Author Topic: Double Eagle CG concerns  (Read 2114 times)

Offline Dan_

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Double Eagle CG concerns
« on: January 06, 2016, 09:25:30 AM »
Friends,
I have been asked to put up some information on the D.E. CG.  

MrG from down under in Australia and Tom Hubbach from down under in Louisville, in particular wanted to give builders a heads up to perhaps save some grief regarding the CG.   Both have built beautiful examples of the Double Eagle and are flying them.

The evidence is pointing to the CG on Leonard's prototype being close to the forward most end of the CG range for the Clark Y airfoil, and we know Leonard builds light.  So if you add any weight at all ahead of the pilot you are headed for a CG headache.

I have attached 2 PDF files.  One from Tom, and one from EAA on the Baby Ace.  The Baby Ace is a popular design and the DE shares the airfoil and essentially the same chord, so it would seem valid for comparison.

I am not an engineer or aircraft designer,  I do watch a lot of T.V. and stay at Holiday Inn express every chance I get.

In my opinion...  you should only tack the cabanes or leave them off until you know where your cg is going to be, because if you add weight up front with that starter or bigger/different engine you are looking at moving the wing forward to get to the 30% chord cg the airfoil needs.

I am including some comments Tom sent to me recently below and may add to them if I get time to do more research on the old Yahoo group...

Edit:  attached 3rd EAA PDF file on stability, control and trim...  Scroll all the way to bottom of post to download. Also a link to this msg has been placed in the news fader on the forum page.

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With me flying, DE Stubby is around 27%.  As I have said, it would probably be better more aft.  This is based on power off stall tests and landing experience.
The battery is located in the tail.  It's a motorcycle batt, maybe 8 - 10 lbs?

Of course, canting the gear forward tends to move the CG forward, too, which did not help us.

We've done everything reasonably possible (and not too reasonable, i.e., moving the firewall back 7") to get the CG to where it is now.  About only thing left is to install a heavier tailwheel, but that means more weight, and we don't need any more of that.
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To

  • Stephen Kiblinger
  • Dan
  • Grant Booth

CC

  • Tom Hubbuch

Message body
Steve, your spreadsheet has you flying with CG at 17.", about 30-31% .  Good info.
Tom






From: Stephen Kiblinger <>
To: tom hubbuch <>; Dan <>; Grant Booth <>
Sent: Tuesday, January 5, 2016 11:12 AM
Subject: Re: DE issues

A rule of thumb I'm familiar with is a CG up to but not beyond 1/3 of chord on a majority of airfoils...

All the Eagles have the same airfoil... As I grew heavier LE #33 was flown right at the the first third boundary... Scroll down here to see the Wt & Bal. : http://angoraaffaire.com/leu/whatdoesitweigh_.htm

On 1/5/2016 7:16 AM, tom hubbuch wrote:

We added weight to the tail as we experimented with flying CG.  We added weight a bit at a time, to carefully move CG more aft.  We never got to the 30% point, but others have, and I think Stubby would be good at 30%.  But, I don't know about 35%.  Use care.


Tom H



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Dan, I have attached a pdf of the latest W&B sheet for Stubby, in case it is of any use to anyone.  Also, I found some more data I had and some other info from other builders of the Eagles.  It is copied from a spreadsheet so I hope it comes out in a readable fashion:



CitabriaCG range is 21 - 36% at lowest loading
CG range is 24 - 31% at max loading
Cessna 150CG range is 24 - 34% at lowest loading
CG range is 26 - 34% at max loading
From FAA Aircraft Weight and Balance Handbook, page 1-3 and 1-4:
1If the CG is too far forward, the downward tail load will
have to be increased to maintain level flight. This increased
tail load has the same effect as carrying additional weight;
the aircraft will have to fly at a higher angle of attack, and
drag will increase.
2A more serious problem caused by the CG being too far
forward is the lack of sufficient elevator authority. At slow
takeoff speeds, the elevator might not produce enough
nose-up force to rotate and on landing there may not be
enough elevator force to flare the airplane. [Figure 1-3]
Both takeoff and landing runs will be lengthened if the CG
is too far forward.
Steve KlibingerLEU flying CG at 17"; 30% wing cord, per chart:
http://www.angoraaffaire.com/leu/whatdoesitweigh_.htm
Jimbo Stevens"Hawg", LE, chopped the throttle coming over the fence, trying my short field landing.  The nose pointed down and no amount of elevator would pick it up.  I tried the weight on the tail trick, which made no discernable difference, and finally surmised that if I have no power I will carry an extra 5 mph to make up for it.
LeonardIn a note to Steve Kiblinger, I wrote:
I recently saw a post stating that Leonard had a near nose-over with the straight gear, thus generating the canted gear design.  Replicating Leonard's W&B calculations in the plans, and adding 30# fuel and my weight (225#) (these arms based on our DE, but probably close to Leonard's DE arms), his airplane flying CG calculates to be about 14%.
Gil DevaultFor what it is worth, My DE was built according to the drawings. All of my weights and measurements all came out to what they were supposed to. My plane flew just fine, no adverse tendances at all during normal flying. The problem I had was at low power settings on landing. It did not want to flair with out a prop blast across the elevator. I would suggest that special attention should be paid to this. It could present a problem with an engine out situation.
My DE did not flair during a dead stick landing and I had an IAS over 50 MPH
TWHand I do not think our DE can be in its safest condition if flown with a CG at the forward limits as shown in the DE plans (that is, at 10.9% of cord).  All evidence that I have seen suggests that forward CG tends to encourage nose down pitching.  CGs in the 29% area (as in our LEU and Spencer's LEU) seem to reduce the nose down pitch when power is reduced.  So, we are testing CG points moved closer to 25-30% to understand the effect on landing flare.
John SteereBodacious, modified XL
CG is right where I want it, so it rotated nicely for the landing flair at an indicated 45-mph
design intent for my CG range is 22% to 34% of chord, or a target of 28% ± 6%.  The empty CG is at 28.05% of chord, and the fully loaded (5-gal of fuel, full oil, 187-lb. pilot, and removable instrument panel) is at 28.345% of chord, or 52.88% of the CG range