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Old Oct 23, 2013, 06:31 PM
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2 blade vs 4 blade prop - More braking?

A friend just built a Troybuilt Mustang. He's wondering whether a four blade prop would cause more drag at idle than a two blade, thus slowing the plane down a bit for landing.

Opinions?
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Old Oct 23, 2013, 10:01 PM
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Flaps !
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Old Oct 24, 2013, 10:02 AM
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He has flaps but is afraid to deploy them completely after flying the P51 on Real Flight because of how much the plane dipped down and became hard to control (his R/C Corsair does the same thing). On Real Flight, it seems that 1/4 flaps slows the plane a bit but remains controllable. How realistic are the flaps on the P51 in Real Flight?
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Old Oct 25, 2013, 12:34 AM
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Quote:
Originally Posted by IatricSB View Post
A friend just built a Troybuilt Mustang. He's wondering whether a four blade prop would cause more drag at idle than a two blade, thus slowing the plane down a bit for landing.
Very likely, but depending on your actual idle speed. We really see this with the electric planes, where we can turn the motor off completely.

A spinning (windmilling) prop creates more drag (slows the plane) than having the prop stop in one position. Most Electronic Speed Controls (ESC's) used for throttle control in electrics have a "brake" option that can be used to stop the prop from spinning to avoid this added drag.

Multi-bladed props are usually used to increase or maintain thrust while reducing the diameter of the prop, so with glow power the amount of drag will depend on your actual idle speed, is the idle low enough that the blades are no longer producing any thrust? Or low enough to reduce the thrust and provide a braking action?
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Old Oct 27, 2013, 09:31 AM
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Yes, the idle speed is enough to help with braking. He's just debating whether the additional cost of the 4 blade prop would help with a short runway (600 ft). We're thinking the optimal set up for slowing it down and still maintaining control will be 1/4 flaps and a 4 blade prop, if Real Flight's Mustang is a good indication of how this one will fly.
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Old Nov 21, 2013, 09:58 PM
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Hi,

Just happened to be passing by this thread, so please excuse me butting in.

I would strongly suspect that a two blade prop would give the better braking assuming equal pitch on both. Same reason that a two blade gives better acceleration given equal pitch but larger diameter to match the horsepower available.

Most people who have problems with flaps deploy them at too high a speed.

The idea is to slow down, then deploy flaps, them slow down some more.

You can't use them to slow down from cruise speed without getting wild pitching.

They aren't air brakes, you don't use them to slow down, you use them once you have slowed down to give a steeper approach without speeding up, and a shorter landing.

Try full flap at altitude, you may find a flap-elevator mix helps if there is a strong pitch.

HTH

Dave H
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Old Aug 03, 2014, 09:31 PM
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All the planes I have flown with flaps make the plane balloon up not pitch down. The fastest plane would only have 1 prop. He just needs to learn to fly up higher and test out the flaps at lower speeds. Full scale planes had to use multi blade props to help absorb the power to make use of as ground clearance wasn't going to allow a 2 blade prop big enough to get the most out of the engine.

Yes 4 blades would help brake better than two as a static blade becomes a wall. 4 blades have more surface area for that.
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