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Old Mar 08, 2013, 09:31 AM
Tim
United States, AR, Fayetteville
Joined Sep 2012
47 Posts
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How does a "raised prop" affect flight dynamics?

Not sure if "raised prop" is the correct term. An example of what I mean is illustrated here: http://www.beginnerrcairplanes.com/p..._rc_planes.jpg

I assume the advantage of raising the prop up is to make landings easier/safer. But I don't have a clue as to what that does to flight dynamics. It SEEMS like it would pitch the plane forward, since the force is now being applied away from the cg.
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Old Mar 08, 2013, 11:35 AM
agnotology
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Joined Jan 2007
3,652 Posts
A high-thrust line can certainly add some interesting dynamics to an airplane. As you noted, having a thrust line that passes above the CG would have a nose down moment at high throttle settings that would disappear or reverse (prop drag) at throttle off. All the trim settings will also be speed dependent, since the trim required to counteract the thrust will be less at high speed than low speed.

The model you linked to actually has a big angle on the the thrust line, so despite the high motor position, it looks like the thrust line passes through the CG. That should eliminate a lot of the thrust effects.

Kevin
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Old Mar 08, 2013, 11:51 AM
B for Bruce
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The 'Wack, BC, Canada
Joined Oct 2002
11,528 Posts
That particular design has a crazy angle to the thrust line. I suspect it was done as much to aid in using a bigger prop as it was to alter how it flies.

Generally the designs that use highly offset thrust lines tend to not be all that overpowered. As such they can get away with considerably less thrust angle to compensate for the distance of the thrust line from the true 3 dimensional center of mass. Otherwise you are correct and locating the thrust line a considerable distance above or below the model's true CoG does result in considerable pitching force. But USUALLY 5 to 10 degrees of angle to counteract this is enough.
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Old Mar 08, 2013, 11:59 AM
Tim
United States, AR, Fayetteville
Joined Sep 2012
47 Posts
Thanks for the replies guys - very educational! :-)
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Old Mar 08, 2013, 12:02 PM
Ascended Master
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Palmdale, CA
Joined Oct 2000
13,499 Posts
With a more rational mounting of the motor, the higher thrust line results in a nose-down moment when the plane is tossed at full power.. The Easy Star has the motor mounted properly, and with a more powerful motor than stock can be launched at less than full power, and go straight out instead of dive.
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Old Mar 08, 2013, 12:44 PM
agnotology
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Joined Jan 2007
3,652 Posts
I flew a similar Cox "Sky Cruiser" for someone last year, and made the mistake of trying to launch it with the mighty motor at full wail. It nose dived into the ground from the combination of the high thrust line and low airspeed. Fortunately it did prove to be virtually unbreakable.

http://www.coxmodels.com/airplanes/c...ser/index.html

I had to launch it at part throttle, with a slow increase until the airspeed increased and the trim and controls became effective.

I would think the highly angled thrust line of the other one above would avoid that, and make it easier to launch with more power on. I really doubt they did it for prop clearance.

Kevin
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Old Mar 08, 2013, 03:12 PM
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Joined Aug 2003
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All my models except my first have been high mounted pushers. I was told to try to point the thrust through the CG when held by the nose and I think I have managed that, but I've only done it by TLAR. What I end up with is it slightly angled. The proof is how it flies and so far so good. It climbs with power and doesnt do anything dramatic when power is adjusted or cut. I'd be looking to change something if I was constantly fighting it.
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Old Mar 09, 2013, 09:22 AM
Registered User
Wilson, NC
Joined Aug 2009
224 Posts
I had a foam flying wing, 47" WS, with landing gear, that had the prop thrust line parallel with the flight line, and well above the vertical CG. The plane was very sensitive in pitch; I could never relax with it. Eventually it was totaled. I have redesigned it to lower the thrust line so it goes thru the vertical location of the CG. It should be noticibly faster and much more stable in pitch.
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Old Mar 09, 2013, 11:58 AM
Registered User
Blacksburg, VA 24060 USA
Joined Feb 2000
3,216 Posts
I have a high-thrustline scratch-built model, with motor mounted on a pylon in front of the wing. It takes forever to roll off the ground, though it shows a good bit of speed once it's in the air.

I also have a Dynam Hawk Sky, which has the motor mounted on a pylon behind the wing. It takes careful throttle management before take-off.

For the reasons explained, I don't find either airplane comfortable to fly.

Jim R.
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Old Mar 10, 2013, 12:28 PM
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United States, UT, Salt Lake City
Joined Oct 2007
7,169 Posts
Just like airfoil and aspect ratios etc.,
thrustlines are the result of compromises that have to be made .
perfect or ideal (calculated) placements /shapes etc.,are just good starting points - in most cases .
A friend flies hard core aerobatics for fun.
His latest ride just about got him the other day- It winds up in spins My advice was to sell it The accumulation of compromises which make for wild aerobatics, also made for a craft where stability is elusive
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