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Just in case anyone is not aware, the main gate post at the field entrance is broken at its base just at ground level. ( the bolt side post) The post is only supported by the fencing attached to it. If the fencing fails then the post will fall and …

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Props

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Props

Post by Stubbsy on Mon Aug 29, 2011 9:25 am

Okay, I know the weather is pretty crappy this bank holiday weekend so here is something to keep your brains warm!

How do props work? (anyone who says they spin and move a plane through the air gets a punch in the nose! Suspect)

summed up I suppose here is my question: - a 7x4 prop is 7 inches long with a pitch of "4", that bit is straight forward enough, but if you compare that to a 6x5 prop for example, which is an inch shorter but with a greater pitch - at a set rpm, which would pull the plane faster?

I always understood that you can think of the pitch of a prop as the "cut size" of the air. A higher pitch prop will cut through more air per rev, and pull the plane further forward per turn, so a 6x5 prop at a set rpm will be faster than a 6x4 - but how does the length of a prop compare to its pitch? A 7x4 prop has a bigger disc size than a 6x4 so pulls more air, so that would be quicker (I presume), but what happens when you increase pitch but decrease blade length or vise versa? scratch

Feeling pretty dumb at the mo cos this is basic stuff - maybe all this time off is making my brain rusty!

Cheers chaps!
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Re: Props

Post by Brian Colclough on Mon Aug 29, 2011 10:02 am

Ben the two figures of a prop size ie 10x6 are really 10"x6". Ten inches is as you say the diameter and the six inches theoretically is the distance the "airscrew" will move forward per revolution. Choosing the right prop for an engine/motor is always a compromise between the power and torque of the power unit the "style of flying" eg 3D flyers tend to go for lower pitch as that allows the unit to throttle or spool up more quickly. Guys who are racing pylon will go for much more pitch as this relates to more airspeed.
Another important factor is the tipspeed of the prop, the bigger the diameter the higher the tipspeed will be and once the tipspeed approaches the speed of sound the prop will start to produce a lot of noise. All this really means that prop choice will always be a compromise and at the end of the day very much down to personal preference. Wink

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Re: Props

Post by Andy Sayle on Tue Aug 30, 2011 9:24 am

What Brian says.

For general sports type models (like that Unicorn wing of yours) you want to be aiming at a pitch to diameter ratio of around 0.6 (eg, 6x4, 10x6 etc). For models where you want more thrust for low speed manouevres like on 3D and aerobatic type aircraft, you would typically aim to have a lower ratio, say 0.35-0.4 (23x8 on my 50cc SU26). For faster aircraft, you aim for a higher ratio, nearer 1.0 and sometimes even above that like 1.2 (my little green pylin racer has a 5.5x5.5 prop on it).

If I swap the normal prop on my green pile-in racer from a 5.5x5.5 to a 5x4, the difference in flight is huge. The top speed is waaay down, but it has better acceleration, a lower current draw, and it is a bit quieter. If I go up to something like a 5x6 prop, I have to be careful on the launch as the acceleration is down, but once it gets up to speed, the top speed is just ridiculous.

Once you start to get prop pitch to diameter ratios up around 0.8, you need to watch out for the blades being stalled at low airspeed (usually indicated by a shedload of noise, and not much thrust!). It is the equivalent of pulling too much angle of attack on a wing....


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