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Propeller Dynamics

Essential reading for model aircraft contest fliers. This is the only book on the market explaining propeller theory in non-mathematical terms. A rattling good read, I know, I wrote it.


Return of the Prop Doctor 


Those who bothered to read last months PANIC epistle may like to hear the sequel, now that I am back from the South Western Regional C/L champs in Dallas. 

First, my Profi counterweights were still not quite right, just grazing the inside of the spinner. Jed and Scott in LA set me right on this so the next lot should be right on. They were happy with the new "click" in feature, although at first they thought it should be filed off! 

Didn't get to meet George Aldrich, he went into hospital to have cataracts removed. I phoned him; he is doing very well and delighted with the results. So to George, take it easy and we in Oz wish you a quick recovery and want you back in harness right away! I gave the new 10X5's to Bob Lipscombe to pass on to George, so now we must wait for the maestro to test fly them. Incidentally, Bob thought Joe Parisi was living in Perth! Hey, where are you Joe?

The Doppler sequel is that my FM Tx's failed to have sufficient range at our secret base in the California desert. They were swamped by traffic on the crowded FM band in California. I was getting airline pilot chatter instead of engine noise! So I bought a little book on aerials and the next step is to build some Yagi antennas to beam the signals direct to the Rx., rather than spray my signals all over the place with the original vertical dipole. 

Even worse, my land-lines also failed on the old Castle Air Force B52 SAC base. This was baffling, as they tested out fine when I first laid them. The rig comprised two microphone units connected by 300' of cable in a T configuration to 1600' of wire back to base and the tape recorder. With no signal other than a country music station (!) arriving at the recorder, I set about testing. At the recorder end, I picked up about 40 ohms which seemed about right. Disconnected at the T, the line was open circuit, also right! 

You guessed it. Not bugs, but rabbits. They ate the cable, leaving one end shorted and the other open! Next time I lay 1080 as well. The black helicopters were also circling in the distance, but this was definitely Bugs Bunny, that wascally wabbit.

Moving right along, I have to tell you the folks in Dallas were so friendly I could scarcely believe. 

My reservation at the Sleep Inn was done with a phone number the guys sent me, but I got the address off the Internet. So I arrived at the Sleep Inn Richardson to find no booking! Hell, that was bad. It transpired there were 3 Sleep Inns in the area, and the one I wanted wanted was neither on the net nor in the motel guide book! So imagine my joy when, after traipsing over to the Sleep Inn at Plano, Rosemary the receptionist managed to find my hotel, and then drove me for an hour through heavy traffic to get there! 

What a woman!. She started her career as a C5 jet engine mechanic in the USAF, and ended it as an administrator working for the Air Force Chief-of-Staff. Rosemary, I salute you, and promote you to Honorary Man.

I think I figured out why that Sleep Inn was not in the guide book. I woke up at 2am one morning to find ants marching over the top of me. When I presented some ant corpses and my dishevelled self to the front desk, the guy just asked me where were the ants going to? Lucky they weren't Fire ants, which are slowly spreading into Texas and wiping out the native wild life as well as motel patrons. They even kill deer by biting their eyes and sending them blind.

Must also thank Steve Moon who helped me with all the details and getting to the field, and also Jim Cooke who very graciously took me to dinner with his family and showed me around the Model shops. These latter were every bit as you would dream a model shop to be. Most had a good range of C/L gear, including reproductions of the original Nobler kit, Fox 35's and fancy carbon control linkages with titanium ends and machined clevices (Bobbye Hall's Hobby House, bobbyehall@aol.com). 

Outstanding in my memory was the MAL (Model Aircraft Laboratory, www.flash.net/~malhobby) hobby shop. Run by Edgar Seay, both junior and senior, this shop has an accumulation of aeromodelling history going back to WW2. There are ancient kits, engines, cut-on-site balsa, F/F rubber kits, magazines, you name it. A real museum, a must-see-when-in-Dallas. If you want Texas Timers for F/F, you should enquire there.

After all this, the contest was a bit of an anti-climax. It was about the size of a NSW State Champs, with jet, stunt and various forms of Goodyear the most popular.

Jet was interesting, as Mr Bailey of Bailey jets was present. He mentioned all the different problems he came across trying to weld thin sheet metal into tail pipes and combustion chambers for pulse jets. These problems are not yet solved: he is looking at laser welding at present. Among these problems were titanium contamination of the stainless steel, and oxygen embrittlement along the edge of the weld. Mr Bailey had spent two years just trying to get one decent weld out of his new welding machine.

The Bailey jet has a larger combustion chamber than the Dynajet. Evidently the Dynajet gives most power when its pulse frequency is lowest, so that the model emits a moaning sound. Since the tail pipe diameter is fixed by the rules, it is hoped that the larger combustion chamber may have the same effect.

Incidentally, I had a portable computer (Acer) with built in microphone with me, so I was able to do a Dopplers analysis on the jets. One could see from the Spectrogram trace that it took the jets about 3 laps to get up to speed. The fastest there showed a pulse rate of 13800 ppm at 200 MPH on the Doppler. Stopwatch was a little less, probably due to the slow wind-up. This was an extraordinary model, by the way. Its appearance was distinctly alien, enhanced by the jet-black finish. The outer wing was raised above the out-slung jet, while the inner was level with the jet. Fuel was 80% methanol, 20% propylene oxide.

Tough work

I noticed this chap polishing his valve seats on a sheet of glass covered with fine abrasive paper. Starting was all done with the traditional hand pump. Real tough work in the heat and humidity of Texas at that time of year. There were some 6 jet entries, and lots of flying.

The Stunters were mostly well flown, although I noticed that many were too slow to handle crisp overhead 8's in the varying wind conditions. Many Eather/Firecracker props were in use on the piped engines, while the unpiped engines wore a whole range of props, with B-Y&O props most in demand. Also saw some Stalker carbon-fibre/wood and carbon stunt props. These were reported to work very well on the Stalker engine, which has a great reputation for performance and reliability. I noted that the Stalker props had a lot of pitch at the tips. The chap who owned them claimed that stunt props work best if they have a roughened surface finish. 

On the subject of pipes in stunt, I noticed one piped model flew well through the pattern but leaned horribly and screeched around the circle for about 10 laps before the motor cut. Some guys had done temperature probe measurements in the pipe and found that when the motor went lean the temperature went right up; then it was impossible to needle the motor rich again.

Also of interest, there were many profile-fuselage stunters present.

Finally, some insight on fuels.

Bob Lipscombe filled me in on NACA work on water in fuel (methanol). This is a problem we have experienced in FAI F3D pylon. The organisers supply the fuel, and it is often contaminated with water. The result is detonation, and compression has to be lowered, a very annoying problem for the contestants. Adapting to the fuel is one problem we don't need.

Methanol absorbs water from the atmosphere quite readily, so just opening your tin is a problem. NACA found that this water can form microscopic droplets which, when in the combustion chamber, become super-heated, then flash into steam and cause detonation. (I assume from the increased compression) 

Now it happens you can test for water in your methanol using silica gel. You dry out the dyed variety, which I think is blue when dry, and lower some into the methanol. If it turns pink, you have water in your methanol, and problems if you run high compression or want consistent needle settings. 

But all is not lost. There is a second form of silica gel, which looks like sand and is white. It is used for drying out flowers. You dry this out and leave it in your methanol for several days and this removes the water from the methanol. Retest, and repeat if the test fails. You need 500g of gel for 4 litres of methanol.

If the fuel is already mixed with oil, I do not know what happens. Bob says it ruins the silica gel. However, it would be interesting to know if the mixed fuel could be tested with the blue gel, even if it ruins the gel, as this could save a lot of heartache. Nitro does not appear to absorb water. 

There is some controversy about this, check out what Lance Smith has to say: Comment 1  Comment 2  Comment 3  Comment 4

That's it for this month, keep those cheques rolling in, I am now destitute from all this travelling.

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