Kitfox-4-1200 : 2180 VW power Performance Data Available !!!!!

[Av8r3400]

At what RPM requiring the use of what diameter prop?

That is the entire argument against these motors' performance in a Kitfox. Low speed thrust works most efficiently with a large diameter prop.

[rogerh12]

Though props from different makers are measured different, and things like blade width and how the trailing edge of the prop tapers down effects the effective pitch, in general the props that seem to work best on the VW (with aircraft having large frontal areas like the kitfox) have 60-62 inch diameters, with a pitch somewhere between 28-34 inches, depending on engine HP and desired cruise rpm and projected cruise speed. Of course, your mileage may vary !!!!
The main problem in figuring pitch is that you need to factor in prop slip, or not. In some props its as much as 10-12%, on others it’s zero (or maybe even less than zero). Once you have the pitch selected, you can size the diameter (or you can just guess) Prop selection is a black art, but I do have software that lets me take an existing prop, enter it’s size and full power rpm and it will tell me how changes in diameter and pitch will affect it’s performance (fairly accurately). So if my existing prop needs to pickup 200 rpm, it will tell me how much to cut off the diameter, or flatten out the pitch to get those 200 rpm.
Hope this helps
Roger

Roger
Oh, I think I didn’t answer the question. Cruise speed on the VW is not set by rpm, but by manifold pressure. 22-25 inches is the cruise range, and this can correspond in a prop rpm between 2800-3400, depending on pitch and diameter of the prop. I like to size my prop to cruise at 3300 @ 25 inches myself (in my zodiac, which as a large frontal area), as this gives me a max power setting at 29 inches of about 3500 rpm (3400-3600 is peak power in most VW conversions, but peak torque is around 3000 and gives the best economy cruise compromise with speed). 22 inches will get me back down around 3000 rpm in the cruise, which slows the plane down a bit (but mostly reduces the fuel bill a lot)
Hope this helps
Roger

[akflyer]

I think you missed Larrys point. VW conversions have been done on the KF and Avids before. Bottom line is, if you want to retain the STOL performance you need to swing a big prop.

You say your a modeler. Look at the electric planes and how far they have come. Take a speed 400 motor. Direct drive you use a 6" prop and the plane will go fast but it wont climb for crap. The same speed 400 geared will swing a 12" prop and give you TONS more thrust for climb. None of the electrics really gained popularity because until the advent of a reliable gear box all that little prop did was make lots of noise but would not really pull the planes around.

One last test for ya on the modeling thing. Take any old tired worn out .25 engine you have and put your 8X7 or 9X5 prop on it. Use a digital scale used for weighing fish and get an actual thrust measurement. Now put a 10X 3 1/2 on it and take another measurement. I know for certain which one will produce the most thrust. Granted the plane wont be as fast top end, but it will go straight up, spool up quicker etc. That is why APC came out with the line of "fun fly" props that were weird diameters and pitch (like the 12 1/4 X 3 3/4) to use on a .40-46 size engine instead of a 10X6.

[HighWing]

Roger,
One sincere request please. I think the topic of this thread speaks volumns about your project. It has been done before many times and very little actual data about the performance. Is there only one of these guys willing to talk about it? Please, after you are finished and flying, post the numbers. What I think we would all like to see is empty weight. Prop pitch and diameter. Anything that had to be done to manage W/B. Take off distance, Climb solo and at gross. Cruise at 75% and max comfort level. Also, of course fairings etc. that can greatly affect certain of the performance numbers. Let's have at least two detailed reports of VWs in Kitfoxes. What we need is data. If it is great, people need to know of a real viable engine option. If it is not so great, in fairness, people need to know that too.

Historically, what we typically have in these situations is lots of thought and research. Lots of discussion. Great anticipation, then the 40 hours begins and lots of silence. It would be great if history proves all of the skeptics wrong.
Lowell

[rogerh12]

Hi-wing;

I would be happy to provide the requested data, but it’s going to be a while !!!!

What I can provide is data from the flying kitfox 4 in St. Louis , as outlined below, and of course my own personal experience with the Zenith 601HDS flying behind a VW. Though you might think it not a good analogy to the kitfox, in some ways it is. The cowling used on my 601 was the BIG one used with Subaru engines having Re-drives, so it was way bigger than needed and had significant frontal area. Also, though this plane is a taper wing, the wings themselves are very thick, crazy thick really and had a lot of drag. The landing gear used round tubes and also had a lot of drag.

What is my point? Well the 601 flew well with the 2180 cc 75HP version of the VW, and climb was better than the ercoupe I trained in, even with it’s short tapered 23 foot wing span, so I think that in the kitfox the climb would be better and cruise speed about the same (which is what the recorded numbers showed below).

Certainly the Rotax 912S is better for the kitfox all the way around, but the VW does appear to provide a fun and flyable compromise on performance at significant cost savings (I saw a brand new 2180cc VW aero long block on Trade-a-plane for just 3000 bucks !!!!) and with rebuild cost of around $500 bucks in parts, continual cost savings. Could the VW provide the kitfox with stall performance? Not like a rotax, but a flatter pitch prop would give a better static thrust for STAL flying, but this thrust will certainly drop off quick as the airspeed builds and not give much of a cross country cruise speed, if that’s what you are looking for too. I personally don’t need to get off the ground quick in Kansas, as that’s what we have lots of around here, ground, -flat long- ground.

Hope this helps
Roger

[rogerh12]

FYI:
This was the original post, and since then one of the members of our forum flew in it a few months back and thought these performance numbers were basically correct with respect to rate of climb and cruise speed (though he didn't weight it). Note: it does have the speed wing mods and vortex generators, it is also a tri-gear with spring gear, so my plane will be setup almost exactly the same as this one as mine has already been converted to Tri-gear (but will run the larger 2300cc engine on car gas)
Here is the post:
Ok, I finally was able to get the performance numbers off a flying Kitfox-4-1200 with a 2180 Great Plains VW motor installed, it‘s the one in St. Louis, currently listed on Barnstormers.
The great thing about this plane is it’s a known quantity. As VW aero engines can be anything from a 1600 cc 45HP engines to a 2300cc 100+ HP monsters (with water cooled heads), this engine has been identified as a 76 HP Great Plains 2180 cc engine turning a Sterba 62x32 prop at 3000 rpm in cruise. This engine requires Avgas at this HP rating and develops peak power at about 3500-3600 rpm, but typically cruises at 3200 rpm, which is close to where the peak torque is created.

N53RJ KitFox IV Speedster

Weights:
Basic Empty Weight = 700lbs
Max Weight = 1200lbs
Usefull Load = 500lbs
Fuel Capacity = 27 Gallons
Wing span = 28.8 ft.
Fuel Burn = 4 - 4.5 Gallons an hour
Cruise speed (economy setting) = 90 to 95 mph at 1500 feet (3000 rpm).
Cruise speed (high setting ) = 110 mph at 7500 msl.
Max speed (At full power) = 115 mph at 7500 msl.
Rate of Climb (two people) = 700 fpm
Rate of Climb (One ) = 1000 fpm
Take off roll at Gross Wt = 300 feet
Landing roll at Gross Wt = 300 feet
Stall speed (with vortex) = 37 mph
Approach speed = 60 mph

Final note, I think the cruise speed could be improved with a prop diameter 2 inches smaller, though the rate of climb maybe negatively affected (or not). I would call the rate of climb good for a small cheap plane, better than the cessna 152 I flew at gross weight !!!!

Roger

[rogerh12]

FYI;

Looks like another VW powered Kifox is on Barnstormers.
Too bad it has not flown yet, I would like to get the performance data from it. Judging by the large frontal area, it has the larger VW cowling.
Roger