Prop help needed

[Guest shroomer]

Hello all.

 

I fly an ultralight trike fitted with a verner 133 engine, 175 cm or 69 inch kievprop/ hotprop (same prop different names) and reduction gearbox 2.29. Prop is pitched for max engine rpm of 5000, giving a prop speed of 2180. 5000 rpm equates to about 75 hp, but take off performance is very sluggish, I use over 200m of runway flying solo, and if I take a passenger this increases a lot. I feel the prop is working very inefficiently.

 

As a comparison there is a trike flying at the airfield fitted with a rotax 582 (66hp) which runs a similar prop (make and size), but the max prop speed is almost 2500 on takeoff. That trike can get off the ground in about 70m solo. There is a slight weight difference between the trikes, but not enough to account for this disparity.

 

The only thing I can think is that my prop is producing less thrust due to being much less efficient at the lower rpm. Verner used to sell a 1:2 reduction gearbox which I would have thought would increase my prop speed and increase efficiency, but when I contacted them they tell me they no longer make that gearbox and it would not be suitable for a 69 inch prop anyhow.

 

The prop is rated for 3000 rpm, and can run a lot faster than it is. I can only think an increased prop speed will increase its efficiency, but verner tell me different. All thoughts welcome!

 

 

[Guest OzChris]

Hi Shroomer...

 

You may be comparing similar props, but definitely vastly different engines. The 582 max RPM (depending on model) gives a max rpm of around 6500rpm, thus giving a far better prop rpm. Your motor, even though more horses, has less max rpm, thus reducing the prop rpm in comparison to the 582.

 

I drive a 3 blade IVO (adjustable) on my Thruster with a 582 and have adjusted the pitch to give near max rpm - I get 6200rpm max power, giving amazing performance. You may need to play with your pitch ever so slightly to achieve the best results for the engine...it is SO important that you are matching your prop with your engine. If you are using a compatible prop, you may decide to test it with just SLIGHTLY more bite in your pitch...that may help give you a little more oomph...but be careful not to over work your engine, Im talking a slight pitch change, followed by some high speed taxi tests. Is your cruise speed also sluggish, or is that ok? With Ground adjustable props, sometimes its about finding a compromise between good climb out and cruise performance...of course, in-flight adjustable props are the best solution.

 

 

[Guest shroomer]

Hi Chris.

 

Thanks for that. It seems it is as I suspected then, the prop speed is too low. It seems strange then that verner stopped making the 1:2 gearbox reduction, that would have brought the prop speed up to 2500 for takeoff, giving much better efficiency. I have played around with various pitch settings, but it is as you say getting the compromise right between cruise and takeoff, the thrust while flying is good. I see kievprop have brought out an in flight adjustable - will have to take a look.