Some good technical info

Dean Pappas' "If it flies..." column in the April 2009 AMA Model Aviation gives a good basic description of what the aerodynamic center (AC) of a wing is, and how to find it.

He describes starting to design around it with CG at the AC, or even a bit aft of AC. That may work for large RC Pattern (Dean's primary interest) but sounds a bit aft to me. RC servos are essentially irreversible - at least they are supposed to be. No tactile feedback, IOW.

In CL, we definitely have tactile feedback. It's one of the best reasons to fly CL!

Another 'adjustment' to keep in mind while reading Dean's excellent info is that our airspeed isn't constant from tip to tip. It increases from inboard tip to outboard tip, because the span adds to the line length radius. If the entire model completes a lap in the same time (Oh, I HOPE so!!) the outboard tip has gone further than the inboard tip in that time. Distance divided by time is speed, right?

An example: 5' wingspan, lines 60' eye to eye, flier's 'arm' factor two feet. The inboard tip flies a 62' radius. The outboard flies a 67' radius.

That is - the outboard tip flies (67/62 =) 108% as fast as the inboard tip. Airspeed increases steadily across the span betwen those two values.

Lift varies with the square of the airspeed, so the outboard tip generates (1.08^2 =) 117% as much lift as the inboard tip. The lift inceases steadily between those values from inboard tip to outboard tip, but by the airspeed squared.

With these points in mind, Dean Pappas is usually worth reading, anyway. This is just a bit more like something us yo-yo's can use.

Being I was an R/C pattern flyer for 25 years, I still enjoy reading Deans stuff. Always very interesting. I think he understands the trimming of R/C pattern planes better than anyone. At least I sure learned a lot from him.

>(snip)
>An example: 5' wingspan, lines 60' eye to eye, flier's 'arm'
>factor two feet. The inboard tip flies a 62' radius. The
>outboard flies a 67' radius.
>
>That is - the outboard tip flies (67/62 =) 108% as fast as the
>inboard tip. Airspeed increases steadily across the span
>betwen those two values.
>
>Lift varies with the square of the airspeed, so the outboard
>tip generates (1.08^2 =) 117% as much lift as the inboard tip.
>The lift inceases steadily between those values from inboard
>tip to outboard tip, but by the airspeed squared.
>(snip)

There aren't too many 5 foot wingspan planes flying on 60 foot lines, but your assessment here is correct. Now some want to "solve" this problem by increasing the span on the inboard side of the fuselage, but in reality the wing doesn't care where the fuselage is. It still has more lift outboard than inboard and all we are doing is moving the fuselage outboard to use as tip weight (which is why an equal span wing needs more tip weight). The lift differential still remains, however. And now we have the thrustline moved outboard further which some are now trying to counter by adding outboard offset to the engine.

But the lift differential is not a bad thing. It is not a bad thing at all. In fact, many designers are resorting to increasing the flap area on the outboard side--especially as it reaches the tip to give added lift here to counter the tendency of the airplane to dip its tip in the corner (which I attribute to the lag in the leadouts not catching up with the airplane as it turns and Bret attributes this to the fact that the stablizer/elevator is experiencing the same phenomenon as the wing only with reverse effect--we could both be right). If we didn't have this increase in lift towards the outboard panel we would have to increase the flap size even more out there because of this lag thing in our lines. Good to have it at times.

Lots of stuff going on that we don't always think about.

To Leonard's reply #2...

I was reasonably careful NOT to bring up the different persuasions regarding physical offset of the fuselage location. Dean's info works best for straight-flying aircraft, so I mentioned the variation of lift across the span that we observe in our flat-skid circular level flight.

For me, dynamically (i.e., considering the forces that result from the motion of physical physical pieces through the air), it does make sense to estimate the point across the span where lift (and wing drag) is equal to the inside and outside. Seems to me a sensible place to hang the fuselage, tail, etc., masses and their drag.

And, in terms of Dean's article, we still gain from a good understanding of where the AC is and what it means.

Yes, Dean seems aware of CL, CLPA in particular. It won't come up in his columns in FM or MA, because those are for serious RC Pattern fliers. Dean, Bill Werwage and Bob Hunt were involved in developing the concept and hardware of the first CLPA tuned pipe setups. The FM article on that is a classic!

My impression from reading Dean's Pattern Columns is that he still keeps a strong connection to CL PA.

He also is our Electric Forum moderator over on Stunt Hangar.

It's all deranged rantings by a lunatic ...

Look at the initials: D. P. that stands for deranged person!
Just ask my wife. There: proof positive.

Thanks kindly for the compliments, when I do write specifically about CL in IIF, you will have no problem identifying it. Expect it to start out quite basic, though. It will still be a mind-blower for the average stick-twiddler.

Dean

"Thanks kindly for the compliments, when I do write
specifically about CL in IIF, you will have no problem
identifying it."

What is IIF ?




Allan Perret

If It Flies:
my column in Model Aviation Magazine.

best regards,
Dean Pappas

How long have you been writing IIF ?
Seems like it's something fairly new ??

Allan Perret

AY, Dean,

Jes' among us yo-yos... A quick question, with -probably- an unnecessarily long and complicated answer. (Ain't that always the way???) -

Given the dynamics of circular, flat-skid flight (CL) just how important is planform in estimating dynamic AC spanwise...?*

Good to see you in here! Appreciated your writings even before the magnificent FIRST CLPA TUNED PIPE ARTICLE in FM...

*- probably answered already in your second geometry-problem column. Perhaps just call this a 'trailer' for the forthcoming epic?

Eternal verticals, with no fall-off in yaw or pitch...!

Hi Lou, Hi All,
Yeah, it's a can of worms, but Worlds have been won with both equal panel and unequal panel wings. There are probably so many prop related effects that this subtlety gets lost if and when the tip weight is just right.

take care and thanks for the kind comments,
Dean

Lou, Bill Netzeband's articles had a reasonable formula for estimating how far off center the MAC of the wing is due to circular flight. Depending on the span, root chord, and tip chord, the offset runs from around .4 in. to .7 or so. The main reason I've found to do the calculation is that you want to make sure that the thrust line is to the left of the MAC and the CG is slightly to the right of it. Those two factors make sure that the plane takes off solidly(the CG offset) and doesn't turn left when the centrifugal force falls off in maneuvers(the thrustline is always trying to turn the plane out).

You can usually trim out any problems caused by improper location of the thrustline, MAC, and CG, but it is better to get the right relationship from the get go. It's much easier with equal panel wings and a full fuselage. Everything automatically lines up in just about perfect order. If that setup takes "too much" tip weight, go to a sidemount engine, ala' the Czech RTF models. All profile models put the thrustline outboard a significant amount, eating up most of the calculated offset. One good reason we used to always put a couple of washers under the front bolts of the engine mounts. A little offset on a profile usually helps, as long as the method used actually moves the thrustline to the left at the MAC. Some of the wedges I've seen actually do more harm than good because they are way too thick.

The classic example of waaay too much inboard wing is the DeBolt All American. Takes a fair amount of coaxing to keep the thing from turning in and running across the circle on take off. But they can be trimmed to fly pretty nice once in the air.

Thanks, Phil...

Bill N's massaged formulas and nomograms used to drive me crazy. What would a nomogram with three term lines look like after 50 years of drawing lines from value to value? ...Even if it were sharply printed on archive quality paper? (It wasn't, was it...) I prefer to do the calculations directly.

Since we have computers to do calculations precisely and quickly, I prefer going that way. Bill N's basic ideas inspired me to the approach I use: One idea is that the "spanwise dynamic center" of the wing is where the lift and drag forces are equal to each side... That can be calculated.

And, oddly, that point on a given model doesn't vary much with static weight or model airspeed. The V terms, in effect, drop out. Same, pretty much, for line drag and rake angle, unless you're talking ultimate CL Speed, perhaps.

With today's well powered, aft-CG, very large tail stunters, we have been spared much of the "design for line pull at all costs" paranoia of the '50s. We see recommendations for zero engine (structural) offset, little or no rudder offset and minimal line rake at the leadouts nowadays. The relationship of CG, MAC and thrustline still holds, but it doesn't seem to be as often mentioned as way back then.

Whatever we believe works for us, works for us. If it doesn't, it's sometimes hard to shift to another explanation. It is a hobby - we can do it as we wish. That's what keeps it interesting for each of us. Comparing notes, without anger or heartache, usually improves what I understand, and I hope others enjoy that experience, too.

And, agree, of course, that Hal DeBolt's AA,Sr. has way too much asymmetry. He got the right concept, IMHO, but the wrong numbers...

I've heard comment that the AA, Sr., is hard to take off. I guess it CAN be, if you expect it to motor on out like a modern stunter. For some reason, I learned taking a model off was an interactive participation sport. Many of us seem to think of it more as a spectator sport, with everything happening at the fast end of the lines. ...That works with modern designs... The AA,Sr., is of a different era. It appreciates a little attention, a readiness to 'lead' slightly, to step back some, if necessary... That's all.

A good light AA,Sr. is an enjoyable flier - with enough potential that a decent pilot can finesse corners to avoid roll wig-wag and still make tight turns.

Now, landing one cleanly is a problem...