Airplane stability (engineering notes)

[David Lewis]

davidchoate wrote: ...Horizontal stabilizer size in relation to wing area. I recently converted a Dumas Aeronca 30" WS. To RC. I built it exactly to scale. Except for adding right and down thrust. I had an expert pilot at my RC Club maiden. It flew, but very. Unstablely. He said. He was not going to be able to Land it properly. Also, what is a good decalage to set the tail and wing. I have been told 2 deg positive is good.

David Lewis wrote: First I would move the CG forward (add nose weight). Reducing elevator throw also may help.

I don't know of a simple "stab area-to-wing area" ratio at very low Reynolds numbers but here are my starting points:
Wingspan = 36" to 24", enlarge stab area 15% over scale on a high wing trainer airplane
Wingspan much below 24", enlarge stab about 20%

Smaller R/C airplanes need more stability because, according to a law of dynamic similitude, time speeds up as the square root of the scale factor, yet the pilot's reflexes stay the same.

Low wing airplanes need more stab area because they don't have pendulum stability working for them. If stability is marginal then it helps to confine flying to calm weather.

Recall that decalage is the difference between wing incidence and stab incidence. If the wing loading is low, I like 2 degrees of wing incidence because the fuselage stays closer to level in cruising flight. Otherwise the tail hangs down, which makes the airplane look like it's struggling. The stabilizer incidence on R/C models I arbitrarily set at zero degrees but other designers may have better advice.

[dirk gently]

The "pendulum effect" seems to be very poorly understood, refer to the following article:
http://www.djaerotech.com/dj_askjd/dj_q ... nbeny.html
It says:
"Contrary to popular belief, there is no such thing as "pendulum stability" for a high-wing arrangement. When the plane is banked, the wing's lift vector banks with it, so the vector's relationship with the C/G does not change, and therefore creates no restoring effect."

What actually happens, high wing designs (but not parasol wings) have an added "dihedral effect" or more scientifically "roll-yaw coupling". http://www.djaerotech.com/dj_askjd/dj_q ... lflat.html explains this effect.

Also, please note that "dihydral effect", "roll-yaw-coupling", "pendulum effect" or whatever you call it, affects roll stability, not longtitudal stability, and has little to do with stab area. So I would say the statement "Low wing airplanes need more stab area because they don't have pendulum stability working for them" is incorrect (regardless of the "pendulum effect" vs. "roll-yaw coupling" debate).

I personally don't enlarge the stab if I can get the CG where it was on the full scale aircraft, even on very small models.
Here is a recent example, 22 inch wingspan, scale size stab, flies just fine:
http://pfmrc.eu/index.php?/topic/50857- ... ntry568266

I believe the practice of enlarging the stab comes from either
- inability to get CG where it's supposed to be without a rubber motor becoming to short (this is tricky though, as larger tail surfaces are also heavier and move the CG even further back), or
- wanting to get the total lifting surface as large as possible.

Also, I sort of doubt (I may be wrong, however) if the Reynold's number affects longtitudal stability in a significant way. I checked with my modelling books, and the formulas to calculate the CG vs. stab-to-wing-surface-rate vs. tail arm do not take the scale into consideration.

[Steve Blanchard]

The need to enlarge the stab obviously is going to be based on the relationship of the stab to the wing size in the full scale ship. The example you gave just now looks like it has a pretty generous stab for a low wing aircraft especially if it is indeed scale in size. It looks larger than that to me.

Steve

[dirk gently]

It is scale, and the CG is not too far towards the nose, about 30% of the mean chord.

[Steve Blanchard]

With a scale stab that large I would believe you don't need to increase it for stability and duration.

Steve

[davidchoate]

Thanhttp[url][/url]ks for the infhttpo.httphttp://balsamodels.com/phpBB/postin ... =3406#tabs

[David Lewis]

Sometimes you want the CG farther back than on the full scale because, as you move the CG back, you get a higher lift-to-drag ratio and higher maximum lift coefficient (due to less stabilizer download). Here are some ways to increase longitudinal stability:
Increase stab area
Increase tail moment arm
Move CG forward (i.e. increase static margin)
Reduce nose chord (distance from prop to CG)
Reduce airfoil camber
Reduce wing chord
Reduce moment of inertia
Increase vertical distance from CG to aerodynamic center

[woundedbear]

As most of you know I have a tendency to ramble in my comments and questions on this form. I am making an effort to stop doing this, as nobody wants a history lesson every time I write so, here's the short version. What is a Reynolds number ? There are so many terms that I don't understand, mostly because of lack of interest in math as a kid, and "I was always getting caught drawing race cars and airplanes" I guess there was a bit of laziness on my part as well.
Then there is all of the short hand, like LOL which I now know means "laugh out loud". One needs to take a class just to understand all of the Yahoo-legns on line. Heres one of my own "BITD" I call that one "back in the day". I do have something to add to the subject at hand. After WWII ended the DeHavilland company had all of their orders for the new Hornet F.1 canceled. However they built a navylized version with larger horizontal stabilizers to help with lower landing speeds. The vertical stabilizers had an added fillet, to give the aircraft better directional control when bringing it aboard a carrier's pitching and rolling flight deck... Ah' shoot there I go again with the unnecessary historical information. being dyslexic is such a pain !
OH yeah, I think the DeHavilland Beaver one of the new Guillow's 300 series kits. Comes with a scale and a larger free flight horizontal and vertical stabilizers.
Ya' know guys, when the teachers tried to tell me that designing airplanes would require an extensive knowledge of mathematics. I would always reply. I know what an airplane is suppose to look like,"if it looks right it will fly right". I managed to win the argument by taking one of my designs and built it. Guess what "it flew perfectly". I was just lucky and now I know that if you want to even talk about airplanes you have to know that math. So now how do I learn what I should have learned as a youngster ? I am going into my 58th summer here on this earth, and I have to learn something that a dyslexic person should not be able to learn. When I was tested,"over a two day period". The people that my Psychiatrist payed,"out of her own pocket", to do the testing. Were perplexed by my high level of reading comprehension. They could not explain how someone who's math, grammar, and spelling were so bad, could possibly be able read on the second year college level.
Everyone has had that one teacher who made all the difference in their life. Mine was Mrs. Steed, I saw her one day at a gas station, and ask her if she remembered me. Her response was of course,"she called me by name", I had not seen her since the fifth grade ! I was about 44 years old at the time,"truly a long shot her being able remembered after so many years". We talked for about 35 minutes, and I thanked her for instilling a love of reading in me. About 3 months after that chance meeting, I read her obituary in the news paper. Lots of dyslexic people have been put into mental institutions by their families, by doctors, by the state. I had an evil teacher who tried to get my mother to put me away.
I have from an early age had this love of flying and airplanes, the old tail dragging warbirds have always had a special place in my life and even in my heart. Well when I began typing up this I thought that I could stay away from the,"know it all" historic, and personal stuff, but once again I've failed. Sorry for all of this, so model on dudes !!!

[WIDDOG]

In the book Flying Scale Models. They talk about pendulum controlled model.

[davidchoate]

I dont mind the History in there. I enjoy building "scale" flying models for the History. Not unlike You, I have been fascinated by airplanes and History. Unfortunately the way the cards fell for Me, I wanted to be an aircraft Mechanic, but ended up an Auto Mechanic. Its not a bad job. Doing it 30+ yrs and still am interested. But must satisfy My Need to understand Flying by way of Hobby. Oh, and My Father was a dyslexic accountant His entire life. My failures are plentiful, and My ignorance widespread, but every Year I get a bit closer to not just learning how to make a controllable Plane, but understanding how it works. Guys like David Lewis, You, and all the peole who share this interest are invaluable. In society We seem to be few and far apart. Not unlike Star Trek fans. And I'm doubly alone as I'm one of them too. Seriously though, the info herer is invaluable and somehow better than in a lot of other Forums. Also joining an Airplane Club was one of the best things I did. Definite Good advice I took from Mitch Here.

[davidchoate]

Here is My current RC build. Alien Aircraft Taylorcraft 42" Taylorcrafts are one of My favorites. I know what the scale one looks like. This smaller version has obvious modifications, but still is definitely Taylor Craft by its characteristics. The tail moment, tail area, and aileron size seem larger. It was put into perspective when some one said that the plane gets smaller, but the air molecules do nt. That helped Me. I had to resize this so small.

[davidchoate]

How does washout help? I heard its to redoce stalls

[sandman]

Washout makes it so the tip of the wing will still provide some lift in a stall since its at shallower angle. The tip will stall last and make your plane more stable in the turn.

[David Lewis]

Tip washout may not reduce stalls but rather is intended to make the stall more docile. At high angles of attack, we'd like boundary layer separation to start at the wing root TE, and then gradually progress out to the tips. Why? Because if the tips stall first, almost certainly one tip will stall before the other and then you've got a bunch of asymmetric lift.

As Dirk Gently correctly pointed out, washout is important in full scale engineering because if every part of the wing stalls at the same time the pilot gets the rug pulled out all at once with little warning. The disadvantage is that washout reduces the maximum lift coefficient and L/D. Sometimes instead of washout the wing uses an airfoil at the root that transitions to an airfoil with good stall characteristics at the tip.

I've noticed washout is more important at high wing loadings. Secondly, airfoils operating at low Re (<60k) usually exhibit a stall that is gentle and gradual anyway (boundary layer reattaches as angle of attack increases). In most cases the wing still develops usable lift after it stalls and little altitude is lost. Some designers claim that, when it comes to small rubber power models, the performance penalty imposed by tip washout is not worth the tamer stall characteristics.

[dirk gently]

Not only will you get asymmetric lift, but you will also loose lift out where the ailerons are, and that will make you loose roll authority.
Full scale airplanes are often designed with aerodynamic devices on wing roots to make the stall point on the root very pronounced, so that the pilot is warned (with vibrations that result from wing root stalling) about approaching the stall point of the whole wing.