Model III Weight and Balance - Need 300 in-lbs fwd

[LSaupe 123]

Performed weight and balance today. Adding the larger rudder/vertical stab, elongated elevator and rear header tank has taken me out of W&B for the most aft CG condition. I need 300 in-lbs (fwd) to get me back in. Hate to add ANY weight, but not much I can do now (nothing to move). Battery and ELT are forward under the dash (behind the firewall). Are there any good places to add in weight forward? Probably distributed would be better than concentrated.

[EDMO 1078]

Three questions:  What is your most aft CG?  Leni says his flys best at somewhere about 17 - 18" aft - confirmed by Dean Wilson.   I think Kitfox has more narrow CG ranges by about 2% than Avid, and they are the same wing.

                           Are you figuring Pilot and fuel?

                           What would heavier tires do for you?

Sometime we overlook the obvious, like having someone else check our figures.

Warp Drive props are a lot heavier than your GSC.

EdMO

More:  What is you EW and Loaded weight, and is GW 1050?

Edited 23 Aug 2015 by Ed In Missouri

[LSaupe 123]

Hi Ed:

 

Using leading edge as a datum, most aft CG is 14.71 inches.  Max allowable from the build manual is 14.28" (28% MAC).

 

Yes, figuring pilot, copilot and fuel.  14.61 inches (no fuel) vice 14.71 (full fuel - 13 gal)

 

Heavier tires won't gain much at all because the arm is so dang small (3.25").

 

However, I am using the book values for arms of pilot/copilot and wing tanks.

 

P/CP = 15.8"

Wing Tanks = 15.97"

 

This may sound odd, but would a small weight disc at the prop hub make sense?  Gets it out front and keeps the MOE reasonably low, though do hate to make additional weight dynamic.

 

Yes, max GW is 1050.

 

Maybe Doug can chime in here as well on where his Model III sorted out during W&B.

[EDMO 1078]

Larry,

      I think you are needlessly worrying about a fraction of an inch. 

      I believe as the "manufacturer" of your plane you can set your own cg limits. 

     Phase 1 has you list the limits you flew it at. 

Most designers use 30% Chord which would be 15.3", and some even use 33% which would give you almost 17".  

     I think you need to have a conversation with Leni (Akflyer) about this.  

I believe with elevator changes your aft cg can be increased, and Avid already is increased.

      I am going to check both my Kitfox and Avid manuals to see what % they used, and get some figures in inches for each.

I will get back to you - working on my wings today since the weather finally turned cooler and humidity went down.

EdMO

Edited 23 Aug 2015 by Ed In Missouri

[TJay 1041]

Isn't that great ED when you step outside in the morning to 53 degree weather, really makes for a better day.

[TJay 1041]

Every Kitfox 3 I have ever seen with a 582 on it has a 20lbs chunk of led hanging under the front two bolts of the engine mount.

[Av8r_Sed 82]

No 20 lb chunk of lead on mine, but I do have an Ivo med 2 blade and Rk400 clutch up front.

[EDMO 1078]

I think a 3-blade Warp Drive or an IVO IFA up front would allow him to put some baggage in the back.

TJay,

We don't have 53 degrees wake-up here yet, maybe 60 - but the temps are a lot better than the 90's with 100% humidity we had.  Could be worse - I have seen it 112 to 115 here in July and August!

LarryS,

      I think Leni's Avid C/MkIV plane may be about the same as your KF3/elevator mod - except his heavy prop.

I suggest you try a PM to Akflyer and talk to him about W/B.

     

EdMO

Edited 23 Aug 2015 by Ed In Missouri

[LSaupe 123]

Found this in the Kitfox service bulletins.  However, it applies to the Model IV's (shown), however the website seems to indicate that it applies to M1 through M4 (see right side column).  

 

http://www.kitfoxaircraft.com/Service%20Letters.htm

 

 Anyone seen a pressure distribution comparison between the two wings?  Curious were the center of pressure point is between this two wings over the flight range.

 

*************************************

 

 

SERVICE LETTER #30A

DATE:                    May 1, 1994   revision  September 7, 1994

SUBJECT:                              Aft CG Limit

APPLICABILITY:                All Model IV-1050 and Model IV-1200 Kitfox's

FROM:                   SkyStar Engineering

This service letter is intended to provide you with information regarding the operating center of gravity range for your aircraft.

We have completed a series of flight tests on our factory Model IV-1200 Kitfoxâ„¢, and our findings support the expanding of the allowable CG range.

The new aft CG limit for Model IV-1050 and Model IV-1200 Kitfox's and Model IV-1200 Speedsters is 16" aft of the leading edge of the wing.  The old limit was established at 14.78", but that has been found to be too conservative.  At 16", you will find that loading your Kitfoxâ„¢ within the CG range to be much easier.

Please make the appropriate changes in your builders manual and/or flight manual referring to this Service Letter.

We are also aware that many of the Model IV-1200 Builder's Manuals were shipped with weight and balance data sheets that incorrectly stated the aircraft gross weight as 1050 pounds, and that some builder's had problems getting their aircraft registered at the correct gross weight of 1200 pounds.  We have re-written the section and can send a corrected version on request.  

Edited 24 Aug 2015 by LSaupe

[EDMO 1078]

Not too sure about the earlier models with smaller tails, which you have changed, but this confirms what I was saying about designers using 30 to 33% chord - 32% would be 16.12 inches aft cg, and still think this may be conservative.  Talk to Leni.

I think my personal cure for weight and balance would be to go on a diet !

EdMO

Edited 24 Aug 2015 by Ed In Missouri

[Av8r3400 1179]

Here's the lik to Leni's post on CG issues:  Link

 

Summary:  The wing flies awesome at 16-18".  20" is workable but you may need the help of flaps to get the nose down in case of a stall.

 

My friend Dave built his hybrid with an extended Avid C fuselage with late model Kitfox wings.  His empty CG is at 16-1/8".  The extended tail gives authority to control pitch with the aft cg, without issue.  My Mangy Fox, with the enlarged tail surfaces will be more aft than my stock yellow Kitfox.  I have no worries about loading my extended baggage area to an 18" CG.

[EDMO 1078]

Good Post Larry3400,

     He may learn something from reading the posts in Akflyer link.   I went back and read it too.

Marshawk,

     What numbers did you finally come up with?

EdMO

[LSaupe 123]

Might be splitting hairs here, but one sensitive spot in the measurement is the tail wheel measurement.  Long arm with low weight.  Gonna check this with another scale. a 2 lb error would get me there, however, this could go either way right?  I have never seen anyone indicate an incertainty value with the measurement, so it might be warranted.  Though the seat of your pants is most likely the best measument.

 

Also, the leading edge location.  Looks like the leading edge added former is an optional install.  Mine has these installed, which causes the reference point (leading edge) to be further forward than if it did not have these. So... what are the Kitfox limit numbers based on (with or without the forms on the leading edge)?  I have seen a few without them, but assume they used the same procedure to check CG?

 

My numbers are as follows:

 

Empty CG: 13.84"

Most forward: 14.29"

Most Aft:14.71"

[LSaupe 123]

Interesting that the original Model IV aft CG is 1/2" further back than the Model I, II,III.

 

Also that the leading edge forner is also 1/2".

 

Maybe the Model IV this was standard (having the former) and they added that 1/2" in to the datum?

 

"If" I can take credit for that 1/2" I am fine.

[EDMO 1078]

Tie a helium balloon to the tail and re-measure - I think you are too concerned about a fraction of an inch on an outdated W/B because of the smaller elevators on the early planes - Your plane will gain weight as time goes by anyway.   Keep the letter and references from Kitfox on file, and use that to verify larger cg range.  The inspector will not weigh your plane - and probably will not read your W/B sheets - just verify that you have them.

     Sounds like you need a heavier prop like a 3-blade Warp Drive that most use, then you could haul a bag in back.

EdMO

Edited 24 Aug 2015 by Ed In Missouri

[LSaupe 123]

I hear ya.  I am not trying to beat a dead horse, but unfortunately the letters don't appear to apply to the Model III.  Add to the fact that I will not be doing the functional check flight (EAA Flight Advisor will be doing that for me).  Just looking for something a bit more concrete here that indicates this increase in aft CG is acceptable/reasonable to increase the comfort factor (or at least allow for an educated decision).  The only documentation I have, that covers the Model III, is the Model III build manual with regard to CG limits.

 

The arguments sound very convincing, but unfortunately no paper trail appaers available until I do my own testing.  I have contacted Kitfox themselves to see if they can clarify the letter 30A applicability, so maybe that will be what I need here.

 

By chance Is there anything on the Avid lmits with the under camber wing?

[1avidflyer 1983]

Avid W/B envelope is forward: 11.185", Aft. 16.5" That is with the MK IV Heavy Hauler wing. It is nearly identical to the Kitfox 3 wing. If you are really worried about the guy doing the check flight not wanting to do it, add just enough weight as far forward as you can to get it into the Kitfox limits for his flights, and you can remove it later if you like. Jim Chuk

[1avidflyer 1983]

I tried to edit my last post after I sent it. Second sentence should say "It's profile is nearly identical to the Kitfox 3 wing". Having trouble editing for some reason.... The heavy hauler wing uses thicker walled spars, more ribs, and shorter length, but the shape is very similar. Jim Chuk

[EDMO 1078]

Jim,

      I am probably the least knowledgeable about Avids, but I am almost certain that Leni said all Avid wings are the same cg range - maybe not counting the Airdale and Magnum - but I think my Magnum book lists about the same cg range as the other Avids - I checked on that, and the Magnum cg is 11.185 to 17.5 - guess because it's longer.

The main thing that changes is the gross weights.

LarryS,

      The Kitfox 3 W/B uses the front spar as the leading edge for all measurements - Would you be OK in aft cg using that measurement?   The plastic leading edge addition is a Kitfox 4 or 5 modification you made, and not by the manual, so you might get by with front spar measurements and be "by the book"? 

EdMO

Edited 25 Aug 2015 by Ed In Missouri

[1avidflyer 1983]

Ed, I think you are correct, my memory says it's the same for all Avids (A,B, C, MK IV). As far as measurments, I would think front of wing is front of wing. A leading edge extention would move the front of the wing forward, and so that is where I would measure from. Jim Chuk

[LSaupe 123]

Ed, Yep, I would make it if I corrected back to the spar.  Ultimately it may bear down to if the cuff changed the center of pressure on the wing at all.  Pressure being more set by the local wing geometry vice the leading edge (just a guess).

Edited 27 Aug 2015 by LSaupe

[EDMO 1078]

Jim

     I agree - as a practical measurement should be from front of wing -

    However his cg limits, as listed in the manual, are written as measured from the front of the spar, as are all dimensions for fuel, pilot, wheels, etc - if his manual is like some others I have read.

     I don't know if Larry measured or changed all the dimensions, or went by what was listed in the manual.

The KF3 manual does not include any additions to leading edge or tail, as none were available at the time the manual was written.

     If you change the datum line location, you have to change all the other dimensions, and that is not "by-the-book".

EdMO

     My plane is a Foxy Flapper, a different size wing, with no kit or manual, and I have to figure and write my own limits -

but that is a whole different story.

Edited 25 Aug 2015 by Ed In Missouri

[Av8r3400 1179]

Both of my model IV airframes use (as do all kitfoxes) the leading edge of the wing as the datum point.

[EDMO 1078]

Larry3400,  Yes, you are correct.

       I was pointing out the fact that there was no plastic leading edge extension when the manual for the Kitfox 3 was printed, and the leading edge was the front of the spar, and all figures were taken from that point. 

The leading edge extension started with the model 4 with the Riblett ribs, and they never changed the wording that the datum was the leading edge, although the leading edge was extended, as was the tail dimensions, and the cg range was changed.

EdMO

Edited 25 Aug 2015 by Ed In Missouri

[akflyer 1427]

I talked to Dean Wilson at length about the CG issues.  The original limits were established based off some one who had never flown before building the plane and learning to fly in it.  Dean said they did factory testing and it flew best with the CG around 18".  He did caution me to not stall the plane without full flaperons if the CG was behind 20" or 21", I don't remember off the top of my head.  I like mine at 18-19" and I have flown past that.  The plane never got too squirrely on me but I did have to use some flap input to keep the nose down.  I would not think twice about flying your plane where the CG is now.