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Old Mar 11, 2015, 02:46 PM
...design-build-fly-publish...
eye4wings's Avatar
Ware, herts. U.K.
Joined Sep 2008
3,563 Posts
Sorry Mat,
Should have explained, although I did say ignoring it!
A turning propeller moves the air backwards yes - that's what we want - but because the blades of the prop are angled to the airfow (less for a fine pitch more for a coarse pitch) There is also a side effect. The air coming off the blade passing above the wing will be moving slightly to one side in the direction the prop is moving while the blade passing below the wing will have the same effect in the opposite direction. This is the helical effect because the airflow is not coming off the pop straight backwards but in a corkscrew pattern.

Because of this effect the wing meets the airflow at a different angle on one side of the propwash from that on the other side. One side will be effectively at slightly more incidence, the other less.

But ignoring this effect the greater effect will be when the motor incidence is different from the angle of attack. If it is angled downwards then the blast of air puts greater pressure on the underside of the wing increasing the lift because it is in effect increasing the angle of attack of the wing section that is in the propwash.
Obviously a motor angled upwards will exert pressure on the top of the wing decreasing the AoA of that part of the wing and decreasing the lift.

For a seaplane the blast of air will also be hitting the water in the take-off. This will increase the 'ground effect', but I was wondering if you - or anyone - had noticed any difference in trim change under power because to my mind an upward pointing motor should decrease the lift of the wing more at higher revs and should therefore mean less trim change through the speed range.

I hope that's explained it well enough.

Robin
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Old Mar 11, 2015, 11:32 PM
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Wormboy's Avatar
Australia, WA, Madeley
Joined Jul 2011
736 Posts
Quote:
Originally Posted by eye4wings View Post
Sorry Mat,
Should have explained, although I did say ignoring it!
A turning propeller moves the air backwards yes - that's what we want - but because the blades of the prop are angled to the airfow (less for a fine pitch more for a coarse pitch) There is also a side effect. The air coming off the blade passing above the wing will be moving slightly to one side in the direction the prop is moving while the blade passing below the wing will have the same effect in the opposite direction. This is the helical effect because the airflow is not coming off the pop straight backwards but in a corkscrew pattern.

Because of this effect the wing meets the airflow at a different angle on one side of the propwash from that on the other side. One side will be effectively at slightly more incidence, the other less.

But ignoring this effect the greater effect will be when the motor incidence is different from the angle of attack. If it is angled downwards then the blast of air puts greater pressure on the underside of the wing increasing the lift because it is in effect increasing the angle of attack of the wing section that is in the propwash.
Obviously a motor angled upwards will exert pressure on the top of the wing decreasing the AoA of that part of the wing and decreasing the lift.

For a seaplane the blast of air will also be hitting the water in the take-off. This will increase the 'ground effect', but I was wondering if you - or anyone - had noticed any difference in trim change under power because to my mind an upward pointing motor should decrease the lift of the wing more at higher revs and should therefore mean less trim change through the speed range.

I hope that's explained it well enough.

Robin

Hey, I learned something! Thanks Robin.

In my situation with the S39. Originally both the wing and motor incidence were at 5+ so given that they were both the same I don't think I would have been in a position to observe what you are describing. What I did observe was serious pitching with increased revs. probably exactly what would be expected.

In contrast. Now I have the motor at 0 to aircraft datum, while the wing is still at 5+. This creates the exact opposite situation whereby I should see greater lift at higher revs. However, in practicality, higher revs= higher speed which acts on the entire wing, not just that portion in the propwash. The outcome is that between 0 and about 70% throttle I get nice increase in lift without the pitching. Above 70% the pitching tends to start again, which I suspect is more to do with the 5+ on the wing rather than anything else. What all this means for me as a pilot is that I can ROW with very little rotation around the step, it just rises up easily at about 70% throttle at which point I can back off to 40-60% and pootle around without worrying about pitching. Drop it down to <40% and she will start to sink for landing without pitching up or down too much.
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Old Mar 12, 2015, 03:27 AM
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eye4wings's Avatar
Ware, herts. U.K.
Joined Sep 2008
3,563 Posts
That all makes sense to me Mat, but the important thing so far as pitching is concerned is the differnce between AoA of wing and tailplane. As far as the actual flying is concerned once trimmed out every plane can fly straight and level. The AW Whitley had a lot of angle on the wing, but so did the tail so it flew with a noticeably nose-down attitude.

I had therefore assumed that for rising off water there was some reason such as ease of take-off that dictated that higher angles of attack were required. Your experience does not support this so I am still left wondering why the angles on the Widgeon for instance were as they were.

Robin
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Old Mar 16, 2015, 06:30 AM
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Sopwith Mike's Avatar
Christchurch,England
Joined Aug 2004
2,894 Posts
Very nice-looking Twin Otter, Ray! It must have sneaked into the Hall of Fame a few days ago. Will you be coming to the Ivan P Fly-In at the end of May? It would be great to see the Matrin Mars as well as the Twin Otter.
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Old Mar 16, 2015, 12:23 PM
Which means "what" to me?
invid66's Avatar
United States, FL, Havana
Joined Jun 2007
87 Posts
My otter so far, and the color scheme I hope to use.
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Old Mar 16, 2015, 12:54 PM
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Victoria B.C. Canada
Joined Aug 2004
105 Posts
Nice color scheme!! Looks like major washout on those wingtips??
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Old Mar 16, 2015, 03:19 PM
Which means "what" to me?
invid66's Avatar
United States, FL, Havana
Joined Jun 2007
87 Posts
Yeah, it didn't like its time in the sun. I think most of that should come out when I finish the sheeting on the leading edge and the center section... I hope.

I also have to finish sheeting the upper and lower fuselage surfaces and build the hatch, nacelles and cowlings.
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Last edited by invid66; Mar 16, 2015 at 03:23 PM. Reason: Not thinking ahead.
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