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Old Dec 12, 2012, 07:52 PM
MrE
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Originally Posted by Cap_n_Dave View Post
Except he pretty clearly stated that "the dive test is stupid."
He did pretty much say that, but you didnt finish reading the rest of his comments. You let that one, seemingly outrageous, statement stop you from following along on the rest of his comments.

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You've heard of the 'Dive Test'... probably the dumbest thing anyone ever came up with for checking balance. I hear modelers talk about it saying that when the plane is nose heavy, it will cause the model to pull up quickly and one that is neutrally balanced or tail heavy will 'tuck' or increase its dive angle as speed increases.

All probably true but goofy to be talking about in the context of Balance.
I added the emphasis. Gordy says its dumb - but he also says its probably true - the dive test does what it does. He admits that. But he doesnt think its the best way to check balance because it only indicates the models trim state.

He goes on to say that another reason he doesnt like the dive test is that its conducted at hi speeds. Boom and control rod flex, wing flexing etc all effect the dive test results at hi speeds.

His test on the other hand is conducted at speeds closer to how the model is flown in a thermal.
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Old Dec 12, 2012, 07:53 PM
MrE
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I think this is the most fun Ive had all week
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Old Dec 12, 2012, 07:56 PM
MrE
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By the way - Gordy isnt the only one poopooing the dive test. Ive heard many many people saying pretty much the same things Gordy is saying - although perhaps not in exactly the same way
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Old Dec 12, 2012, 07:57 PM
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Originally Posted by MrE View Post
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.
Gordy's explanation doesn't make sense. If it works for him, then great ...

Drela's explanation makes sense in the context of stability and control.

A question ... it appears to me that Gordy thinks we should fly our planes much closer to the NP, Drela says find the NP and then setup the CG so the plane flies the way you like (presumably I could find the NP and then move the CG forward). What's the difference?

And Gordy made a direct statement about the veracity of the dive test. In other words, (as far as I can tell) Gordy disagrees with Drela.

BTW, nice use of the "appeal to authority" logical fallacy.

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I disagree
No worries.
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Old Dec 12, 2012, 07:59 PM
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Originally Posted by MrE View Post
I added the emphasis. Gordy says its dumb - but he also says its probably true - the dive test does what it does. He admits that. But he doesnt think its the best way to check balance because it only indicates the models trim state.

He goes on to say that another reason he doesnt like the dive test is that its conducted at hi speeds. Boom and control rod flex, wing flexing etc all effect the dive test results at hi speeds.

His test on the other hand is conducted at speeds closer to how the model is flown in a thermal.
And it would appear to me that Gordy is trying to make a distinction between trim and stability. They are not the same.

Further, the "dive test" is not supposed to be a screaming dive. It is supposed to be an elevator pulse which upsets the airplane for a very short time. If you're reaching crazy airspeeds where your carbon fiber creation is flexing under aerodynamic loads, then you've missed the point.
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Old Dec 12, 2012, 08:01 PM
MrE
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One more comment about dive tests. I had a model just recently that early in my trimming cycle would pull out fairly quickly from a shallow dive. However I discovered that if I let it get too fast in the dive it would tuck instead.

That dive test did not in any way indicate CG.

Thats the kind of thing Gordy is talking about when he says the dive test is dumb - for checking balance.
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Old Dec 12, 2012, 08:03 PM
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By the way - Gordy isnt the only one poopooing the dive test. Ive heard many many people saying pretty much the same things Gordy is saying - although perhaps not in exactly the same way
So what you're saying is that Gordy (an expert), and a bunch of other people, disagree with Drela (another expert).

Fascinating.

No wonder people are confused.
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Old Dec 12, 2012, 08:03 PM
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This IS fun!
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Old Dec 12, 2012, 08:10 PM
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One more comment about dive tests. I had a model just recently that early in my trimming cycle would pull out fairly quickly from a shallow dive. However I discovered that if I let it get too fast in the dive it would tuck instead.

That dive test did not in any way indicate CG.

Thats the kind of thing Gordy is talking about when he says the dive test is dumb - for checking balance.
And to what did you attribute the aircraft's response?
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Old Dec 12, 2012, 08:23 PM
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The dive test is certainly dumb if you don't do the first part of it, which is to trim for level flight at normal thermal speeds. Then you do the dive part to check the model's trim state. If it pulls up you're carrying extra up elevator trim. Why are you carrying extra up elevator trim? Because your plane was nose heavy when you did the first part. So the whole test, parts A and B together, is a way of checking your balance....at least that's my understanding of it.

But at this point I'll just throw my hands up in the air and say "I don't know anymore". This stuff is supposed to be science, not magic. I thought the truth was out there. I wish I had a wind tunnel to test the pitching forces on an air frame in different states of trim while varying the airspeed. Maybe we need to get Mythbusters involved.
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Old Dec 12, 2012, 08:32 PM
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maybe we need to get mythbusters involved. :d
yes!!!
Best idea so far!
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Old Dec 12, 2012, 08:42 PM
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This IS fun!
Indeed.

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Old Dec 12, 2012, 09:01 PM
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But at this point I'll just throw my hands up in the air and say "I don't know anymore". This stuff is supposed to be science, not magic. I thought the truth was out there. I wish I had a wind tunnel to test the pitching forces on an air frame in different states of trim while varying the airspeed. Maybe we need to get Mythbusters involved.
The wind tunnel thing, of course, has been done.

In 2008, Matthieu Scherrer put a F5D racer model in the ISAE wind tunnel, and measured (among other things) Cm as a function of AoA for two different airspeeds (20 m/s and 40 m/s). The results show that Cm varies with AoA as you would expect from a stable aircraft (negative slope of the Cm vs. AoA curve), but there appears to be no difference in Cm when airspeed goes from 20 m/s to 40 m/s, all other things being equal. As expected.

The Excel file with the results is available from this link: http://pagesperso-orange.fr/scherrer...easurement.xls
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Old Dec 12, 2012, 11:29 PM
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Who said it was dumb? And what are his credentials?

All of you have obviously watched the Radio Carbon Art Masters of Soaring Videos, heck you guys likely own every copy right?

The dive test is mentioned and done in one of the videos... can one of you repeat how its done and what one of our countries best contest pilots looks for in the dive test he does?

Tom K read an excerpt posted by Glider Jim of an article I wrote...not the article.

Miami Mike, are you saying that I don't know more than you about rc sailplanes?

I was there in Calgary when Tom K flew for our country at the F3J Worlds....You?

And one more thing....I'll remind you guys who want to be 'right' about balance....

Full flying stabs allowed guys to set their own balance points, add lead add up trim, because 'stability' is the main thing...at least it is to guys who don't want to do the work to learn how to control the high performance models they paid for.

But it became clear that balance and cg were two different things and only one was important to task soaring...on the slope or thermal. (oh yes I have flown more slopes (and slope models) than anyone else in this thread...in the world).

Because of that understanding the latest contest planes are starting to be produced with fixed stabs (incidence to wing)...the only way you can get away with that is if the balance point is static...not adjustable. The LATEST designs.

Not only X tails but we are seeing Vtails returning now that task balance has been figured out.

What will you guys do when all that is offered is fixed stab ships that won't let you hobble aircraft and have them fly straight?

Who was it that quit posting advice and comments on the RCSE when he posted an explanation of why it made no sense to dive planes to test balance? The one guy who has more soaring achievements in our hobby. The one guy who realized that many of you were more interested in arguing than learning...and we never saw another post from him again.

That toss test is about the end of the flight not the the control of it. Thumbs off the sticks or feeding up trim ...if the nose drops when the model stops, gravity had something to get a hold of that was being held by the elevator up trim.

The fact is if you balance at about 45% of root chord with a full flying stab ship, its balanced close enough. Why, because the stab will be at about zero incidence, the model will not indicate airspeed changes by pitching nose up or down, instead the model will change speed when it enters lift or sink.

This thread is about rc sailplane guys learning more about the topic. While I haven't added up the flight hours, likely I have thousands of hours flying with guys like Tom K, Phillip K, JW, DP, BC, LJ, AB, MS...Dr Drela. and others of their skills around the world. Visalia, Southwest Classic, Heathcote Cup, Tangerine, Nats, TNT, MidSouth, Mid Am, Isle of Wight, Great Pumpkin Fly...heck I have even flown...with most of you.

So the question is this, do you want to learn more about the hobby or do you want to show how brillant you are?

Read my stuff, go to the field and test it...prove that its wrong by doing - not talking....like I have...then and only then did GordySoar...write about it.

And Capn Dave and the rest...
You'll note that each of you who have typed my name in comments...used my name. Are you afraid or ashamed to let everyone know who is behind your comments?

Gordy
Today I flight tested the Pioneer flying wing sailplane, and worked on Wil Lipscomb's new SupraX 4 ship...you? ;-)
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Old Dec 13, 2012, 02:38 AM
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I ve read and read about that 45% of root chord, but i am still afraid To implement it. Not because i doubt about you saying that, but because of distance of numbers.

On my Topmodel Albatros root chord is 24,3cm and thus 45% gives a starting point of 10.9cm. Now, mfg gives a cg of 71mm To 73mm, which is of course nose heavy. I placed the cg at 80mm and its flying like never before. But hey, 80mm To 109mm is quite a distance and makes me hesitate.

Should I go To it, or going mm by mm and see what happens?

I am currently rebuilding it as thungs inside had To be moved, in order To achieve a cg further back of 80mm

Thanks
Friday
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