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## Counteracting the slipstream effect

• Posts: 61

### HPOUND created the topic: Counteracting the slipstream effect

Hello experts. I've got a query re Aero / Revision Questions Section 4 / Question 5:
The designer can eliminate slipstream effect by:
(a) off-setting the engine thrust to the left
(b) biasing the rudder to the right
(c) biasing the rudder to the left
(d) off-setting the fin to the right

Now (a) and (c) are unambiguously wrong if we assume the prop is spinning clockwise as viewed by the pilot. The answer solution says (b), and I agree this is a valid option, but I'm wondering why (d) is wrong. Is it because we assume an off-set to the fin would involve moving the leading edge of the fin to the right, thus encouraging a left yaw?

When I read this distractor, I interpreted moving the aft edge of the fin to the right, thus producing a right yaw. This is the only explanation that makes sense if (d) is to be correct, and perhaps option (d) is wrong because we don't know what effect the offset will have, unless we know if it's offset from the front or back edge.

Thanks.

• Posts: 2297

### bobtait replied the topic: Counteracting the slipstream effect

That's an interesting interpretation, but if you were holding a model aircraft with a moveable fin and someone told you to off-set that fin to the right, would you not move the leading edge to the right? That was the intention of the text in option (d). I do see your point though. In the case of the rudder, because it is hinged at its leading edge, movement left or right has to refer to the trailing edge.

Bob

• Posts: 61

### HPOUND replied the topic: Counteracting the slipstream effect

Thanks! I'm at that stage of my revision where I'm going into every little thing, so I appreciate the clarification.

I pulled out my model airplane and pretended to bend the fin. My inclination was to grab it from the back. Then I asked my (non-pilot) wife to do the same (she had not seen me do the first trial), and she also grabbed from the back. I concede others may well grab from the front. Of course, grabbing from the back means grabbing the fin forward of the rudder.

The point is that (b) is unconditionally correct, while (d) requires an assumption. Thus, (b) is the better answer. Perhaps the distractor could specify "off-set the front of the fin to the right"?

Cheers.

• Posts: 2297

### bobtait replied the topic: Counteracting the slipstream effect

The fun part about multi-choice questions is playing with words. Going back to the model aircraft, if you looked at it from the side and I asked you to rotate it upwards, would you lift the tail or would you lift the nose? There seems to be no problem with up and down, but it gets confusing when you think of left and right. Now I'll be up all night contemplating this!

Keep having fun. Bob

• John.Heddles
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• ATPL/consulting aero engineer
• Posts: 702