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Old Apr 28, 2012, 06:06 AM
Flying Leatherneck
Japan, Okinawa Prefecture
Joined Jan 2012
2 Posts
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Impacts of running too large a prop on a Saito gas engine?

Mulling ideas. I have two Hangar 9 Fokker DVII kits and two Camel kits I am going to start building this summer. Seriously thinking about putting Saito FG gassers in one each. FG-14 should fit in each without cutting the cowls. But I'd like to run a large prop for scale'ish purposes, even in flight appearance. However, running a 16 or 17 inch prop exceeds the recommended size for the engine. All the reading on these kits and engines trend that there is rarely a need to run an .82 size at more than 1/2 throttle. Conventional wisdom states the FG-14 equates to about a .72 power output. Am I cumulatively setting myself up for engine failure, diminished performance, or premature wear with using a slightly less powerful engine equivalent to glow, with hanging a larger than recommended prop?

Flying behavior is not to scorch around like a modern tacair jet, but to try to maintain scale like speeds and maneuvering to the degree can be achieved.

Also had the bright idea to see if an FG-17 can fit with minimal exposure through the cowls, still swing a big prop, but keep the RPMs down to sound scale'ish. I did the math (though I can't find the scrap of paper) and I think I figured a 4 stroke at around 4 or 5 thousand RPMs with give out the same exhauste note as a 9 cylinder rotary. But I may be off. Need to recompute that and revise this post to confirm my idea...

Grateful for any info on the prop issue.

Regards

Ben
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Old Apr 28, 2012, 06:48 AM
222 km/hr Parkjet flyer
solentlife's Avatar
Latvia, Ventspils pilsēta, Ventspils
Joined Jan 2010
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Without entering the arena of engine size to model ... fitting larger prop to a fuel engine is not as bad as with electrics.

I regularly overpropped my fuel planes in 1980 ... 1990's ... when they were WW1 bipes etc. For exactly the reason you state ... scale speed and better sound. My Neiuport 28 on a 60 Glow had a 15" wood prop that was way over the rec'd ... and with a dustbin silencer sounded and flew wonderful. But engine size was the recc'd.

What I did find though was that fuel consumption was higher, greater amount of carbon and crud out of exhaust ... and throttling was not so crisp. It usually meant that I was flying at or near full throttle most of the time ... same as the real ones !! Despite Hollywood ........ many early planes only had on / off switches on their engines with crude speed control.... so I'm told !

As I understand it - UNDERpropping is more problem for an engine than over ... as then you don;t have the flywheel effect, engine revs too easy and can overspeed, fuel draw is less so less cooling effect of fuel and oil mix etc. etc.

My personal view is never to underpower a model with smaller than max recc'd motor..... prop then is another matter.
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Old Apr 28, 2012, 07:47 AM
I bail out, anywhere, anytime
Taurus Flyer's Avatar
The Netherlands, OV, Almelo
Joined Nov 2010
2,973 Posts
Ben

Look for props with a very low pitch value is of course very important
Some time ago I did look for these and that we can do again because there are!!
See the pictures.
Second solution, make your own, see first picture but that is a little bit more complicated.

I can calculate a big prop when you find a low pitch example. (HP with a solver program.)

Cees
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Old Apr 28, 2012, 08:14 AM
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u2builder's Avatar
USA, NH, Alstead
Joined Oct 2007
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I tend to run 4 pitch Xoar props on my Saitos. I want a big prop and don't care about the speed. It is like having a climb prop rather than a cruise prop. They fly more than fast enough for me even at part throttle. Of course I am talking about bipes and 3D type planes with relatively lower wing loading than speedsters.
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Old Apr 29, 2012, 11:54 PM
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Canberra Australia
Joined Jun 2009
735 Posts
I would not recommend overproping the Saitos (eg above what is recommended). From my experience, the engine can overheat and will start to stumble. Having said this, you can, of course, go up a size in diameter (and drop a size in pitch) over the max diameter, but I wouldn't try any more than one extra inch. Further, you would want to be able to fly the plane at three quarter throttle, with WOT just for take off and every now and then.

I had an overpropped FG-20 on a 14lb VQ Pilatus Porter which needed WOT most of the time, but I could only get about 5 minutes before the engine started to overheat. I switched to the max recommended prop and it has been fine. As far as I can tell, flying performance has not been affected. I realise you ae just after the scale effect, and not performance, but you should be able to get a brand of prop that suits your purpose (eg perhaps a heavier prop).
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Old Apr 30, 2012, 12:19 AM
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United States, CO, Dacono
Joined Nov 2010
695 Posts
We have 2 Saito 80's and one ASP 80, all three will run hot, buck and spit the prop off with 13" props, all three planes run 12.7's and they run fine.
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Old Apr 30, 2012, 04:35 AM
NM2K
Ringgold, GA, USA
Joined Jan 2006
1,045 Posts
Quote:
Originally Posted by cherokeenut View Post
We have 2 Saito 80's and one ASP 80, all three will run hot, buck and spit the prop off with 13" props, all three planes run 12.7's and they run fine.
r br


My standard prop for my Enya 60 and 80 four-strokes is a 13x6 or a 14x6, if ground clearance permits. The engine(s) need to be retuned for any change of propeller (even other props of the same size, but different brands), glow plug or fuel.

A gear or belt reduction unit would solve the problem for those wishing scale rpm and prop sizes while running the engine within its recommended power band.

Don't forget to add a small cooling fan and ducting for the cooling air, in which case a slightly larger than recommended engine would be the best choice.


Ed Cregger
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Old Apr 30, 2012, 09:37 AM
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United States, CO, Dacono
Joined Nov 2010
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Quote:
Originally Posted by Artisan View Post
r br


My standard prop for my Enya 60 and 80 four-strokes is a 13x6 or a 14x6, if ground clearance permits. The engine(s) need to be retuned for any change of propeller (even other props of the same size, but different brands), glow plug or fuel.

A gear or belt reduction unit would solve the problem for those wishing scale rpm and prop sizes while running the engine within its recommended power band.

Don't forget to add a small cooling fan and ducting for the cooling air, in which case a slightly larger than recommended engine would be the best choice.


Ed Cregger
We are wondering if it's our 6,000 ft altitude, less air. I can't even think of running a 12" prop on my Enya 60, it will kick the prop off.
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Old Apr 30, 2012, 09:49 AM
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South Africa, WC, Cape Town
Joined Dec 2009
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A chum of mine had a Saito 1.50 GK in a Showtime 90. He over propped it for better 3D...on the 4th flight it dropped a valve. He just about made it back to the runway. It just needed a new valve but the piston was damaged as well. He didn;t bother to replace it and after rebuilding the ening he sold it after a further 20ish flights and bought a gasser.

Don't over prop too far, it could cost yo your model. Of course if you're f lying scale like in a WWI bird then the risk is pretty low of a failure but its higher than if you stay within the recommended prop range
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Old Apr 30, 2012, 10:48 AM
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Chicagoland
Joined Feb 2000
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Running spark ignition you can run virtually any size prop you desire. There are some concerns, however. With the reduced fuel volume there is reduced oil volume. Engine wear could be an issue. Cooling can be another concern due to the slower air flow around the engine. Lower RPM means smaller volume flow through the carb and you could find it difficult to tune properly and the throttle response will not be linear.

I've run an Enya 46-4C with a 18x10 prop on ignition. It ran fine after some modifications. I did not put a lot of time on it, so I don't know the long term effects.

Tim, how would reducing the RPM cause a valvetrain failure? It seems to me completely unrelated.

Glow engines over propped are another issue. Ignition timing is essentially controlled by air fuel mixture.You really have to pay attention and change plugs, reduce compression ratio, etc., if it's to work well.

Greg
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Old May 02, 2012, 12:54 AM
Flying Leatherneck
Japan, Okinawa Prefecture
Joined Jan 2012
2 Posts
All. Many thanks for the insights and info. I'll take this one cautiously, especially since these hangar 9 ARF Fokkers and Camels are not on the shelves anymore and I spent the better part of a year even finding the kits in the first place... for a bit more than original cost. But such is the "lack of spare time to build vs threshold of spending" equation... Time is money, in the inverse....

Regards

Ben
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Old May 02, 2012, 01:11 AM
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South Africa, WC, Cape Town
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Quote:
Originally Posted by gkamysz View Post
Running spark ignition you can run virtually any size prop you desire. There are some concerns, however. With the reduced fuel volume there is reduced oil volume. Engine wear could be an issue. Cooling can be another concern due to the slower air flow around the engine. Lower RPM means smaller volume flow through the carb and you could find it difficult to tune properly and the throttle response will not be linear.

I've run an Enya 46-4C with a 18x10 prop on ignition. It ran fine after some modifications. I did not put a lot of time on it, so I don't know the long term effects.

Tim, how would reducing the RPM cause a valvetrain failure? It seems to me completely unrelated.

Glow engines over propped are another issue. Ignition timing is essentially controlled by air fuel mixture.You really have to pay attention and change plugs, reduce compression ratio, etc., if it's to work well.

Greg
For a gasser the chance of under rev causing valve train failure is less likely but if the extra load wears the front bearing prematurely and excessively?
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Old May 02, 2012, 11:48 AM
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Joined May 2004
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Be very careful with overheating a saito gasser. Gasoline leaves a lot more carbon in the combustion chamber than methanol, and I've had a cowled FG-20 burn a valve on me.
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Old May 03, 2012, 03:01 AM
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Well that is a problem. The extra load has to be absorbed somewhere and it usually means additional heat from high peak pressures (think of the extra load as a brake - i.e Brake horse power...) but lower attainable piston velocity. So theres less power available to carry the load.
Hotter valves means poor seating when they're closed so there more blow by etc so there the possibility of good damage to the valve train if the situation persists. The main problem is at WOT but the average temperatures are higher due to the higher average load
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