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Old 03-10-2012, 05:44 PM   #21  
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Most texts explain that torque causes a left yaw during takeoff roll due to more pressure/drag on the left tire as a result of the left rolling tendancy. There aren't a whole lot of other forces that can explain why a conventional twin likes to "go left" so much on the takeoff roll, definitely not "spiraling slipstream". The spiraling slipstream, if significant to any extent, will go "backwards", how's that going to be some kind of major influence with a twin?

I also highly doubt spiraling slipstream (being significant) due to how much negative p-factor can affect an aircraft (it's not directly related to the aircrafts AOA, it's directly related to the aircrafts flight path vs props flight path, which are very different obviously).
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Old 03-10-2012, 07:38 PM   #22  
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You have most participants of these boards on a limb as far as the proportion of the classic four left-turning forces on a typical SEL airplane. P-factor is a big one, no doubt. Torque has to be large for high-torque airplanes. For a tricycle airplane, I would give slipstream a third place ranking. Several users said they have seen soot or spray indications that support spiral slipstream theory.
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Old 03-10-2012, 08:39 PM   #23  
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Most texts explain that torque causes a left yaw during takeoff roll due to more pressure/drag on the left tire as a result of the left rolling tendancy.
Regarding torques effect on rollout, I'm not sure, I can't claim one way or another. What I know is I've not read a statement like the one made in PHAK from any source I'd consider reputable on this subject. Though the concept makes intuitive sense, I wonder if its magnitude is negligible.

Where I run into a mental block with the scenario PHAK presents is that full aileron for a crosswind departure doesn't exhibit an appreciable effect on the need, or lack of need, for rudder during the rollout. I would guess at the later portion of the takeoff roll, with speed on the order of 40-50 knots, full aileron would yield a far greater rolling force than engine torque forces.

Quote:
The spiraling slipstream, if significant to any extent, will go "backwards", how's that going to be some kind of major influence with a twin?
If the props are counter rotating, as many are, then spiraling slipstream can be ignored. However, so could the effects of torque on wheel weight and p-factor.

If the twin has a critical engine, then slipstream and torque effect would come back into play. As well as p-factor when in flight.

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due to how much negative p-factor can affect an aircraft (it's not directly related to the aircrafts AOA, it's directly related to the aircrafts flight path vs props flight path, which are very different obviously).
That is why I said fuselage AOA. Which isn't exactly accurate as most engines are canted downward a few degrees. In other words, p-factor at the initial start of the rollout, prior to raising the nose, would present with a right yawing tendency.
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Old 03-11-2012, 07:19 AM   #24  
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If the props are counter rotating, as many are, then spiraling slipstream can be ignored. However, so could the effects of torque on wheel weight and p-factor.

If the twin has a critical engine, then slipstream and torque effect would come back into play. As well as p-factor when in flight.
That's just my point, the engines are mounted away from the fuselage in my "conventional twin" scenario, how does the spiraling slipstream from the left engine "jump" backwards and right and "hit the tail"? Why doesn't it go "straight back"?, and if not, then is it really widening that much as it's moving backwards (and maintaining the same magnitude?). I just don't see that as realistic. I don't see them behaving the "same" with such radically different engine mounting configs (single eng vs conventional twin), unless those charateristics are due to something else (torque during takeoff roll and P-factor during flight).

But I agree with the torque on takeoff, and I think I found myself turning the yoke a little in that direction during takeoff roll in a high powered single.
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Old 03-11-2012, 09:16 AM   #25  
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That's just my point, the engines are mounted away from the fuselage in my "conventional twin" scenario, how does the spiraling slipstream from the left engine "jump" backwards and right and "hit the tail"? Why doesn't it go "straight back"?
I cannot say with any degree of certainty. I will make note that most of the books I've been reading do mention slipstream in a twin. However, it is not expanded upon since asymmetrical thrust is nearly always the critical design consideration.

Do recall, though, that it's not about air from the slipstream striking the tail and pushing it. It's about the slipstream creating a sideways component relative to the free stream flow. Thereby causing a slight change to the effective relative wind interacting with the vertical tail.

With that said, some thoughts/questions come to mind: Is the slipstream in a twin still close enough to change the rudders local AOA? Does the slipstream hold a uniform diameter, or does it widen as it travels backwards? Consider that the both engine in a critical engine configuration will move air to the right at the top of the slip stream.
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Old 03-11-2012, 12:28 PM   #26  
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P P-factor
A Accelerated Slipstream
S Spiraling Slipstream
T Torque

It ain't rocket science folks...
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Old 03-23-2012, 11:12 PM   #27  
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Single engine:

The propwash is absolutely rotating when it leaves the blades. One way I came to that conclusion because the exhaust leaving a jet engine is not rotating due to stators, among other reasons.

If the plane was parked, breaks on and engine speed high and constant, would that mean the left-turning tendencies of "gyroscopic effect", engine torque and p-factor would not be realized? Any stresses on the airframe and gear would be from spiraling slipstream only.

(Got an argument for engine torque? Line two planes up, one behind the other. Have the propwash from the front plane wash other back one, which isn't running and take the stress/strain measurements)
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