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erbus - n/a
3-Sep-06, 02:06 PM
... finally got the hover in the water after centralising the engine in the duct and sorting out my clearances as well as fitting new longer blades ... and its worse than ever.



I noticed after all the work the engine will not really rev past 5000 (used to go 6500) is this the new blades?

the new blades are only about 10mm longer and are 4zrs (the old ones were also 4zrs but a slightly different profile)

the blade angle is the same as the old but with the extra length and all clearances tightened up is the blade angle to severe a load for the engine?



lift and thrust were also poor but as i couldnt get to max power it was hard to tell the cause.



i dont now what the angle is but can get a photo later but angle remains unchanged before and after.



any help on this would be most welcome



dave

erbus - n/a
3-Sep-06, 03:28 PM
have checked the blade angle and it looks to be 40 degrees.



(the 4zrs have 4 small dimples on the root and it is the 3rd dimple that is lined up with the hub centre line.

Ian Brooks - n/a
3-Sep-06, 03:36 PM
Hi



The blade loading will be a little larger due to the exra length, and if they have a different profile (?) then the angle will probably be different anyway.



Back the blades off slightly, a bit at a time, until you reach 6500 rpm - then have another go...



Ian

erbus - n/a
3-Sep-06, 03:58 PM
typically what should the blade angle be for integrated craft?

are there pros and cons to having blades too steep/too shallow?



the blades can be adjusted in 5 degree increments if i use the locating piece. if i drop to 35 what would i lose/gain? apart from less engine load.



thanks

dave

Hovertrekker - n/a
3-Sep-06, 05:41 PM
Dave,

Don't use the pins or ball bearings. They're not needed. If you can measure accurately from the leading edge of the blade root to the center of the hub, then use those measurements to get all blades the same. Make a mark on your duct and make sure they all track evenly. It's a trial and error process. With the fan on the craft, Loosen up all the fan hub bolts so that you can just move the blades by hand. A rubber mallet can help http://hovercraft.org.uk/images/icons/smiley_icons/icon_smile.gif Make the adjustments, tighten all bolts and run it up on a trailer. After a few iterations you will get max RPM. 37.5 degrees might be a good place to start. Good luck.

erbus - n/a
3-Sep-06, 06:07 PM
dave,

thanks for that, will get rid of the pins as it will be easier to adjust. was 40 degress excessive?



the engine max rpm is 6800, should i be reducing the angle until i reach this - in theory, is this what blade angle is all about - matching max power of engine to max blade angle?



thanks

dave

hovercad - n/a
3-Sep-06, 07:45 PM
Hello,

I have just gone through what you are about to.

Regardless of the engine the principals are the same.

My personel preference was not to go max revs of engine but to have it reving in peak power range.(EG max revs of engine 7000.Peak power 6500.)

Simple way of getting all blades the same.

Clamp a hacksaw blade (or something simular) to the frame so the end just touches a blade.

If all are the same they should all just touch rotating around to each blade.(I take the plugs out for this its easier.)

Mark hacksaw blade.This is a reference point if all goes wrong.

Presuming revs are too low.

Remove hacksaw blade.

Measure about 1.5mm away from fan side.(IE hacksaw blade now sits more towards the back of the craft.)

Loosen all blades and reduce pitch.

Attach hacksaw blade to the same position on the frame at the new mark.

Now adjust all blades to just touch the hacksaw blade.

Clamp hub.DONT FORGET TO FIT FAN GUARD FIRST

Test.

If too high same as above but in between old mark and knew mark.

Test.DONT forget fan guard.etc etc.

If revs are too high to begin with mark the blade the opposite way.



For me i could leave the hub bolts done up and loosen the outer bolts to loosen the blades.(Multiwing 5z blades 6 in a 9 blade hub no pins used)

Hope all goes well.





Craig

Ps I learnt due to stupidity tested without fan guard.

carb filters got sucked off went into fan and broke every blade.

Not nice having sharp fragments of blade whizzing past your head.

Plus it costs more money to replace the blades rather than spend a little while fitting the guard.DDDDDDDDDDDDDUUUUUUUUUUUUUUUURRRRRRRRRRR

Hovertrekker - n/a
4-Sep-06, 02:51 PM
dave,

thanks for that, will get rid of the pins as it will be easier to adjust. was 40 degress excessive?



the engine max rpm is 6800, should i be reducing the angle until i reach this - in theory, is this what blade angle is all about - matching max power of engine to max blade angle?



thanks

dave




Dave - every craft/fan/hub/engine is different. 40 degrees on my craft is too little! Yes - you want the fan to absorb the available horse power. Follow Craig's advice and fit the fan guard. I left out those little safety details.