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Old Jun 03, 2012, 07:05 PM
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Yes, that's what I meant. The downthrust force is applied behind the CG which raises the nose as power is applied.

I believe, but am not sure, that the Neptune applies upthrust in front of the CG, which also raises the nose as power is applied.
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Old Jun 03, 2012, 09:30 PM
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If you search this thread "motor angle" you get lots of info- heres what thomas B -The guy who started this thread-says "The Seawind actualy needs a little bit of up thrust, due to the high thrust line...it does not have any down thrust. The stock thrust line has a little bit of up thrust, but the model performs better when a little more upthrust is added to the stock amount.


-
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Originally Posted by fabric8 View Post
So looking at the polaris, I clearly see that the motor is angled downward - which from googling the term, is downthrust.

Bob, is this right?

Fairlane, are you saying I should do the opposite and have the motor point upward for upthrust?

Confussed.
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Old Jun 03, 2012, 09:35 PM
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I tend to get my info from others who have more experience than I do and then I try to pass that along. Heres a little discussion from some veteran flyers on this topic-although this is specific to the Neptune and seamaster. Check out the link

A high-mounted engine like that tends to lever the nose down when you add throttle. Minnflyer says 1-2 degrees, I lean a little harder toward three degrees up. And since the prop is now facing a pretty significant incidence up, the right side of the prop takes a bigger bite and tries to turn the plane left. Add 3 degrees right thrust as well. What ypu DON'T want is for the thrust to force the nose down just when you goose the throttle in an aborted landing.

_____________________________

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http://www.smilesandwags.com/Floats.html
Quote:
Originally Posted by bob93447 View Post
Yes, that's what I meant. The downthrust force is applied behind the CG which raises the nose as power is applied.

I believe, but am not sure, that the Neptune applies this force in front of the CG, which also raises the nose as power is applied.
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Old Jun 04, 2012, 03:01 AM
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OK, so prior to seeing the latest reply, I setup the seawind with some downthrust using a couple of dubro #4 washers (used for 4/40 screws) in between the motor mount and firewall. I headed back to the field for a test flight before sunset. With the downthrust, the plane had a very hard time taxiing on the grass. The nose kept wanting to dive into the grass even with full up elevator (with a LOT of deflection (D/R turned off). I decided to just keep increasing throttle, and it started to pick up speed. Eventually, it took off and I was airborne. I don't remember if I let the elevator stick go back to neutral as it climbed. I was just just focused on gaining altitude. When I was high enough, I decreased throttle to half stick and flew around just to get familiar with the plane. I had to add up elevator trim to keep it flying level.

Unlike my bixler, this plane is VERY sensitive to aileron input. I was flying with 50% aileron D/R, but it's still too much. I'm going to move the links 2 or 3 holes into the servo control horns (they're at the end holes now). It can do rolls like crazy.

But other than that, the plane flew great. It's fast and very very responsive. I was flying in gusty wind, but was able to handle it fine. Did some rolls and loops. Also tried a setting on my TX where I mixed some rudder with aileron. I killed throttle and glided it in for a very gentle landing.

I'm not sure it'll be able to take off on water with the nose wanting to dive so much with the added downthrust. But I also forgot to move my CG a bit back. I'll probably try flying it again the way it is but set my CG at the spar, or 1/4" inch before the spar.

Then, I'll move the washers to the bottom to give it some upthrust instead of downthrust to compare how it behaves.

Here's a pic of my thrust angle now.
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Old Jun 04, 2012, 11:59 AM
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Let me first say that the Neptune/Seamaster thrust adjustments are VERY different from the SeaWind thrust adjustments. The Neptune does have a pylon mounted motor, but the motor is AHEAD of the Center of Gravity. The SeaWind motor is BEHIND the center of gravity and thrust adjustments are very different.

Just like fabric8's picture shows, downthrust angles the motor downward. On the SeaWind, adding Downthrust causes the nose to rise under power and causes you to put in more "Down" elevator trim for level powered flight. When you reduce power, the nose may pitch down, but when you jam the throttle wide open, the nose will stay level or pitch up slightly.

The best of all worlds occurs when you have the elevator trimmed at the neutral point and the motor thrust adjusted so that the plane flies level power on, or power off.

Your setup with downthrust should work fine. Fly it that way and get it trimmed for straight and level flight under power first. If you find that the nose drops too much when you reduce power, then take one washer out and try again.
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Old Jun 04, 2012, 11:29 PM
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Yes- let us know how this works especially when you give it throttle in the air. It would be interesting to know. Like i said earlier, my seawinds mount should be at the same thrust angle as stock and it flies as well as it did with the original mount.

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Originally Posted by kingsflyer View Post
Let me first say that the Neptune/Seamaster thrust adjustments are VERY different from the SeaWind thrust adjustments. The Neptune does have a pylon mounted motor, but the motor is AHEAD of the Center of Gravity. The SeaWind motor is BEHIND the center of gravity and thrust adjustments are very different.

Just like fabric8's picture shows, downthrust angles the motor downward. On the SeaWind, adding Downthrust causes the nose to rise under power and causes you to put in more "Down" elevator trim for level powered flight. When you reduce power, the nose may pitch down, but when you jam the throttle wide open, the nose will stay level or pitch up slightly.

The best of all worlds occurs when you have the elevator trimmed at the neutral point and the motor thrust adjusted so that the plane flies level power on, or power off.

Your setup with downthrust should work fine. Fly it that way and get it trimmed for straight and level flight under power first. If you find that the nose drops too much when you reduce power, then take one washer out and try again.
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Old Jun 05, 2012, 02:27 AM
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Ok, so today I setup the motor with upthrust. I decided to go somewhat aggressive and went with 2 washers on the bottom, so the motor was tilted up about 4 degrees. It was late in the day and the winds were gusting pretty bad. I probably shouldn't have flown, but I wanted to see how it would do, so I did.

In addition to the upthrust, I moved my CG to almost at the spar. I launched from grass and with the upthrust, the plane taxiid on the grass without nose diving this time. I didn't need to give anywhere close to the amount of throttle to get it going forward as I did when it had downthrust. Withint a few feet, it took off but was an ABSOLUTE handful in the air. The gusty wind didn't help. As soon as I cut power to about half, it became more stable, but I quickly realized the tail was dropping and sliding all over the place. I actually lost control a couple of times, but was able to regain it and glided in for a safe landing.

I moved the battery forward and set CG back to spec. Took off again and this time it flew much better, but it was still unstable with higher throttle. I couldn't tell if the upthrust was causing it to pull left or right, or if it was the gusty wind. Regardless, I didn't like the way it was flying. I landed it and was going to reduce the upthrust at the field, but the wind just kept getting stronger, so I decided not to push my luck.

I got home, removed one washer which reduced the upthrust, and decided to take it to the pond (for the very first time) to see how it floats, and tracks in the water. The first thing I noticed was that there's no rudder control in low speed. The little fin on the rudder just doesn't dig deep enough to turn. With power and some up elevator, the tail sinks and has better rudder control, but its still very dissapointing. I saw a thread on wattflyer where someone glued on a bigger piece of plastic to the tail fin which he claimed helped. I'm going to try something like that, but won't glue it because it'll prevent me from grass take offs.

In anycase, I wanted to see how this slight upthrust angle behaves on water so I throttled up gently with some up elevator (not full) and the plane pitch itself up, and skipped up off the water twice after it picked up speed. I immediately cut power because I didn't want to fly it so late with wind. But I can tell it'll take off very easily and smoothly from the water with this setup

I'm hoping to get a flight early tomorrow from the grass before I head to work to see how the mild upthrust behaves. So far, having the upthrust made taking off from the grass MUCH easier than the downthrust which caused the nose to dive down hard. I'll report back with how it actually flies in the air, and what effect upthrust has when I throttle up as it fly's by.
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Old Jun 05, 2012, 11:49 AM
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UPThrust

Interesting data. It looks like a second order effect, in this case the propwash deflecting off the stabilizer/elevator, is overwhelming the primary effect, upthrust. Because upthrust alone should be pulling the tail up and driving the nose into the water.

I'll be very interest in your future results.

Bob

PS The old "clean" data I have seen regarding downthrust has been someone on RCU who put substantial downthrust into an airframe called Laddie's "ARROW" to substantially improve performance. The Arrow is a Laddie M. redesign of the Northstar that eliminates the stabilizer and elevator to there is no "second order effect.
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Old Jun 06, 2012, 12:34 AM
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Quote:
Originally Posted by bob93447 View Post
Interesting data. It looks like a second order effect, in this case the propwash deflecting off the stabilizer/elevator, is overwhelming the primary effect, upthrust. Because upthrust alone should be pulling the tail up and driving the nose into the water.

I'll be very interest in your future results.

Bob

PS The old "clean" data I have seen regarding downthrust has been someone on RCU who put substantial downthrust into an airframe called Laddie's "ARROW" to substantially improve performance. The Arrow is a Laddie M. redesign of the Northstar that eliminates the stabilizer and elevator to there is no "second order effect.
Bob, I think your theory is spot on. Unfortunately, winds were blowing at 12mph this morning, so I couldn't test fly. Hoping tomorrow will be calm.
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Old Jun 06, 2012, 03:07 PM
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Upthrust it is!!!!

Upthrust it is!!! With a weather.com repored 0mph wind this morning, I rushed to the field to test the SWEP. I found a dry patch of grass (everywhere else there was morning dew and the hull wouldn't slide) to take off from and gradually increased power. The nose did pitch down a bit at first, but with up elevator and gradually increasing throttle, the plane leveled off, picked up speed, and was airborne by about 3/4th throttle. I eased off on the elevator (instead of letting it go) and noticed the plane started to pitch down, so I gave it more elevator and held it there until I gained safe altitude to play with the trims.

When I was high enough, I backed down to 1/2 throttle. The plane needed a few degrees of up elevator to fly level, and a few clicks of left aileron. But with that, it few beautifully. I got myself familiar with it's flight characteristics and got comfortable with it very quickly. At 1/2 throttle, it's still somewhat fast, but flies straight and true, and tracks nicely like a low wing. I did a few flyby's at about 12ft. It's such a nice looking plane, but looks nicer in the air flying by

I thought I noticed a bit of down pitch when I mashed full throttle from half once, but when I gradually went to full, it didn't seem to do this. I'm going to test this again if it's calm enough during my lunch break to confirm.

I'll probably also try and add some left thrust so I can center my aileron trim.

Landing it was really really nice. I cut power and it descended pefectly, with a very nice long flared touchdown. Can't wait to fly it again and try it on the lake this weekend. Will have my wife to get it on video then.
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Old Jun 06, 2012, 03:54 PM
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I know for me, i have to hold full aft stick up the plane gets off and climbing, then I can ease up. If I let of the stick too soon on takeoff, back onto the water she goes.
The PZ109 takeoffs....right rudder, right rudder, right rudder
This plane...hold it back, keep it back, waaaay back.
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Old Jun 06, 2012, 04:01 PM
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UPThrust

Interesting result. Glad that you have it flying well after all your setup experiences. Looking forward to your water takeoff results. If it flys well off the water, ie. takes off without porpoising a lot, than you really have something.

Bob
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Old Jun 07, 2012, 12:19 AM
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Hi guys,
Bob sent me a note about this thread and asked if I had any thoughts on the upthrust vs downthrust issue. I think there are a few things I can add.

There are other things that affect pitching moment with throttle on a model like this besides just the thrust line. These include the incidence angle of the horiz stab relative to the thrust line (plus the airfoil used on the stab--cambered or uncambered), as well as the incidence angle of the horiz stab relative to the wing. From a discussion Bob and I had a couple months ago, it appears the Seawind has a significantly positive angle of incidence on the stab--as much as +5 deg relative to the wing. That's not good, and is probably why the Seawind has problems with takeoff and with thrust-induced pitching moment. Ideally, on a model like this it would be better to have the wing at a slight positive incidence angle and the thrustline and horiz stab at zero to slightly negative incidence angles.

With that much incidence angle relative to the thrustline, every time you add throttle the prop slipstream increases the lift on the stab and pitches the nose down. And the more downthrust you add, the more you're increasing the angle of attack of the prop slipstream relative to the stab, which makes the problem worse. This could explain why upthrust actually helps in this case--it reduces the angle of attack of the prop slipstream relative to the stab.

Frankly, I think the best solution would be to remount the horiz tail with much less incidence (probably around zero deg, assuming it's an uncambered airfoil and the wing incidence is slightly positive (1 or 2 deg)). Then put the thrustline at zero. That should provide better overall handling qualities.

On my Polaris design, I was lucky enough to stumble on just the right combination of wing incidence (0 deg), horiz tail incidence (-2 deg), and thrustline angle (-2 deg) to virtually eliminate pitch changes with throttle. The Seawind configuration is somewhat similar to Polaris but also different, so the perfect setup for it would likely be a little different. I'd have to have a good three-view of the model to make a better judgement call on the best angles to start with.

Hope this helps!
Steve
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Old Jun 07, 2012, 12:58 AM
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Very interesting!
Thanks for the into!
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