Tweeking a Belko Prop?

[frank williams]

Can you heat tweek a Belko prop?.  They are thick sections and I think I've heard they are foam filled or else balsa cored.  I'd like to alter the pitch just a bit but I don't want it to come apart completely.  Any experience out there with this?  Specifically its a three blade, ~11.5 diameter, pitch around 4.5 to 5.

[roy cherry]

yes they are foam filled don't do it  roy

[Randy Cuberly]

Can you heat tweek a Belko prop?.  They are thick sections and I think I've heard they are foam filled or else balsa cored.  I'd like to alter the pitch just a bit but I don't want it to come apart completely.  Any experience out there with this?  Specifically its a three blade, ~11.5 diameter, pitch around 4.5 to 5.

Hi Frank,
The answer is a resounding NO.  I tried to tweak one when I first started using the Belko engines about 10 years ago.  I used heat and gently tried to twist and the prop simply de-laminated into a terrible mess.
They are constructed with the blue type foam and then laminated with carbon and very thin epoxy.   They are very stiff and light and simply cannot be twisted without destroy them unless you can come up with some magic that I couldn't.  Serguey (sp) Belko told me not to even try but I thought it would be worth finding out for myself...BAD IDEA!  They are very light and efficient.  They were made in two pitch sizes.
I'm not sure which size I sent to you but there is only about 1/4 inch difference in pitch between the one for the 56 (that you have) and the 60 like the two I have.  In my opinion the 56 is the better of the two engines and uses the all black prop.  I don't remember which pitxh is which but will measure one tomorrow (It's just after midnight here now and being an old "fart" of 79 I'm too sleepy right now to trust any measurement I might make tonight).  I found the black prop to be by far the best for the Geo XL that I flew it in for about 5 years before the airplane and engine was stolen from the flying field (never to be seen again).  The Geo XL was very clean aerodynamically, but fairly large at 710 sq inches,  It also was very light at 54 oz.  The Belko engine was absolutely perfect for it and I'm currently building another one for the Belko 61 that I have.  The 61 is a bit heavier and uses a slightly large prop of the same type as the one you have.  Unfortunately it will likely be a bit heavier due to the unavailability of 4 lb balsa.
Belko told me when he came here to America for several contests, that his engines with the very long cranks need to use light props to overcome the extra weight of the very long crankshaft.
I've been wondering how the Belko engine was working for you.  Lacking a light three blade I would go to a light wood two blade prop with about 1 inch longer blades and a 5 or 6 pitch.  That was Belko's suggestion also.  The Belko engine should run in a 2-4 that is very closely regulated so that there is very little actual increase in prop speed with the change. 

Randy Cuberly

[frank williams]

Thanks all for your inputs .... I was pretty sure that it was out of the question to try to tweek them ...... how in the world does someone make something like that.  I have enough trouble making solid carbon props, much less a two sided construction.  Real craftsmen.

Randy, I have had some health issues and have just got back to running the Belko.  No nitro is new to me.  Steve Moon in Dallas does it quite well and has some excellent motor runs.  So  far the motor seems to be "over muffled". I''m runnng the muffler that came with it, so I assume it should be the right size.  With the black three blade you sent me, the lap time is a bit slow.  I have one of the kevlar-carbon three blade of Belko.  I'll check what the pitch is on it.  The motor starts and runs nicely but it might need a head shim or two since it struggles a little over the top.  It may be a little over-compressed and advancing the timing too much.  Of course the cylinder is mounted inboard.  The fuel I mixed is 9-9-82.  I'll try a two blade with a bit more pitch.  Still working with it every now and then.  Too many fun things to do.

[Randy Cuberly]

Thanks all for your inputs .... I was pretty sure that it was out of the question to try to tweek them ...... how in the world does someone make something like that.  I have enough trouble making solid carbon props, much less a two sided construction.  Real craftsmen.

Randy, I have had some health issues and have just got back to running the Belko.  No nitro is new to me.  Steve Moon in Dallas does it quite well and has some excellent motor runs.  So  far the motor seems to be "over muffled". I''m runnng the muffler that came with it, so I assume it should be the right size.  With the black three blade you sent me, the lap time is a bit slow.  I have one of the kevlar-carbon three blade of Belko.  I'll check what the pitch is on it.  The motor starts and runs nicely but it might need a head shim or two since it struggles a little over the top.  It may be a little over-compressed and advancing the timing too much.  Of course the cylinder is mounted inboard.  The fuel I mixed is 9-9-82.  I'll try a two blade with a bit more pitch.  Still working with it every now and then.  Too many fun things to do.

Hi Frank,

I tried several different mixes of fuel when I first started using the Belko.  I definitely would recommend a bit more oil.  I settled out at 22% half klotz and half castor.  I mounted and ran mine inverted in a fairly conventional manner.  It's also my opinion that you need a very open and unrestricted air flow to the venturi. 
I believe the kevlar/carbon prop was intended for the 61 engine, but I never tried it on the 56 that I used in the GeoXL.
What RPM are you launching at!  I used about 9200 (from memory)
Also the tank position was fairly critical.  I'm not sure how your mounting arrangement would affect that.  You'll notice in the photo's I'm sending that I used a functional scoop on the nose that opened directly to the venturi.  That was also the recommended situation by Belko.
Incidentally, the engine you have has very little time on it.  It was totally rebuilt by Belko, and only run by me for approximatly 2-to 3 minutes by me.  It originally belonged to The very nice gentleman from Switzerland whose name I cannot remember right now...I do remember that his lovely wife's name was Monipue!  They actually lived here in Tucson for a short time!
Basically the engine was the twin of the one I used for a number of years in my Geo XL .
I'm having a bit of trouble reducing the photo file to fit on the forum.  I'll send it to you when I can get it together

Randy Cuberly

PS:  I also had some health issues for approximately 2 years that stopped my flying for awhile.  When I was ready to start flying again the wonderful Chinese gifted us with the current Pandemic and I'm still waiting to get back to it.  I'm building another GeoXL and will probably use the Belko 61 in it!  The Geo XL in the Photo was about 8 years old and had many flights on it probably over a thousand and the engine was still running great!  Too bad it was stolen!  Partly my fault!  I was not feeling well and forgot and left it on the apron at the flying field...when I returned about 45 minutes later it was GONE!  I was the last one at the field when I left!

If I can do anything further to help please let me know!  The airplane you have the Belko in is beautiful!

[dale gleason]

Capt. Rene Bergere

[Randy Cuberly]

Yes...

Thanks for the name Dale!

Sometimes my 80 year old brain just drops names.  It can be very disturbing.  Especially when it's someone that has been a very good friend!

Randy Cuberly

[Lauri Malila]

René Berger.