Letters to the Editor
All letters will be carefully considered; those of general interest will be used. Send to Model Aviation, 815 15th St., N.W., Washington, D.C. 20005.
Matching Pitch to Operating Conditions
It is very simple to get a rough idea of the propeller pitch required to match a particular velocity/rpm situation. Take the velocity of the airplane in inches per second and divide by the revolutions of the prop per second; the result will give the required pitch in inches (for any diameter prop) to set the driving face of the prop at zero degrees angle of attack. Add a couple of degrees angle of attack along the blade elements and you will be very close to the ideal pitch for this operating condition.
If you examine Kit Bay's (Aug., 1978, Letters) conclusions regarding pitch/velocity/rpm you will find the method described above gives identical values of pitch. However, it is wrong to assume that once a propeller begins to operate with its blade elements at a negative angle of attack that this means it is acting as an air brake. The governing factor here is the actual zero-lift angle — not to be confused with the term zero angle of attack.
What kind of an rpm/prop combination would we need to use if we wanted to soup up that Austrian speed glider to hit 300 mph?
300 mph = 5,280 in/sec ÷ rps
- 333 (20K rpm) = 15.85" pitch — Maximum Diam. 10.9" — P/D Ratio 1.45
- 416 (25K rpm) = 12.69" pitch — Maximum Diam. 8.72" — P/D Ratio 1.45
- 500 (30K rpm) = 10.56" pitch — Maximum Diam. 7.26" — P/D Ratio 1.45
- 583 (35K rpm) = 9.05" pitch — Maximum Diam. 6.22" — P/D Ratio 1.45
- 666 (40K rpm) = 7.88" pitch — Maximum Diam. 5.45" — P/D Ratio 1.45
Looking at the results above it seems we are in a bind. Because of the limitation of diameter to keep tip speed at 950 ft/sec and because the pitch/diameter ratio is very high, the ratio is too high for maximum efficiency (which occurs at a value around 11). Decreasing required pitch by increasing rpm doesn't help since the shrinking diameter keeps the ratio the same. It looks like it will take a lot of brute horsepower driving a very inefficient prop. The propeller design for these requirements will look odd. Because of the extreme blade angles most of the blade area should be concentrated toward the tip and the pitch should be non-helical for the same reason, with the pitch increasing toward the tip and having its highest value near the radius 70% position. The high tip speed calls for very, very thin airfoils on the outer half of the blade and they should be around 5%–6% thick at the tip. There would be some arguments for going to a three- or four-bladed propeller. Whatever the number of blades, the props would have to be fabricated using epoxy/graphite/carbon.
I promise never again to get hooked by an RC subject to the point I'm tempted to write letters about things I know nothing about. I'm disappearing forever with my favorite FAI Power model.
Bill Gieskieng, Jr. Denver, CO
Free Flight Famine
For the fourth consecutive month Model Aviation has failed to present any construction articles for free flight models.
I would not be writing this letter if MA were an independent publication, but this magazine is represented as the "Official Publication of the Academy of Model Aeronautics." There are numerous model airplane magazines that cater to the interests of radio-control modelers. I would hope the Academy's publication would try to represent all its members. As things exist now, there are four (4) monthly sections dedicated to Control Line, six (6) features about Radio Control (including Oldtimers, which deals as much with radio control matters as with free flight), and only two (2) sections for free flight.
I find it difficult to believe that with all the excellent free flight competition modelers around, you cannot obtain worthy subjects for publication.
I feel it's time the AMA stopped considering free flight as second-rate and started giving us as much consideration as you do the other aspects of our hobby.
Joel Chesler Malverne Park, NY
Editorial note: Model Aviation is often criticized for alleged undercoverage of this or that event. Radio people say we don't give comparable material (quantity) to other magazines — but we do, and more in some cases. Control-line people sometimes say exactly what Joel just said of Free Flight. MA has four free flight columnists, not two: Warner (Sport/Scale), Meuser (Duration), Haught (Oldtimers), and Tenny (Indoor). As an old free flighter, the editor is conscious of weak coverage of powered free flight — though Meuser is constantly presenting insights on the subject.
Wheel Landings
Regarding Lee Taylor's article "The Art of Thinking Big" in July's magazine — generally an excellent article — I must take exception to his technique for making wheel landings. The proper technique for full-scale aircraft is to make a normal flare, except that you allow the wheels to touch before the stall; as soon as they touch you push the stick forward, and not before!
If you push the stick forward with the airplane still in flight, the nose will go down, airspeed will build up and you've got a potential crash. I think the technique applies equally well to 1/4-scale or full-scale aircraft. I just test flew my new Lazy Ace and used a wheel landing as I have described and it worked perfectly — no scary stall! The rest of Lee Taylor's instructions are OK — as speed drops off, come back on the stick to avoid nosing over.
Let's see more on 1/4-scale models! Keep up the good work.
Doug Mac Brien Granby, MA
More About "Quickie"
Regarding the "Quickie" controversy, the opening letter in Letters to the Editor (August, Model Aviation) points up the diverse interests in model aviation. Whereas the Quickie article by Don Berliner did not appeal to Mr. Walter Wirths, it turned me on completely. I would think that Model Aviation magazine is doing its civic duty to modelers by presenting in-depth articles with 3-views of exciting new designs of which there are no plans or kits available.
As you know, I built a Quickie, 1/4 scale, from the Berliner article supplemented by an information kit from Quickie Enterprises. The information kit included a CG location for the real aircraft which I did not rely on entirely because of scaling factors. I built several gliders and rubber versions of the Quickie to establish better CG information, and also, important, decalage. With some refinement in CG and decalage the 1/4-scale model now flies fine and I will submit a construction article to you for your consideration.
Workable CGs are 50% between the root quarter-chords for gliders, and 40% aft of the foreplane quarter-chord for the peanut and 1/4-scale RC versions. These numbers will get you in the air. As important, though, is decalage (the angles between the wing and foreplane chords) which must be a healthy 5 degrees.
John Hunton Annandale, VA
Regulations for "Big Models"
I noticed your "For Openers" editorial in the July issue of Model Aviation and was certainly relieved to finally see a word or two of warning on the "big" RC models. I am very much concerned about how these models can be safely operated and the thought of pylon racing with 16 lb. airplanes is frankly appalling.
I believe the AMA should immediately initiate a detailed investigation to determine if these monsters can be safely flown under the same conditions and procedures that we have used so successfully for smaller models. At the very least we must have absolute weight and power restrictions that will limit the kinetic energy developed in flight.
So far, most of the large designs appear to be floaters. Everyone knows, however, that a Quadra engine can produce 5 hp with a mild rework and a dose of nitro, and it's only a matter of time until 20-lb. models are capable of very high speeds. The energy release on impact of such missiles could be readily calculated but the effect of a direct hit on another flier or bystander is more difficult to evaluate. It is obvious, however, that the risk of serious injury or worse from such a collision is sufficient to warrant very careful consideration by AMA.
Please review this question and publish additional information and comment on the subject as it becomes available.
Glenn F. Gresens Baton Rouge, LA
Editorial response: The matter is under study. The feeling seems to be that it would be better to so govern ourselves that "big" models do not create a situation which could lead to the FAA determining that they should impose restrictions. Further information will be printed in MA when it becomes available.
Reply to Gieskieng
It should be realized that a great deal of the information given in my propeller articles has been gleaned from plain engineering data and theory as applied to propellers in general. As was stated in the original article, there has been very little research done with propellers for our models in a scientific manner. The objective of the articles was to show modelers in what areas research could be done with hope of improving the breed. While I personally have been doing some of this research I feel that I have only scratched the surface, and in fact recent work indicates that this may very well be true. When the diameter of a Formula I propeller can be increased from 8-1/2" to a full 10" and have the new prop competitive in a race, then something good has obviously been found. The next objective would be to take advantage of the findings and develop them to their full potential. It appears realistic at this point to say that much can be gained through propeller research.
The points which Bill has brought out are all valid and can make worthy additions to what has already been presented. One point is prop pitch relative to the zero-lift angle of various airfoils. One of the problems encountered when delving into something as well established as propellers is being able to describe and relate the design in a manner familiar to the average modeler. If these people are confused by what you say or do not understand, then the work is wasted. It can be a bit difficult to consider all ramifications.
The pitch of propellers, for any number of reasons, has always been measured on the back side of the blade and stated in inches. Inasmuch as most airfoils used have been very flat-bottom, this has been a good reference point. The thrust of a propeller is created by the lift of the airfoil and the windmill effect (leverage) from the rear side of the blade. Bill suggests that we should measure pitch from the zero-lift angle of the airfoil rather than from what is the back of the blade as is customary.
This would help in applying theory as comparisons are made between various types of airfoils. However, what else has to be considered is that thrust not only comes from the airfoil but also from the windmill effect. So, the actual pitch would not be the angle of the back side nor would it be the true lift angle of the airfoil; instead it would be a compromise of the two. So it would appear that the descriptive number we use to differentiate between various propellers, and call pitch, would be better called a model number. In practice it is used more for comparative purposes and if our work is to remain comparable to the "norm" then we too must follow the procedure, even though it may not be theoretically correct.
It was interesting to learn that the NACA had done some work with streamlined airfoils and propellers — apparently using the same reasoning suggested in the prop articles. It was good to know that they had seen an improvement through the use of some sort of a symmetrical-type airfoil.
For Bill's info: yes, it is true that some of the racing props do reach 27K rpm in the air. It is also true, as explained in the article, that this may not be very good practice due to reaching the speed range of compressibility. There are probably a number of areas that can be explored to find improvements along this line, such as multi-bladed props. Unfortunately for competition purposes, where all this sort of thing is usually worked out, we are restricted to 2-bladed props. For this practical reason, work would have to be done in other areas. Suggestions would be the use of high-speed airfoils and/or sweepback in the portion of the blade subjected to the Mach speeds.
As the prop production portion of the article indicated, the use of templates for the prop blocks will put you in the ballpark with the pitch and blade widths. Obviously, the final finishing should be done with some sort of pitch gauge. If you have a pitch-gauge instrument it is obviously useful; otherwise your suggestion of making template angles for each station will work well also.
It was also interesting to know that the NACA had done some work with the location of maximum blade width and found some improvement by using 0.38R.
Something else to try! I hope that the readers will see how Bill's letter has added to the worth of what was previously offered and in this way "pooled knowledge" can make investigation not only easier but even more interesting.
For my part, I believe my recent prop work has discovered the basis for what could be a super prop for Formula racing at least. The results of a drastic change have been very surprising and enlightening to me. Should our dear editor see fit, perhaps we will be able to pass these findings along in the near future.
Harold deBolt Buffalo, NY
Editor’s note: Mr. deBolt's letter responds to an earlier letter from Bill Gieskieng — he had not seen Bill's letter in this month's column. And the "dear editor" is waffling. He wonders if the world is ready for Harold's latest findings! The "unloading" figures are unbelievable — differences between on-ground and in-flight rpm. The editor is also a bit "terrified" of those sharpie Pylon guys — as he is of sailplane designers. We would feel better if some of these positive Pylon guys would try some of deBolt's earlier article revelations. How about it — should we carry on?
Transcribed from original scans by AI. Minor OCR errors may remain.
