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Old Jun 17, 2012, 12:50 PM
Registered User
Joined Dec 2010
36 Posts
Help!
Extreme thrust angle on pusher

Just finished my first scratchbuild and proved to myself I'm still a noob.After reading many conflicting opinions decided to mount my motor with prop pointing up.This plane was originally intended to be an edf pusher so prop clearance wasn't an issue before.When assembling I realized I had to mount my motor at an extreme angle(16 degrees or so), in order to get prop clearance.When doing static pitch tests it pushed the nose down pretty hard at half throttle so I added the T-tail section with negative incidence hoping to give me a little downforce on the tail.This is my first scratchbuild so I was just wondering if anyone has any similar builds or experiences.Most commercial pushers have the prop pointing up a bit it's just that I had no choice but to mount the motor at this angle because of the tail boom.I could lessen the angle but that would mean raising the motor pod,and everything is glued already and not adjustable I followed the formulas on adamone rc and used CG calculators so Im pretty sure she"ll fly.I just don't want to nose in on maiden.Will she fly?Any thoughts,suggestions,or opinions are greatly appreciated
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Old Jun 17, 2012, 02:42 PM
An itch?. Scratch build.
eflightray's Avatar
South Wales U.K.
Joined Mar 2003
13,805 Posts
Fit a smaller prop ?

You shouldn't try to compensate for the thrust angle with aerodynamic changes, what happens when the motor stops ?

Please video the first attempt to fly it.
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Old Jun 17, 2012, 03:30 PM
Rookie Hopeful
air4prs's Avatar
Massillon, Ohio
Joined Sep 2007
116 Posts
Thrust Angle

I have the Mutiplex Easy Star that has a similar rear mounted pusher stye motor mount. Measured the Easy Star and The down thrust angle is approximately 20 degrees and I spin the APCe 6 x 5.5 prop with a 3s 2300 mah lipo.

This combo works very well . With your angle at 16 degrees is fairly close to the EasyStar 20 degrees.

You can trim with your elevator and then modify your angle to suite.


Paul



Just finished my first scratchbuild and proved to myself I'm still a noob.After reading many conflicting opinions decided to mount my motor with prop pointing up.This plane was originally intended to be an edf pusher so prop clearance wasn't an issue before.When assembling I realized I had to mount my motor at an extreme angle(16 degrees or so), in order to get prop clearance.When doing static pitch tests it pushed the nose down pretty hard at half throttle so I added the T-tail section with negative incidence hoping to give me a little downforce on the tail.This is my first scratchbuild so I was just wondering if anyone has any similar builds or experiences.Most commercial pushers have the prop pointing up a bit it's just that I had no choice but to mount the motor at this angle because of the tail boom.I could lessen the angle but that would mean raising the motor pod,and everything is glued already and not adjustable I followed the formulas on adamone rc and used CG calculators so Im pretty sure she"ll fly.I just don't want to nose in on maiden.Will she fly?Any thoughts,suggestions,or opinions are greatly appreciated[/QUOTE]
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Last edited by air4prs; Jun 17, 2012 at 03:32 PM. Reason: Mistake
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Old Jun 17, 2012, 07:14 PM
Shelter Kitty "Orange Death"
bartricky's Avatar
United States, FL, Monroe
Joined Jan 2008
3,165 Posts
Most pushers I have seen has some down
thrust.

Down thrust if behind the CG translates into raising the nose.

http://www.hobbyking.com/hobbyking/s...dProduct=16542
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Old Jun 17, 2012, 08:58 PM
Wake up, feel pulse, be happy!
C₄H₁₀'s Avatar
United States, AK, Fairbanks
Joined Aug 2009
12,576 Posts
Quote:
When assembling I realized I had to mount my motor at an extreme angle(16 degrees or so), in order to get prop clearance.
Bad course of action, as Ray mentioned. It'd be better to run a smaller prop (maybe a 3-blade to keep thrust) or make the pylon taller.

Ideally, you should set the plane up so that it'll glide straight (not pitching up or down) with the power off, then adjust the motor's thrust angle so that it flies level with the power turned on. Generally it's just a matter of slipping a few washers or shims between the motor mount and firewall to point the motor where you want it.
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Old Jun 18, 2012, 06:49 PM
Registered User
Joined Dec 2010
36 Posts
Time to experiment

The motor and prop on this are the same ones that come with the AXN.I think I can go a little smaller,I'll have to crunch some numbers and see.I haven't even given this beauty a glide test yet so I can't speak of what she does in the air.When perched on the CG stand the plane will pitch forward with throttle.Less throttle=less pitch forward.I will probably do a glide test this weekend and hopefully it will glide straight with no throttle and I'll be able to tame the nose down effect with a little reflex or maybe some elevator/throttle mixes.From what I've read, high mounted pushers in general can be a pain to setup correctly.I'll start with the easiest adjustments and if that doesn't work I guess I'll be performing surgery I really don't want to pry off the motor pod to raise it considering it's already glued.I always thought the higher I raised my motor pod, the bigger the moment arm between CG and thrust line.Therefore pointing the motor like mine would lessen that distance and not create such a pitch change.Apparently that didn't turn out the way I hoped.Wouldn't making my thrust angle more parallel to the wing make the nose push down more?I've also read that having horizontal stab in the wash of the prop has more of a nose up effect in high mount pushers than just pointing the prop end up.Hence my idea for the small T-tail section.Sorry it's just that I've already read many conflicting threads.Still learning, and once again thoughts or opinions are welcome
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Last edited by nomadic80; Jun 18, 2012 at 07:26 PM.
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Old Jun 19, 2012, 10:21 PM
Registered User
Joined Dec 2010
36 Posts
Still confused????

After studying some more I think that giving this plane a fully functional T-tail could possibly solve my pitch issues.I was told putting the elevator in the prop wash would make the elevator immediately effective therefore giving me pitch control the moment the plane leaves my hand.I was starting to think my elevator wouldn't have much authority until i got decent airspeed which would explain why some real life pushers have T-tails in the prop wash.Then I came across this picture.I'm suprised to see a more extreme thrust angle than on my scratchbuild.Yet I'm guessing it flies fine.(Sigh)...it's like the more I research the more confused I get.Anybody have any theories...(crickets chirping)
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Old Jun 19, 2012, 10:30 PM
Wake up, feel pulse, be happy!
C₄H₁₀'s Avatar
United States, AK, Fairbanks
Joined Aug 2009
12,576 Posts
Think about this: Your T-tail bit is relatively small compared to the rest of the horizontal stabilizer and elevator (the moving part).

If you knock your little T piece off, you can still achieve exactly the same thing using just the elevator control that you already have without potentially compromising the plane's aerodynamics. All you've done is "hardwired" the plane for a bit of up-elevator trim.
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Old Jun 19, 2012, 11:10 PM
Registered User
Taiwan, 北市
Joined Dec 2010
1,215 Posts
I'd suggest making the curve smooth first on the up side of the wing on yours, better to put the two servo leads to the down side of the wing. Then try to make the leading edge a little sharper. These will increase the performance of this wing.

If you still worry about the thrust angle you applied, just make it adjustable by insertting a piece or two of 1mm washer between the motor and the mount, then screw it up for adjusting the angle when needed. I'm sure you know where to insert the washer for the angle needed.
Try to apply half throttle first when launching your plane to see how it flies, if it's ok, then throttle it up a bit by a bit to see how it climbs up. It'll nose down right after a quick WOT if the angle is not good enough. I'm sure you know how to correct back a nose down by giving up elevator. Always maiden it on calm day.

Chen
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Old Jun 20, 2012, 12:55 AM
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Joined Dec 2010
36 Posts
I mainly added the t tail section to basically hardwire a very slight downforce onto the tail, so that way I could possibly lessen the amount of elevator trim needed.Now i think i will remove it and do glide tests.If it needs a lot of up elevator trim I'll put it back on and maybe make it bigger.If that doesn't do the trick Im going full T-tail.I still think she's going to need a lot of reflex(up elevator)in order to fly level.In fact the more I think about it, a full T tail seems more effective and logical.As far as the wing goes it's a standard kfm2 airfoil with supposed good lift and stall characteristics.I am hoping wing lift plays a major factor in counteracting my nose down force, so improving lift with better airfoil might be solution,but that would be a lot of work.Washers usually work but I can't point my prop end down and I don't think I should point it up anymore.I guess I have some choices to makeThanks for the info so far folks
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Old Jun 20, 2012, 01:20 AM
Rocket Programmer
jasmine2501's Avatar
United States, AZ, Mesa
Joined Jul 2007
25,460 Posts
I would put some wheels on that sucker and just give it a go. Hand-launching is risky when you don't know the characteristics of the plane, but a rolling take off should tell you instantly if it's right or not, if you have the skills to feel that.
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Old Jun 21, 2012, 09:19 AM
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Joined Dec 2010
36 Posts
Not a bad idea!

Jasmin, the tought of putting landing gear on this bird did occur to me. The bottom does have some slippery tape and it does slide nicely on carpet with a little over half throttle.I was hoping be able to do ROG but I haven't tested yet in grass.This would make nosing in on hand launch a non issue. At this point in the build, landing gear would be a hassle,but I have contemplated making a takeoff dolly.Thanks for the suggestion
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