Pulls size issues

[G1sammoms]

First off I have a 2 engine surveyor
16 on the lift and 20 on the trust
72" evo
My problem is in the trust area , 6" on the engine 14 on the prop shaft
I'm turning 3000 rpm at full throttle sitting still
I noticed the plan set call for a 15" pulley on the prop shaft ...
How much of a difference is the Gona make if I were going to change to the 15" pulls?

[G1sammoms]

Here a craft pic

[G1sammoms]

Sorry pulley not pulls

[AJW]

What size is the small / driver pulley on the thrust engine? divide the size of the larger pulley by the smaller pulley to give you the gearing ratio i;e 12" large / driven pulley / 6" smaller / driver pulley = 2:1 reduction. I have only used that as an example too early in the morning here for me to use my brain I recently had a twin surveyor that had a 118mm driver and a 220 driven pulley, "this is from memory so maybe slightly out" That craft had a 13 hp for lift and a 35 hp for thrust it used a multiwing Z7 fan for thrust. That craft flew very well for a twin engined craft.

You will always be limited by the amount of thrust you can achieve because you only have 20 hp pushing the craft. The secret is to get as much thrust efficency as possible from the engine without increasing the RPM of the engine too much. That engine will run all day long at 3000rpm but start trying to get max + revs out of it as some hovercraft manufacturers do and then reliability could be an issue. Increasing engine/prop RPM will increase noise.

You also have to take into account air flow into the prop. there will be parts of the prop that get limited amounts of air, because there will be things in front blocking that air eg a 20 hp engine etc, so some gains can be had by cleaning up the air intake to the prop, (may not be possible) maybe repitch the prop if it is adjustable to maybe get the rpm up to 3300 max.

I'm sure someone will be along shortly who can explain it better than me. If the surveyor flies ok as it is I wouldn't mess around with it too much.

[John Robertson]

Wind the pitch down half a turn on the Ivoprop (instructions on the Ivo website) to get the RPM to around 3200/3300 MAX (it'll increase further at cruise speed). If RPM doesn't increase then check throttle travel or governor. Won't make a lot of difference to thrust whatever you do.