Static thrust of Ivoprop

[AJW]

One thing I had with one of my Surveyors was too much material on the rear skirt to the splitter bar. When on the water it was creating a scoop so making it difficult to get over hump. John Robertson noticed it one day when I took him out on the Surveyor as we were trying to find the cause of my problem. You would need two onboard one to pilot the craft the other to look through the lift duct at the rear skirt.

[bphillip2]

Hello Sumner. I am assuming you have 1TB48 which measures 5.15 od, measured on the pulley metal OD, not the belt. I note though you previously said the belt OD with the a144 was 5.25". The A belt rides low in groove on these pulleys so your statements are in conflict.
The effective ratio in this case would be 2.95:1 or 1016 rpm if your loose belt does not slip at 3000 rpm. This would be about 15.2 hp total at 3000 rpm with about 8 to the prop. This drive is not correct for that engine, or you are not giving me the correct pulley size on the engine. How sure are you the fan is set to 25°?

[SumnerJ]

Hi Bryan. I'll be dipped: I just went out and rummaged through some old hovercraft parts and I found the little cardboard box that the pulley came in. It is a 1TB52. I looked online and the OD is specified as 5.55".

It does seem that there is too much lift and not enough thrust during starting. When I'm trying to get up on plane, I'll wind the engine up to 3000 RPM or over that and there is plenty of spray from the lift fan pushing a whole bunch of air into the cushion and out the sides, but not enough thrust to get over the bow wave quickly.

For the lift fan, I followed the instructions in the plans which said that it would make it 25 degrees and are as follows:

1. Put bushing in drive pulley and place bushing head down on flat surface.

2. Place fan hub half with the large center hole on top of drive pulley.

3. Note 4 grooves inside the fan hub blade end receptacle. Note righthand receptacle and place white plastic ferrule into second closest groove.

4. Note four grooves in fan blade butt end opposite the convex surface of the blade. Place blade into the rightmost receptacle so that the plastic ferrule in the hub half engages the second closest groove in the fan blade butt end.

5. The fan blade should be in the hub so that the convex surface is up and the inner radius trailing edge of the fan blade hangs just below the the fan hub half. (When I assembled the lift fan, I verified this.)

..... repeat for other blades.

8. If properly assembled, the fan should rest on the flat surface with the hub supported on the blade trailing edges just above the flat surface by H (approx 3/32 in).

I followed these instructions. It seems to have the right amount of lift when cruising at 2300 to 2500 rpm engine RPM.

[bphillip2]

Hello Sumner. I was going to suggest a 1TB54 and a BX belt. Changing to a B from an A belt will increase prop speed by 5% resulting in 14% change in power to the prop. They also significantly increase traction, but getting the right length can be tricky, as they ride higher. Maybe the 52 could work with extra prop pitch.

It sounds like you have a good handle on the fan.

Let me know what you think. It would be helpful if we which pulley you had right know to estimate the length of the new belt.

[SumnerJ]

Hi Bryan. Any experience in how much higher the BX belt rides in the pulley groove, compared to the A belt? From that I could calculate what length the BX belt would need to be. If you don't know off-hand, I can calculate it from the angle of the sides of the groove on the pulley. (Do you know what that angle is? I didn't find it looking online in pulley catalogs. I could just measure it also.)

I'm not sure what you meant by "It would be helpful if we which pulley you had right know to estimate the length of the new belt."
Right now I've got the 1TB52 drive pulley and the Browning BK140H prop pulley. Is that what you were asking about?


Regards,
Sumner

[SumnerJ]

Bryan, I'd like to try the BX belt and the 1TB54 pulley. If those combined increased the prop RPM by 10%, that would be about 1282 prop rpm for 3000 rpm engine speed. From what I've read, thrust is proportional to rpm squared so increasing prop rpm by 10% would increase thrust by approximately 20%. Does that sound about right?

[bphillip2]

I thought you had two TB pulley's.

They ride flush.