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Old Nov 05, 2012, 01:18 PM
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I am curious how a 3 blade prop would work as a pusher with limited air flow? The 2 blade is said to be more efficient which may mean used as a tractor. Would the 6-3-3 blade replace the 6-4 two blade on my new design? Grayson sells both and I may learn something.

Dereck, The Easy Star canard is cool looking. I have wanted the standard version in my stable since it came on the scene. Our club president has one and loves it. The steep DOWN thrust appears to pass through the CG which makes the model drop a bit on sudden full throttle. Since the CG is directly under the motor on my planned model, I would hesitate to raise the thrust line.

Charles
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Old Nov 05, 2012, 02:24 PM
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The more I read of Don's words of wisdom, the less I want to go flying in real aircraft ...
"Aviation in itself is not inherently dangerous. But to an even greater degree than the sea, it is terribly unforgiving of carelessness, incapacity, or neglect."

Airplanes are precisely as safe as you make them, no more, and no less. That is true whether you're talking about model airplanes, hang gliders, or airliners.

I used to be involved with hang gliders, in fact I still have my 18' Standard Rogallo rolled up out in the barn (it's grounded because I declared that its "landing gear" has exceeded its age limit). I was one of the founders of the club at the university where I was going to engineering school. There was also an all-city club, that I was less involved with. The university club put over 60 people through "basic training" while it was active, and the worst injury we had in all of that was an aggravation of an already sprained ankle.

Meanwhile, the all-city club lost two people to fatal accidents in one weekend. Both of them were friends of mine. In both cases they were experienced, competent pilots, who each had one bad habit. In both cases it was the bad habit that got them.

Model airplanes have their fatalities too, although fortunately those are rare. One I recall involved a man who put a plastic prop (NOT fiber-reinforced) on a .60 engine (the rule then was never put a plastic prop on anything bigger than a .19). He was leaning over it to adjust the needle valve (avoid putting yourself in the plane of or slightly in front of the plane of the propeller, because that's where a broken blade is going to go). The prop threw a blade, which pierced him through the heart, killing him instantly.

The DC-10 that crashed in Chicago in 1979:
http://en.wikipedia.org/wiki/America...nes_Flight_191
... went down because of shortcuts taken by a maintenance crew in Oklahoma. There were two large shear pins that held each of the pylons of the wing-mounted engines to the wing structure. The forward one did most of the work, and needed to have periodic inspections for cracks. There was an elaborate hoisting jig used to support the engine at its C/G while the forward pin was pulled and inspected. However, the mechanics got in the habit of saving time and trouble by just supporting the engine with a sling and a forklift at its forward end. Unfortunately, the support was not on the engine's C/G, which put excessive loads on the rear shear pin. These improperly done safety inspections initiated cracks in the rear shear pin. When that broke, the engine (at takeoff thrust) pivoted around the forward shear pin and went up and over the leading edge of the wing, destroying the hydraulic lines for the leading edge flaps in the process. This caused the leading edge flaps on the left wing to retract, causing an asymmetry of lift that the ailerons could not overcome. The plane was banked a little past vertical when the wing tip began to drag a long trench in the ground, then the plane went in, making a huge fireball and a lot of very tiny pieces. No survivors. All because someone got in a hurry, and took a shortcut.

The 1989 crash in Sioux City:
http://en.wikipedia.org/wiki/United_Airlines_Flight_232
happened because someone reassembling an engine during an overhaul used a pocketknife to cut the plastic tape holding an inspection tag to one of the engine rotors. This caused a scratch, the stress concentration from which initiated a fatigue crack. When the rotor (a large titanium disk a couple feet across) burst, it tore through the aft fuselage and managed to damage all three of the redundant hydraulic systems for the flight controls. The only control the pilots had left were the mechanical linkages to the two wing-mounted engines. Using the effect of the thrust on pitch trim, and differential thrust to control yaw and roll, the pilots miraculously managed to put the plane down on an airport, which saved the lives of almost two-thirds of the people aboard. ONLY 111 people died, the other 189 survived. The plane was destroyed, but at least the pieces were mostly fairly large and recognizable, thanks to some skilled, intelligent and resourceful pilots who refused to give up.

All because of a pocketknife, some plastic tape and a little carelessness.

Airplanes, OF ANY SIZE, and especially the high-speed parts of them (such as propellers) demand your constant attention, care and respect. The moment you forget that is the moment you're most likely to become a statistic.
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Old Nov 05, 2012, 02:33 PM
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Originally Posted by canard addict View Post
I am curious how a 3 blade prop would work as a pusher with limited air flow? The 2 blade is said to be more efficient which may mean used as a tractor. Would the 6-3-3 blade replace the 6-4 two blade on my new design? Grayson sells both and I may learn something....
A common error folks make when switching to a prop with a different number of blades is to change the pitch. Prop pitch is a function of a number of things, particularly airspeed, but number of blades is not generally one of them.

Adding blades does help deal with the problem of inadequate diameter. However, if you also go to a flatter pitch, it will probably put less load on the motor, causing a reduction in amps, and therefore a decrease in power. A 6" 3-way with the same pitch as the 2-way it's replacing might help, but I have my doubts about a 3-way with reduced pitch.

Props have induced losses (the by-product of making thrust, analogous to induced drag of a wing), and profile losses. Adding blades hurts the profile losses (more "stuff" being waved through the air), but helps the induced losses. In the vast majority of model applications the profile losses dominate, so the fewest practical number of blades (two) works the best. However, if you are trying to absorb too much power in too little diameter, the induced losses gain in importance, and this MIGHT favor three or more blades.

As far as the detrimental effects of interaction with the fuselage, both a 2-way and a 3-way are going to suffer from that, probably about equally.
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Old Nov 06, 2012, 05:19 AM
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Sorry to interrupt with such an off-topic subject. But whilst we're on the subject of propellers, I'm planning to build a scale-ish model of the 1925 Dornier Wal. For the sake of scale, I need 4 blades. With a 6' wingspan I should have room for 6" blades. She doesn't need to fly fast, of course, so I favour 3S1P batteries with 1100Kv motors. Now, since the aft prop will be operating in accelerated air, perhaps it could use a higher pitch. Perhaps 6 x 5" for the tractor and 6 x 6" for the pusher. How does that sound?

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Old Nov 06, 2012, 06:46 AM
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The aft prop does need to have a little more pitch than the front one. The exact amount depends on a number of factors, but you're probably not too far off.

You can also reduce the diameter of the aft prop. As the air going through the slipstream accelerates, the diameter of the "stream tube" also necks down, so that the "mass flow" along the stream tube stays constant. If you look at any cross-section along the stream tube, the cross-sectional area times the air velocity at that point must be the same as at all the other cross-sections along the stream tube.

Based on that, if your 5" fwd to 6" aft pitch ratio is correct, then the aft prop should also be about 5.5" diameter, not 6".

A while back, I did some consulting for a 24' scale model of Rutan's "Voyager" round-the-world airplane. He originally had identical 8" pitch props on both ends. The plane would not take off, and the aft motor was pulling almost no watts, just windmilling in the breeze from the forward prop. Switching to a 10" pitch prop on the aft motor fixed the problem.
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Old Nov 06, 2012, 08:16 AM
What could possibly go wrong?
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Thanks Don.

Any size of a model of the Voyager must be an ambitious project, especially 24'. Was it a success eventually? Getting one to fly at all would be a thrill, but after that I imagine the objective would be to make sure that nothing very exciting happened at all.

Dornier went on to develop the system of push-pull twin props. The Dornier Pfiel was the fastest combustion engine plane of its time, I think, possibly of all time. 474 mph, how does that compare?

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Old Nov 06, 2012, 09:07 AM
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http://en.wikipedia.org/wiki/Fastest...riven_aircraft

Fastest propeller driven aircraft is the Tupolev Tu-114 at 540 mph. Four 15,000 HP Kuznetsov turboprops, each driving a pair of 18 ft 4-way contra-rotating props. With that much power, even an 18 ft. prop has a high disk loading, so lots of blades and contra-rotation are both justified.

Fastest piston-engined aircraft is "Rare Bear", a highly modified F8F Bearcat, at 528 MPH.

The Do335 was very fast for its time (fastest prop-driven fighter during WWII), but that was a long time ago.
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Old Nov 06, 2012, 06:31 PM
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I've got 3-bladers on 3 of my planes and they work fine. What I have read is that yes, 2-blade props ARE more efficient, but not radically so. If you can't get a 2-blade prop on the plane for space restrictions, (often the case in a pusher application), or if you just want more of a scale look, the 3-blade props work good. I buy mine from Master Airscrew.

The rule of thumb on 3-blade props is that they are either the same as a 2-blade with the same pitch and 1" larger diameter, or else the same as a 2-blade with the SAME diameter but with 1" more pitch. Don't know if that's true...that's just what the pros at Master Airscrew say.
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Old Nov 06, 2012, 08:42 PM
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Gross oversimplification, to the point of being not true in at least a significant number of cases.
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Old Nov 07, 2012, 07:07 AM
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Nick, if I was building the Val, I would not worry much about rear props running in the turbulent air. As you said, the model will be flying at relatively low speed, so I wouldn’t worry extracting every ounce of thrust from the rear motor. I the rear, I could always install higher RPM motor if I need to. I never had to.
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Old Nov 07, 2012, 09:17 AM
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It's not the turbulence, in fact the slipstream behind a typical prop is mostly laminar flow. There are thin helical sheets of turbulent flow representing the wakes of the individual blades, but the large regions in between are still smooth.

The problem is that a prop makes thrust by speeding up the air going through it. A second prop positioned directly behind the first one will therefore see a higher airspeed in the inflow. How much faster depends on the details, but the worst case will be at low airspeeds and high power settings, such as takeoff and climb. It might be in the ballpark of as much as 50-70% faster in some cases, only 10-20% faster in others.

This increase in inflow reduces the angle of attack on the blades of the aft prop, reducing the torque load it applies to the motor, which means that motor does not need to pull as many amps to reach its volts times Kv RPM. This means the aft prop is not drawing as many watts, and so in effect the aft motor is operating at reduced throttle. If you have enough power in the forward motor to fly the plane without much help from the aft motor, then your plane might still fly OK, although climb rate in particular will be reduced (nothing else responds more strongly to even small changes in total power than climb rate does). However, in a plane with marginal surplus power to begin with (such as the Voyager model I helped with), it could mean the difference between flying and not flying.
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Old Nov 07, 2012, 10:52 AM
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Gross oversimplification, to the point of being not true in at least a significant number of cases.
Then don't use them Don. I'm just saying they work fine.
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Old Nov 07, 2012, 11:06 AM
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Sorry, can't resist...

Whatever the principles behind the DO-X, fly it apparently did. Efficiency - not going to argue that it wasn't likely to be brilliant.

Noise - don't want to think about that My first 60-odd flying hours were done in Vickers Varsity aircraft - a close cousin to the Wellington twin engined bomber of WW2. Even with headsets and a helmet, the noise of two big radials and four bladed props was, well, very loud, to be nice. We only flew for around four hours too.

Sitting in the DOX for any length of time must have been pretty deafening.

At least our electric models would not sound so bad

And now back to discussing the merits of having the tailplane at the other end...
(does that make it a 'headplane'? )
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Old Nov 07, 2012, 11:11 AM
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Just to prove I do a little more 'canardly' than read this thread avidly.

It's eyeballed off the 'Flash', around 30" span on the horizontal flying bit at the back, and aimed at mostly flying around in circles, right side up, on nice days.

D
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Old Nov 07, 2012, 11:35 AM
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Then don't use them Don. I'm just saying they work fine.
The point is that as a "rule of thumb", that technique will NOT get you an equivalent match in the vast majority of cases, especially if you change the pitch. It also ignores some important factors, such as total blade area. And, no, it will not always "work fine". It just happened to seem to work fine in the particular cases you tried. You got lucky, something to be grateful for when it happens, but not something you want to count on as a reliable design tool.

Of course if the original 2-way prop was not sized properly in the first place, then trying to match it with a truly equivalent 3-way really doesn't matter much, either.

If using those methods does result in a plane that flies reasonably well, it just means that you've re-proven the old adage "Anything will fly if you put a big enough motor in it." The Saturn 5 had a terrible L/D, but it still managed (through absolute brute force) to put a man on the moon.

OTOH, if you take the time to optimize the complete prop/motor/battery system, you will get more performance for less weight and cost. Good things come to those who do their homework.
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