downthrust on engine mount

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downthrust on engine mount

Postby mr157ifhz » Mon Sep 06, 2004 9:00 pm

Another question here for my plane to fly more efficiently. My plane is high wing, tractor engine. I see most engine installations are with a certain amount of downthrust. I understand the need for this to counter the 'swing' action acting from the high mounted wing. But if the amount of offset is too large, or not enough, would this effect cruising speed to a large extent? My offset to the side is bang on as I hardly need any rudder input at all on takeoff (is taildragger). Also for efficiency; I have a fairly blunt cowl, and no spinner. The prop is a warp drive, with a fairly large hub (10"). I know a spinner would reduce drag. Would the reduction in drag be noticable on a plane that currently cruises at 60mph?
-Matt
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Postby Sean_Caranna » Wed Sep 08, 2004 1:43 am

Beyond the drag improvement a spinner will make your prop more efficient as the airflow should be less turbulent at the prop root.
Shaping the spinner to blade junction is the key here. When you couple this with the reduced drag you will find that you are burning less fuel AND going a bit faster.

Offset of the engine is really dependant on the design. Offsetting the engine slightly up can improve stability along the lateral axes if it is a problem. This problem is more pronounced in aircraft with a lower CG, high wing, with low wing loading.

One factor is that by offsetting the engine you are changing the prop blade angle to relative wind. See P-factor. Obviously you are creating your thrust in a slightly different direction. While this can help your plane fly straight, it is inefficient. A bit like flying in a constant crab. That's why most commercially available aircraft use 0 offset and control the flying qualities with careful design and the use of trim tabs and such.


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Postby mr157ifhz » Wed Sep 08, 2004 2:03 pm

Good to know. A spinner is on the way. I think it will help smooth the flow through the two air inlets on either side of the prop hub, further reducing drag. Over the winter I intend to make a much more streamlined cowl.
-Matt
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Postby mr157ifhz » Tue Sep 28, 2004 8:31 pm

The spinner is on, wonder why I waited so long. Makes quite a difference. Previously I cruised (solo) at 50mph and 3700rpm. Now, I can pull the rpm back to 3500rpm and see 60mph (subaru ea81, geared 2.3~1) I know most subaru drivers are cruising around 42~4500rpm. I am around 80mph at that point, but I am never in too much of a rush to get back down, so I like the lower revs.
-Matt
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Postby Johnny luvs Biplanes » Wed Sep 29, 2004 7:17 pm

What size spinner did you use?
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Postby mr157ifhz » Thu Sep 30, 2004 1:51 pm

Hi John. Is a 10", fiberglass. The whole thing is 1.5lbs, compared to aluminum ones that were twice or more weight.
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