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Old Dec 17, 2012, 06:48 AM
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Quote:
Originally Posted by Phil.Taylor View Post
Sorry, but you can do the dive test from any state of trim you like - so long as you return to that state of trim after changing the CofG. Re-read Daryls posts & think about it. What you are actually testing for, is dynamic stability/neutrality - at that particular state of trim.

Phil.
Okay, fair enough. The model at least needs to fly at a constant speed hands off for extended periods. If it is neutrally balanced, it should be able to do this for a variety of trim speeds. I suppose the benefit of setting trim for a slow glide is you know within a reasonably short period that the model is trimmed for constant speed flight. With a fast or even moderate cruise, a pilot may think that the model is maintaining constant speed, when in fact it may be gradually accelerating. So in Daryl's case where it tucked, either his model was not stable or it was not trimmed for constant speed - correct? (I actually want an answer to this to confirm my understanding). Also, it seems to me the dive test is really testing for static stability and not dynamic stability (the initial response to the down input and not the long term damped behavior of the aircraft).

Tom
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Old Dec 17, 2012, 07:31 AM
Daryl Perkins's Avatar
United States, VA, Falls Church
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Tom,

It was trimmed for constant speed. My example was simply that Joe was on one side of "slow speed cruise" and I was on the other side. Always within one or 2 clicks of trim. Slow speed cruise is somewhat subjective. I wasn't consciously trying to trim for a slightly faster cruise, that's just what felt right to me at the time. And it felt.. OH MY GOD slow....

I think I've gone that far the other direction now, and am probably trimmed too slow for the dive test to mean anything.
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Old Dec 17, 2012, 09:45 AM
Phil.T-tailer
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Quote:
Originally Posted by Kiesling View Post
With a fast or even moderate cruise, a pilot may think that the model is maintaining constant speed, when in fact it may be gradually accelerating. So in Daryl's case where it tucked, either his model was not stable or it was not trimmed for constant speed - correct? (I actually want an answer to this to confirm my understanding).
Tom
If your model is accelerating on its own, when it should be cruising - you've got problems !

Phil.
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Old Dec 17, 2012, 10:00 AM
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Ireland, County Kerry, Kerry
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Quote:
Originally Posted by Phil.Taylor View Post
If your model is accelerating on its own, when it should be cruising - you've got problems !

Phil.
... unless it's got a tiny fan or turbine on board ?
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Old Dec 17, 2012, 12:17 PM
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Michigan, USA
Joined Jul 2006
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Quote:
Originally Posted by Daryl Perkins View Post
I think I've gone that far the other direction now, and am probably trimmed too slow for the dive test to mean anything.
So to clarify, you're saying you have the CG where you want it, but because you're trimmed for slow flight, doing a dive test would probably result in the plane pulling out rather quickly?

Also, when you say "push" the plane around the sky, I assume you mean having to give it a little bit of down now and then to keep it moving. With a rearward CG setting is there a tendency for the plane to get into a "mushing along" state if you don't give it a little down once in awhile to keep it moving?

Jim
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Old Dec 17, 2012, 02:48 PM
IT'S NOSE HEAVY!!!!!!!
cityevader's Avatar
United States, CA, San Jose
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Seems like he's pushing it because there's a bit of up trim, not necessarily because of the cg, per se, right?
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Old Dec 17, 2012, 04:09 PM
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Originally Posted by cityevader View Post
Seems like he's pushing it because there's a bit of up trim, not necessarily because of the cg, per se, right?
Right - at least that's what I'm understanding him to say.

My 2nd question should have been separated.
When I've messed around with moving the CG back it sometimes seems like the plane is just mushing along. With a more forward CG it seems like that doesn't happen. It'll settle into a groove, or if I slow it up too much it'll drop the nose and speed itself up. Just wondering if others have experienced this or if I'm just imagining things.

Jim
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Old Dec 17, 2012, 11:27 PM
MrE
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Seems to me that if its mushing after you moved the cg back, then you didnt re-trim it to fly at the same speed as before you moved the CG. Sounds like you need another click or two of down.
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Old Dec 18, 2012, 01:55 AM
IT'S NOSE HEAVY!!!!!!!
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I'm laying in bed, mind whirling...

I'm picturing a mobile hanging from the ceiling. You know, different shapes and weights and moment arms...yet all in perfect balance.

Obviously a heavy/short arm balances a light/long arm and any air movement causes it to rock and sway. Now give each end a "perfectly" flat and level horizontal surface...then provide a "perfectly" horizontal and level wind. In a "perfect" world it would remain motionless regardless of wind speed. If say, the shorter arm had it's flat surface moved back to be right at the ("mounting") string, and then re-weighted to remain perfectly balanced, it should still stay motionless with a "perfectly flat" wind...regardless of speed.

If it were then allowed to "dive" at an angle, it would have a vertical vector force (gravity) and a horizontal vector force (wind).

Could it be surmised that in a "perfect" setting, it would balloon into the wind (due to horizontal vector forces on the lighter/longer "tail") and conversely tuck with a tail wind? (Perfect world and no other forces involved)

I'm not sure where I'm going with this, but I've lost enough thoughts at bedtime because I didn't write them down.
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Old Dec 18, 2012, 02:02 AM
IT'S NOSE HEAVY!!!!!!!
cityevader's Avatar
United States, CA, San Jose
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Quote:
Originally Posted by GliderJim View Post
Right - at least that's what I'm understanding him to say.

My 2nd question should have been separated.
When I've messed around with moving the CG back it sometimes seems like the plane is just mushing along. With a more forward CG it seems like that doesn't happen. It'll settle into a groove, or if I slow it up too much it'll drop the nose and speed itself up. Just wondering if others have experienced this or if I'm just imagining things.

Jim
Just thinking out loud...if it "settles into a groove", isn't that an indicator all by itself that it's nose-heavy? ...If it "wants to" fly at a particular speed, that is?
Then, your second description is what I call porpoising...which is (to me anyways) an adverse flight characteristic...whether thermaling, sloping, or power planes...or maybe it's just me.
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Old Dec 18, 2012, 02:08 AM
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Quote:
Originally Posted by cityevader View Post
Could it be surmised that in a "perfect" setting, it would balloon into the wind (due to horizontal vector forces on the lighter/longer "tail") and conversely tuck with a tail wind? (Perfect world and no other forces involved)
If that were true, wouldn't that mean that the glider flying in the air has a way to sense the airmass movement even it takes place at constant speed? Or am I mis-interpreting what you mean by tailwind/headwind?
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Old Dec 18, 2012, 02:14 AM
IT'S NOSE HEAVY!!!!!!!
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I was thinking that in one direction, the "wind" would hit the top of the "tail" and in the other direction the bottom of it.

On second thought, for actual flight, that would require a super steep dive in order that the horizontal "flight speed" would be less than the "tail-wind speed" in order for the wind to affect the underside of the tail whist going downwind...nevermind.

That's what I get for "bedtime physics"!
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Old Dec 18, 2012, 02:35 AM
I don't want to "Switch Now"
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Upwind and downwind have no impact once the model is free of the ground, so any analysis of supposed differences between the two conditions is moot.
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Old Dec 18, 2012, 02:38 AM
IT'S NOSE HEAVY!!!!!!!
cityevader's Avatar
United States, CA, San Jose
Joined Mar 2012
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not if the mobile is diving steep enough! but of course, it really isn't "flying" at that point.

Yup...bedtime physics indeed. I too tired to post, not tired enough to sleep.

Edited to add: I turned the computer back on to add that my above response was in an emotional fashion, to prove "I'm right" (even if I'm wrong) as prolly most of us have done thus far (and I don't know who's right or wrong)...
If said perfectly balanced mobile was traveling straight down, a 90 wind would affect the "tail" and push it to some degree, would it not? Thereby angling it more into the wind. Tail feathers of an arrow effect. re-edited: arrow is a bad example because of connotation of speeds which I wasn't intending....and you're still talking about a PLANE. I was talking about a ceiling MOBLIE.

We can both be right! We can both be wrong!
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Last edited by cityevader; Dec 18, 2012 at 07:42 AM.
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Old Dec 18, 2012, 02:55 AM
MrE
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United States, WA, Gig Harbor
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I think its interesting to note that we have gone from:

The dive test is the way to check/set cg
to
The dive test is stupid (when your talking about cg) - it only tells you the trim setting
to
No its not stupid - it DOES tell you CG because its physics
to
It may or may not be stupid but it doesnt tell you squat about CG

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