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View Full Version : TakeOff RPM on 912uls



kitfox812
12-09-2009, 01:33 PM
I have 485 hours on a Kitfox s-7 and I have always thought I had low RPM on takeoff.
I have a Powerfin-F prop which is designed for the 912uls.
I have readjusted the pitch numerous times and still end up with a pitch setting that gives about 4700 RPM on lift-off,WOT. Any less pitch will red line at half throttle in cruise. The pitch setting I have set now goes to 5000 about 10 seconds after lift-off depending on my climb rate then up to 5200 in about 15 sec and is good at cruise. Climbing at 5200 or 5400rpm and 600-900fpm is no problem. My WOT is above 5700 RPM in flight so I can't decrease the pitch any more.
Does anybody know if this is normal or could it the Prop or even the Engine.
Any ideas will be greatly appreciated.

Rich

Slyfox
12-09-2009, 05:25 PM
I originally had a powerfin on my 912ul and that sounds just about right, I wouldn't change it.

Av8r3400
12-09-2009, 06:04 PM
At only 4700 rpm, you are leaving a lot of takeoff power on the ramp. And it will still over-speed in level flight?? Wow, that's weird... Sounds like this prop may not be the optimum item for your plane.

Dorsal
12-09-2009, 06:14 PM
I am interested in the responses to this as I had assumed that if you pitch the prop for best take-off you would have to back off the throttle in level flight and that this is a viable option.

RobS
12-09-2009, 10:11 PM
These are not my words, but they confirm your assumption:

Fine pitch allows the engine to reach maximum speed and hence maximum power at low airspeeds. Vital for take-off, climb, and for a go-around on landing.

Coarse pitch allows the desired aircraft speed to be maintained with a lower throttle setting and slower propeller speed, so maintaining efficiency and improving range. Coarse pitch will ensure your engine does not overspeed while the propeller absorbs high power, producing a higher top speed.

In a fixed pitch prop, fine pitch is better for take off performance but results in higher RPM for cruise.

jrthomas
12-10-2009, 07:16 AM
It seems to me that static rpm should be around 5200 to 5300. If you're only getting 4700 on take off, what's you static rpm? I've seen props do some strange things. A friend of mine had a fresh 447 Rotax that would turn up 6000 static on the 1st run up after warm up but wouldn't go past 4000 or so a few seconds later. After every conceivable remedy failed he gave up and installed another 447. Guess what! It did the same thing. He finally tried a different prop with success. I think I'd call Powerfin. Let us know how things work out. Good luck, James Thomas

kitfox812
12-10-2009, 09:18 AM
The SS-7 was DAR'ed in Aug of 07 and I called Powerfin shortly after with the problem. They took it back and shortened it a hair and rebalanced it for me for free. It has operated flawlessly otherwise and I really like the prop. For the price of changing props, I have to. I will give them a call again though.
With Rotax just coming out with the SB about not less than 5200 on take-off,
I was getting a little concerned again. The plane will take-off in a 100 feet or so and stand on the prop. I cruise at 5200 and 115mph so it is great if I don't look at the RPM. Thanks for all the input.

horsepower
12-10-2009, 12:12 PM
Not for sure but I think Powerfin may be out of business. I live in North Idaho about an hour from their facility in Spokane,Wa.I have a Powerfin prop on my 582 and I called them to ask a question and got a message that the phone is no longer connected.Bummer if they are just another business to go under with our sinking economy.Hopefully it is only rumor. Randy

kitfox812
12-10-2009, 02:10 PM
We have been flying to St Maries for lunches and fly-ins for about 9 years now....great place.

I went to Powerfin.com and there is a msg that they moved to Texas and the 800 no. will be up the end of Dec.. The old number on their site goes to Time Life?? I hope they are still going. I sent them email regarding this problem.

JimS
12-11-2009, 07:10 AM
WOW!

If your getting airborn in 100' and cruising at 115 mph at 5200 rpm, I'd say you have the prop dialed in as good as possible.

JimS

av8rps
12-11-2009, 04:28 PM
A friend of mine had a similar issue once with a ground adjustable prop; lower than desired rpm for climbout, but yet would very easily redline in straight and level flight.

We flew together half way across the country trying to figure out why his plane performed so poorly compared to an identical one just like it with the same prop and engine. What we ultimately discovered was that one blade was adjusted with more pitch than the other (that specific prop had a very sensitive and difficult to adjust hub). After setting both the same, his airplane performed just like the other one we were comparing it against. It made a huge differnce.

So I would make absolute certain that all prop blades are adjusted EXAXCTLY the same. We used at that time an electronic level to insure the most accurate measurement. Then we played around until we learned the best technique for tightening the hub bolts. Most importantly is checking the prop not only for proper tracking, but to assure both blades are pitched exactly the same.

Might sound trivial, but I would say having one blade off more than the other(s) would be the equivalent of having one wing on your airplane having a different angle of incidence than the other. Imagine how poorly that would work? Well, a prop is in effect just a wing...

Paul S

Dave S
02-14-2010, 03:40 PM
All of you guys are an inspiration to me - between this thread and the one on "warp drive Pitch settings" you got me thinking about my prop settings; and, decided to go back and experiment a bit. My biggest problem was trying to figure out which of the two threads to reply to...oh well...

1) Our airplane has the 912ULS with the Warp 70" tapered tip - (FYI - also have the air intake plenum which is suppose to be good for 5 horsepower - everything else being equal. The warp folks told me over the phone that the pitch range for this application in a Kitfox is 10 to 15 degrees. Also - Warp said the tapered tip behaves differently than the square tip once the airspeed picks up at cruise.

2) First setting we tried was 12 degrees - great acceleration and climb - but in cruise - ran out of green RPM before I ran out of throttle.

3) Set the prop to 14 degrees - Climb out rpm was barely 5200 - going to a little less than best rate was necessary to keep above the 5200 RPM but cruise at 5400 resulted in 118 - MPH true. Have been running with 14 degrees for a while.

4) This weekend - went and reset the prop to 13 degrees - Conditions = altimeter 29.96, OAT 19 degrees F, Altitude 3000 MSL Results = Full throttle @ best rate held a smidge over 5200 RPM - noticable improvement in acceleration (not that it was bad before) - a bit better climb and a cruise of 115 TAS under the above listed conditions and 5400 RPM.

I know I need to do some additional testing with more presicion with the new setting - but this appears to keep the Rotax SB happy and not interfere with the cruise too much - It is real obvious that a person will run the RPM into the yellow if not throttled back some now at cruise with the 13 degree setting.

However - all of this and the discussion on the list sure makes one lust for a constant speed or flight adjustable pitch prop. I never used to think about stuff like that when I was flying a rental 152......

Also think I am going to take Steve's advice about cutting the idle back a bit and just get better at using the throttle during landing.

Sincerely,

Dave S
KF7 Trigear
912ULS Warp

Slyfox
02-14-2010, 04:59 PM
I have the same engine. You mentioned the inflight adjustable. I tell ya, if you did go with an inflight, it will totally spoil ya. But you will need to anti up the denaro:(. Cutting back the idle is a total blast. I did a drop in on an air field. It was one that has 150' trees on one end with most being to one side, I came in from the side that had the worse trees, I did a 60mph over the trees than made a sharp turn to the right and pull back all power, with flat prop also, and literally dropped onto the runway. This was a base to final right over the runway, I didn't slip any, but with a slow speed already and by the way the turn was mainly rudder, I only turned maybe 15degree bank, swung the tail around and pulled the power, down like an elevator, pushed in a little power down at the runway and did a beautiful touch down, wheel landing on the runway, way fun. Truelly amazing what these kitfox's can do.:D