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Thrust ???

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Old 11-20-2005, 04:35 PM
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Torqued-Up
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Default Thrust ???

Ok, I had a lengthy discussion with a flying buddy at the field this morning... He was complaining that he had to hold about 1/2 stick of up elevator when hovering a new airplane to keep it vertical... The dicussion came down to whether he needed to add up thrust or down thrust...

Given the example, which would you add to the engine thrust line, up or down thrust???

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Old 11-20-2005, 05:58 PM
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Default RE: Thrust ???

hover is not a good indication.... because balance is such a big part of it......

when flying level, does the plane need lots of trim??
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Old 11-20-2005, 10:52 PM
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Default RE: Thrust ???

The test for engine thrust is: in level flight at full bore and trimmed for level flight chop the throttle to idle. If the thrust is correct the plane will nose down gradually as the speed bleeds off. If it vears up you need up thrust, if it vears down you need down thrust.

I agree with the earlier poster it is probubly a ballence problem, most likely nose heavy. be carefull!
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Old 11-21-2005, 04:51 PM
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Default RE: Thrust ???

The method paladin described is commonly used to check thrust; however, I’d substitute “cruise speed” for “full bore.” Thrust, like everything else, is not a linear function, and the amount of down/right thrust will vary on airspeed. Ideally you want to set neutral flight for your cruise speed, not full bore. I realize for many, cruise speed is full bore, but I just thought I’d point out the potential issue.

As others have posted, reaction in a hover is not enough information to determine if thrust is set “properly” in the traditional sense, and what is proper is subjective. Dang near everything is dependent, from CG to incidence to thrust which only complicates everything.

You need to distinguish between a zero airspeed hover and a forward hover. A forward hover can be stationary with respect to ground if pointed into the wind. With a positive forward airspeed right side up hover, needing up elevator to maintain is probably normal. However, in a zero airspeed hover, needing some up elevator may also be normal. Often, pilots get into a forward moving hover and don’t realize it. If the plane is truly in a zero air speed “hover”, the plane should start to torque roll unless gobs of aileron are input to prevent.

Since everything a plane does reacts about the center of mass, if the thrust line is above the CM, you will have a natural down pitch when hovering due to thrust, i.e. you will need up elevator. Consider most planes have down thrust built in, either by having the engines angled down or by having the engine above the center of lift (high mounted engine.) Think about a typical plane, high mounted engine and heavy landing gear. If you try to balance the plane straight up on the tail, it will fall to belly… same thing happens in the air. There are a bunch of other factors I won’t get into, such as downwash from wings, incidence, drag, etc. But one thing that may help a lot, esp. if it is way out, is moving the center of mass up such that the plane balances close to vertically on the thrust line. You can check this by hooking a string on the prop/spinner, suspend the plane from the nose, and note the attitude it takes compared to engine thrust. Ideally, you want the plane’s center of mass directly in line with the thrust vector.

Cheers.
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Old 11-21-2005, 05:20 PM
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Default RE: Thrust ???

ORIGINAL: JohnW

Since everything a plane does reacts about the center of mass, if the thrust line is above the CM, you will have a natural down pitch when hovering due to thrust, i.e. you will need up elevator.
Cheers.

It isn't that simple. In the vertical, perturbing effects due to the center of mass location are much less significant than they are in more level flight attitudes.

The location of the centers of lift & drag (they frequently don't coincide) must be considered along with the center of mass. Even in a true hover, there are still substantial lift & drag effects due to the powerfull propwash over the flying surfaces & other airframe components in the propwash (landing gear for example). In some circumstances, they can have more impact than the center of mass.

Trim you model for hands-off level flight at full speed, or at least a high cruise speed, then pull into the vertical at speed and observe the direction in which the plane deviates from the vertical line as it decelerates. Repeat the test, but kill the throttle after entering the climb & observe the result.

Repeat the test again, but enter with closed throttle from a high speed dive, then open the throttle in the vertical climb & observe the result.

From those three evolutions you should be able to clearly see the effects of thrust line.
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Old 11-21-2005, 05:36 PM
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Default RE: Thrust ???

After checking the setup on my buddies airplane, I noticed that he had to trim in considerable up elevator to fly straight-and-level at cruise speed... When inverted, it dives for to the ground... He has CG set at roughly 2" behind recommended, but it still acts nose heavy... Strange... Could be an incidence issue, but I will advise him to try the techniques you guys have outlined to isolate the problem...

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Old 11-22-2005, 03:52 PM
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Default RE: Thrust ???

BritBrat... nothing is ever simple. I tried to hit at that with my initial disclaimer that everything is related, dependent, etc., but I probably over simplified. You explained a good test, I often use a similar test on pattern ships. I have found that many pattern ships need a small mix, maybe 2%, for throttle to elevator offset to the low side of throttle to prevent pitch moments at low vert speed with low throttle, such as the entry to a stall turn. In a hover, the stab is basically 100% effective due to prop wash, but the wings are not. 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.
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Old 11-23-2005, 02:09 PM
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Default RE: Thrust ???


ORIGINAL: JohnW

BritBrat... nothing is ever simple. I tried to hit at that with my initial disclaimer that everything is related, dependent, etc., but I probably over simplified. You explained a good test, I often use a similar test on pattern ships. I have found that many pattern ships need a small mix, maybe 2%, for throttle to elevator offset to the low side of throttle to prevent pitch moments at low vert speed with low throttle, such as the entry to a stall turn. In a hover, the stab is basically 100% effective due to prop wash, but the wings are not. 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.
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Old 11-23-2005, 03:21 PM
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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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Old 11-23-2005, 04:22 PM
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Default RE: Thrust ???

I was hoping for some new magical insight... but you basically confirmed my understanding. The tests you described would indeed detect an issue with thrust/CG/incidence, but it wouldn't necessary indicate which are off, just that the combination is off. Is that correct? I often struggle with other people's planes on this issue. I'm at a disadvantage because I have to take their word that they set the CG properly, built the model as per the plans, etc. and I often cannot verify this for them at the field. I've encountered a few planes that just acted screwy and I could not easily detect what was off, my guess was all three (thrust/CG/incidence) such that the plane trimmed for level flight OK, but had all sorts of other issues. I know any power off test will remove thrust from the equation, and any vertical test should remove balance point (CG) from the test. What I struggle with is making senses of the result especially when something is way out. I.E. a test may show down thrust is too much, but is that because it is set wrong or is it the CG or incidence that are set wrong that make the model act like it has too much down thrust when the thrust is actually set as per the plans. Any thoughts on that? Cheers.
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Old 11-23-2005, 05:29 PM
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Default RE: Thrust ???

That's an invitation to chatter for days & I doubt if I know any more about this than you do. When you start to include all of the perturbations possible from three-axis flight control inputs, plus thrust line issues & physical structural variations, it can be quite daunting to sort out.

With problem airplanes, I always start with a physical structure check-up. Following a good eyeball check for alignment, plus a measurement of same, I twist & flex wings, tails & control surfaces to ensure that they aren't distorting excessively in flight -- & fix any suspected weak areas. I check for warps, slack in the controls, even throws, differential, loose skin surfaces, excessive throw (particularly elevator), incidence, C-of-G, lateral balance,etc. -- standard stuff, but it eliminates many problems straight away.

Thrust line issues are relatively easy to determine and to correct. That is usually where I start in-flight investigations -- if I suspect too much, or too little thrust setting, I test it by adding or subtracting thrust angle with shims & observe what happens. Following that, I start to systematically eliminate the remaining problems -- if I can.

Speed-dependent trim issues (not related to thrust line) are the next easiest problems to fix, so that's where I go, & for normal level flight (students airplanes) it isn't too hard to get things close.

It is for coupled aerobatic misbehaviour that things get interesting & frustrating. The problems are designed-in at that point & it is damned difficult to fix some of them (assuming that I can actually isolate a particular causitive agent), but I try it one step at a time.

I guess that's not very helpfull, because I'm sure that is what you do as well.



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