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On 10/1/2010 1:20 PM, Ed Anderson wrote:
A number of you have heard us that made it to Paducha about the difference in Sam Hodges Prop on his RV-7A and my prop. Both are suppose to be 74 x88. Well, here you can compare for yourself - for some reason, Sam just laughed when I referred to my "Monster" prop. Even though Sam and I had corresponded several times about his lower than desired engine RPM and I had asked several times about the size of his prop - which he always responded .."...same as yours, 74 x 88" which indicate the prop was not the problem. That is until you saw the chord of his prop blades they measured over 8" whereas mine measure around 5" - big difference.
Ed
Edward L. Anderson
Anderson Electronic Enterprises LLC
305 Reefton Road
Weddington, NC 28104
http://www.andersonee.com
http://www.eicommander.com
Now might be a good time to talk about how to order a prop for an experimental, for those who haven't had the pleasure in the past.
Sam's problem was almost certainly due to a massive mixup; maybe to the point of getting his data plate on the wrong prop. Having said that, it might have helped Sam to know the following:
While diameter should be measured the same no matter who we buy from (although in this case even *that* was suspect), the pitch means almost nothing at all. Yes, that's what I said.
Each prop carver may well use a different airfoil (meaning different lift, and different lift/drag profiles), and may have different methods of measuring pitch (there are at least 3 different ways to measure that I'm aware of). This means that a 72/74 from one carver might be a 72/78 from another (even with the same airfoil, chord & blade planform: identical props).
The good carvers will want to know (realistic) sea level horsepower expectation, prop rpm at that power level, (realistic) cruise/top speed of the airframe using the engine of choice (defining the drag of the airframe), & whether we want the prop biased for climb, cruise or compromise between the two. Though I don't have any experience with turbos, I'm sure that they would want that little tidbit of info, as well.
If we give them honest figures, the good carvers will come surprisingly close to optimum on 1st try. But if you just tell 'em we want a 72/73 because our buddy got one with those measurements from another carver, we will likely have 'issues'.
Charlie
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