You may recall that in the last issue I detailed the design and construction of a sort of giant pattern plane, made from an EU-1A fuselage and an 80 inch span, 1200 sq. inch wing. I mentioned that one of the things that I wanted to do with my new Turbo Hawk, as I have called it, was to try some 4 bladed propellers. A 4 bladed prop would be shorter and allow a shorter landing gear. The shorter gear would allow a 3/16 wire gear leg to be strong enough so that light weight, small plane retracts could be retrofitted or built into a later version. Well, the prop testing is done and here are the results of the tests. First, the set up I used:

Super Tigre 3000
Super Tigre can type muffler
OS 7D carb
10% nitro, 10% oil "Big Tigre Blend" from Red Max

The procedure that Terry Green and I use for prop comparison testing is to fly the plane with a baseline prop that we think is the best and subjectively compare the other props to it for level flight speed and climb. It may not be a scientific as tachs and thrust meters, but when your thumbs are on the sticks and you pull the nose up, rpm or not, you know which one climbs better. A summary of our method is: Spend the money, buy a lot of props, fly 'em and try 'em.

All right, how did we make the 4 bladers? First, I used 18 inch props cut down to 16 inches. If you will compare 14, 15, & 16 inch props to 18, 20, and 22 inch props, you will notice a large difference. Props of 18 inch diameter and up are much heftier and wider in the hub and also the blade. I figured that a 16 inch prop with a 10mm hole that was notched would be pretty weak, so I used 18 inchers and shortened them. I would not recommend that any of you try this procedure with props smaller than 18 inches for safety reasons. If you sling off a prop blade on the ground, you can injure yourself or someone else. If it happens in the air, it can shake your plane apart. Be safe, stick to the big hefty ones. Besides, those wide blades just look good.

Next, I took the simple way out and notched both props, one on the front, and the other on the back. I could have used metal disks and 4 separate blades, but notching was easier. Even though the 18 inch props have a pretty hefty hub, I decided to leave a 1/2 inch thickness in the center. This means that the blades would not be in line. The flight tests I have flown indicate that it doesn't seem to make any difference.

The props were notched on a radial arm saw, although you could use a table saw or a drill press with a router or shaper bit. For the saw, I set the blade height to 1/2 inch and then I held the prop with both hands while Terry pulled the saw to make the cut. I slid the prop over a little after each cut until I had the size cutout I wanted.

The original test flights were flown without the two props glued together. The roughness of the saw cuts made it impossible to move the props relative to each other with the prop nut tight. Later I secured the joint with a squirt of thin Jet.

Naturally, I sanded them and did my best in the way of balance. Finally, I gave the prop a coat of black Super Poxy, which I thought looked more scale-like on the military finished plane. Actually, the hardest part was cutting out a 3 inch spinner for all four of those blades.

I used 3 sizes for the tests: 18-8, 18-10, and 18 6-10, all cut to 16 inch diameters. I figured that 6 big props were enough dent in the wallet, considering that I might end up junking all of them. (Actually, I had the 18-10's and 18 6-10's laying around, so it wasn't that expensive.)

THE BASELINE: The 18 6-10 is a good prop for the 3000 with the OS carb so we decided to use it for our baseline prop. It was the one I had used for my original test flights of the Turbo Hawk, so I took it up and we calibrated our eyeballs for level flight speed and climb.

CUT DOWN 18-10: Next we bolted on a four bladed 16-10 (cut down 18-10's). This is a pretty hefty prop and the 3000 didn't turn up anywhere near what it did with the 18 6-10. The Turbo got airborne quickly, but the level flight speed was slow speed, I expected the plane to poop out in the top of maneuvers. I was surprised when it pulled through a big loop with very little speed change at the top.

Next, I flew past and pulled up into a vertical climb. I was surprised, very pleasantly surprised. The Turbo must have gone up to a thousand feet before it quit. The guys at the field were amazed at the climb. I then spun down and set up for some practice acro. I tried a square loop with half rolls, a maneuver which requires good vertical performance. Chug, chug, chug, the speed seemed to be the same all the way around the square. Square eights went the same. Sustained knife edge was a little weak, due to a lack of speed, but point rolls were still good. I decided that I could live with the slower speed and great climb of the 4 bladed 18-10 prop.

CUT DOWN 18 6-10: The next prop was a 4 bladed 18 6-10 cut to 16 inches. I would guess that it's now a 16 6-9 wide blade, or something like that. This turned out to be a better prop than the cut down 18-10's. The level flight speed was faster, nearly up to that of the 18 6-10. The climb was still great. Maybe it was a little less than the stock 18 6-10, but still very high. The climbing portions or square eights and the square loop with half rolls were easily done. This one, overall, seemed to give flight characteristics very similar to the stock 18 6-10.

CUT DOWN 18-8: Well guys, I was going to do this one, but I bashed the plane. The Turbo Hawk was a good one, but it is pretty well totaled. It happens. I plan to build another one, but not soon. You may have read a couple of my articles in RCM a few months ago. They are going to publish some more, and I am working on the plans for a small plane for possible publication. If you have never done a construction article, you need lots of pictures all through the building process. This tends to make it longer and more of a pain. After that little plane, which I plan to give to Chad (college kid step son), I'll get working on the new giant twin. Boy, I really hated to lose that Turbo Hawk.

Actually, if you must know, I went three for three the week I bashed the Turbo. I had a 45 powered fun fly plane with a phenolic tube for a fuselage and I managed to twist it off doing snaps. Score 1.

Next, I was going to do a little night flying. The glider guys use light sticks or Cylumes on the wing tips and fly at night. I had an old timer, Playboy Sr., so I thought I would get it out and fly power at night. I was testing and the rubber bands cracked the pylon (wood was old) and the wing came off. Talk about flying an arc to the ground! It looked like a powered bomb, nice ballistics. Score 2. Well, I thought, I'm sticking to giants. No more of these little flimsy planes. The next day I bashed the Turbo. Score 3, and I'm in shock.

ADDITIONAL 2-BLADE TESTS: Mixed in with the tests of the 4 bladed props, I also tested some more two bladed props. The 18-8 Zinger and 18-8 Dynathrust both wound up to a pretty high rpm on the 3000. They both sounded fast, but did not pull the plane any faster than the 4 blade or the 18 6-10. They both, however, failed miserably in the climb area. Climb was blah. It went up a couple of hundred feet and topped out. After the out of sight climbs of the 18 6-10 and the 4 blades, this was a disappointment. These props are adequate, and they are good to have in reserve in case I break the others, they just aren't the correct size for getting the maximum performance from the 3000 in this plane.

THOUGHTS: I have been thinking about the performance of the 4 bladed props, especially considering the low rpm the engine is turning. I also noticed that the guys with 2000's & 2500's used smaller props, 15-10, 16-8 & 16 6-10 compared to the hefty 18 inchers that 3000 fliers normally use. I finally got out the spec sheet that comes in the engine box, you know, the one we never read. It had the specs for all three of the big Tigre engines. The rpm's at which these engines get their max horsepower and torque are interesting. Forget the amount of power or torque for a moment, and look at the rpm at which the maxes occur:

RPM for:

	Engine 	Max HP 	Max Torque 
	2000 	13,000 	8200 
	2500 	10,000 	7900 
	3000 	7900 	5000 

You can see that the 3000 is a low rpm engine, a torquer, if you will, compared to the other two. The 18 inch prop and my 4 bladed props pulled the rpm down to where I was close to the max torque range. This is the reason that the engine kept on pulling even though the speed of the plane was slow. It was an interesting test with interesting results. The 4 bladed props were relatively easy to make, perform well, give more ground clearance, and they look really great on the plane. Very turbo-propish. Now I may try one on a Zenoah.