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Old Apr 19, 2012, 07:05 PM
Full Scale Piper Cub Driver
Piper J3's Avatar
Hinckley, Ohio
Joined Nov 2008
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Question
COG question...

Tonight was a beautiful night for trimming. Approximately 2 mph wind, 60F, and no clouds. Flew my E-Pulsar 3.2 REF about 45 minutes before sundown in what appeared to be zero lift. I tried different elevator trims and I think itís about optimum for speed above stall and flatness of glide. Sink rate (L/D) looks to be in the 90 fpm range which I think is nearly twice what other folks have been reporting. I havenít done dive testing for COG yet Ė Iím a little scared. Love the way this plane flies. Does COG position affect L/D or is COG position mainly used to have sailplane single weak lift? Open to suggestions. Thanks in advanceÖ

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Old Apr 19, 2012, 07:29 PM
Ascended Master
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Palmdale, CA
Joined Oct 2000
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A properly located c.g. will have the least trim drag for the speed.
Dive test... be gentle.
Get trimmed, push over lightly, not full down! Watch what happens.
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Old Apr 19, 2012, 07:31 PM
Full Scale Piper Cub Driver
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Am I right that 90ish fpm sink rate is a bit high for Pulsar 3.2?
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Old Apr 19, 2012, 09:38 PM
"...certainty is absurd."
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Min sink speed will be slower than best glide speed by a couple of kph. Min sink speed is just above stall speed, so you may have to try slowing it down more if you are flying at best L/D.

CG position, within the range usually used on RC sailplanes, will make very little difference to performance. Depending on the exact configuration, minimum trim drag should be at about a 5% static margin for a conventional lay-out with a stab about 20% the span of the wing. This is reasonably pitch stable. If you get the CG back near the neutral point, all the control deflections required to try and hold a speed, and speed changes, will cause the performance to get worse.

Turning will really increase the sink rate. Most sink rate tests are done by flying as straight as possible. Either fly straight out as far as you dare and make a very gentle turn back, or fly a large rectangular circuit with very gentle turns.

Kevin
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Old Apr 20, 2012, 07:44 AM
Red Merle ALES
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From the master:
http://www.polecataero.com/handlaunchu/cg-location
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Old Apr 21, 2012, 10:34 AM
"...certainty is absurd."
kcaldwel's Avatar
Joined Jan 2007
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This is a good illustration of glider polars and the various optimum speeds:

http://www.5c1.net/Glider%20Performance%20Airspeeds.htm

Kevin
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Old Apr 22, 2012, 09:57 AM
Registered User
Mobile, AL
Joined Jul 2008
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? for cloudyifr

Mark Drela's commentary on CG is his usual carefully crafted, lucid explanation.
As I understand it, the dive test results depend on the position of the elevator trim in cruising level flight. If the decalage is correct and the CG is at the rear limit of neutral stability, then the elevator should be in neutral in that flight state; is that correct. Should the decalage be incorrect, then the whole concept is screwed up. I am having a chicken or the egg problem with this.

Thanks
Lloyd
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Old Apr 22, 2012, 10:02 AM
turn, turn, turn.
Athol, Massachusetts
Joined Oct 2005
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Quote:
Originally Posted by lloyd3521 View Post
Mark Drela's commentary on CG is his usual carefully crafted, lucid explanation.
As I understand it, the dive test results depend on the position of the elevator trim in cruising level flight. If the decalage is correct and the CG is at the rear limit of neutral stability, then the elevator should be in neutral in that flight state; is that correct. Should the decalage be incorrect, then the whole concept is screwed up. I am having a chicken or the egg problem with this.

Thanks
Lloyd
Imagine that the elevator is a full flying elevator... then your problem disappears.
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Old Apr 22, 2012, 10:43 AM
Ascended Master
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Palmdale, CA
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Quote:
Originally Posted by lloyd3521 View Post
Mark Drela's commentary on CG is his usual carefully crafted, lucid explanation.
As I understand it, the dive test results depend on the position of the elevator trim in cruising level flight. If the decalage is correct and the CG is at the rear limit of neutral stability, then the elevator should be in neutral in that flight state; is that correct. Should the decalage be incorrect, then the whole concept is screwed up. I am having a chicken or the egg problem with this.

Thanks
Lloyd
.
Whatever the decalage, from a trimmed condition, a sufficiently nose heavy plane will self recover from a dive test.
You can play around with c.g. and decalage to get a different response.
But don't push into a vertical dive! The wingtips can meet over the center of the airplane!
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Old Apr 22, 2012, 10:48 AM
"...certainty is absurd."
kcaldwel's Avatar
Joined Jan 2007
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Quote:
Originally Posted by lloyd3521 View Post
Mark Drela's commentary on CG is his usual carefully crafted, lucid explanation.
As I understand it, the dive test results depend on the position of the elevator trim in cruising level flight. If the decalage is correct and the CG is at the rear limit of neutral stability, then the elevator should be in neutral in that flight state; is that correct. Should the decalage be incorrect, then the whole concept is screwed up. I am having a chicken or the egg problem with this.

Thanks
Lloyd
"Decalage" and elevator position are the same thing. If the trim speed is correct, through either setting the elevator or stab angle, then the dive test results will get you in the ball park for the CG. The elevator deflection to get to the trim speed doesn't matter, and makes essentially no difference to the performance either, as long as it isn't huge.

There are bunch of secondary effects such as the MAC and drag centre being raised way above the CG by dihedral, low Re effects, stab airfoil hysteresis, etc., that make the dive test less than completely reliable.

Adjust the CG position for the pitch stability you like from the glider pitch behaviour. Don't forget the reduce the elevator throw as you move the CG back, because leaving large throws can make the glider seem skittish in pitch with aft CG when it is really just too much elevator throw. The elevator gets more effective with aft CG. Trim speed is then set with the elevator. If you end up with large elevator deflections for trim, you may want to change the stab or wing incidence to get a more neutral elevator at trim. Unless the trim deflection is quite large, it makes no difference to the glider performance, and it makes no difference to the glider behaviour unless there is a secondary problem of a poor stab airfoil or something.

Kevin
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Old Apr 22, 2012, 10:59 AM
Red Merle ALES
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United States, Mt, Helena
Joined Apr 2002
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Kevin,

Nice explanation! I often hear where folks think that small elevator deflections for their straight and level flight make a big difference but as we know they do not.

Here is a document from Dr. Drela I saved that confirms this:
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.
Curtis
Montana
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Old Apr 22, 2012, 11:02 AM
LSF303 / AMA Life Member
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USA, IL, Wheeling
Joined Jan 2003
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If a reasonable dive results in a very slow recovery and it takes little down elevator to fly inverted, the CG is pretty close.

But then CG is like spicy foods, it's pretty much to each individual's taste.
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Old Apr 22, 2012, 01:52 PM
Registered User
Mobile, AL
Joined Jul 2008
168 Posts
To Kenny Sharp

Tried to engineer a flying stab and consistently fail to get the lateral rigidity I find acceptable. Should have studied engineering or else quit trying to design my own.
Thanks all.
Lloyd
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Old Apr 22, 2012, 03:52 PM
Zor
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Ontario,Canada
Joined Feb 2007
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Quote:
Originally Posted by lloyd3521 View Post
Tried to engineer a flying stab and consistently fail to get the lateral rigidity I find acceptable. Should have studied engineering or else quit trying to design my own.
Thanks all.
Lloyd
If a cantilever main wing can be engineered to stand the full lift forces including high G forces then a flying horizontal tail surface can be using the same techniques.

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