britbrat

Full scale or model?

No to full scale (but at the aft limit) & yes to the model.

How did it fly -- impressively pitch sensitive & did stunning snap rolls -- great fun.

paradigm

Oh come on britbrat. Do you mean fun or scary. (I guess they can be the same thing)

My point is this, I think you would be surprised how large the static margin of a typical r/c airplane is. Even putting the c.g. behind the manufacturer's specification doesn't mean it is unstable yet. If you truly flew an aircraft at a unstable c.g. location, then it would naturally diverge from straight flight. Just based on the small moment of inertia of an r/c aircraft I'm guessing the divergence time would be very small (less than half a second). The average human response time is 0.2 seconds. This means, that just to keep even a marginally unstable airplane in the air you would have to input elevator commands between 2 and 5 times a second. If you can do it, great, but that doesn't sound like fun to me.

I believe that there is a common misconception between marginal stability and an airplane actually being unstable by definition. My original point with this discussion was that if you add too much anhedral, then the airplane will continually roll away from straight and level and if you can control it, you will have to put forth a lot more effort. (The snap rolls might be worth it though.)

Mesae, I appreciate your comment and I agree. We are usually saying the same things, just in different ways. My goal with my posts was not to browbeat anyone. Just to add a little knowledge from my area of expertise.

mesae

Well taken.[8D]

Ed_Moorman

My Feedback: (1)

I've had close to a dozen high wing planes with anhedral. I put it in on Sticks to counter the roll due to yaw. Works great. About 3 degrees on each side is what usually works for me. If I am lazy, I just flip the dihedral brace over, but many times this isn't enough to get the plane completely neutral in roll coupling. I have yet to find a plane I put anhedral in to be unstable.

If you want to try a plane with too much anhedral, fly a trainer, or a Stick with dihedral, upside down. Now you have a low wing plane with anhedral. You can control it pretty well in straight inverted, but there will be a tendency to roll out of outside loops and you'll have to watch ailerons closely or you'll barrel roll out. This is unstable in roll, but it isn't all that hard to fly.

In that same vein, I have flown planes with the CG back as far as 50% of the MAC. I had an 80" sport giant that I changed from a Quadra 42 to a Super Tigre 3000 to get the Q-42 serviced. The CG went way back. The plane had a large stab so I flew it anyway. Yes, it was unstable. I could bump up or down and release and it would finish the loop on its own. I had to really keep an eye on it or it would go off where ever it wanted to. On throttle reduction for landing, the tail would drop and I had to "prop it up" with down elevator for final approach. I've had 3D planes with the CG back that would oscillate in level flight, but this plane was really out there. It was way past oscillation to where it would diverge. After 6-8 flights I fixed the CG because it would tire me out just keeping up with it.

I guess the point that was made above is that unstable in roll or pitch isn't really that violent in RC planes unless you ham fist the controls. A good RC flier can handle unstable even though it is uncomfortable and tiring, but a novice or someone who isn't as capable would have trouble with pitch. Unstable in roll would be much easier.

mesae

All good. But whether a human (even a highly skilled one) can fly it depends on HOW unstable. Even in roll, I can envision an extreme anhedral angle that would be so unstable that it would be difficult or imposible to fly upright, or vice versa with dihedral. In fact it occurs to me that I have done this using Aerofly. I put a mad dihedral angle on one of the models (60 or 70 degrees, if memory serves) and flew it around. It was profoundly stable in roll while upright. So stable, that I found it extremely difficult to fly inverted. If I stopped making corrections even for a short time, it would immediately roll 180 degrees to upright (sometimes despite my best efforts to prevent it), and pendulum for a few cycles. It also required a lot of power just to stay up, and was hard to land because of that, since so much of the vertical component of lift was lost to the extreme dihedral angle, and I did not lengthen the wings to compensate. It made me realize how undesirable excessive stability can be, because ANY turn was difficult. And it looked REALLY funny.

britbrat

Why do you think that I don't know any of this stuff? I have two engineering degrees, several patents, nearly 200 publications & several thousand hr in full scale aircraft. I have been designing, building & flying models since the late 1940's. I flew air defence missions in first gen jet interceptors over the very lethal far north, so that you could safely grow up in a nice warm home and have the right & means to earn your degree. While I will fiercely defend your right to express your condescending opinions, I'm not impressed with them.

I have very definately flown models in highly unstable configurations -- both with intent & by accident, & I can assure you that with the C-of-G at the true aft limit, anhedral is definately destabilizing in pitch. Try it yourself.

You stated that my comment about anhedral destabilizing pitch was "completely false". The reality is that it is entirely correct & that you were wrong -- period. You are simple quibbling about magnitude & applicability.

I will accept your appology & we can start over -- or we can seriously cross swords -- & don't bet on the outcome. I'm an old man now, but I can still corall enough neurons to bite your butt pretty damned hard.

paradigm

Britbrat, I think I would rather keep my butt unbitten thank you. I will reluctantly retract my "completely false" statement. The statement you made is true, but it is comparable to saying something like, "If my shirt caught on fire, I would be sweating a lot." Well, that's true, but I think I would have other problems to think about first.

My problem with your post, and maybe I didn't get this across, was that while it is technically correct, it is misleading to other people that don't know as much as you or I. Anhedral can be unstable on a low wing aircraft at high angles of attack if the anhedral is large, but if you did that then the lateral stability is going to be so bad that the longitudinal problems are almost irrelevent. I wasn't trying to attack you with my post, I was simply trying to eliminate confusion among other members. So if my comments came across as being a little aggressive, I'm sorry.

P.S. Which interceptors did you fly?

britbrat

CF-100 Mk 3B & CF-100 MK 4B

paradigm

Ahh, the canuck. Very cool airplane.

britbrat

Cool alright. There was never a happy balance in the heating system, & at 40K+ either the pilot or the RO would freeze, or alternatively, roast -- at least in the examples that I was exposed to.

I liked the aircraft, but it was not particularly popular with many pilots -- they usually wanted Sabres. I liked getting home.



Now that I can be more objective (cooled-off), I appologize for being pedantic & aggressive. As my abilities wane with age, I have become quite defensive, not to mention forgetfull. However, I still like to fly models that scare the living snot out of me.