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Engine thrust Angles

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Old 11-27-2004, 11:00 AM
  #1
Wizard61
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Default Engine thrust Angles

I am hoping someone can help me with a question I have. I have noticed on smaller (60 Sized) plans the use or incorporation of side thrust and down thrust when mounting the engine. I also have a few larger scale plans (80+ inches) that the designers do not incorporate any of either. As the model aircraft get larger, is the need for side and down thrust lessened, or are the designers of the larger scale aircraft more interested in scale appearence than flight performance? I also wonder if the larger aircraft do not need these adjustments because usually the engines for those aircraft turn at a lower RPM.

Thanks for the help.

James Andrews
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Old 11-27-2004, 09:44 PM
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Default RE: Engine thrust Angles

Just a guess but are the bigger plans you have checked of aerobatic planes with symmetrical airfoils, mid mounted wings with no dihedral - while the smaller ones are more likely to be high wing trainers? I think that could account for this but that's a guess.
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Old 11-27-2004, 11:07 PM
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Wizard61
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Default RE: Engine thrust Angles

Hello Alan thanks for the reply.

Specifically, the plans I am looking at are all scale aircraft. The 60 sized are mostly Royal/Marutaka, and Top Flite. The larger sized I am looking at are from Dave Platt and Don Smith. I am just curious if the offsets are necessary in larger models.

James
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Old 12-06-2004, 06:04 PM
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Default RE: Engine thrust Angles

James:

The incorporation of side thrust and down thrust has nothing to do with the size of the airplane.

I don't know of any full size airplane that has side thrust, and I do know that some full size airplanes have up thrust as well as downthrust, but most do not. There are trim tabs in the cockpit that the pilot uses so that he doesn't have to hold control stick forces for steady state flight at different speeds, such as changing from cruise to approach speeds.

The flight handling of model airplanes is different from full size airplanes since you are flying "looking outside in" as opposed to being in the cockpit "looking inside out", so model designers make greater use of thrust angles for making flying easier; especially on high wing trainers to minimize trim changes when flight conditions change. On aerobatic models, you are constantly doing maneuvering flight and 0 - 0 thrust alignment is necessary to maintain flight symetry in pattern maneuvers.

Hope this answers your questions.

feihu
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Old 12-06-2004, 07:25 PM
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Default RE: Engine thrust Angles

In many full sized aircraft the downthrust is disguising itself as lots of wing incidence angle. A high positive incidence angle will ensure the airplane flies with the fuselage reference line nose down and that has the effect of introducing downthrust in normal cruise flight.

An example where the downthrust is VERY obvious can be seen in this T28 drawing from another thread in Aerodynmics...



I also remember that the Grumman Bearcat uses lots of obvious downthrust as does the Hellcat.

So you need to look at the overall thrust angle, wing incidence and tail incidence and know what angle of attack the wing will fly at in cruise to say if there is any actual downthrust or not. Since some full sized craft set the engine at 0 and the wing and tail at high positive angles the scale plans may follow that practice.
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Old 12-06-2004, 11:59 PM
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Default RE: Engine thrust Angles

Actually, down thrust and right thrust are both used in model aircraft pattern designs. Both are used to keep the flight at 0-0 during maneuvers. The propwash and torque effect will make a difference from maneuver to maneuver. The idea is to optimize the design so that trim deflection is minimized. Right thrust compensates for engine torque, and is dependent on the engine, prop, fuselage side area and rudder. Down thrust may be needed to minimize trim changes with power changes. This depends on CG and engine thrust lines relative to the wing and stab centerlines, and their incidence. This was true for pattern planes of the 70's and 80's like the Dirty Birdy and UFO. Whether it's still true for modern pattern, I don't know for sure.
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Old 12-07-2004, 12:03 AM
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Default RE: Engine thrust Angles

If I may add a point. Since most scale models are flown with overpowered 2 stroke engine , the torque from these engines pull the model hard to the left. Especially the smaller ones since their (involuntary) movements are faster.

The model designs that put the engine offset from zero are normally trying to minimize the challenge of take off torque as well as to minimize level flight problems. My 50" BF 109 has a visible left thrust built into it. However my 80" BF 109 has some, but difficult to detect.

As Dave Platt once said "large models fly, small models flit".

Smaller true scale models need all the forgiving qualities they can get.

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Old 12-07-2004, 12:09 AM
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Default RE: Engine thrust Angles

Also you may notice some Verticle stabs are offset to ease in flight control. Bf 109, Corsair ect.

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Old 12-07-2004, 06:11 AM
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Default RE: Engine thrust Angles

Everybody thanks alot. I appreciate the info.

Regards,

James Andrews
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Old 12-07-2004, 11:38 AM
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Default RE: Engine thrust Angles

To me right engine thrust would seem a better solution than offsetting the vertical stab like i have seen on some aircraft too, since the engine torque, p-factor and the corkscrew slipstream created by the prop acting on the fuse are all the forces trying to yaw the plane to the left which is directly related to how much power the engine is producing at the time.
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