Setting The Thrust Line
Most models require the engine to be set at an angle relative to the fuselage centreline and is typically 2½° right thrust and 1° down thrust. The engine mounting will also be offset to the right when viewed from the front. For a clearer understanding of thrust lines, see our Thrust Angles section. Setting the thrust line correctly is probably the single most effective thing you can do to improve the handling of your model. It is also easy to test fly for and easy to correct - a real "no-brainer".
Simply put, right thrust is required to compensate for the spiralling airflow from the propellor hitting the tailfin while down thrust stops the model climbing when the throttle is increased. (Unlike a trainer, where more throttle equals more power equals more height, the aim now is for more throttle to equal more power. With today's high drag or "constant speed" models, a considerable increase in power only gives a modest increase in airspeed.)
To get the thrust line set properly, you need to eliminate pitching and yawing with throttle use.
It is essential that you have decided on the engine/propellor combination that you will use and that the engine is "on song" and well run in. It is also essential that the model is trimmed to neutral. If this isn't the case, you'll be wasting your time.
Establishing The Side Thrust.
Fly your model straight into wind at full throttle and pull up into a vertical climb. The theory here is that as the airspeed bleeds off, a continued vertical climb is now fully dependent on engine power alone. Do not use rudder (or rudder trim) to correct for any deviation from the vertical. Your model will do one of three things.
- If the model continues to climb vertically, the side thrust is correct.
- If the model gradually, then more noticeably, veers to the right, the model has too much right thrust.
- If the model gradually, then more noticeably, veers to the left, the model has too little right thrust.
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This test should then be repeated but this time start by heading downwind. If you're happy that you know what you have to do for side thrust, you can go straight on to check the down thrust in the same flight. Repeat this process a few times to ensure consistent results.
Establishing The Down Thrust.
Fly your model straight into wind at full throttle and horizontally. Abruptly close the throttle and keep it closed for 2 or 3 seconds. Open the throttle fully again. Do not use the elevator (or elevator trim) to correct for any variation in height - unless of course it's screaming straight for the ground! The model will do one of three things.
- When the throttle is closed, the model slowly arcs nose down (as the airspeed drops) and when the throttle is re-opened, the model should continue towards the ground in a straight line. Recover (Duh!) and congratulate yourself that the down thrust is correct.
- If when the throttle is closed, the model arcs nose up and when the throttle is re-opened, the model goes nose down, you have too much down thrust.
- If when the throttle is closed, the model arcs nose down and when the throttle is re-opened, the model goes nose up, you have too little down thrust.
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Repeat this test a few times to make sure you're getting consistent results.
Changing The Thrustline.
Whoopee! My favourite - an engine out job... This can usually be done at the flying field.
Do not attempt to change the thrustline by sticking things under the engine mounting lugs. The correct way is to alter the angle between the rear face of the engine mount and the front face of the firewall. There are several ways of doing this.
Perhaps the easiest way is to place a washer or washers on the bolts that hold the engine mount to the firewall between the mount and the firewall. Most mounts have four bolts holding them to the firewall so a few minutes thought and you should be able to work out what goes where.
- To increase down thrust, add washers to the top of the engine mount.
- To decrease down thrust, add washers to the bottom of the engine mount.
- To increase right thrust, add washers to the left of the engine mount (when viewed from above).
- To decrease right thrust, add washers to the right of the engine mount (when viewed from above).
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Down thrust and side thrust can be changed together.
Check that there is still adequate clearance around the engine and exhaust then ensure everything has been re-tightened correctly and try re-testing. There should be a marked improvement. If it's not perfect, it's engine out time again! Keep repeating the whole process until the model successfully and consistently "passes" the thrust line tests.
Once you are happy with the thrust line settings, you can make the installation permanent by machining the rear of the engine mount to the appropriate angle. If you have had to add a considerable amount of side or down thrust, you will probably have to move the position of the engine mount to bring the propellor/spinner back onto the fuselage centreline
Remember, if you change the propellor to a diffent type or size or change the engine, you will almost certainly have to alter the thrust line again.