Thrust angle question

• posted

I am rebuilding a crashed park flyer with my own designs, and would like to understand the 'thrust angle' better.. the WHY of it.

Vertical Thrust angle:

Attach an engine to any object, if the thrust line passes through the C of G, the object is dragged in a direction. But if the line passes over it or under it, the object is also rolled forward or backward in the direction of the pull. So I take it the thrust angle is to get the thrust line to pass the CoG (as viewed from the front or side of the airplane). The engine in this case will point down only if it is located closer to the fuselage than the CoG. If it is higher than the CoG from the fuselage, it should point upward so the thrust line passes through the CoG.

Secondly it regards the AoA of the main wing. There will always be a positive AoA on the main wing (in straight flight). Which means that either the fuselage will point slightly higher and the engine should be mounted 'lower' so in a straight flight it is level. Or the wing itself will be mounted on the fuselage with a slight (sufficient) AoA so the engine can be mounted parallel to the fuse.

Third, is the drag of the airplane, and the thrust line should ideally pass through the middle of the drag face. View a park flyer from the front, and the main wing's thickness is the biggest 'face' of the airplane. Everything else is relatively thin. Which means the center of the drag 'face' is slightly higher. Again mounting the engine higher and yet parallel should be equal to lower and pointed down. This one should be taken with a grain of salt since the drag face changes with speed as the AoA on the whole aircraft changes.

Of these three issues, the CoG is the most significant. Do I have it right?

Horizontal Thrust angle:

I dont get this one. Other threads have said the prop coming down is more efficient than going up, which is why the thrust angle is 2 degrees to the right. But in a level flight if the motor is pointed into the direction of motion, why would the prop going down be any different than when its going up? I'm sure its not gravity. If the angle of the motor is slightly different from the air velocity, shouldnt the motor be adjusted so it is in a theoretically level flight horizontal; so that the prop is efficient and no sideways adjustment is required? I cant come up with any reason why a prop going up or down would be any less efficient.

• posted

Er. I think you've missed the boat by a bit.

Your observations on thrust vs. CG are more or less correct in the first paragraph, but then you start to diverge.

The point of pointing the engine is to make the plane stay more or less trimmed as the power is changed. Down thrust (on a front-engine plane) pulls the nose down more with more throttle, which tends to keep the plane from ballooning with throttle. Right thrust (on a front-engine plane with dihedral) pulls the nose to the right, which makes the plane roll ever so slightly to counteract torque. It is true that engines with high thrust lines tend to need less down thrust, this has much more to do with drag than with CG.

R/C pattern planes need to have the same handling characteristics whether they are right side up, bottom side up, top side up or left side up. They are usually set up for zero down- and right-thrust.

• posted

So that has more to do with drag than C of G (drag is high but theres no acceleration at higher speeds, so no difference in CofG's effects). Clears things up for me.

That is all I need to understand. I think I'll add that to 'thrust angle' in wikipedia.

Does this mean the angle is to the left on low-wing aircraft, and theres no angle in a highly symmetrical thru-fuselage wing aircraft? I take it the aeleron trim is more useful to counter the prop of a cap-232 than the thrust angle

Thanks again

• posted

A couple more thoughts:

The propeller slipstream spirals back and hits the vertical fin, yawing the airplane left.

At high AOA, the downward-moving blade is also forward-moving, while the upward-moving blade is also rearward-moving - sort of like a helicopter rotor. The downward/forward-moving blade has higher airspeed and greater thrust, yawing the airplane left.

• posted

No, the right thrust is a function of the direction that the motor turns, not whether the wing is high or low or in between. Note that as soon as you roll inverted right thrust becomes left thrust, and up thrust becomes down thrust -- this is why a stunt plane sets the engine to 0-0.

• posted

Thrust relates to the C of G only during acceleration. Thrust lines are determined by drag centers, not CG. At any steady speed the thrust can't tell where the CG is. Angling an engine down reduces the left-yawing tendency caused by the higher AOA of the downgoing blade when an airplane is at higher AOAs. And it's the AOA of that blade, not its airspeed, that causes the yaw. Pitch changes during throttle changes are due to prop slipstream over the horizontal stabilizer. A noseheavy airplane will have more down-angle on the stab to counteract the noseheaviness and will pitch more when the throttle setting is changed. Balancing the airplane properly minimizes this.

Dan

• posted

It is actually WAYmore complex than that. Aerodynamic drag and lift characteristics will have far greater impact on thrust angles.

• posted

That's what the FAA says too, it's the AOA of the blade. They even publish a drawing of a pitched-up airplane to illustrate it, with erroneous dotted lines and shaded areas that show the downgoing blade still going straight down. As if the crankshaft didn't pitch up with the rest of the airplane.

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