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Old Dec 12, 2012, 02:42 PM
Detail Freak
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Old Dec 12, 2012, 02:59 PM
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Originally Posted by Kiesling View Post
If launched at the trimmed speed why would the model slow down (regardless if is nose heavy or not)? This test sounds flawed to me . . .

Tom
Drag.
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Old Dec 12, 2012, 03:03 PM
Red Merle ALES
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Subscribing for entertainment purposes!

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Then let me entertain you!

I don't understand what a crooked airplane is. Do you mean having an articulated tail that when the model is flying in straight and level unaccelerated flight that there is a little up or down elevator trim?

Dr. Drela wrote:
Quote:
A more precise and technical answer is reprinted below from Mark Drela in the Allegro-Lite YAHOO group, message #6005; The importance of decalage is way overblown. First of all, if you have an all-moving tail, then obviously "setting the decalage" has no meaning. You just set the elevator trim to whatever it has to be for your CG position and desired trimmed speed. It turns out that for a hinged elevator, decalage should also have little effect. You can change pitch trim either by moving the elevator or moving the entire tail. There should be little or no difference in pitch stability or elevator response between the two types of changes. This assumes that the tail airfoil is reasonably good, and the elevator deflection is modest so that the tail's cl(alpha) and cl(elevator) curves are still linear. The only reason to change the decalage (angle of the front stabilizer) in this case is to null out the elevator trim deflection. This will give more symmetric elevator range, reduce elevator servo load at high speed, and perhaps reduce the tail's profile drag very slightly. If there IS a difference in handling from a change of decalage, then you surely have poor airfoil flow over the tail, with some combination of dead band or hysteresis. This also indicates the presence of large separation bubbles and high profile drag. A simple slab airfoil with a semi-circular LE is very common, and is prone to these types of problems. Making the LE more elliptical rather than round should give a significant improvement.
End Quote.

If this is the case then the fundamental understanding of what the horizontal tail does aerodynamically that Dr Drela who is wicked smart and immensely well educated in aerodynamics is trying to tell us. The "wicked smart" was a quote from a friend of mine who sits in the back room with Dr Drela at the main table during meetings.

See the attached drawing:
There are six airplanes all balanced the same. Three have a full flying horizontal tails and three have articulated horizontal tails. All of these planes trimmed to fly at their best L/D. All of these planes are flying at the same angle with respect to the main wing. i.e. Their main wings angle of attack are the same.

The L/D of all these planes are different, perhaps not very significantly, but different. Now why is that so? Is it because of the differnet horizontals? Nope. Is it because of the displacement of the fixed vs moveable elevator deflection? Nope.

So lets assume that the articulated horizontal tail has a resonably good airfoil and the elevator deflection is modest as Dr. Drela infers above.
So comparing airplane B - B, to examples A and C, is there a difference in drag across the wing and tail? Nope.

So earlier I said that there was a difference in L/D. So why is that?
Hmm, could it be fuselage drag? Yup. I'd guess A-A would be the worst but possibly, if the fuselage is shaped properly that C-C may produce less drag than B-B.
This is what Paul Naton has shown us with his Radian mods. It has little or nothing to do with his model flying with a little up/down elevator trim.

Also, I haven't read where full moving vertical stabilizers instead of a fixed fin and moveable rudder are particularly necessary or valuable on high performance sailplanes. Even though they are generally coupled with ailerons and so are moving every bit as much as the elevator does.

If so than wouldn't adjusting the the trailing edge of your wing i.e. camber or reflex create "crooked" sailplanes?

A hinged elevator is just like a camber flap on the wing - it simply adjusts the camber of the surface.

Thanks for listening.
Curtis
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Old Dec 12, 2012, 04:09 PM
Detail Freak
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Quote:
Originally Posted by GliderJim View Post
Drag.
I don't think so...
If its launched at the trim speed, the drag will remain constant, not increase, so the speed should remain contant, not decrease. RIGHT?

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Then let me entertain you!

Curtis
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I haven't yet made a statement, other than the one above...
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Old Dec 12, 2012, 04:32 PM
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Originally Posted by target View Post
I don't think so...
If its launched at the trim speed, the drag will remain constant, not increase, so the speed should remain contant, not decrease. RIGHT?
You know, I haven't actually gone out and tried this yet, but I guess the idea is to get the plane flying level to the ground, not slightly nose down like we normally cruise around. Slightly nose down means you're flying "downhill", so gravity keeps you going. Perfectly level should mean that you're slowing down the entire time.
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Old Dec 12, 2012, 04:41 PM
Detail Freak
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Then you aren't at a "trim speed".
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Old Dec 12, 2012, 04:56 PM
MrE
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Originally Posted by target View Post
Then you aren't at a "trim speed".
From Gordy's Balancing Method - see attachment above.

Quote:
Go do a few hand tosses to get the plane trim, so that hands off, it flies flat and level, not diving not ballooning…just a long flat glide. If your glide is heading downward, it’s not a trimmed for a long flat glide!

Now, once trimmed as stated, give it a good toss, get it flying straight and keep your hands off of the elevator stick! The only important part of this system is the last 10’ of its glide, so watch what the nose does very carefully. If at the end when the sailplane slows, the nose suddenly drops to the ground, GET THE LEAD OUT.
Seems to me if you're doing this test and the model is nose heavy, then you will be forced to feed in up as it slows. To the point where the elevator cant hold it. Still, it is different from what he said in this thread. On the other hand - it doesnt really change the end goal, so to me it doesnt matter.
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Old Dec 12, 2012, 05:14 PM
Detail Freak
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I'll stick to what the guys with the engineering degrees tell me....

And, I understanding that we are saying the same thing. Its just not possible to have a sailplane fly "level". Its flying on a down slope of some kind, or its bleeding off extra speed until it does, one or the other, but eventually it has to fly at some slope towards the ground (in neutral air).

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Old Dec 12, 2012, 05:26 PM
MrE
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Nit-picking some ones exact word usage and the minute details of aerodynamic theory must be an incredibly FUN thing to do - we sure do spend a LOT of time doing it in these forums dont we? Note that I am included in the "we".

However, its easy to loose track of the main points when we get side tracked that way.

The main point Gordy is trying to make it that - in general - we are flying our "TASK SAILPLANES" too nose heavy. His method will get your model very close to the point where there is zero force on the elevator at all speeds. Which by the way, is the point where Dr Drella has the CG marked on the Bubble Dancer plans. See attached. I dont remember for sure about the Allegro but I think its on there too.

His second point is that the dive test in and of itself, does not directly test or indicate CG. It only indicates model trim.

How many times have many of you seen someone flying a plane who claimed it was nose heavy or tail heavy when the trims were grossly off and had never been set at all? Doing a dive test in that situation is useless.

I happen to think there is a place for the dive test IF you understand how all this works AND trim the model. But thats just me
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Old Dec 12, 2012, 05:28 PM
MrE
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Originally Posted by target View Post
I'll stick to what the guys with the engineering degrees tell me....

And, I understanding that we are saying the same thing. Its just not possible to have a sailplane fly "level". Its flying on a down slope of some kind, or its bleeding off extra speed until it does, one or the other, but eventually it has to fly at some slope towards the ground (in neutral air).

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target, I dont think Gordy is debating ANY ones theory at all. He is haveing you set the model that way to do the test. In normal flight you would ofcourse be flying as you describe.
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Old Dec 12, 2012, 05:32 PM
Pompano Hill Flyers
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Quote:
Originally Posted by GliderJim View Post
You know, I haven't actually gone out and tried this yet, but I guess the idea is to get the plane flying level to the ground, not slightly nose down like we normally cruise around. Slightly nose down means you're flying "downhill", so gravity keeps you going. Perfectly level should mean that you're slowing down the entire time.
It also means that you're constantly increasing up elevator and angle of attack to compensate for loss of lift due to constantly decreasing airspeed, until your wing finally stalls and you drop to the ground. In short, no, I don't think that's what the idea is.

Right now I'm leaning heavily toward Tom Kiesling's view that the test is flawed. If a glider is in stable flight at a certain speed and attitude, it should remain that way until it reaches the ground. There's no reason that it should slow down by itself as if it knew somehow that it was about to land.
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Old Dec 12, 2012, 05:47 PM
Detail Freak
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Originally Posted by MrE View Post
Nit-picking some ones exact word usage and the minute details of aerodynamic theory must be an incredibly FUN thing to do - we sure do spend a LOT of time doing it in these forums dont we? Note that I am included in the "we".

However, its easy to loose track of the main points when we get side tracked that way.

The main point Gordy is trying to make it that - in general - we are flying our "TASK SAILPLANES" too nose heavy. His method will get your model very close to the point where there is zero force on the elevator at all speeds. Which by the way, is the point where Dr Drella has the CG marked on the Bubble Dancer plans. See attached. I dont remember for sure about the Allegro but I think its on there too.

His second point is that the dive test in and of itself, does not directly test or indicate CG. It only indicates model trim.

How many times have many of you seen someone flying a plane who claimed it was nose heavy or tail heavy when the trims were grossly off and had never been set at all? Doing a dive test in that situation is useless.

I happen to think there is a place for the dive test IF you understand how all this works AND trim the model. But thats just me
I agree with most of this....
The dive test is useful, but all of the instructions must be followed.
I personally use it for crude adjustments, then look at flying inverted, then nit pick the overall handing characteristics of the plane.

For me, having a plane that radically changes pitch with speed is most annoying. A little is OK and tolerable....more than a little? No way.

CG back is good until the opposite happens and the plane tucks with speed increases. Every bit as annoying, and likely more dangerous.

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Old Dec 12, 2012, 06:48 PM
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Originally Posted by MrE View Post
The main point Gordy is trying to make it that - in general - we are flying our "TASK SAILPLANES" too nose heavy.
Some proof from Gordy would be nice, but I suppose guesswork, conjecture, and magical unicorns will be the order of the day.

Quote:
zero force on the elevator at all speeds
I'm fairly certain that the "no load on the elevator" is only possible at the design airspeed ... which is the point of trim, which is to compensate for "off design conditions." Would be nice if Herk chimed in but I suppose he bailed out of the thread.

Quote:
His second point is that the dive test in and of itself, does not directly test or indicate CG. It only indicates model trim.
Which is 100% incorrect.
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Old Dec 12, 2012, 06:49 PM
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Originally Posted by MrE View Post
target, I dont think Gordy is debating ANY ones theory at all. He is haveing you set the model that way to do the test. In normal flight you would ofcourse be flying as you describe.
Except he pretty clearly stated that "the dive test is stupid."
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Old Dec 12, 2012, 07:43 PM
MrE
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Cap_n_Dave;23512587]Some proof from Gordy would be nice, but I suppose guesswork, conjecture, and magical unicorns will be the order of the day.
Advice on CG placement from an LSF5 constitutes magic unicorns? Then I suppose Dr Drella is guilty of the same as he recommends starting at exactly the same place Gordy does.

Quote:
I'm fairly certain that the "no load on the elevator" is only possible at the design airspeed ... which is the point of trim, which is to compensate for "off design conditions." Would be nice if Herk chimed in but I suppose he bailed out of the thread.
Again - thats Dr Drellas recommended starting CG on the Bubble dancer - a task sailplane if there ever was one.

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Which is 100% incorrect.
I disagree
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