UKEY STAB DIMENSIONS

[Guy B Jr]

I have a UKEY 40 and a 15. For repairs sake, I am recording all the dimensions. The stab and elevator on both planes has a 12" span. This produces a stab and elevator area for the UKEY 15 of 16.4%, whereas the UKEY 40 is 9.3%.

I would appreciate it if some of you that own UKEY 40's and 35's would measure the span of the elevators and let me know.

Thanks.

[Steve Helmick]

I posted "over yonder". But I shoulda axed, are these J&J Ukey's or Jim Pearson's?    Steve

[john e. holliday]

Is there a difference???

[Guy B Jr]

Steve --- They both were mfg by Jim Pearson.

I just pulled out some plans of non flapped planes and checked the percent of horizontal tail area compared to the wing area. The spread is all over the place.

UKEY 40                 9.3%
UKEY 15               16.4%
Coyote                 20.75%
Medic                    9.1%
Plugger Super Slow  8.4%
SC2                     13.9%
Challenger 15         10.3%

Maybe the answer is "whatever works".

[Tim Wescott]

Stab area isn't the whole of the story.  The area of the stab, times the moment arm from the mean aerodynamic center of the wing to the mean aerodynamic center of the stab means a lot more.  You can make this into a dimensionless number (and hence one that lets you compare apples to apples across all different sizes of planes) by using the "tail volume coefficient", which compares tail area x moment arm (the tail "volume") with the wing area x mean aerodynamic chord (the wing "volume").

Google tail volume coefficient for more info -- it's out there.

Or, just wait for Howard Rush to see this topic and explain how I'm kinda right, but leaving out a whole bunch of important information.

Flapless planes are a lot more forgiving as far as TVC goes -- basically, if the tail is small then the CG position gets more critical, but it never gets to the point where you can't actually control the thing.

[Guy B Jr]

Tim,
The UKEY 40 is balanced and quite controlable, even with the smallish tail feathers. This is evidenced by the fact that Mike Donovan flew the entire OTS pattern with it.

[Steve Helmick]

Is there a difference???

Quality of construction of the Jim Pearson U-Key (the one example I've seen, vs. at least a half-dozen plus from J&J) was incredibly nice. I'm talking about the fit and finish of what comes out of the box, as it comes out of the box. Just could not believe it!