Vne for KF model 4/1200

I'm currently using a Vne of 125 mph flying my 4/1200 with 32 ft wing span. What's the groups thoughts on this? I'm currently cruising at 110-115 mph indicated.

My first question is this: How accurate is your ASI? Second is: What does Kitfox say the VNE supposed to be on the Kitfox 4? JImChuk

My pilot handbook, if I remember correctly, says 120 mph for the standard model 4 1200 and 140 mph for the Speedster. I assume the airfoil shape of the tail surfaces of the Speedster handle higher airspeed a little better than the standard model 4's flat surfaces. James Thomas

My understanding is it has more to do with the thickness of the windshield material.

Thanks for the replies. My ASI is correct. I don't plan on faster than 120mph but when I got my kit vne was listed at 125 in my limitations (skystar). Of course we keep speeds down in rough condions.

You're probably right DesertFox4 about the windshield thickness. I just figured the airfoil tail surfaces might be less likely to flutter at higher speeds. I do remember that the Speedster called for a thicker windshield. I suspect we're both right. James Thomas

Just as a comparison, the Avid MK IV had a VNE of 135. They never did use any wood ribs in the tail feathers on the Avids. Avid wings are 2' shorter than the standard Kitfox 4. I have noticed the .060" thickness windshield start to buckle in at about 100 MPH though. JImChuk

My IV-1050 stated 125 mph.

My IV-1200 Classic has listed 140.


I agree with Steve, it is all about the windshield. The earlier kits had a thin (.062"?) Lexan (polycarbonate) sheet the w/s was made from. This would begin to stove in at around 115-120 mph. The later kits had a thicker piece (.087'?) or the molded acrylic piece from LP Aeroplastics which was much more rigid.

Is the 140 mph for the standard wing span Larry?

I don't know that the wingspan has any effect on the VNE.

FYI:

Denney Aerocraft Company Model 4 brochure from June of 1992 lists VNE for standard Model 4 at 125 mph, and 145 mph for the Model 4 Speedster.

There is however mention in the brochure about the Speedster having a thicker windshield for the higher airspeeds it flew at (which was advertised with a cruise of 125 and top speed of 135. fwiw, I flew that plane and can verify it actually did those numbers or more, and all on a wood 3 blade GSC prop no less!)

Relative to this discusssion, it is my opinion that maximum structural cruising speed is much more important to the everyday Kitfox pilot than VNE.

The Vno is 98 mph for my Model 4 -1200. When dealing with lots of turbulence I adhere to that number like religion as it is a lot more likely exceeding that airspeed number will potentially cause structural damage to your Kitfoxes airframe :eek:

Ironically, my Kitfox is the first plane I have ever owned that I had to pay so much attention to that number, as my other planes aren't fast enough to get close to their Vno number. And even though my Kitfox is a float plane, there are times I have to work to keep it under that number :)

Paul, how did they get 135mph out of the speedster? I know it has one bay shorter wingspan, right? Did that account for the speed increase?

You know that is a good question. :rolleyes:

So many times I read about peoples reported speeds and wonder how it is that we have so many slow Kitfoxes? My long winged 80 hp 912ul Model 4 on amphib floats will top out (in perfect conditions with WOT) at just a tad under 125 mph. I've demonstrated it many times to my float plane buddies.

So when thinking about getting over 130 from a M4 Speedster it really shouldnt be that difficult. (Fwiw there were many aviation articles that confirmed those speeds). Besides having a shorter, less draggy wing, the factory Speedster was well built and pretty slick aerodynamically with a radiator fairing, well fitting wheel pants, and all streamlined struts (wing, jury, tail, etc). Overall it is just a good example of how to make a Kitfox faster IMHO.

I heard the first 914 powered Model 4 even with the long wing could hit VNE straight and level, and at altitude was 150+ mph. Later the Model 6 proved those speeds again on a long cross country from Idaho to SunNFun, but at altitude. (As I recall 185 mph on one stretch at 21,000 ft).

Our Kitfoxes are more capable speed-wise than most know...