|Aug 25, 2004, 11:43 PM|
The prop numbers are usually a set of 2 numbers. The first one is the total prop diameter. The second is the pitch of the prop blades themselves.
For instance..I use APC props.
I am using a 9x6 prop on my GWS Spitfire.
The total prop diameter is 9 inches across.
The pitch of each blade is 6 which means the prop would pull the plane forward 6 inches for each prop revolution.
The pitch is not absolute, however, due to prop slippage. The blades do not pull air with 100% efficiency..they slip in the air a bit. So you will end up with a bit less than a full 6" pull forward.
That's my limited take on the prop sizes.
Also...GWS props use a slightly different designation. Their 9x7 prop is listed as 9070. You can see where they are going with that!?
Some props are 9x7.5 or 8.5x7 GWS uses zero's if there is no decimal.
|Aug 26, 2004, 07:02 PM|
Red Lake Falls, Minnesota, USA
Joined Jul 2003
It's kinda tricky though because a GWS 9070 9x7 is NOTHING like a top flite 9x7 wood prop except in pitch and diameter. He he that makes no sense but i mean if your changing props you kinda gotta see them side by side or atleast good pics.
|Aug 26, 2004, 08:51 PM|
It would follow that, to a greater degree, a smaller diameter prop with a greater pitch on the same motor under the same loads would deliver similar thrust? I am sure that this is not totally hard and fast given the same RPM would result in a slower tip speed on a smaller diameter prop possibly allowing a higher operating RPM, and so on, blah-blah, quack-quack.
I guess the best fallacy in this example can be illustrated by the efficiencies of the Ducted Fan systems...
Anyway, my first foamie design will be a twin-tailed photo ship not unlike the Cessna super skymaster:
Only with no front powerplant. Thats where the shooting platform will be.
The design concern that I have is keeping the distance between the booms reasonable in order to fit both the pusher prop and the control linkages safely in the same space -- rudder on one side and elevator on the other. Perhaps a 3-blade or 4-blade would work?
BTW, I spent much of my childhood taking cheap balsa gliders and assembling them Twin Mustang style.
One can dream, scheme and cut foam...
|Aug 26, 2004, 09:58 PM|
Decreasing diameter of the prop (actually it's the length of the prop, since it's not circular in shape ) and increasing pitch and rpm can net similar thrust, up to a point. It's really not that simple, however.
Generally, a larger diameter, slower turning prop of a relatively low pitch is more efficient aerodynamically and will produce more usable thrust than a smaller, high pitched one turning at high rpm. The larger prop disc moves a larger volume of air per revolution, but the lower pitch results in a lower inflight maximum airspeed (aka pitch speed). Smallish, high pitch props are more intended for fast, sleek airframes, and in extreme circumstances, the blades can actually be stalled when rotating, at low or zero airspeeds of the plane. The pitch on them is such that the AOA of the blades is too steep to produce much lift (thrust) until the plane is moving forward.
It's really a matter of experimentation to find the most efficient prop for a given application, WRT thrust, pitch speed, and amp draw. All three parameters play a role, so it's a balancing act. In general, you should choose large, shallow pitch props for slow-flying planes (like trainers and WWI planes), and small, higher pitch props for fast ones (like WWI warbirds and such).
|Aug 26, 2004, 11:25 PM|
I apprecciate that last post...some good info I have been pondering on.
I am feverishly trying to understand all this prop size, amp draw, watts stuff...but still feel pretty dumb.
Maybe you can help clarify some points for me???
I am flying my GWS Spitfire and use several different sized APC props. I ran some tests with my Whattmeter and came up with more questions than answers.
GWS 350/C 7.4v 1200 mah lipo Static test runs
No rpm data...night and tach would not read!!!
9x6 E 6.6amps 50.1W
9x7.5 E 7.6amps 51.2W
10x7E 8.5amps 55.3W
Using this data, I am trying to understand where the power comes from. Do I want higher watts? Lower? Amp draw? I know the GWS motors work well around 7-8 amps ....but what relationship am I looking for between amps and watts? What is a good reading relationship? This is for a faster warbird as you mentioned in previous post.
I was surprised at the amp jump between the first 2 props, yet watts remained close. I repeated the runs to verify the data.
Is that extra amp worth it for 1 watt power...is that good or inefficient?
I know...pain in the butt questions. I hope someone can help clear the muddy water (brain matter) for me.
Thanks for any help.
That should be a interesting project. I like those planes. Keep us up to date.
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