Radio Control: Sport/Aerobatics
Ron Van Putte
Just after sending in my last column, I received a long, delightful letter from Greg Lovick (Grapevine, TX) on the subject of propellers. Since several of my previous columns dealt with propellers, many readers wrote either to agree with me or to "straighten me out" on what was written. I still believe that everything which appeared in print is technically accurate. Anyway, most of Greg's letter is printed here to "cap off" the discussion on propellers. If any of you still prefer to continue the discussions after reading the following letter, feel free to write me.
Letter from Greg Lovick (Grapevine, TX)
Your February 1981 column in Model Aviation promised a discussion of the percentage of prop thrust due to the front and rear blade surfaces, to be printed in March. Maybe we already agree with each other.
The first line of reasoning that might be applied is based on the Rankine–Froude momentum theory, which assumed that the propeller acted as a magical, bladeless disc which could act on the air passing through it to produce an increase in velocity. Textbooks will show, with all the attendant math, that one-half of the air velocity change produced by this disc (propeller) is gained before the air moving through the disc reaches the disc, and the other half of the velocity change shows up behind the disc. Thus, if a model is flying at 60 mph, and the speed of the prop blast (measured at the tail) is 80 mph, then the air velocity seen by the prop is 70 mph. Going a step further, the drop in air pressure just ahead of the prop is equal to the rise in air pressure just behind the prop. This builds a pretty good case for saying that the front and rear surfaces of a prop each produce 50% of the thrust.
The problem with this explanation is that we can't fly with just a bottom surface of a prop, or just the top; they are unavoidably inseparable, so we must have been asking the wrong question.
Perhaps a better question, and one which can be answered, is this: Which side of a propeller has the greatest potential for destroying the lift (thrust) produced by the prop? Just as with a wing, if the lower surface is distorted, the drag produced will increase, but the lifting ability is virtually unchanged. However, if the upper surface is distorted, the airfoil will stall at a lower angle of attack and the lifting ability will be severely reduced, in some cases more than 50%. This argument says that the front of the blade can cost us more than one half of our thrust, but it does not say the front surface produces more than one half of the thrust.
Author's conclusions
Let me cut through all of my own baloney and toss out my own conclusions:
- As long as we insist on flying aerobatics with throttled engines and $2.00 fixed-pitch propellers, we are going to be stuck with lousy efficiency. Even a million-dollar research program to make a technical-overkill molded prop cannot provide a prop that will be highly efficient in all flight modes, even on a single engine/model combination.
- Any method of measuring pitch is OK if it is used to compare each blade of a new prop to a good standard design. The modeler's real problem is variation in manufacturing. We can all afford to test-fly a dozen props on a model and discover which brand/pitch/diameter works best. Our problem is that no two individual props are really identical. (For $2 each, they may never be.)
- When balancing a prop, screwing up the front of the blade can hurt worse than screwing up the back of the blade — and a touch-up with fine sandpaper all over the blade wouldn't hurt.
- A little more attention paid to weight and general aerodynamic cleanliness of the airplane will do a lot more good in increasing apparent engine performance than all the Zinger-sawing and Rev-Up-bashing you can do. My advice to potential Pattern propeller-modifiers is to leave a half-pound of paint, glue, and sawdust on the ground, and use stock props.
Footnote
- The Gossamer Penguin solar-powered airplane weighs 170 lbs and flies with twin Astro 25s — we have a lot to learn.
Greg did a beautiful job of capturing the important items relative to propeller performance, and I for one really appreciate it.
I have also been getting a lot of letters about the "Agony of Defeat" letter which was published in the February 1981 column. Much to my surprise, most of the letter writers don't have much sympathy for the author. The consensus is that the author is a novice who expected instant success without the effort required to get it. A typical letter included these lines: "I have never met a modeler who was unwilling to help if approached with the right attitude. If this person has only been able to get '30 minutes on the sticks in two years of trying,' I can only wonder about his attitude."
Okay, I'll buy the claim that some of the reasons for the guy's lack of success probably lie with him, but until you've visited another club as a stranger or taken a good look at your own through a stranger's eyes, you probably haven't seen just how we really relate to beginners. Take a good look at your club. How do you treat someone who drives up to the flying site and stands behind the flight line just watching for an hour? Many club members will ignore him, right? Would some be even less cordial than that?
I can remember one trip that I made for the government. I was stranded in a strange city for the weekend with nothing to do, so I found a local flying site and spent some time just watching without introducing myself. To my amazement, a club member came up and told me that I might as well leave, because they didn't allow strangers to fly at their field! Can you believe it?
Let's try a scenario. Pretend that you are a beginner, and you drive up to the club field with a poorly constructed model containing a bargain engine and a discount-house radio. How do your club members treat you?
- Did you just hear one of the club experts make a joking comment about "a wing that looks like a propeller?"
- Did another laugh and point out your "S12 engine" or your "El Cheapo radio?"
- Did you get ignored by the people who could really help you because they were too busy?
If you can say no to all of these questions, then I'd love to be a beginner in your club! However, if you said yes to one or more of the questions, your club is probably not all that unusual. Next time a stranger visits the club field, take a good look at how you and your fellow club members treat him. If you'd be happy with the treatment, congratulations. If not, an affirmative club policy change is in order, and the Golden Rule is a good place to start.
Ron Van Putte 111 Sleepy Oaks Road Fort Walton Beach, FL 32548
Transcribed from original scans by AI. Minor OCR errors may remain.
