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different props on same engine

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Old 07-12-2014, 01:14 PM
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wingbolt
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Default different props on same engine

Installing slightly more powerful engine (saito 180) than required on extra 330 L (72" span) I usually use a 16x6 or 16x8 on both my saito 150 and 180 engines. I really never pay much attention to prop specs. If I use a smaller prop on the 180 will it reduce the thrust somewhat ? Say instead of the 16x6 I install a 15x8 what effect would this have, if any. Or am I just confusing myself which is getting easier each season. There is very little difference in size and weight of saitos through the 120 150 and 180. I have used several 150 and several 180, no 120`s. The 150`s have been good to me and my models.
It seems silly to go to a 150 for this model when I have the 180 taken from a 1/4 scale H.9 taylorcraft. I always appreciate opinions (hopefully from the smart) I always thought a smaller prop on same engine turns faster and flies model faster duh. Thanks and may all your landings be "beauty shooters"
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Old 07-12-2014, 07:29 PM
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Every internal combustion engine ever made has a particular RPM range where it makes the most power. The torque peak with be at an RPM a bit lower than the horsepower peak in every case I've ever seen. That's why the manufacturer has a set of prop recommendations, to get your flying with the RPM where the engine will perform best. If you pick a prop toward the less diameter and less pitch end of the recommended props, you'll see quicker acceleration, more in flight RPM, and therefore make the most power. It may not necessarily be the best flying prop, but for your engine that will be the result. A prop on the higher end won't let the engine unwind as much, but will usually make better use of the torque peak. All of this is secondary, of course, to the real world experience of seeing how the plane flies with a given prop.

On a 4 stroke, you have another concern: letting the engine turn too many RPM and float its valves. Underpropping a 2 stroke just gives you an engine that screams in the air but doesn't pull very well. Underpropping a 4 stroke results in costly repairs. Your manual should say what the RPM on the ground should be for your engines, so choose among the props that load the engine enough to keep it from over revving.
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Old 07-13-2014, 10:55 AM
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Thank you Jester, would never use a prop not recommended. Found your info. helpful however. Evidently there is something to what I suspected about using a smaller diameter prop. I am not happy with a small "edge" arf . Have a 91- 4 up front and at a distance it appears inverted. Using a 14x6 and thought of going to 13x8 which I have now decided not to do. I am of course talking about K series props. Believe a smaller engine would have been fine with this 60" span edge as I do not shoot holes in the sky.
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Old 07-13-2014, 06:40 PM
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I just reread your original post and see you were talking about making a tradeoff between diameter and pitch. When I first read it, I thought you asked about the difference between a 16x6 and a 15x6. In general, making the change you asked about will give you more straight line speed and less vertical pull. Of course, there is also the matter of how the two props unload differently, so one would likely find that there is more overall power to be had from one than the other.

So you're losing orientation with your Edge? What is your trim scheme like? With that big rudder, I'd think it should be easy enough to pick that out against the sky assuming it's a dark color.
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Old 07-18-2014, 06:31 PM
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If you like scale speeds i'd be running an 18x6/18x8 on the 180.I run a 15x8 on my 115,it's to small a prop for the 180 you'll be valve bouncing all over the place.
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Old 07-19-2014, 02:27 PM
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Originally Posted by Old Fart View Post
If you like scale speeds i'd be running an 18x6/18x8 on the 180.I run a 15x8 on my 115,it's to small a prop for the 180 you'll be valve bouncing all over the place.
When this post first came up I questioned it but didn't reply. I run 15 inch props on my OS 91s and even my YS 1.10s. I use 15 or 16 inch props on my 1.20s and questioned the use of small props on a 1.80? I have never had the 1.80 so not sure what there prop range is or there rpm range. I agree, the 15 and 16 sounds just a tad on the small side. With a 15 inch prop I would think valve floating would be a problem. I would also be testing out different brands of props.
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