down and right motor

[Beginner!?]

hi there

please does anyone know why some of people use from down and the right thrust for their models?

what the right thrust means really?does it means that the motor must be acock at the right or must be abit at the right side of the model?

thanks for any input

[vintage1]

The answers.

To get them to fly straight and at a constant glide /climb on and off power, both.

Right thrust is the angle of te thriust line to the models center line.

Right thrust only works if there is some dihedral.

[Beginner!?]

:confused: :confused: :confused: :confused:

[HarryC]

Right thrust only works if there is some dihedral.
Right thrust is not related to dihedral, it opposes yaw on the fin caused by spiral flow.

The airflow behind a propeller does not go straight back, due to drag it gets moved slightly around with the prop so that it flows in a very slight spiral down the plane. When it flows over the vertical fin then it does not hit it edge on but at a very slight angle, giving the fin an angle of attack. That produces a force which causes the tail of the plane to yaw out a bit and the plane will then try to skid slowly around a turn. The amount of force (sideways lift) is related to the speed of the flow which in turn is related to the throttle setting, so the more you open the throtle the more it tries to yaw. By pointing the motor slightly off to the side it will produce a thrust that also yaws the plane in the opposite direction, helping to balance the yaw from the fin. As you apply more power the prop thrust and thus the yaw force increases, just as the yaw force from the fin increases, so the idea is the two will remain roughly in balance and you keep going straight. Without it, the plane will yaw left on full power and yaw right at idle, assuming you have trimed the rudder for cruise power and speed. In reality, side thrust only helps reduce the yawing, it does not stop it. The reason is that the yaw force produced by side thrust is related only to the throttle setting, but the yaw force produced by the fin is related to throttle setting and airspeed. As airspeed is increased, it's straight flow straightens out the spiral so less fin yaw is produced, but fly more slowly at the same throttle setting such as climbing steeply and the spiral flow is more dominant so the yaw is stronger and the plane will yaw off to the left whereas at the same throttle in level flight it went straight. Thus sidethrust is not a cure, simply a reducer of the yaw caused by changing throttle. Note that the direction of yaw from the fin is determined by the direction of rotation of the propeller, if the prop goes the other way the spiral will be in the opposite direction. The direction that all model i/c engines turn will give a left yaw from the fin so the engine must be angled slightly to the right. Full size planes tend to have engines that rotate the same way but not always, one of the most famous examples was the Spitfire when it changed from the Merlin to the Griffon engine which rotated the opposite way and quite a few experienced Spitfire pilot who were used to pushing in a boot of right rudder when opening up the power to take off, just did that by habit on their first Griffon take off thus helping rather than countering the Griffon's right yaw leading to a most embarrassing ground loop. The yaw from the fin would be balanced if the fin had equal area and span above and below the fuselage for then one would yaw left and the other would yaw right and there would be no need for right thrust and no change in yaw as power or speed changed. It follows then that yaw is most pronounced on planes whose fin is all on top of the fuselage such as a Chipmunk, and least where some fin goes as low as possible such as aerobatic planes.

Ignoring fly-by-wire fighters that are unstable, a normal stable plane is nose heavy and needs the tail to make a small downforce to balance it. Even zero rigged models still produce a small tail down force because the tail's airflow is affected by downwash from the wing. The force generated by the tail, like the fin, is related to the airspeed over it so it is heavily influenced by the propwash. Assume the plane is trimmed to fly level at medium power. If you open the throttle the propwash over the tail increases immediately so it makes more down force and pitches the plane up. It wants to make the plane climb to slow it down until the flow is slow enough again that equilibrium is reached. If you throttle back the propwash over the tail immediately slows down so it makes less downforce and the nose drops and the plane wants to speed up until the tail force is restored. This is how planes tend to be speed stable - the elevator trim is a speed trim. Very stable beginner's planes will be most nose heavy so the tail will be creating large balancing forces and will react vigorously to changes in the speed of flow over them from the prop. Some trainers pitch up and down far too much as the throttle is changed. To help counter that we angle the motor downwards so that it produces a nose down force whose magnitude is related to the throttle setting, so as you power up and the tail tries to pitch the nose strongly up, the increased down thrust is pulling the nose down a bit to counter the tail. Planes that are not so stable such as aerobatic models (which are still stable, just less so) have only a little bit of nose heavy balance thus the tail down force is small and is much less reactive to changes in airspeed over it. Ultimately, all stable planes will climb if power is applied, as the plane speeds up more lift is produced so it starts to rise, that causes the tailplane's AoA to reduce thus pitching the plane slightly nose up, and the opposite is the case if power is reduced. Downthrust can not be used to counter that because downthrust force changes immediately with the throttle whereas changes in the plane's overall speed can take some time. So there are two separate pitching effects when the power is changed - the immediate effect from the change in airspeed of the propwash over the tail, and the delayed effect of the change in lift caused by a change in overall airspeed.

H

[vintage1]

Not necessarily.

Torque moment can roll the plane left and the yaw moment due to sliptsream is not necessarily turning the model left.

The yaw moment induced by right thrust will not translate into a rolling moment without dihedral.

Likewise the yaw induced by the spiral sliptream will not translate into a bank with no dihedral either.

[HarryC]

Banking is a secondary effects of yaw. A plane that yaws will turn, albeait slowly and out of balance regardless of whether it banks or not. Dihedral is irrelevant to the yaw induced by the spiral flow, the yaw will exist regardless of any dihedral. If the plane does bank as a secondary effect of yaw the side thrust will have no effect on the bank.
Roll as an effect of engine torque is extremely rare due to the aerodynamic damping of the wings and their great leverage. Torque rolling is limited to aircraft with enormous power at low airspeeds. Trust me - I have flown the full size P51 Mustang with a 1500hp Merlin engine and there is no change to lateral trim with changes in engine power (unless you are daft enough to slam open the throttle near stall speed), but there is a considerable change in rudder trim all through the flight envelope. If you are ever over in Gloucestershire give me a call and come up in a full size plane with me, I can demonstrate the marked yawing as power is changed, the plane constantly changes heading around a very large circle yet there is no bank despite the generous dihedral.

[Treetop]

OK, how about some easy answers,

Downthrust, keeps the plane from trying to loop continually.

Sidethrust, keeps the plane from turning continually.

So on my MPX Pico Cub, looks like downthrust is built in, should I have a bout 3 degrees of right thrust to offset the torque roll, or whatever it is called?

I had a question once that was moved to this forum and became befuddled. Althought the answers are correct and good in many cases, these techno talk wars that begin forget the status and the question of the newbie asking it in many cases. Thanks, tt

[Beginner!?]

i think i am confused abit because i couldnt understand all posts here

i think at some of models the angles are measured based of the thrust line,if so,how this is posible if there is a down thrust???

you dont think the up thrust is better than down thrust!?because the plane will go up with that,or there is anything that i dont know?

how i can see which angle is good for a down thrust?

[himeros]

I think they are trying to tell you what is happing to the plane without off setting the motor. What you need to know is how much down and right angle you need to put in your plane. Some think 2 or 3 degree down and right will work, measured from the center line of the airplane. Most models already have this built into the plans. Good luck.

Himeros :)