Letters to the Editor
All letters will be carefully considered, those of general interest used. Send to Model Aviation, 815 15th St., N.W., Washington, D.C. 20005.
Oldest-Youngest RCers?
Just as a note, I think I read that your magazine was looking for the oldest and youngest flyers. My wife's grandfather is 85, and flew just two weeks ago, and has been in it for several years. My daughter has flown my 2-channel gliders with my help for a year this March, and has flown by herself several times in the last few months, and she is only four!
Jim Talbot Chico, CA
Scale Effect—Thorny Subject
John D. Preston's article "You and Your Scale Model" in the January, 1978, issue of Model Aviation contains an error.... I refer to his statement, "For example a P-39 or P-51 modeled at a 2-inch-to-the-foot scale would have to fly at 50 mph to represent full-scale flight at 300 mph."
The basis for my comments comes from Bradford W. Powers' article "About the Size of It" in the same issue, and my own experience with dynamic similarity and scale factors while doing graduate work.
Quite simply, Mr. Preston has made the common mistake of assuming that everything scales down the same; he assumed that a 1/6 scale model would have to fly at 1/6 the speed of the full scale aircraft to represent full scale flight. As Mr. Powers points out, the actual model velocity should be:
Full Scale Velocity 300 mph Model Velocity = 300 / √6 = 122.5 mph
According to Mr. Preston, the speed of the average pattern model is approximately 100 mph. Then, contrary to Mr. Preston's assertion, a heavy P-39 which flies much like a pattern airplane would not be at a disadvantage if the flight realism rule change takes place. Mr. Preston's subsequent implications that big, light-weight, and slow equate to realism are also inconsistent with the facts.
While I am not a scale modeler, I read Mr. Powers' article with interest; the "scale effect" has intrigued me from the engineering standpoint ever since I was first exposed to it. In fact, writing an article such as Mr. Powers' is one of those little things that I never got done. However, I'm glad he did, since he is much more qualified than I.
The point of my letter is not to ridicule Mr. Preston, but to raise a red flag of warning: If AMA is going to add 10 bonus points for realism of flight presentation, let them be sure of what realism is. I also raise the question, "How many other scale experts are laboring under the same misconceptions as Mr. Preston?" Mr. Powers' article is one that should be read by every scale modeler, scale model judge and Scale Contest Board member.
Sheridan L. Miszklevitz Benton Harbor, MI
The recent article by Bradford Powers, About the Size of It, has generated other letters to this column, much too long and complex to be reproduced. Scale Effect, obviously, is a subject to be handled with kid gloves, as this exchange of letters indicates. Accordingly, we encouraged John Preston to reply through this column and, since Powers is quoted or referred to in both Powers' and Miszklevitz's letters, we invited Mr. Powers to make comment. Mr. Preston's letter appears immediately below and, after it, the Powers letter.
It appears that an unfortunate choice of words on my part has led Mr. Miszklevitz to confuse "dynamic similarity" with "visual similarity." The paragraph of concern in the article begins with the sentence "Such a scale model should, in my view, not only look like the prototype, but also fly in a similar manner." Had I said "fly at a visually scale speed" I think the point I was making would have been clearer to the reader.
To the eye, realistic speed is measured by the number of aircraft lengths covered in a unit of time. We cannot change our perception of time when actually observing a model fly, but we can cheat the eye when shooting movies of models. As Mr. Powers' article states, the dynamic scale speed of a 1/4 scale model of a Citabria travelling at 150 mph will be 75 mph. Mr. Powers goes on to say that to take real looking movies of this model in flight it will be necessary to shoot them at twice as many frames per second as the projector speed. Thus when projected back the movies of the model in flight will give the illusion that it is travelling at a visual scale speed of 38 mph. This is 1/2 the dynamic scale speed and 1/4 the full scale aircraft speed. Unfortunately I happen to agree with Mr. Powers that we generally do not achieve a visual scale speed in real life. However, the contestant with a large and/or light model can certainly put on an impressively realistic flight performance compared to, say, one whose model requires the speed of a pattern ship just to remain airborne. As proof of this point, consider the Brandenburg seaplane, modeled by Joe Tschirgi, which has a very low wing loading but which earned the highest flight score in AMA Scale at the 1977 Nationals.
Perhaps judges should acknowledge Dynamic Scale Speed when awarding points for realism, but for myself, I will continue to reward the contestant who has made the effort to approach Visual Scale Speed.
John D. Preston Falls Church, VA
I think we can say that they are both right. Preston, in his article was talking about "visual" scale speed, which looks like the real to the observer, but is actually unattainable. It would be 1/6 that of the speed of the prototype as Preston says, if it could be attained. (Editor: We note that Preston said 1/4, not 1/6.)
Miszklevitz is talking about "dynamic" speed which is what one would actually get with the proper power. In my article I tried to make this point by referring to "observers" vs. "Mother Nature."
To elaborate further... If I build a 1/6 scale model of a Citabria which has a 150 hp engine and flies 150 mph, it would look right in the air if it could be flown at 1/6 of 150 or 25 mph. Actually, however, it would fly at 150 times the square root of 1/6 or 150 x 1/2.45 or 61 mph.
Now... if I have a 1/6 scale model of a high-powered fighter with maybe 1500 hp that flew at 300 mph, my model would "look" right at a speed of 1/6 x 300 or 50 mph as Preston says... but actually the model would fly at 122.5 mph, as Miszklevitz says. But in order to get such a high speed, which is greater than most mod-
Letters to the Editor
continued from page 7
els ordinarily fly at . . . the power would be 1500/529.1 or 2.8 hp, which is over two to three times the power of a .60 engine used in most models built by hobbyists.
Bradford Powers San Diego, CA
Ghoulies and Beasties
Could you help? First, I would like plans on the "R/C Yardbird," the flying lawnmower — besides being a member of an R/C club (Quincy Flying Falcons), I am also a member of a U-control club (Quincy Glu-Devils) in which I spearhead by always flying something different. I was very much impressed by the Zonker being of canard design and was wondering why not in a U-control . . . I have not been able to obtain any info on U/C canard designs. I think the Zonker design and size would be alright if I knew where to put the bellcrank. Also where it would balance . . .
Tom Martin 718 Adams Quincy, IL 62301
Yard Bird plans cost $10.50 when ordered from Ted Teisler's Yard Bird, 941 Sherwood Dr., La Grange Park, IL 60525. Flying lawnmower plans also were published in RCM. Concerning a CL version of an RC canard like the Zonker (March, '77 MA), we suggest moving the CG forward, say 10%, from what was shown on the Zonker plans, this to reduce control sensitivity. The bellcrank should be pivoted an inch or so to the rear of the CG, to cause enough yaw to maintain line tension. The plane could be so constructed that the CG and control pivot location can be changed, if found necessary. A guide for the lines should be provided by extending a boom forward from the wing tip. Why not try an all-sheet profile Half-A testbed? Tom's full address is given in the hope someone with experience in flying CL canards will help him out.
FF Records—A Puzzle
I am confused about how those records were established for unlimited rubber, or Mulvihill as it will be called now. The 1977 regulations stipulate in Category I that the maximum flight limit is 5 minutes, and that 3 official flights are allowed, except for fly-off flights. Thus 15 minutes would seem to be a perfect score. However, the open record is 68 minutes, 54 seconds. Does this mean that fly-off flights
Letters to the Editor
are counted for record purposes? This situation would seem to indicate that a record is not only dependent upon the modeler and his skills, but also upon the quality of competition resulting in a number of fly-off rounds.
Could you please explain this situation to me?
William G. Carson Quantico, VA
Fly-offs are counted in FF national records, such as Mulvihill. But quality of the competition isn't necessarily a factor in these high figures—such as 68 min. and 54 sec. The reason is that a guy can keep going for record purposes until he misses a max flight—even if his competition dropped out several flights back.
Transcribed from original scans by AI. Minor OCR errors may remain.
