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 The Trouble with NACA-4digit airfoil sections

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.


Travelling Dynamometer 


Some time back, Charlie Stone mentioned in one of his reports on the state-of the-modelling-art in Western Australia that he had seen a wonderful sight cruising our radar-polluted streets. The object he had spotted appeared to be a wing protruding from a car, the said object being tested for the aerodynamic forces acting thereupon. No doubt the perpetrators of this exercise our now relaxing at Her Majesties leisure in Casurina prison, as our bobbies rack up points for their promotion to the lucrative drug squad. Objects protruding from vehicles is a definite no-no, and this is what so attracted me to the proposition: so I filed this gem at the back of my mind, ready for recall in one of my relapses from advanced old-timers disease.

Then, about 3 months ago, out of the blue, I was invited to submit an actual propeller for wind-tunnel testing: indeed, it was a competition to design the most efficient propeller for operation at 77 knots and 500 watts (.7 HP) at the shaft. Now that is definite model aeroplane territory, and I figured it was a shoo-in.

Wrong. You wouldn't believe the stupid mistakes I made in the design. First off, I refused to believe the results from my vortex software. Then I miss-read my Bolly chart for pitch at different RPM and airspeed: not only that, but I looked it up for 77 MPH!! 

Further, the vortex results confused me, suggesting very large chords for what I mistakenly thought the pitch should be. I became more and more embarrassed, as nothing was working out. So I made my best guess, since any guess is better than none, and sent off my item, and 11X8.5.

Using the prop design theory was like swimming in white-water...you drown.
My only hope was to measure my prop performance before the competition guys ran their tunnel tests, and thus preserve my rapidly declining reputation. At this point Charlies missile popped up into my conscious mind: the answer was to stick the prop out of the car window and do the aerodynamic tests as we raced down the freeway.

I had all the equipment. Two neat little walkie talkies with plenty of range. All I needed was for Rosemary to tear along 3k's in front of me in her little Spanish-made Holden Barina and give me a squawk when she spotted the Feds. That should keep me out of Casurina, or worse, Graylands happy-home. 
As for the dynamometer, I should be able to whip that up in a week.

I had a pile of nice strong 2024 aluminium bar I picked up from Ryersons in Salt Lake City, Utah; it was lying in my shed unused. Those people at Ryersons were very nice, but a little strange, like most people from that part of the USA. They are located in a huge industrial complex, which was, in an earlier time, a Naval Base during WW2. These Americans really know how to organise wars.
Now a Naval Base in Utah is an interesting concept. The only water within 500 miles is present only when the Great Salt Lake floods, and I don't know when that last was. But you know, when in Rome. I figured I could get the good folks there to cut up some 2024 into lengths that would fit in my back-pack: that would let me go thru customs back home without copping import duty or bond store charges: good move.

So I drove unchallenged in thru the gates, waved to the guard as if I owned the place (confidence is everything in the US) and set off past 300 semi-trailers to Ryersons. I walked in just as I do back home but couldn't find the cash-customer counter, just a bunch of offices. Clearly another idiosyncrasy of the place. Terrible service, but at last a sweet lady divined my need for service and came out to ask what it was I wanted. Rather odd, the place was full of aluminium, you would have thought she could have guessed. So I told her I wanted some 2024 cut up to fit my back-pack and since I was paying cash, could I have a discount.

Well from the look of astonishment on her face, I figure she had taken me for Joseph Smith the Second (big honcho Mormon), or L.Ronald Hubbard (big honcho Scientologist) or even Clint Eastwood (big honcho Honcho). I have a lot of trouble with people wanting Clints autograph: I find it easier to sign and grab a kiss if the girl is pretty, than explain that I'm actually an Australian crocodile farmer from Kununurra.

So I slowly repeated my request, thinking maybe she was a Mexican and had trouble with my accent. I once wanted to order an engine direct from the States and stupidly tried to phone my order thru. I literally had to spell every word I said, and they still wanted to put me thru to San Francisco Zoo.

This still didn't help, but I thought I saw comprehension dawning. She was figuring out that maybe I wasn't American. Maybe that accent wasn't from the Appalachians after all. They certainly have a problem with our accent. One time I went into a bar just south of Beale airforce base, home of the SR71. It was set up just like Top Gun, great photos, a BBQ out the back, nice Juke Box. I put on "Desperado", really love that one. It was a good choice too, because not 20 minutes later a cowboy came in and put that on again.

I was with Number 1 daughter Beth, and since we chose to barbie some steak, I figured to skip the Bud and go for a nice Californian white. So up to the bar, and ask for 2 glasses of Chardonnay. Well the 2 girls behind the bar just looked at me, like I was Clint Eastwood, the same old problem. So to break the spell, I said "skip the Chardonnay, give us some Reisling". No response. So I tried for white wine. W-I-N-E. Poor things were thunderstruck, just stood there with their mouths open. I had to give up and get Beth to place the order. Lucky they weren't lezzos or we'd still be there.

Now back at Ryersons. So I got ready to spell out 2-0-2-4 when she started to speak. And what she said stunned me. "We don't sell aluminium here".
You could have knocked me over with a feather. I could see great piles of the stuff everywhere. I only wanted all that I could carry on my back. Turns out you had to buy it by the semi-trailer full, and nobody walked in off the street to buy it, you had to have an account and get your purchasing officer to order it. Well, I wasn't taking no for an answer; I told her, look, I'm an Australian, I'm on my way home and I won't have an opportunity to get this stuff again; that I'm not only the purchasing officer, but the CEO, the accountant, the machinist and the sweeper-upper as well.

Something in there did the trick, probably the CEO bit, because she came over completely on side, took my order and my money, but couldn't give me the 2024 right away because it had to be cut in Chicago of all places, then shipped direct to me in Perth! Bugger, I was sure to be caught for Customs and Bond store, but now I was a Desperado too, so I graciously accepted her proposition, signing my autograph as always: Clint.

Now back to the Dyno. While wandering thru Target 6 months back, I spied a GMC 700 Watt Router that looked useful. Big mistake. 

The motor easily detached from the wood-working bits, and I could see it held great promise for testing F2B props. The price was a steal at $70, so I added it to my store of useful-in-the-future items.

Since I needed to measure the power being absorbed by the prop, the motor had to be supported on a large ball-race to permit rotation under torque. After all, power is just the product of torque and RPM. The rotation was resisted by a spring, and the deflection of the spring could be converted to Newton.metres, which are the metric units of torque. The race set me back $30, but after I peeled off the dust seals and cleaned out the grease gunk, it turned over freely enough so I was happy.

Now the prop had to be tested in an airstream if I was to get efficiency measured, so it had to be out the car window on a beam. Further, the beam had to be pivoted if I was to measure thrust and drag, so a second race was brought into play for the pivot. Most of this can be seen in the photos. 

Now, just for a moment, this is the theory; prop efficiency is the product of thrust times velocity, divided by power absorbed by the prop. You have to get the units right, of course. American units are horrendous, so I always work in metric. Power is Watts (just the same as in electricity), velocity is in metres per second and torque, as already stated, is in Newton.metres. I only labour this point because there are no extra constants in the equation to get the numbers right. That is why metric units are so nice to work with.

With the mechanical work done, it was just a matter of setting up the electricals. Power was from a 600 Watt inverter I picked up in Singapore some years ago, at less than half the price in Australia. It not only supplied 240V from a 12V car battery, but could be used as an uninterruptible power supply for my computers. And of course, I could power-up my Doppler data acquisition computer on the field with it. You need a big inverter, because a 17" color monitor soaks up 300 Watts on its own. Turns out there was a reason it was cheap.

So you see I had a 240V power source that could run around with me in my Laser. Incidentally, that Laser once belonged to number one daughter Beth. It pays to keep in sweet with daughters, she now does web pages and keeps my website up to date as well.

The Router motor has another good feature, a speed control switch. I rewired this so that I could operate it from within the car, with just a pot and a few wires.
RPM had me bothered for a while, until I found that Jaycar had sensors that matched the sensor in my old Skyward Tachometer. I rewired it with a switch, so that I could use the internal sensor for engine testing and the external sensor for the dyno. The latter sensor is mounted in a tube behind the prop. Seems like a simple trick, but it had problems.

Just to be thorough, I also wired an AC current meter and a voltmeter into the 240V circuit. Not all economy priced multimeters can measure AC current, so I was lucky to find a really good UpTek meter going for less than half price at Altronics, only $40. A real bargain, so I put in a nice expensive NiMH battery to jazz it up a bit.

By this time the unit was ready for bench testing. You know, like 5 weeks later!
I pulled the strain gauge out of one of those nice Tandy 500g electronic scales, to do the torque measurement with zero deflection. Unfortunately, when I turned on the motor, it vibrated a bit and hammered the strain gauge to death. Another $100 down the drain. So back to the spring-and-pulley, very cheap, and it works well.

With everything finally tarted up, I ran up the prop on mains power. Yikes, 10000 RPM with only 2 G-clamps holding it down: scary stuff.
Reading off the AC amps and the voltage, I was horrified to find that the motor was only 35% efficient. So much for the advertised 700 Watts, the thing would have made a good hot water heater! Never mind, I could live with that. Hell, I could live with Germaine Greer if she could measure prop efficiency!
Next up I fired up the battery/inverter to see what happened. Well, I can tell you that the 10000 RPM mains sting was gone. All I got was 7000 flat out. What was wrong? 

Well, that nice inverter had really lousy voltage regulation. It gave the advertised 600 Watts OK, but it started at 190V on low load and struggled up to 220V at 6000 RPM. Not what you'd call worlds best practise. But I could live with that. Hell, I could live with Monica Lewinsky if I got lucky!

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With everything working, it was time to mount it in the car. A couple of pine beams across the rear seat looked business-like, so all was ready.
It takes a few seconds to set everything up when taking the torque and thrust readings, so I needed a nice long stretch of straight road. Also I needed a steady driver who didn't know this was all illegal. Not only did Grant Lucas know where there was good road (he used that road for cycle-racing time trials), but he also knows to ask no questions when things look a bit dodgy. So it was off to the boondocks, otherwise known as Mundijong. We installed Rosie at one end of the road, with her knitting, a bottle of water and her Walkie Talkie. Things were looking good.

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Except for the tachometer. When turned on, it read 3000 RPM! Hell, this was the Sun we were under, I didn't know it shone at mains frequency! Probably it was the power lines by the roadside doing this; luckily the effect stopped when the prop was rotating.

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Then it was up and down the road, with everything working well. Well, nearly well. We lost touch with Rosie for a while, and when we got to her she was shooing off a 7' Dugite. You can't depend on Women; what if the road patrol mafia had come up, we would have been gone for sure!

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Well, that about covers it all. We got a figure for prop efficiency, 43% at 60 kph and 6500 RPM. This is just the beginning; we now just need to get some anti-venom to keep Rosie on side! 

See the comment on this article from a fellow inventor!

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