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<HTML> <BODY BC-inOLOR="FFFFFF" TEXT="000000" LINK="000099" ALINK="ff0000" VLINK="000099"> <HEAD><TITLE> For Bob </TITLE></HEAD> <br><br> <img src="P1010061.JPG"height=150> <img src="P1010062.JPG"height=150 ><br> <img src="P1010056.JPG"height=150 > <img src="P1010057.JPG"height=150 ><Br> <b>Guarding The Prop</b><br><br><br><br> <img src="P1010059.JPG" height=250 > <br> Four Inlet Guards - call them "A-in", "B-in", "C-in", "D-in".<br> Top row: A-in (8 mm wide holes). B-in (4 mm wide holes)<br> Bottom row: C-in (6 mm gridwork - rabbit fence) , D-in (1.4 mm gridwork -- insect screen) <br><br> <img src="P1010060.JPG"height=250 > <br> Three Outlet Guards - call them "B-out", "C-out", and "D-out".<br> From left to right: B-out (4 mm wide holes), C-out (6 mm gridwork), D-out (1.4 mm gridwork) <br><br> Data<br> <br> Data collected using a standard duct, motor, and propeller, powered by a constant 5 volt 2 amp AC->DC power supply. To measure thrust, the ducts were set up upside down on a bracket attached to a scale with an accuracy of +/- 1 gram and powered on. There were multiple runs for each configuration - the average of each run is shown below. <pre> Inlet Outlet Avg Thrust % of standard Guard Guard in g ---- ----- ----- ------ None None 40 100% None B-out 31 77.5% None A-in 36 90.0% (presumably, the hole helped.) A-in None 39 97.5% A-in B-out 31 77.5% B-in None 37 92.5% B-in B-out 27 67.5% C-in C-out 34 85.0% C-in None 39 97.5% C-in tight* None 37 92.5% D-in None 38 95.0% D-in D-out 30 75.0% </pre> Conclusions: The presence of any outlet guard at all will limit performance significantly. Surprisingly, the four inlet guards tested showed similar performance. The presence of an inlet guard will limit performance, but not drastically. <br><br> *Also: Leaving a gap between the guard and the bell mouth intake helps performance. The gap between the duct and the A-in guard is shown below.<br> <img src="P1010063.JPG" height=150 > <br><br> Data is only available for the C-in guard, although the same effect was demonstrated with all guards. With the gap, using the C-in guard on the inlet, the thrust was 39, but when the guard was brought down tight, closing the gap, the thrust dropped to 37. <br><br> All images can be viewed at higher resolution - select and view separately. <br>