Sparrow Hawk 1 Pre-rotator

[Colin Gibson]

Help.
Can anyone supply data an the efficiency of their Sparrow Hawk 1 pre-rotators.
Mine has the three small clutch pads and has essentially been a poor performer right from the beginning.
Have these clutches been improved and if so, what and how were they improved.
I have learned to fly it as originally built but its time to improve the efficiency of this component. It would certainly improve my take offs to have it working better.
Colin

[twistair]

I assembled 3 SHs with this setup, all them showed stable 180-190 rrpm though I was able to make 210-220. What is your number you call it poor?
If it's lower than 180 then check that outer clutch disk is as close (when disengaged) to pads as possible (less than 1 mm, better less than 0.5 mm), that cable line has no sharp turns in small radii and that cable moves OK in its sleeve.

[twistair]

In addition: this setup performance also may be poor for some more reasons:
1) pads glued so that they three don't form flat surface (say, one is higher from the disk than others)
2) pads were overheated by too long slippage
3) pads were not original but cut from other material

[Colin Gibson]

Originally Posted by twistair

I assembled 3 SHs with this setup, all them showed stable 180-190 rrpm though I was able to make 210-220. What is your number you call it poor?
If it's lower than 180 then check that outer clutch disk is as close (when disengaged) to pads as possible (less than 1 mm, better less than 0.5 mm), that cable line has no sharp turns in small radii and that cable moves OK in its sleeve.

I get 80rpm then struggle after that for more. Never ever had the rpm's you quote here.This tends to put a light even shine on the pads indicating they are flat.
I rough them up occasionally which helps for a while.
There were not pads supplied with the kit, only material to make the pads out of which mine are made of. They are glued and riveted.
Colin

[Colin Gibson]

Originally Posted by twistair

In addition: this setup performance also may be poor for some more reasons:
1) pads glued so that they three don't form flat surface (say, one is higher from the disk than others)
2) pads were overheated by too long slippage
3) pads were not original but cut from other material

The pads were machined today and one was higher than the others.
Will be installing back on the Gyro Sunday.
Expect a lot better performance.
Thanks for the advice which was backed up by my friend John who machined them for me.
Colin

[MichaelBurton]

We made a few changes:

changed friction pads to aircraft quality pads.
Changed the spring as it was binding against the key.
used larger actuator cable size to decrease stretch.

Excessive rpm will prevent a good pre rotation.

One gyro that came in had the clutch plate coated with some high temp ceramic coating to prevent rust and discoloration. It worked well for that but was so slick that it could not get over 100 rrpm. I used a disk sander to remove the coating from the clutch face to solve the problem.

I have also seen problems with the cable routing, drive cable lubrication, assembly of upper can, assembly of lower clutch drive, etc. All of these things can cause a poor prerotation.

[Colin Gibson]

Michael,
Thank you for your reply.
The drive cable moves freely, an I'm happy with the lube.
Routing has no sharp bends.
Bendix works as expected.
If performance is still lacking I will try different pads but the test will determine that.
I heard about the ceramic option but thought it was only to be on the outside.
I do not have a ceramic coating.
Looking at the surface as John machined them, I are surprised the thing worked at all.
Colin

[Rusty Russell]

How did they perform after the re-surfacing?

[Steve McGowan]

A few updates of the pre rotater clutch was that the disc and padding area was made 6".

Not good for the inner-cable guy's,
Remember the inner 1/2 " drive cable isnt as strong as you would think.. It can and will twist apart.

you can get 200rpm on the pre-rotation,, a few times.. and the next it may break on Ya..

Im gonna wait on my ground roll until 120 rpm and then release the system at 180 as specified. on the take-off roll..

Thats just me guy's.. Your Gyro,, Your Take-off..

Steve

ps.. easiest way to Re-Surface the disc,, scuff it on concrete.. REALLY

[Coen]

No hurry, was just a bit worry'd about that barbed fence at the end of the short and soft runway that one may encounter someday or in my case more often than others.

Regards Coen

[Aussie_Paul]

Originally Posted by Colin Gibson

Michael,
Thank you for your reply.
The drive cable moves freely, an I'm happy with the lube.
Routing has no sharp bends.
Bendix works as expected.
If performance is still lacking I will try different pads but the test will determine that.
I heard about the ceramic option but thought it was only to be on the outside.
I do not have a ceramic coating.
Looking at the surface as John machined them, I are surprised the thing worked at all.
Colin

Colin, if all else is ok, the machining should do the trick. My 3 RAF pads were pop riveted and this distorted the pads and they were only touching at the corners. Make sure you begin pre rotation at idle until the clutch is not slipping, then very gradually increase engine rpm trying not to make the clutch slip, 200 is easily possible.

Aussie Paul.

[Colin Gibson]

Paul
Sorry I haven't been checking my posts.
The machining did help but still didn't produce the rpm I was expecting.
I tend to avoid runway situations where I don't have 500meters. The Sparrow Hawk gets off two up in 400 and I like a 100 spare in case it doesn't.