ENGINE THRUST LINE

- by Clay Ramskill

Last month we talked a bit about the effects your planes prop generates - except for the thrust. This month we'll feature the thrust itself.

For all practical purposes, (excluding those prop effects) we can consider that the engines thrust acts along the centerline of the engines crankshaft. The plans for your plane show whether or not that coincides with the centerline of the plane itself - it may or may not, depending on the plane and how well the designer did his job.

From the prop effects story, you know that all of our planes, to some degree, want to turn to the left when slow, at high power, and at a high angle of attack. This is not always a good thing; not all pilots have the experience to correct with just the right amount of right rudder at the right time. Let's face it - we'd rather the darn thing wanted to go straight ALL the time. And we can do that, by using some right thrust on the engine.

By shifting the engine so it points a bit to the right, the engine will tend to pull the nose of the plane to the right, turning the plane. And that turning tendency will be proportional to the amount of power applied, as is the prop effect tendency for the plane to turn left. So by putting in a degree or two of right thrust, the prop effects may be pretty much cancelled out. The only down side to this procedure is that the right thrust will be left thrust when the plane is inverted, ADDING to the prop effect.

The thrust line may also be shifted to help out with pitch (up and down) problems. Flat bottom wing trainers are notorious for wanting to go up with full power - we trim them for loping about at 1/2 power, and when we goose the engine, up goes the plane. To a large degree, this tendency can be cured with DOWN thrust; with the thrust line pointed down a bit, the more power we apply, the more the nose wants to go down, counteracting the tendency for the plane to go up at the higher speed. The plans for my Seniorita include 6 degrees of downthrust, which is a LOT - but that contributes to the fine flying qualities of that design.

Shifting the thrust line is relatively simple - you can just stick a washer or two behind the engine mount, or you can buy plates with the angle manufactured in. Better yet, build it into the plane; the Goldberg Extra 300 kit has allowed for 3-4 degrees right thrust, shifting the rear of the engine mount to the left so that the spinner will line up properly.

So how do we know if changing the thrust line will give us a better flying plane? If your plane has a vicious tendency to go left when its slow and you add full power, you would definitely be better off with some right thrust. The same applies if the nose always drops in left turns and comes up in right turns. Experimentation will tell you how much for the plane involved and the way you fly. And if your plane always wants to climb at full power, or if when you trim hands off at full power, then chop the throttle, the plane dives, some down thrust will help.

The above techniques are a part of the trimming process. Don't be afraid to make changes if your plane doesn't fly the way it should; and thrust line changes are relatively easy to do, and can always be changed back if the results are not what you want.

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Last Update: 10/12/97