What is factory Vne on Model-4 and Speedster ???

[rogerh12]

Howdy;
What is the Vne on the Model-4-1200 and Speedster type, as defined by Kitfox? Is there a difference between the two versions? What is the typical physical limiting factor that established this factory specified Vne? Are these Vne's at all flexible upwardly, too conservative, or is that just a crazy concept to bring up?
I guess this last question has to due more with a temporary overspeed during a dive in smooth air, not a sustained overspeed in cruise or through turbulence in a long decent.
Thanks
Roger

[Av8r3400]

My IV Classic-1200 has a Vne of 145 mph.

My IV-1050 has a Vne of 125 mph.

My understanding of the limiting factor was the windshield. The earlier planes used a thinner material that would begin to stove in above 125 mph. The later kits had a thicker material supplied. I don't know this is the only reason, though.

The IV-1200 has two counter weights on each flapperon. The 1050 only has one (of twice the mass). This may have been to prevent a twisting flutter in the flapperon to start?

I have a molded plexiglass (LPAeroplastic) windshield on my 1050, so it has never been a concern for me. I also have one for my 1200 now, too.

[HighWing]

Roger,
I tried to verify the numbers with my collection of documents but couldn't find anything and the Kitfox Aircraft site doesn't have the IV specs posted, but my memory tells me that the Vne of the IV kit as delivered was 125 MPH. The Speedster was 135 MPH.

All the talk then to now indicated that the difference was totally dependent on the windshield thickness. The long wing IV had the windshield thickness at 3/32" - .090" Lexan whereas the Speedster had 1/8" - .125".

The .125 Lexan caused problems because Lexan that thick doesn't like to be bent to the tight radius at the door posts and would readily craze requiring frequent replacement. Typical fabrication guides place a minimum cold formed bend radius of .125" Lexan at 12.5"

As a side note, the .090 will craze as well (9" minimum bend radius) and some went to the .060" - I did. It would oil can a bit approaching red line but not at the typical cruise speeds.

[Geowitz]

The official Skystar Kitfox "Pilots Operating Handbook" that came with my kit says...

Model 4-1050 - 125mph
Model 4-1200 - 125mph
Model 4 Speedster - 140mph

Ditto on the windshield comments.

[rogerh12]

Once I asked the designer of the Zenith line, how the Vne was determined on his planes. He said it was just a speed that they tested the plane too, and not a speed at which anything above would cause permanent damage (the the G-rating on a wing). So I asked what speed would cause permanent damage to my 601HDS, and he said he didn't know, and remarked that they have not calculated the damaging Vne speed, or even test the plane to determine this "True Vne" speed.

So I was wondered if the Kitfox had a "True Vne", something to be avoided at all cost, so I could mark it on my Airspeed indicator as a double red line. And also wondered what would get bent first, which sounds like the wind screen. (Easy enough to see bending, so who needs a redline on the indicator anyway !!!)

Thanks for the input guys
Roger

[DesertFox4]

(Easy enough to see bending, so who needs a redline on the indicator anyway !!!)


The DAR signing off on you airworthiness certificate.

[jdmcbean]

POH - Published numbers: VnE
Model IV-1050: 125 MPH
Model IV-1200: 125 MPH
Model IV Speedster: 140 MPH Speedster had the Acylic windshield

Early Model 4-1050 actually had 95 MPH published but was changed by SkyStar later on. Speculation was a carry over from the Model 3.

[Geowitz]

There ya go.

[rogerh12]

Since my new Airspeed indicator stops at 130 mph, I will just use 125 as my marked Nne. I doubt my Porsche powered plane will go much faster than 100 mph anyway, with a climb prop attached (beats 95 mph anyway).
Roger

[HighWing]

I finally found what I was looking for. See attachment.

One note, though, I guess we will have to agree to disagree about the availability of the acrylic windshield in 1992.

[jamesbdunn]

What would the Vne be limited by if the windshield were not an issue?

I originally thought the wing design might cause excessive stresses using normal pilot induced control forces. But after reading this thread, I was surprised the windshield (a simple structure) is the main factor behind the present Vne.

My Vari-eze cruises all day at 200 mph. So if the Kitfox firewall were horizontally extended to include part of the top of the engine cowling area to maintain the firewall integrity. And, a canopy were devised similar to faster aircraft. The windshield shouldn't be a significant modification.

What is the next limiting factor for Kitfox Vne?

With engines up to 200 hp being mounted on a Kitfox, 140 hp engines commonly available, and windshields that certainly can handle higher air speeds, why isn't the Kitfox flying at higher cruising speeds?

Possible thoughts are:

a) the wing sail area at 125 mph in relation to the shear forces on the folding wing critical structures?

b) wing profile related pilot induced control forces?

c) aileron induced instability?

d) .... ?

But these are not supported in the provided literature.

Given the aerodynamic design of the Kitfox, it just seems like it should be cruising at 160 mph unless wing design is a limiting factor. And wings on the Kitfox can easily be replaced.

One set for STOL, another for high cruise.

Though I'm not at all disrespecting the Kitfox relatively high range in airspeeds between stall and Vne, as compared with other aircraft.

[Av8r3400]

The newer Kitfoxes with the stronger, molded windshield and two balance weights per flapperon have a VNE of 145 mph.

With enough horsepower, I'm sure you could go faster than the plane is designed to go. More horsepower equals more weight. More horsepower equals more fuel. More fuel equals even more weight. Weight is the detriment to speed. This becomes a never ending circle of more power to overcome more weight.

Making a plane like a Kitfox go fast becomes like a dog chasing his own tail, power, weight, fuel, structure, etc, etc…

If you want to cruise at 160 mph, build an RV.

[jamesbdunn]

I'm fickle and moody. Sometimes I want the STOL for playing in the local mountains and rivers. Then less often, I want to go back East 2000 miles to visit family.

The Kitfox obviously has the roots to grow capacity. Simply look at other discussions here:

http://www.teamkitfox.com/Forums/showthread.php?t=5964&page=3

The common Kitfox is no where near its performance maximums with existing 70 hp engines. But there are other turbocharged engines that actually save weight at 140 hp; though durability is questionable (not yet known, not that it isn't).

[Av8r3400]

Sounds to me like you just need two airplanes. :D

[rosslr]

You know James, I used to think the same way - I owned a Mooney and it was great for long distances. But then I realised that the commercials do high and fast much better and cheaper than I could! But they couldn't low and slow and off field... no brainer solution to me!

r

[jamesbdunn]

The acrylic windshield upgrade is too easy a fix for it alone to limit Vne. The FAA testing for the windshield might be far less expensive than another component, so I get that. It is cheaper to stop at the windshield which is easily observable and measured without involving a wind tunnel.

As is shown elsewhere, there are Kitfox flying faster than Vne. Kitfox itself modified the 4 to create the Speedster.

So my question:

After the windshield, what is the next Kitfox feature that limits Vne?

[t j]

I sense a little confusion about V speeds for amateur built experimental airplanes. The FAA doesn't do the testing on them to set Vne, the manufacturer (builder) does that however they want.

To answer your question though, my opinion is the fabric covering is the factor limiting speed to about 150 mph for any tube and fabric airplane. I suspect no one has felt the need for such speed enough to go there.

[Esser]

The fabric isn't the limiting factor. Pitts have a Vne of over 210MPH.

I imagine the limiting factor is the flexible wing of the kitfox. Susceptible to flutter.

[Av8r3400]

A fabric covered Beech Staggerwing would cruise almost 200 mph.

Esser, I believe you are into the main limiting factor being flutter. Either from the flexing of the wing or the flapperons or a combination of both. No matter how you cut it, these aircraft were not designed to or intended to go fast.

The Denny claims of 150+ mph cruise may be possible with a 914 turbo engine at 20,000 feet, as a computed TRUE airspeed. But down low this kind of speed is just looking for trouble using an airframe designed for 100 mph.

In the 30+ years of Avids, Kitfoxes and the rest, there has never been an inflight structural failure of the airframe. This is one of the main reasons I chose to own this aircraft type. Putting enough horsepower to muscle the plane to these higher speeds is just looking to ruin this record of safety.



Just my opinion. It is experimental, so you can do as you please. I just think it to be very ill advised.

[Dave S]

For me, chasing Vne is not that important. Certainly, I did test our S7 to 140 mph to verify the plane was controllable throughout the design envelope.....Knowing the wing loading of the aircraft....I hate to think what kind of whack on the fanny a person would get if they hit a good pothole in the sky going really, really fast - I've never been able to figure when the sky would be guaranteed 100% free of potholes - routinely cruising around in the yellow arc, irrespective of where Vne is planted - just not me I guess.

I never bought a kitfox to see how fast I could get out of the sky and back on the ground :)

Sincerely,

[av8rps]

Unless I missed it, no one has talked about the glue used to hold fabric down. Stits (Polyfiber is new name) used to rate their glue as good up to 140 mph. So, because a Kitfox is built typically without stitching the fabric to the flying surfaces, I'm guessing the 135 to 145 number is mostly because of that, AND the simple fact that this airframe was tested to those speeds and proven safe.

And honestly, considering we started with an airframe originally designed for only 80 mph top speeds, 145 miles per hour is a huge increase over original design criteria. I doubt there are many airplanes that have increased there top speeds by 50 percent through just a few decades of evolution. 50 percent is a really big number in that regard. And it's even bigger when you consider the airplane is still a phenomenal STOL airplane.

And even if you rib stitch the wings and tail to alleviate the glue concern, the standard 32 ft Kitfox wing flex in turbulence at high speeds could easily induce flaperon flutter (ultimately leading to wing flutter). And then there's that tall vertical fin that moves a lot, so it should be made stronger and while you're at it add an aerodynamically balanced elevator and rudder. Unfortunately, that is a lot of re-engineering for such little gain of only 15 to 20 mph. Of course, re-designing a more rigid wing (and tail) is certainly doable, but I'm a bit concerned by the time you were done modifying a Kitfox for 160+ speeds, it wouldn't be right to even call it a Kitfox. It would almost be a whole new airplane.

The gang here knows that I am one of the guys that believes a modern Kitfox could fly a lot faster than it does with a few modifications. However, my "Fast" is a 140-145 cruise speed, not 160 mph.

I know this airframe can high cruise safely at 140+ mph if you clean it up more and add some more thrust. But I think unless you are willing to majorly redesign this airframe, it would be a better goal to keep high cruise goal to current VNE numbers. All of us that love Kitfoxes have enjoyed and benefitted from the designs great safety record.

With all that said, I really do think we can get our Kitfox cruise speed up while still retaining most of its STOL abilities. With all we have learned about these great little planes over the decades, I would suggest you just follow the basic concept of the original Speedster and maybe make a few more minor improvements from there. I think the company had it mostly figured out a long time ago with the Speedster concept, and I know that concept would work again if we took it even further.

Oh, and fwiw, a super cleaned up "Speedster" will be the best looking plane on the ramp, and will look like it goes at least 160 mph to onlookers...

....Only you have to know it's 15 mph less. ;)