Airwaves

Model Airplane News, Nov 1998

LINDBERG SEARCHES FOR SPIRIT OF ST. LOUIS

I buy your magazine at the newsstand and I know that in every issue, you have a plan of the month. I was wondering if you have ever had an issue with the plans for a Spirit of St. Louis.

LINDBERG27@AOL.COM

[email]

We have no Spirit of St Louis plans in our catalog; sorry. Pat Tritle of Pat's Custom Models has a plan for a small (47-inch wingspan), Speed 400-powered model in his catalog; you can reach him at (SOS) 296-4511. Nick Ziroli Plans-(516) 467-1752-has a l/4-scale (138-inch wingspan) plan available. LM

CIRCUMFERENCe VS. AREA CALCULATIONS FOR PROPELLERS

I must disagree with the first section of "Multi-Engine Setup Techniques" in the September issue of Model Airplane News. In the attempt to claim a theoretical speed advantage for the twin-engine configuration, incorrect formulas were used. First, the area of a circle is calculated with pi x radius2, not pi x diameter, which is for circumference. Therefore, Mr. Hahn's 2,000hp twin would have 23 percent more propeller disk area than the 2,000hp single. Thrust is not linearly proportional to the blade sweep area. The twin would have approximately 7 to 8 percent advantage in static- and low-speed dynamic thrust.

The maximum speed of a propellerdriven aircraft can be generalized as being dependent on three factors: power, propeller efficiency and frontal area drag. Power is identical. The twin's props would have their small efficiency advantage. However, it is clear that the single would have significantly less frontal area, while the twin would be penalized with separate engine nacelles in addition to a fuselage. The 2,000hp twin would possess greater climb and low-speed acceleration, while the single would not only be faster but presumably more maneuverable as well.

Other than these considerations, I enjoyed this article written by such a highly skilled craftsman and flyer.

ROBERT COATNEY

Macomb, IL

We received several letters like yours, Robert, and all of you are correct; we blew it. Such a basic algebraic error shouldn't have slipped through and into print, but it did; we apologize. You are indeed correct in your assessment of the proper equation for calculating the area of a circle.

Your second criticism sent me scurrying for the article and again, I have to concur: in that segment, Greg seems to imply that the words "speed" and "thrust" are synonymous, and that is clearly not the case.

Thanks to all who brought these problems to our attention. We print Robert's letter so that our readership might benefit from it as well. LM

TINY BUBBLES IN THE LINE

I am new to flying R/C models. As a senior citizen, I find this great fun, and I'm meeting a lot of nice folks. I have joined the local R/C club and the AMA. I have a new Sig Kadet Seniorita with an O.S. MAX .25 for power. It has a tube running from the muffler to the fuel tank. When I ran the engine for the break-in period, air bubbles kept running through the fuel line and into the carburetor. The fuel tank is built into the model and is not visible. The engine runs but seems to do so unevenly. Is this a problem? Will it be OK to fly it this way? One fellow said just to use it "as is." Thank you in advance for any help.

GENE PARRO

Eugene, OR

Gene, bubbles in the fuel line are not a good thing. Bubbles in the fuel supply lean out your mixture and, as you said, they cause the engine to run erratically. The bubbles come from either vibration or a pinhole(s) in the fuel line. Vibration causes the fuel to "foam" within the tank, and this produces many tiny bubbles in the line. Large bubbles almost always come from a hole or a cut in the line. Do yourself a favor and change the lines and, even if you have to carefully cut an opening in the model, check the fuel tank. Foam rubber packed around the tank will greatly reduce fuel foaming. If this is an old model, I suggest completely replacing the fuel tank and fuel lines. If you avoid one deadstick landing, it will be worth the effort. GY

R/C SHUTTERBUG

I have been in R/C for almost 40 years and have found it a great hobby. In the last few years, I have taken up photography. As an employee of McDonnell Douglas, I have photographed full-size aircraft; now, for my own enjoyment, I am learning to photograph models-a somewhat difficult task.

I particularly liked your Joe Nall coverage; the model photography, especially in flight, was superb. The quality of your photography is always the greatest. To improve my modeling and flight photography skills, could you tell me what kind of camera equipment and techniques you use? Thanks for your help.

BEN LANTERMAN

Bridgeton, MO

Ben, many thanks for your kind critique of our photographs. Actually, most of my everyday photography (as in my BW photos for the "Thinking Big" column) is done with a Nikon 5005 AF equipped with several different lenses. Mostly I use a 28-85 Nikkor 1:3.5-4.5 lens with a Tiffen 62mm Sky IA filter (added just to protect the lens). For the lucky few flight shots I do get, I use a Nikon F5 with a Nikon ED AF 3001:4 lens with a Hoya HMC 82mm UV filter. Before the F5, we used a Nikon 8008 with a fixed 300mm lens. With the 8008, I relied mostly on manual focus for flight shots, as its auto focus (AF) was way too slow. Now, with the F5, I use both manual and AF. AF helps with really fast jets and fighters, while I like to use manual for WW I biplanes, Piper Cubs and other slow "targets." I'm glad you enjoy the magazine, and I hope my explanation helps. GY SEWN HINGES ADDENDUM In my article on sewn hinges for control surfaces in the August '98 issue, stipulated silk thread for the hinging medium. Actually, any of the currently popular threads-polyester, rayon, or nylon-will work just fine and are available in many shades to match almost any color scheme. I have used silk for years and did not realize that my supply was well over 30 years old! Sorry about that! RANDY RANDOLPH