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Avid B model with MK4 Elevator measurements for EDMO

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Posted

Ed I got your measurements today. This was a B model kit serial number 279. The elevator was extended and trim tab added by Highwing LLC. The elevator was done to MK4 specs based off a template that was provided to me by Jack L (logtruckdriver) These are close wags not exact measurements. Sorry the pictures suck I mostly wanted you to have idea where I took the measurements from.

 

Joey

 

The horizontal is 18" at it's widest point.

 

IMG_6257.jpg

 

The elevator just so happens to be 18" at it's widest point as well.

 

IMG_6258.jpg

 

The horizontal is 12" at it's narrowest point out by the end.

 

IMG_6262.jpg

 

The elevator is about the same in the same location.

 

IMG_6261.jpg

 

The trim tab is 21 1/4" at the hinge and 5" at it's widest point. It's powered by a Ray Allen set up and is more than enough trim authority.

 

IMG_6260.jpg

 

IMG_6259.jpg

 

Hope this helps. I did 5 stop and go's today and really concentrated on slow approaches and getting the tail up quickly. There's just no comparison with this set up. Landing with 15 degrees of flaps on and and the back pressure trimmed off makes landing completely different for the better. I'm having to relearn how to land this thing after 350 or so hrs. It flies much more tail high with the flaps on which prevents hitting tail first at slower speeds. On the last one I crossed the numbers at 45 indicated. I used to cross the numbers at 50-55 and would still touch tailwheel first. Took a small burst of power to arrest the sink just before touchdown for a nice three point and a super short roll out. Compared to some other airplanes I still feel like I'm flying too fast. Hoping once I get my bush gear back on here pretty soon and can get the nose higher I can get it closer to 40 and still three point.

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Posted (edited)

Thanks Joey,
I hope these numbers and your flying report on the improvements will help someone considering updating the tail of the earlier models. It looks like your HS/Elevator ratio is 50/50, about the same as a J3 Piper Cub. I haven't calculated the trim tab % yet, but it looks close to my 24 x 3 tabs in size. I could make the trim tab chord wider on mine if needed, but I think it will be OK.  My elevator measurements are real close to yours, and my HS is a little larger - I may be about 55/45%.
Good Flying,
EdMO

Edited by Ed In Missouri

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Posted

Thanks for the measurements Joey.  I am looking forward to not have to hold the stick back with a death grip on a full flaps approach!  Also where do you keep you cg? I believe me with full fuel is around 15.5.

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Posted

I measured my elevator after work today. At the longest chord it's 14 inches. I have a baby trim tab 3 x 8 inches. Mod coming soon.

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Posted

That sounds about like my original size and I had no trim. My CG is forward at about 12.5" with just me and a light fuel load. My next big mod is to get rid of my side radiator and just run a belly radiator to shave some weight off the nose.

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Posted

This topic is of interest to me as well, since I also have the original B tail, and then some big nasty sheet metal extensions that were riveted on, and then another (fixed) sheet metal trim tab that was riveted on after that. After reading about Joey's increased stab/elevator area modifications, I'm considering the bigger elevator as a potential upgrade for me later on (size matters!)

 

The obvious question I have is whether anyone has checked into any negative aspects or increased risks associated with using a larger elevator and/or stabilizer. Meaning flutter, balance, ability to over-stress anything, etc. Do the maneuvering speeds change? If you can apply more down-force at the back, you can generate a lot more bending loads in the wings.

 

Can anyone tell me specifically how these increases are done... do they simply cut off the trailing edge of the elevator, move it rearward a few inches, and splice in more tubing? Or do they build a new elevator?

 

Joey, can you explain why you land more "tail-high" now? I would think that with additional elevator authority, to land at slower speeds you would be able to force the tail downward into a tail-low position.

 

Has anyone in the Avid/KF world experimented with the aerodynamic balance "ears" on elevators like the Super Cubs have?

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Posted (edited)

My elevators have the "ears" - not for aerodynamic balance, but for mechanical mass balance to prevent flutter, and to counter gust forces - same with rudder - not flying yet. Probably an over-design for this type of airplane, but making a useful load of a small amount of lead in the tail!

The aerodynamic part is to decrease stick forces, I think.

EdMO

Edited by Ed In Missouri

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Posted

The down side to a bigger elevator is... there is no down side to it.  If you flew an Avid fast enough to induce flutter the tail would be the last thing on your list of things to worry about because you would have already somehow ripped the wings off and were now the not so proud pilot of a motorized lawn dart :lol:

 

I am surprised that no one has yet made a spring mechanism that couples the flap handle to the elevator push rod and automatically relieves the stick forces needed when you pull on the flaps.  The forces are really not that great, but it makes it where you HAVE to hold the stick or you will be looking straight down in a heart beat. 

 

Putting on a larger "balanced" elevator is high on the list of things I want to do.  Larry has a nice one he built for his project plane.  The only issue that MAY arise is when the wings are folded, the "ear" may smack the wing at full down deflection, but you can fix that by putting a gust lock on them.

 

:BC:

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Posted (edited)

I have made trim tabs on both elevator sides - one is manual, and the other is connected to the flap control to automaticly counter stick forces when flaps are deployed - both are controlled by Bowden cable wire - PIA to hook up manual trim compared to the electric trim, but I wanted manual control.

EdMO

Edited by Ed In Missouri
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Posted

I have made trim tabs on both elevator sides - one is manual, and the other is connected to the flap control to automaticly counter stick forces when flaps are deployed - both are controlled by Bowden cable wire - PIA to hook up manual trim compared to the electric trim, but I wanted manual control.

Edmo

 

 

Excellent idea EDMO! The R/C model guys have a flap>trim mixing switch that performs this function and it works well.

 

Please be careful with the Bowden cables, they have to be tied down very well along the entire length to prevent slop and unwanted movement. If you can move the trim tab up and down like a spring, it can "excite" at some speed and cause a flutter-like situation... even at a much lower speed than you would expect for "high-speed flutter".

 

A Bearhawk built by an Edwards AFB Flight Test Engineer (Russ Erb, "Erbman") had exactly this scenario happen to his trim tab on the first test flight. I dearly want to agree about manual being better than electric, but the electric system is much easier to take slop out of.

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Posted (edited)

Thanks, I have never seen figures for "flutter speeds"...

I will have my elevator mass-balanced anyway. Some have added a spring to take out any free play.

We were allowed to remove the mass balance weights from the ailerons on my Ercoupe as long as the play in the controls did not let the trailing edge exceed a certain limit of free movement - I am thinking it was less than 5/16 inch.

EdMO

Edited by Ed In Missouri

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Posted (edited)

A Note for those thinking of putting "ears" (horns) on their elevators - This can also apply to rudders:
There are two types:
1. Shielded, which do not extend to the forward edge of the horizontal stabilizer -
2. Unsheilded, which extend to the leading edge of the stabilizer - some have run into trouble with this type, as the aerodynamic force was too great to overcome with stick force after deflection - or, caused a reversal of stick forces required - you need to do some research on this.
I made mine as a modification of the two types, being in the form of a "V" pointing forward - mine were designed mainly for mass balance of the elevator.
EDMO

Edited by Ed In Missouri

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Posted (edited)

post-399-0-50699400-1421006357_thumb.jpgFOXY TAIL MODIFICATION DRAWING

Some corners were rounded by welder, and ears made longer. See FOXY FLAPPER FOTOS in KITFOX IV section.

EDMO

Edited by Ed In Missouri

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