Thread: Thrust ???
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Old 11-23-2005, 03:21 PM
  #9  
britbrat
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Default RE: Thrust ???


ORIGINAL: JohnW

BritBrat... nothing is ever simple----

------ Would it be possible for the test to indicate for example a pitch to canopy, but yet a true hover would still pitch to belly? I was thinking along the lines of a stab/wing incidence issue where one out plays the other depending upon airspeed and prop wash conditions? I'm look forward to your thoughts on that issue. Cheers.
That is a very interesting question. To clarify what you are asking -- I presume you are refering to the dynamic test that I described & that it indicated a thrust-driven pitch to the belly, whereas in hover, the model pitches to the canopy. Is that correct?

Pitch coupling is affected by so many things ---



Let's ignore yaw-axis trim changes for the moment.

In the vertical dynamic test with power on, as speed falls, thrust lines eventually predominate if horizontal tail trim forces are low. However, since the horizontal tail is in the strong propwash, tail trimming forces are so powerfull that they could easily overpower thrust line forces. This is why the power-on & power-off test are also required.

For the moment, let's ignore incidence & assume that it is effectively zero-zero. Normally, a correctly balanced model with excessive down thrust would carry up-elevator trim for horizontal cruising flight. This model, trimmed level at cruise speed, would climb with closure of the throttle at cruise, tend to baloon at low speeds with low power & dive with suden power application at low speed. Trimming tests would easily diagnose too much down thrust.

You would expect this model to pitch to the belly in hover, due to the down thrust. However, in a hover, horiz tail trimming forces are still fully active due to the powerfull prop wash, but the gravitational component is removed & those tail trim forces easily overpower the thrust trim forces -- therefore the model would pitch to the canopy.

If the same model had the C-of-G at the aft limit, the tail trim forces would be minimized & it is possible that it would pitch to the belly, or even show little pitch change at hover.

Conversely, if it was nose heavy, the pitch to the canopy would be very strong at hover.

In the case of that same model, but with positive incidence, it would carry little up-elevator trim or even some down-elevator trim, particularly as the C-of-G moves aft. In a hover it would most likely pitch to the belly.
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