Cg question on a mk 4

[Jbird 15]

i just completed the Cg calculations on my mk4 long speed wing I ended up at 16.63 the total weight is 583 . The Cg is a little aft .any advice or suggestions Would be helpful . Thanks Rusty

Edited 18 Apr 2016 by Jbird

[EDMO 1078]

Rusty,  Is that empty or with pilot and fuel?  I think most EW CGs are in forward range, about 9 - 11?   EDMO

[Jbird 15]

Hi Ed that is empty . I think the Cg limits are 11" and 16.5

Edited 18 Apr 2016 by Jbird

[EDMO 1078]

Rusty,  Have you rechecked your numbers?  Is there some weight (Battery?) in the tail that could be moved?   Have you figured out what the CG would be loaded?  I think Akflyer said his flies best at about 17 or 18 which is outside of factory rear limits.  I think empty you should be close to 11.   Kitfox is about the same.  

If you tried to post pictures or W/B, it didn't show when I clicked on the Xs.    EDMO

Edited 18 Apr 2016 by EDMO

[Jbird 15]

I will recheck the numbers . Thanks Ed 

[EDMO 1078]

If you missed my edit, "If you tried to post photos, or WB papers, they didn't show when I clicked on the Xs on your first post.    EDMO

[Jbird 15]

Ed the numbers came out the same, but when I did the loaded numbers I came up with 16.45 

[1avidflyer 1983]

That seems very heavy on the tail.  Are you sure you have the wing leveled at the correct position?  My Avids weight about the same and the tail wheel weight was about 50 lbs. Jim Chuk

[akflyer 1427]

I would leave it as is.  I have talked to Dean Wilson about this at length and he stated they did testing to 21".  He was very comfortable with me going to 18-19" but cautioned against doing stalls at CG further aft than 20" unless you were using full flaps.  I love the way my plane fly's at 18".  All phases of flight are better at the more aft CG. 

His basic words were that when the plane was designed the idea was for people who could not normally afford to fly would build this plane and learn to fly in it.  They kept it very conservative for the rear CG limit so a newby had no issues with the tail dropping out on them.  Everyone I know that has flown with the plane in the 16-18" range has not had a bad thing to say about it. 

I know I have personally flown with a much more rearward CG, but I (like Dean) would not recommend it to the average low time pilot.... A nose heavy airplane fly's poorly, a seriously tail heavy airplane fly's once.

[EDMO 1078]

Ed the numbers came out the same, but when I did the loaded numbers I came up with 16.45 

Rusty,  Something don't seem right here - Your loaded cg should be more aft than your empty cg  

Can you post your figures?  I do mine with pencil and calculator - too old to trust the computer cgs. 

Edited 18 Apr 2016 by EDMO

[1avidflyer 1983]

Did you subtract the tare on the rear scale when you blocked the tail up to level?  Dumb question, but I'm sure it's happened.   Jim Chuk

[EDMO 1078]

Did you subtract the tare on the rear scale when you blocked the tail up to level?  Dumb question, but I'm sure it's happened.   Jim Chuk

Another basic question:  How accurate are your scales?  EDMO

[C5Engineer 1170]

I've never see one that far aft. With fuel and you it's going to be outside the published envelope. What did you use for an arm to calculate the moments? Did you hang plumb bobs off the leading edge to measure? I also think your tail is super heavy. Did you weigh the tail with it level on the third rib out per the manual or in a 3 point attitude? Most 2 stroke powered birds fall in the 10-12 range empty. 

[Jbird 15]

Thanks for the replys . I didn't understand that the tail needed to be level when it was weighed. After re reading the instructions it is pretty clear. I did level the plane to find the arm. It was +1.5   I will re weigh the plane leveled from the 3rd hinge rib. It will be interesting to see the difference. I will post the results . Thanks again Rusty

[1avidflyer 1983]

Bet your tail wheel will weigh between 45 and 55 when it's level.  Jim Chuk

[nlappos 413]

Here is the excel sheet for my Mark IV. I stole shamelessly from Joey, who did the real work. The tail should be 50 ish pounds. I think. I did a spot weight after mods, without leveling, just to be sure the total made sense (it did). With the aircraft in normal tail dragger attitude, the tail weighed 94 lbs. Leveled, it weighed 54 lbs.

 

LAPPOS N4052X Weight &amp Bal.xls

Edited 19 Apr 2016 by nlappos

[akflyer 1427]

Weights on tails vary with landing gear used.  This is why you must level the plane and drop a plumb bob and get the datum for your main and tail wheel.  My new gear is a few inches forward of the stock gear so i have more weight on my tail than most.  when i first swapped gear i also weighed it and i did some serious head scratching when i punched the weights into my previous spread sheet.. once i had that duuuuh moment i leveled the plane again and pulled new measurements on the mains and suddenly things made much more sense.