wing chord, wing area, tail moment, horizontal tail area, and chord. That's it. No AOIs or AOAs.

So the post may have been abrupt, but it did have value. It's quite possible that both posters are pointing to the same cause.

On the other hand, posts that insult or embarrass should be reported. And that one was harsh enough to warrant editing.

There are so many things that can be contributing to the two airplanes behaving differently. Radio setup could be one. Some TX have the capability to have reverse expo but at a glance have the same numbers so is an easy mistake. Different length servo arms, different thrust angles, lots of things at different degrees. I would agree that CG could be playing a part here and disagree that incedence has no part of it. If the stab and wing are both at zero and the CG is slightly aft it can cause a pitch instability that is more severe then if just one or the other is present. The reason is that the wing is " Hunting ". The cure would be to move the CG forward and/or add a tad of positive incedence to the wing.

Jezmo,

Thanks for your kind comments. Relative incidence between wing and stab does have an influence on pitch stability according to Denker.

http://www.av8n.com/how/

An excellent read even if it is directed toward full scale.

Cheers,
Bedford

Interesting article. It's clear where Denker explains how decalage can affect the effectiveness of the elevator, but I missed where he shows it actually affects stability.

Would you help with that and point out what part of the article explains incidence or decalage as part of pitch stability?

tia

I'm expecting a hand slap for this but.......................I have said over and over that these full scale theory papers have very little bearing on our models. We have a fraction of the wing loading, twice the power, a fraction of thier drag and so on. It is unrealalistic to expect all this lititure on full scale to work on our models. For the life of me I can't figure out how a person can take the opinion of somone who wrote a book on full scale theory but has never flown a model over somone who has 33 years of r/c experience. Just seems like apples and oranges to me.

Relative incidence between wing and tail has a BIG effect on the trim of a plane but trim and stability are seperate (though highly interctive) issues. It's very common for people to get trim and stability mixed up and to some extent Denker is guilty of that in parts of his write-up. However, on an RC model incidence is to a large extent irrelevant because you simply adjust incidence with elevator trim until the model is trimmed to fly at the CG and flying speed you have set. The incidence would have to be miles out before it became un-trimmable with elevator. If you struggle with this think about the case of the all moving tailplane where tailincidence angleand elevator deflection are one in the same thing.
<u>It's the CG that determines stability, not the stabiliser/elevator trim</u>. If you think otherwise, take a plane that has the CG much too far back and see if you can make it stable by adjusting the incidence angles

Steve

Models obey the same laws of aerodynamics as full size planes, they fly in the same air!. models being smaller and slower flying do have some slight differences due to Re effects but the basic rules are the same. Your reference to flying for 33 years is interesting.
I'll give you an analogy; my mom has been driving cars for about45 years but she doesn't have a clue how they work or what goes onunder the bonnet. if i was seeking advice on how car engines work or how to diagnose an engine problemi would value the words of a qualifiedauto engineer (who didn't drive) overmy moms45 years ofdriving experience any day.

Of course in the ideal world you would find someone who had both practical experience and a good grasp of the theory, these things are not mutually exclusive!

Steve

WOW! ...I did not expect this much info. but I'm learning a LOT! ....Thanks so much to all who have replied....
Ed

Thanks for looking at my post Rock. I am not nor do I want to be an aerodynamicist (though I have been a licensed Airframe and Powerplant Technician for over 29yrs). I have been flying a pylon racer made by Great Planes called the Viper 500. Mine had a tendency, for whatever reason, to be flying along perfectly straight and level, full throttle running about 120 mph give or take a few, and without warning would just make an abrupt twitch toward the ground. Quite unnerving since pylon racers are generally within 15ft or so of the ground. Any twitch toward the ground will definitely get your attention very quick and make you follow through on finding a cure. As it happens, many Viper pilots have had this phenomenon occur to them. There are threads on this forum about the problem and through those posts, for which I am greatful, I learned the cure is to change the incidence angle of the tail stabilizer assy. After the change in angle the plane no longer has this undesired twitch for the ground. Again, I am no aerodynamicist, and have no clue why the change in stab angle fixes the problem on this particular aircraft but it does. Moving the CG forward helps but then the plane becomes significantly more difficult to land as too much pitch authority is lost to hold the nose up as the speed drops making the landing speeds unacceptably high. Not sure it is the same type of problem but it sure seems related to me so maybe there is some correlation between incidence and pitch stability.

Here is a possible answer
IF the CG is constant then the TOTAL tailplane down force must be the same .
however - the tailplane drag may change withoutchanging the downforce
SO- change incidence of horizontal tailplane and then retrim elevator to achieve original trim.
Tho some of the old school guys will likely doubt it - you can steer a model by simply changing drag
also - if you look at latest aerobatic setups for indoor flying - you may see angled plates on the elevators tipped inward to add drag -AT THE REAR
this moves the fore -aft center of drag rearward - (obviously) Sorry no charts no math - but fly the plane and you will see that moving drag rearward DOES change handling.
I suspect this is what the incidence change on the racer is doing
Ihave a house ful of various setups which don't fit the "std' arrangments - mostly foam stuff but the proof is in the pudding and these little buggers allow for quick mods and the resultant effects can be easily checked - some work- some donts butth results are easy to duplicate or replicate.

rmh,

No doubt drag under certain circumstancescan change the way a plane flies, spoilers, air brakes, drag rudders, etc are all example, as are the drag producing 'fences/spoilers' you see on indoor models. However the drag produced by slight elevator defection is a very small force and as the elevator is usually near the centre line of the plane that tiny drag has no significant effect on the pitch of the plane. Consider the fact that for a typical airfoil lift force is about 50 times greater than drag then you start to appreciate that drag plays a much smaller part than lift in the forcesgenerated by an unstalled airfoil.

I know you have in the past expressed the theory that the tail of a plane controls mainly by drag but you are alone on this one and would need to come up with some pretty solid evidence to convince most people.

in any case i dont think this is relevant to the OP's problem because he didn't say that one model had more elevator deflection than the other.

Steve

</p>

Perhaps you have have a better answer -
The elevator is often not near the center of the center line (Ttail is an extreme example)
My noting that drag can be used as a control device is easy to prove using split control surfaces.
I have done it, as I am certain, have others.
On speed designs -it is easy to reduce the tailplane size -thinking that only small pitch changes are required
If you plot drag from front to rear on the design - the drag becomes smallest at the tail.
small changes here are important
On a racing car (straight line speed) - IF you run slightly lower air pressure in one rear tire - the car will pull to that side
In this case- difference in rolling resistance and or traction.
I am not trying to convince others - I really have no interest in that .
I simply relate what I have tried and observed.
As for the elevator deflection being the same with a difference in incidence and no change in CG?
That simply won't happen.

Hi,
I have attached a file that shows what parameters that changes by incidence 0.9deg and 0,0deg.
What really happens is that Fuselage deck angle changes and nothing else. This is the line between center of motor and center of stab (pull line) The incidence will dictate how low or how high the tail will be flying.

daRock,
You have a future in the diplomatic corps.
I forget what I wrote, but apologise for any hurt feelings.
I wanted to be clear, but you did that too and nicely

As for speedracerntrixie's quote
I have said over and over that these full scale theory papers have very little bearing on our models. We have a fraction of the wing loading, twice the power, a fraction of thier drag and so on. It is unrealalistic to expect all this lititure on full scale to work on our models.

Physics is physics, Until you come down to molecules and sub-atomic particles. We just have to watch the old Renolds Number, but all the full size rules theories and equations are good for most outdoormodels.

Same deal with my Lanier V-Tail Q500 ARF. The wing angle was easier to adjust than the tail, so that's how I got it to stop hunting around in pitch mode.
I think the stab gets preloaded with the wing angle being off by so much and reduces it's effective ability to correct for normal disturbances.
In effect, the plane flies like it doesn't have enough stab area, when the incidence angles are so out of wack.
Experiencing the problem first hand and then finding the cure is the only proof I need.

I just got back from a nice 4 hour flying session- outdoors
we flew a variety of models inc a new small design which is quite different
It is extremely responsive but has a relative wing/fuselage layout which produces about the same lift in any roll attitude
You have to remeber that any turn involves rudder unless you bank to 90 degrees (the plane remains flying level)-then elevator turns it .
A new RTF model is coming on the market which has VTO and landing capabilities
The control system employes a 3 axis gyro assist - the whole model is about 170 bucks
Point being - the flight parameters of new models far exceeds the parameters of the "Piper Cub' types of only afew years back.
The typical setups for stability simply don't apply as they did.