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Old Dec 18, 2012, 04:27 AM
Phil.T-tailer
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Devon, UK
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Quote:
Originally Posted by cityevader View Post
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.
So cut loose - cut that thread thats holding you & your thoughts to the ceiling - and fall, fall, fall free - just the air around you - drifting you off to sleep...

Phil.
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Old Dec 18, 2012, 08:27 AM
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United States, CA, San Jose
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Quote:
Originally Posted by MrE View Post
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

FWIW, it took me from the bleachers to the playing field with my grossly out of whack planes.
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Old Dec 18, 2012, 08:38 AM
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Joined Mar 2008
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For your consideration, an interesting post from Dr. Drela from 2007:

Quote:
Theoretically, the initial trimmed glide speed doesn't matter, as long as the dive is significantly faster. But if you're changing the CG and doing dive tests, it's good to try to start at the same trimmed glide speed so that you can gauge the effect better.

And the real situation is more complicated. From an old post I dug up:

Any given real airplane does not have a fixed
stability margin at all airspeeds. Normally
the stability margin decreases with airspeed.
There are typically three reasons for this:

1) Cm of low Reynolds number airfoils is not really constant.
It typically becomes more negative (nose down) at small AoA.
This looks and smells just like a reduced stability margin.

2) If the CG is below the wing's center of area, like in any
poly glider and most DLGs, the CG will effectively move aft
relative to the wing at lower AoA and reduce pitch stability.

3) Flexibility of the wing, tailboom, tail surfaces,
and linkages tends to have the same effect as imparting
down-elevator trim at increasing airspeeds (reducing AoA).
The net effect is the same as that of lowered pitch stability.

If the glider positive stability in a slow glide, it can still
go unstable in a very fast glide, resulting in a tuck-in.
The dive test, if done at an appropriate speed, is useful in that
it represents a worst-case scenario where all three effects
above gang up.
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Old Dec 18, 2012, 09:11 AM
Daryl Perkins's Avatar
United States, VA, Falls Church
Joined Mar 2007
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Sorry, didn't mean to confuse everybody. The dive test is fine for ballparking. But, realize it's completely trim dependent. Also realize, that for every differing TVC, expect slightly different results.

I don't use it anymore. Especially on a model that's new to me.
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Old Dec 18, 2012, 01:29 PM
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San Diego
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What does TVC stand for? I looked for a definition but all I could find was thrust vector control.

Thanks,
Tom
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Old Dec 18, 2012, 01:33 PM
Red Merle ALES
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United States, Mt, Helena
Joined Apr 2002
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Tail Volume Coefficient
http://www.djaerotech.com/dj_askjd/d...tail_coef.html

Curtis
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Old Dec 18, 2012, 02:29 PM
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San Diego
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Quote:
Originally Posted by CloudyIFR View Post
Thanks Curtis,
I guess I'm use to seeing it written as Vh
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Old Dec 18, 2012, 03:15 PM
MrE
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Quote:
Originally Posted by Cap_n_Dave View Post
For your consideration, an interesting post from Dr. Drela from 2007:

I think its interesting to read the portions of the thread where this quote came from. Taken out of context it doesnt really give the full story. Phil Barnes, Dr Drella and others were discussing similar issues that we are.

http://www.rcgroups.com/forums/showt...ea#post7762862
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Old Dec 18, 2012, 03:54 PM
mostly gliders
liukku's Avatar
Skellefteċ, Sweden
Joined Sep 2003
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Quote:
Originally Posted by MrE View Post
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

Hello
I like your summary of the subject. I feel the same as you describe, but I still have my favorite method to trim my airplane in the best way I can, and it consists of a little of each method.
Think this thread is a very nice Christmas present reading for many of us, whether we are writing posts or trying to learn to be even better glider pilots.
Now I wish everyone a Merry Christmas and Happy New Year!

/Ville
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Old Dec 18, 2012, 04:50 PM
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San Diego
Joined Aug 2004
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Thanks for posting that MrE

That thread lead me to this one http://www.rcgroups.com/forums/showthread.php?t=710453

which explains what I was trying to get at (poorly) with my previous posts.

Here, Mark explains the reason why as the CG is moved closer to the neutral point the range of stable trim speeds becomes very small. This effect becomes more pronounced the further the CG is below the wings area centroid.

So when I asked Daryl if his model was trimmed for constant speed, what I was getting at is was it trimmed for a stable speed. With a very aft CG it doesn't take much down trim to lead to an unstable model. Even if the model is trimmed for neutral stability, any little disturbance causing the nose to point down can force the model into an unstable condition. With a forward CG, the model has a broader range of stable trim speeds and therefore is less effected by disturbances (ie more stable).

In practice I see this effect play out with my various models. For example my Supra's which have the CG far below the wings area centroid will tuck in the dive test (They actually will tuck in the zoom off launch if I let go of the stick). I have the CG pretty far back on these models. My F3B models which probably have a similar margin of stability in level flight, but the CG is much closer to the wings area centroid, go straight in the dive test.

So for someone that flies with a very aft CG, the dive test doesn't mean much because the pilot already knows the model is close to neutral because it goes unstable if the model is disturbed and uncorrected.

That said, it still probably isn't a bad idea to test your models response in a dive to understand when/if it goes unstable. It may give you more motivation to pay close attention when flying far away. I know when I flew my Supra's at distance it was a little bit mentally draining. Also, if you don't feel comfortable with when the model goes unstable, you can change the CG accordingly, or if your model always pulls up quickly from a dive you may want to move the CG aft.

Tom
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Old Dec 30, 2012, 10:40 AM
Daryl Perkins's Avatar
United States, VA, Falls Church
Joined Mar 2007
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Spent some time with my Vixen yesterday. My linkage is super solid. Tail boom is F3B strength stiff.

I have it trimmed rather floaty, not quite min sink, but awfully close. It doesn't tuck at this trim setting until it gets moving very fast. I put a couple clicks of down trim and retrimmed for my interpretation of a medium cruise. (The problem with subjective interpretation)

Boy does it tuck there....

But the plane is perfectly stable and comfortably flyable at this CG setting. Im certainly not going to move it forward because it tucks.

This is why I don't use the dive test as an optimization method. It's ok as some sort of confirmation, but certainly not an optimization method.

Thought you might like a bit of clarification on my CG settings.

Happy New Years everybody, and best of tuck in your CG quests for 2013.

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Old Dec 30, 2012, 10:55 AM
mostly gliders
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Skellefteċ, Sweden
Joined Sep 2003
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Really has nothing to bring this thread, but I just want to once again thank all the talented pilots who sincerely share their experiences. I'm learning all the time.

Happy end to old year and Happy New Year!

/Ville
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Old Dec 30, 2012, 04:12 PM
Bouras Slope Flyers
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Athens, Greece
Joined Mar 2008
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I am still hunting that tuck.

Happy new year to everybody
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Old Dec 30, 2012, 04:55 PM
Sink Stinks
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Orange County, CA
Joined Aug 2004
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I suggest that anyone interested in flying, of any kind, snag this book:

http://www.amazon.com/Stick-Rudder-E.../dp/0070362408

Easy reading, and while focused on powered flight, it still does a great job of explaining a lot of the theory of how an airplane is controlled in flight.
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Old Jan 05, 2013, 08:04 PM
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United States, CA, San Jose
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I'm so glad I started this thread, as it has improved my planes a great deal! Thank you all for contributing!

Being the dummy I am, all the Cl, Cm, theoretical design what-have-you, turned into blah blah blah (for me personally) BUT!! I learned a few things about recognizing a nose heavy plane.
If I could sum up what I personally have learned, it'd be three things: They are mushy. Elevators need to be constantly correcting. They stall badly.

I've only barely flown once since starting this thread, but today I had some fun with my wee Alula in 2mph wind on a tiny dome of a hill about 300ft wide and 50 feet high. Not ideal for flying, but ideal for testing as the flights were a bit extended. The Alula was trimmed well and would launch flat if thrown flat and climb out at a 45° if thrown at 45°, during a super-fast side arm launch. This high speed launch could well be considered a "dive test" because of the speed involved, and it could be "argued" that the plane was "balanced" (by the dive test crowd) because high/low speed flight was similar...BUT!!! it was mushy, needed constant elevator corrections, and stalled badly. Keeping in mind that the dive test is basically to test for trim and not actual balance, I went further...
So I taped a penny roughly two inches behind the CG and did many more flights. The difference was AMAZING!! It no longer HAD to fly fast, and was hands-off-elevator-stable, very responsive, floated much better, and when continually increasing up-elevator was applied, it wouldn't "stall". Rather, it'd merely only lose altitude. No longer did it need time (and 8 vertical feet) to recover from the "stall"! A mere fractional reduction of the purposeful-up-elevator was all that was needed to keep it flying, still level, repeat, still level, and merely bleeding off altitude with the still-heckuva-lot of up elevator applied.
And funny thing is...the side arm launches are still identical. If thrown flat it goes flat. If thrown at 45° it climbs at 45°...same as before, when it was nose heavy.

This is my story, devoid of any math whatsoever and based solely on observation alone, and with the help of many of you here, to get my plane flying saawwwEEEEEtly!!!
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