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Thread: Quick change wing extensions

  1. #1
    tommg13780's Avatar
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    Default Quick change wing extensions

    I was able to obtain a set of quick change wingtip extensions from a new forum member that just moved into my neighborhood, Tony Zelsnack. Tony is building a model 4 Classic and also had a model 4 salvage plane which is where the wing extensions came from.

    I was interested in getting a set of extensions ready for this season and actually had the components acquired and construction in progress. So now all that's necessary to finish up is a set of standard wingtips that are almost in the mail from Kitfox Aircraft.

    I did a search on the forum and found very little for wing extension comments so I'm starting up this thread to see what is out there on this topic. I'd also like to expand the scope of this thread a little to compare the differences in Classic wing versus Speedster. Of course we recognize the Speedster advantage at cruise speed but hear again there's little content out there regarding the difference in slow flight characteristics.

    I'm expecting the weather to improve soon and then I will be full on into the flying season. My intention is to gather specific data comparing the classic wing with speedster. This should demonstrate a fairly accurate comparison because all other aspects of the airplane remain the same except the wing span.

    All comments and suggestions are appreciated.
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  2. #2
    Senior Member Geowitz's Avatar
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    Default Re: Quick change wing extensions

    I'm actually skeptical that the speedster wing is any faster... and it may be slower. Less wing area means a higher angle of attack is required to create the same lift on the shorter wing thus inducing more drag. Everything else being equal of course. I think you'll see faster cruise with the longer wing as the drag created by a longer span is probably less than the drag created by a higher angle of attack.

    I think the speedster looks better and more proportional in shape, with the only other major benefit of higher wing loading causing a smoother ride in rougher air. Aileron responsiveness is quicker though too.

    I've got the speedster wing and while I know I would get better STOL performance from the longer wing it's still pretty good relative to most other planes.

  3. #3

    Default Re: Quick change wing extensions

    That is correct. I have the speedster wing with extensions and the cruise is better with the extensions on. The only advantage I have found to the shorter wing is a quicker roll rate. Everything else favors the longer wing.
    Bryan
    Project Kitfox
    Bowen Aero LLC
    Kitfox Model 5 Lycoming O-290 D powered
    Building a Model 7 Apex powered
    Redding, CA
    http://www.youtube.com/c/ProjectKitfox

  4. #4
    Senior Member av8rps's Avatar
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    Default Re: Quick change wing extensions

    For years I've wanted to clip the wings of a Model IV down to 26 or 27 ft because I l believe it would provide me with the ultimate Kitfox Speedster. But I haven't done it yet, so at this point my theory is still not proven. However, I have done a lot of research, even having gone so far as to ask Dean Wilson his opinion (he agreed with my research fwiw). So based on what I learned from all that, here is what I came up with;

    The shorter wing (the 117 sq ft Speedster wing) should always be faster on the earlier, lighter airplanes. Unfortunately, our little Kitfoxes with wings originally designed for a 400 lb empty weight are now being equipped with heavy engines, heavy paint, etc, etc, and can now weigh more than double the original 400 lb design empty weight.

    So while the shorter Speedster wing has been well proven to work well on a 650 lb airplane, that Speedster wing starts having trouble supporting the higher weights of the newer airplanes, especially when we start seeing 950 lb empty weights, and / or 1550 lb gross weights. There is an apparent point-of-no-return of what either of our Kitfox wings, long or short, can handle. When the wing loading gets too high, the increased angle of attack required to support the extra weight offsets the benefits of the reduced drag of the shorter wing.

    I think the best way to show what that point-of-no-return number is, is to show some average examples of Kitfoxes with long wings and short wings. So here goes;

    - Kitfox 4 Speedster; 1200 LB GW with 117 sq ft wing = 10.3 lbs sq ft.

    - Kitfox 4 Classic; 1200 LB GW with 132 sq ft wing = 9.1 lbs sq ft.

    - Kitfox 5 long wing; 1550 LB GW with 132 sq ft wing = 11.7 lbs sq ft

    - Kitfox 5 short wing; 1550 LB GW with 117 sq ft wing = 13.2 lbs sq ft

    - Kitfox 5 Short wing; 1400 LB GW with 117 sq ft wing = 12.0 lbs sq ft

    So, even with the longer wing on the Kitfox 5, at 1550 lbs the wing loading will be 114% of what the Kitfox 4 Speedster has.

    And if you put the shorter Speedster wing on that same Kitfox 5, you will have a wing loading that is 128% of what the Speedster has, which is apparently too far over the point of no return according to what others have experienced.

    I would speculate the reason the Kitfox company way back then limited the gross weight of the Speedster winged Model 5 to 1400 lbs, as at 1400 lbs the wing loading is pretty much the same as the longer winged version at 1550 lbs.

    And to be honest with ourselves, we really should use the empty weight of our airplanes for more accurate comparisons. As just one example, if you assume you will carry the same load in both models being compared, you will find the newer Kitfoxes that use aircraft engines will have a much higher wing loading than will the earlier Kitfox Model 4 912 powered versions.

    - Kitfox 4 Speedster 912ul - 650 LB EW with 117 sq ft wing = 5.6 lbs sq ft
    - Kitfox Super Sport IO-240 - 950 LB EW with 132 sq ft wing = 7.2 lbs sq ft
    - Kitfox Super Sport IO-240 - 950 LB EW with 117 sq ft wing = 8.1 lbs sq ft

    So comparing the numbers of the heavier Kitfoxes to the earlier 912 powered short wing 4 Speedster, even with the long wing, the IO-240 Kitfox wing loading is 128% of the Speedster.

    And if you dared put a short wing on the IO-240 Kitfox, your wing loading would be 145% of the original Speedster. That number I am certain is well beyond the point of return...and I doubt many of us would find that Kitfox very versatile.

    (Oh and for the record, a 650 LB EW Kitfox 4 with a 26 ft wing would have a empty weight wing loading of about 6 lbs per sq ft, which is about the same as a 800 lb long winged 912 powered Super Sport Kitfox)

    My simple theory regarding short or long wing is this;

    Whatever Kitfox model you have, if you can keep the wing loading of your airplane with short wings approximately the same as the the wing loading number of the newer Kitfoxes using the long wing, you will be fine. It is all about how many pounds per square foot that wing has to lift that determines the wings efficiency. If you can do that you are likely to have yourself a real Kitfox "Speedster".

  5. #5
    Senior Member av8rps's Avatar
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    Default Re: Quick change wing extensions

    Bryan,

    Would you care to share with us your Kitfox empty weight? That would help us to see if my numbers are at all accurate.

    Paul S

    Quote Originally Posted by redbowen View Post
    That is correct. I have the speedster wing with extensions and the cruise is better with the extensions on. The only advantage I have found to the shorter wing is a quicker roll rate. Everything else favors the longer wing.

  6. #6

    Default Re: Quick change wing extensions

    My original empty weight was at 891lbs with the short wing and over the years with upgrades and with the long wing on and the 8.50 tires I'm now at about 920lbs.

    Paul I think your thought process is right on, however the Kitfox is primarily a STOL design, it is dirty and draggy so going to all that effort to shorten a wing to gain speed seems to me like you will be going against the purpose of the design to gain a few knots at the expense of STOL and climb performance.

    For me once I put the wing extensions on they have never come off. I can tell the difference in STOL performance, climb, cruise, and slower roll rate. The longer wing makes more sense for the design. An RV would be a much better design if you want speed, but it's all about experimenting. It would be a lot of work, but a fun project.
    Bryan
    Project Kitfox
    Bowen Aero LLC
    Kitfox Model 5 Lycoming O-290 D powered
    Building a Model 7 Apex powered
    Redding, CA
    http://www.youtube.com/c/ProjectKitfox

  7. #7
    Senior Member av8rps's Avatar
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    Default Re: Quick change wing extensions

    Thanks for the empty weight info. Your airplane is probably a really good example of what I was trying to explain, as you said it gains nothing performance-wise from using the short wing. And even though I never identified exactly what the point-of-no-return is, there is definitely a point where too high of a wing loading will diminish the benefits of a clipped wing.

    And yes, I agree the design was originally created to be a STOL aircraft. But the Kitfox has evolved tremendously since then, and in reality is known today more for its incredible versatility than probably any one particular feature. There are few aircraft that can match the versatility of a Kitfox (and most that do are basically from the same lineage).

    But in reality, the cross country capability of later model Kitfoxes with the Riblet airfoil is still better than most airplanes with similar horsepower. Sure there are faster airplanes, like an RV design. But put one of those on floats, or try to fly it into the bush, and you will quickly recognize its shortcomings. The Kitfox is never going to be a 180 mph airplane with the existing wing, but it will fly about as fast as an average 172 or Cherokee on 2/3rds the horsepower, and faster if it's an earlier Speedster with the 100 hp 912. AND it is way more versatile than either the Piper or Cherokee.

    Personally, the incredible versatility is what I love so much about the Kitfox. They can be good at pretty much anything you want them to do. And you can modify them as needed to have them be better at one particular area if you desire that. Or you can just leave them stock and you will still have an airplane that does most everything better than average. There are few airplanes that can tout those capabilities.

    I once had a guy picking on my "slow Kitfox" at an aviation event, comparing it against his RV-6. I certainly couldn't disagree, and commented that I have always liked the RV designs, and wouldn't mind owning one. But feeling I had to defend the honor of my little 80 hp Kitfox amphib in front of the group, I made a comparison of his RV-6 to my Kitfox in all the other areas it excels in (floats, back country STOL, trailer ability, hangar storage, fuel burn, etc). From that discussion the group concluded that for traveling the RV would be superior, but for most peoples needs, the Kitfox was a more practical choice. Unfortunately the RV owner wouldn't stop with his rant about how fast his RV-6 was, so I asked him why if he liked speed so much that he didn't build something faster? He responded "like what?". So I told him about the time I got to fly a 330 hp clipped wing turbo Glasair 3, and how it would indicate 260 knots at 65% power. And how some guys were getting close to the 400 mph mark at Reno with Glasairs (fwiw - one did 409 mph this year). That finally got him off his rant about speed and made him realize we can pick on just about any airplane if we want to. So we all learned a little something that day, and even though it was a lively conversation at times, we all parted as friends.

    Ironically, the last time I saw my new RV-6 friend he told me he was thinking about building something again. And get this...he thought it might be something on the order of a Kitfox .

    He said he thinks that back country camping and STOL stuff looks like a lot of fun.



    Quote Originally Posted by redbowen View Post
    My original empty weight was at 891lbs with the short wing and over the years with upgrades and with the long wing on and the 8.50 tires I'm now at about 920lbs.

    Paul I think your thought process is right on, however the Kitfox is primarily a STOL design, it is dirty and draggy so going to all that effort to shorten a wing to gain speed seems to me like you will be going against the purpose of the design to gain a few knots at the expense of STOL and climb performance.

    For me once I put the wing extensions on they have never come off. I can tell the difference in STOL performance, climb, cruise, and slower roll rate. The longer wing makes more sense for the design. An RV would be a much better design if you want speed, but it's all about experimenting. It would be a lot of work, but a fun project.

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