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Mar 11, 2015, 03:46 PM
...design-build-fly-publish...
eye4wings's Avatar
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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Mar 12, 2015, 12:32 AM
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Wormboy's Avatar
Quote:
Originally Posted by eye4wings
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.
Mar 12, 2015, 04:27 AM
...design-build-fly-publish...
eye4wings's Avatar
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
Mar 16, 2015, 07:30 AM
Slip the surly bonds...
Sopwith Mike's Avatar
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.
Mar 16, 2015, 01:23 PM
Which means "what" to me?
invid66's Avatar
My otter so far, and the color scheme I hope to use.
Mar 16, 2015, 01:54 PM
www.raysmodels.ca
Raymac's Avatar
Nice color scheme!! Looks like major washout on those wingtips??
Mar 16, 2015, 04:19 PM
Which means "what" to me?
invid66's Avatar
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.
Last edited by invid66; Mar 16, 2015 at 04:23 PM. Reason: Not thinking ahead.
Apr 06, 2015, 04:48 PM
Ron
Ron
Registered User

seagull


Here's a picture of the " Seagull" a little flying boat 60" span designed by my friend Ivan Pettigrew....He and I were out doing " trim" flights this morning...and I must say...if you want a small flying boat...this is the one to build...it is absolutely fantastic....no vices...aerobatic, and safe....the wing is a little more involved than one with square tips, and it takes a little bit to get the thrust line of the motor perfect, but once you do ...you will be rewarded with a wonderful flying little ship...
Other than the two points I mentioned the build was easy, and straight forward...worth every minute I spent on it.....Thanks Ivan for the help and advice on the trim flights...I can hardly wait to get out flying with it again....it does the sweetest touch and goes you have ever seen :-) as Phil says........ happy happy :-)
Ron.
Apr 07, 2015, 01:49 PM
Registered User
cracksmeup's Avatar
Quote:
Originally Posted by Ron
Here's a picture of the " Seagull" a little flying boat 60" span designed by my friend Ivan Pettigrew....He and I were out doing " trim" flights this morning...and I must say...if you want a small flying boat...this is the one to build...it is absolutely fantastic....no vices...aerobatic, and safe....the wing is a little more involved than one with square tips, and it takes a little bit to get the thrust line of the motor perfect, but once you do ...you will be rewarded with a wonderful flying little ship...
Other than the two points I mentioned the build was easy, and straight forward...worth every minute I spent on it.....Thanks Ivan for the help and advice on the trim flights...I can hardly wait to get out flying with it again....it does the sweetest touch and goes you have ever seen :-) as Phil says........ happy happy :-)
Ron.
Very nice plane Ron and its sounds like your enjoying it ! joe
Apr 08, 2015, 07:59 PM
Ron
Ron
Registered User
I have found some issues with the design of the Seagull....they are..... once you start flying it, you don't want to leave the pond.... and on a 2200 mah battery you can only get about 20 or 30 touch and goes...and every 15 minutes or so you need to put in a fresh battery ......
Major problem :-)
Apr 08, 2015, 10:35 PM
Registered User
portablevcb's Avatar
Ron,

I have the same problem with the Chipmunk. And even in a bit of wind it flies great. I think I need to get some more batteries and chargers

charlie
Apr 10, 2015, 02:16 PM
Registered User
Gonzo007's Avatar
Quote:
Originally Posted by Ron
Here's a picture of the " Seagull" a little flying boat 60" span designed by my friend Ivan Pettigrew....He and I were out doing " trim" flights this morning...and I must say...if you want a small flying boat...this is the one to build...it is absolutely fantastic....no vices...aerobatic, and safe....the wing is a little more involved than one with square tips, and it takes a little bit to get the thrust line of the motor perfect, but once you do ...you will be rewarded with a wonderful flying little ship...
Other than the two points I mentioned the build was easy, and straight forward...worth every minute I spent on it.....Thanks Ivan for the help and advice on the trim flights...I can hardly wait to get out flying with it again....it does the sweetest touch and goes you have ever seen :-) as Phil says........ happy happy :-)
Ron.
Good looking plane Ron. I have had those plans for a couple of years now. I should build one.

Colin
Apr 10, 2015, 03:40 PM
Registered User
pentaxman's Avatar
portablevcb

Wait until the Chipmunk shows you really how good she is, then you will be in real trouble.

I have flown mine from football fields both rock hard and waterlogged, taking off and landing between the puddles.
Likewise snow is great fun due to the low weight, no skis necessary.
Mine flies quite happily in 25mph gusty winds with no additional ballast and you can expect zero ground speed take offs with that amount of wind.

Best of all she is almost viceless, goes where you point her just like the full size machine. She will also suprise in just how slowly she can fly even in still air.

Good luck and enjoy her.
Last edited by pentaxman; Apr 10, 2015 at 03:41 PM. Reason: My lousy typing strikes again
Apr 11, 2015, 02:58 PM
Ron
Ron
Registered User
Colin do yourself a favour and do it......here's a picture of old and new...( mine and Ivan's) mine has about 80 water landings...Ivan's has about 12,000

Mickey is a month old...Ivan's is 21 years old this year it's old enough to vote :-)
and he still says it's his favourite sea plane to fly.
Apr 12, 2015, 04:05 AM
Registered User
ivve's Avatar

Albatross


Hi all.

Photos of my IVAN Albatross, allmost ready for first maiden flight.

Ivan / Sweden


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