#1 invicta421 Apr 02, 2003 11:05 AM

[RCSE] Why does dive CG test work?

I have been pondering this question for the last few weeks, but
haven't satified myself with an explaination yet. Most aerodynamics
seem to make intutive sense to me, but this doesn't.

The test I am speaking of is when you put the sailplane into a 45
degree dive and see if it pulls up, flys neutral or tucks under. A
page on how to do this can be found here:

http://www.polecataero.com/articles/cg_art.shtml

But this page doesn't explain why... to me it seems that a nose heavy
glider should tuck not rise when the CG is in front of the average
lift on the wing. Any help understanding this?

Thanks

-Richard

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 #2 Lex Mierop Apr 02, 2003 11:05 AM

RE: [RCSE] Why does dive CG test work?

Richard,

It's all about the trim. When flying a nose heavy plane at regular thermal
cruise speed, you have to add up trim to get it to fly straight and level.
The "up" trim setting is causing an aerodynamic force on the tail is pushing
down to compensate for the extra nose weight. The amount of aerodynamic
down force required to stay level is constant for a given amount of nose
weight. The amount of aerodynamic force created by a flying surface varies
with the speed of the airplane. As it goes faster it is more effective
(better Reynolds numbers).

So what you start with is a nose heavy plane that is trimmed for level
flight at cruise speed. The tail forces balance the nose heaviness of the
model. Now increase the speed, and what happens? The nose heaviness
remains constant, but the downforce on the tail increases, causing the model
to pitch up.

The inverse is true on a tail heavy model. To trim for straight and level
at cruise speed, the tail requires down trim (an upward force on the tail to
compensate for the lack of weight in the nose). Increase the speed, and the
upforce on the tail increases, causing the model to pitch down. That's a
tuck.

-l

-----Original Message-----
From: invicta421 [mailto:richardh@examen.com]
Sent: Wednesday, April 02, 2003 12:51 AM
To: soaring@airage.com
Subject: [RCSE] Why does dive CG test work?

I have been pondering this question for the last few weeks, but
haven't satified myself with an explaination yet. Most aerodynamics
seem to make intutive sense to me, but this doesn't.

The test I am speaking of is when you put the sailplane into a 45
degree dive and see if it pulls up, flys neutral or tucks under. A
page on how to do this can be found here:

http://www.polecataero.com/articles/cg_art.shtml

But this page doesn't explain why... to me it seems that a nose heavy
glider should tuck not rise when the CG is in front of the average
lift on the wing. Any help understanding this?

Thanks

-Richard

RCSE-List facilities provided by Model Airplane News. Send "subscribe" and
"unsubscribe" requests to soaring-request@airage.com. Please note that
subscribe and unsubscribe messages must be sent in text only format with
MIME turned off.
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 #3 George Voss Apr 02, 2003 11:05 AM

Re: [RCSE] Why does dive CG test work?

While I'm not an AE, I'll give it my best shot.

Think in terms of static and dynamic loads. You balance your sailplane
in a static mode using either your fingers, or some type of
deal with.

As the plane speeds up, the dynamic loads have more affect than the
static load, i.e. the elevator trim setting has more authority than the
static balance. Sooooooo, if you have more lead in the nose than
needed, you need to carry some "up" elevator to make the airplane fly
level at normal speeds. As the speed increases, the elevator over rides
the balance and the plane pitches up. The opposite is also true and
that's why the airplane will tuck if the CG is too far out of range.

I hope that helps. gv

invicta421 wrote:

>I have been pondering this question for the last few weeks, but
>haven't satified myself with an explaination yet. Most aerodynamics
>seem to make intutive sense to me, but this doesn't.
>
>The test I am speaking of is when you put the sailplane into a 45
>degree dive and see if it pulls up, flys neutral or tucks under. A
>page on how to do this can be found here:
>
>http://www.polecataero.com/articles/cg_art.shtml
>
>But this page doesn't explain why... to me it seems that a nose heavy
>glider should tuck not rise when the CG is in front of the average
>lift on the wing. Any help understanding this?
>
>Thanks
>
>-Richard
>
>RCSE-List facilities provided by Model Airplane News. Send "subscribe" and "unsubscribe" requests to soaring-request@airage.com. Please note that subscribe and unsubscribe messages must be sent in text only format with MIME turned off.
>
>
>

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 #4 Andy Thonet Apr 02, 2003 11:05 AM

RE: [RCSE] Why does dive CG test work?

This diagram might help.

http://clubedsf.org/html/cg_dive_test.htm

Andy Thonet
KG6MGD
Club EDSF

-----Original Message-----
From: invicta421 [mailto:richardh@examen.com]
Sent: Wednesday, April 02, 2003 12:51 AM
To: soaring@airage.com
Subject: [RCSE] Why does dive CG test work?

I have been pondering this question for the last few weeks, but
haven't satified myself with an explaination yet. Most aerodynamics
seem to make intutive sense to me, but this doesn't.

The test I am speaking of is when you put the sailplane into a 45
degree dive and see if it pulls up, flys neutral or tucks under. A
page on how to do this can be found here:

http://www.polecataero.com/articles/cg_art.shtml

But this page doesn't explain why... to me it seems that a nose heavy
glider should tuck not rise when the CG is in front of the average
lift on the wing. Any help understanding this?

Thanks

-Richard

RCSE-List facilities provided by Model Airplane News. Send "subscribe"
and "unsubscribe" requests to soaring-request@airage.com. Please note
that subscribe and unsubscribe messages must be sent in text only format
with MIME turned off.

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 #5 renselange@earthlink.net Apr 11, 2003 03:02 AM

Re: Re: [RCSE] Why does dive CG test work?

But, doesn't it follow from this very logic that things change with variations in speed? So, a slow dive test gives different results than a fast one - always? Intuitively, I always did the Gordy test ("try to make it fly nice, flat and far from a hand toss") and added or removed some lead to make the plane more or less stable according to my taste. I did do dive tests occasionally just for the heck of it and find that my planes slowly pull up from a dive. But since I fly primarily TD, I never worry about diving much.

Rense Lange

-------Original Message-------
From: George Voss <gavoss@swbell.net>
Sent: 04/02/03 09:23 AM
To: invicta421 <richardh@examen.com>
Subject: Re: [RCSE] Why does dive CG test work?

>
> While I'm not an AE, I'll give it my best shot.

Think in terms of static and dynamic loads. You balance your sailplane
in a static mode using either your fingers, or some type of
deal with.

As the plane speeds up, the dynamic loads have more affect than the
static load, i.e. the elevator trim setting has more authority than the
static balance. Sooooooo, if you have more lead in the nose than
needed, you need to carry some "up" elevator to make the airplane fly
level at normal speeds. As the speed increases, the elevator over rides
the balance and the plane pitches up. The opposite is also true and
that's why the airplane will tuck if the CG is too far out of range.

I hope that helps. gv

invicta421 wrote:

>I have been pondering this question for the last few weeks, but
>haven't satified myself with an explaination yet. Most aerodynamics
>seem to make intutive sense to me, but this doesn't.
>
>The test I am speaking of is when you put the sailplane into a 45
>degree dive and see if it pulls up, flys neutral or tucks under. A
>page on how to do this can be found here:
>
>http://www.polecataero.com/articles/cg_art.shtml
>
>But this page doesn't explain why... to me it seems that a nose heavy
>glider should tuck not rise when the CG is in front of the average
>lift on the wing. Any help understanding this?
>
>Thanks
>
>-Richard
>
>RCSE-List facilities provided by Model Airplane News. Send "subscribe"

and "unsubscribe" requests to soaring-request@airage.com. Please note that
subscribe and unsubscribe messages must be sent in text only format with
MIME turned off.
>
>
>

RCSE-List facilities provided by Model Airplane News. Send "subscribe"
and "unsubscribe" requests to soaring-request@airage.com. Please note that
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>

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 #6 Douglas, Brent Apr 11, 2003 03:02 AM

RE: Re: [RCSE] Why does dive CG test work?

<<But since I fly primarily TD, I never worry about diving much.>>

Hi Rense,

but that's not the point... The idea behind this test is that it shows what
you have to do to make your plane fly. Example, if your plane's nose heavy,
you've got to dial in some up to keep it flying level. That's why it pulls
up in a dive, the up elevator is magnified at speed.

If you always fly about the same speed, you may never see this, but you'll
still be fighting to keep the nose up and maybe not flying as efficiently as
you could be...

At least that's how I read the dive test, good luck!

Brent
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 #7 GordySoar@aol.com Apr 11, 2003 03:02 AM

Re: [RCSE] Why does dive CG test work?

But, doesn't it follow from this very logic that things change with
variations in speed? So, a slow dive test gives different results than a fast
one - always?

You are right... THINGS... do change (too many in fact), but nothing of value
to determining optimized balance. :-)

Gordy

 #8 Jeff Reid Apr 11, 2003 03:02 AM

Re: [RCSE] Why does dive CG test work?

Darn, dive test is meant to help adjust CG?

All this time, I thought it was meant to test the strength
of a glider's nose. So far my best is 40 feet into a very
muddy field. Wow, you learn something everyday. This is
going to save me a lot of repair time.

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 #9 Erich Merkel Apr 11, 2003 03:02 AM

Re: Re: [RCSE] Why does dive CG test work?

"But since I fly primarily TD, I never worry about diving much."

Jeez, what do you guys fly for? What's the point of going to all the work
of finding that thermal, and getting really high? What do you do when
you've maxxed the round and you can just barely see that new molded super
plane that you spent all those hours trimming to perfection? Personally, I
point the nose straight down, and make that sucker sing!!! There's nothing
like the sound of a moldy in a high speed dive!! Sooo sexy... ;-)

Erich Merkel
Colville, WA
Phone: 509-684-0440
Cell: 509-680-1141

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 #10 Douglas, Brent Apr 11, 2003 03:02 AM

RE: [RCSE] Why does dive CG test work?

Actually, I did that kind of dive test with a Paragon a couple weeks back,
and based on the depth of the hole, I think I was right on.

I've done repairs, hope to find a better way to "test" it next time.

Brent

*It's back together - lucky for me the ground was still swampy...

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 #11 Kurt W. Zimmerman Apr 11, 2003 03:02 AM

RE: Re: [RCSE] Why does dive CG test work?

It's not the high speed dive that bothers me... it is the sudden stop, thud
& crunch that does.
Kurt

-----Original Message-----
From: Erich Merkel [SMTP:slopesoar@theofficenet.com]
Sent: Wednesday, April 02, 2003 11:19 PM
Cc: soaring@airage.com
Subject: Re: Re: [RCSE] Why does dive CG test work?

"But since I fly primarily TD, I never worry about diving much."

Jeez, what do you guys fly for? What's the point of going to all the work
of finding that thermal, and getting really high? What do you do when
you've maxxed the round and you can just barely see that new molded super
plane that you spent all those hours trimming to perfection? Personally, I
point the nose straight down, and make that sucker sing!!! There's nothing
like the sound of a moldy in a high speed dive!! Sooo sexy... ;-)

Erich Merkel
Colville, WA
Phone: 509-684-0440
Cell: 509-680-1141

RCSE-List facilities provided by Model Airplane News. Send "subscribe" and
"unsubscribe" requests to soaring-request@airage.com. Please note that
subscribe and unsubscribe messages must be sent in text only format with
MIME turned off.

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