PDA

View Full Version : Pusher Motor Downthrust


donjiskra
07-07-2008, 04:00 PM
I tested my "Sky Arrow" rebuild this friday and discoverd that it kept nosing over. Checking the internet I found that with a pusher motor mounted above the wing the motor, down thrust is required. The down thrust pushes the wing down, which raises the nose upwards.

I had the motor set with upthrust, therby pushing the nose down.
You can see the amouint of upthrust I had in the photo and the flap deployment.

Now I need to get the right downthrust angle set.
BTW, my flap modifcation did help but not enought to lift off the water.

Has anyone else encountered this situation, with downthrust and a pusher motor?

Don

Murocflyer
07-07-2008, 04:10 PM
Don,

I'm sure you have seen this thread (http://www.rcgroups.com/forums/showthread.php?t=888546)? On all of my pushers, I never had to mess with right or left thrust.

I hope your plane was easily retrievable.

Frank

donjiskra
07-07-2008, 05:58 PM
Geez Frank, you sure "get around". Running Wings Across America 2008" and being the "Answer Man" in this forum, what an amazing guy!!!!!

Yes the informational link you posted is exactly where I found the information I mentioned. The downthrust angle makes a lot of sense, doesn't it.

How about this, "for every action there is an opposite reaction", and in photographic lighting,"the angle of incidence is equal to the angle of reflection"........ponder these for a moment and the solution becomes apparent why my plane tipped over.

My plane just tipped over and floated upside down, easily retrieved by the "Suburban Aero Modelers" electric powered rowboat.

Thanks Frank,
Don

Murocflyer
07-07-2008, 06:32 PM
How about this, "for every action there is an opposite reaction", and in photographic lighting,"the angle of incidence is equal to the angle of reflection"........ponder these for a moment and the solution becomes apparent why my plane tipped over.

So you dumb thumbed it then? ;)

Glad to help. Looks like a very nice plane.

Frank

Murocflyer
07-07-2008, 07:27 PM
That thread by Mike was pretty timely. I am doing a beta build for Steelhead Products and I keep forgetting what angle to set the motor.

I am a big fan a pushers for AP planes. Here is how I set up mine. I gave it a couple of degrees down angle. It looks like more because of the sloping wing, but it's not much.

Frank

donjiskra
07-07-2008, 09:36 PM
No "dumb thumb", I was video taping, Ron was thumbing. We discussed what the problem could have been. First attempt was without the flaps and the float tips were plowing under.
Second attempt with flaps deployed, this kept the floats up but when more power was applied, she nosed over.

I had an opposite situation with my first float plane, a twin AventuraII. It was a twin tractor, (motors at the leading edge), electric. It kept "submaring" when power was applied. I placed a 1/8" shim under the front of each motor. Voila, no more problems.

I see in your photos the downthrust, looks correct, let us know how that amount worked out. I suspect I'll need a lot more, consiidering the floats and weight below the CG.

Don

Murocflyer
07-07-2008, 09:56 PM
Don,

Has that plane flown yet? How does she handle in the air?

Frank

donjiskra
07-07-2008, 10:43 PM
Frank,
Yes it has, I've been working on it for three years now.
Many modifcations have taken it into a new realm, beyond what Hacker had in mind for it. It was sold by Hobby Lobby for awhile then discontinued a few years ago. Anyway, here's your answer if it has flown.
http://www.youtube.com/watch?v=7nKBnljHHH8
This video was made last year, this year's mods have been the flaps and a 4s battery, which increased the Watts/lb from 55 to 100,a huge difference in power.

Here's a link on the "Sky Arrow" rebuild:
http://www.wattflyer.com/forums/showthread.php?t=8060&highlight=sky+arrow

Don

Murocflyer
07-07-2008, 11:17 PM
Cool! Very nice, I like it.

Now back the the thrust angle which still seems to be somewhat of a mystery to me. Which way is yours going right now? Is the motor nose up or down?

Thanks,

Signed,

Always looking to learn something in VA

donjiskra
07-07-2008, 11:39 PM
Frank,
The motor nose was pointing up, which caused the problem.
It should be pointing down, like you have shown in your photo.

Pointing the motor down, (downthrust), will make the planes nose rotate upwards.

I never stop learning, no matter where I am.

Don

Murocflyer
07-07-2008, 11:50 PM
Thanks Don. That sure sounds like it should be opposite doesn't it?

Frank

donjiskra
07-08-2008, 12:58 AM
Frank, That's what I thought when I did the last mod, thinking the upward motor angle would push the plane up. While in reality, the upward angle pulled the tail of the plane up causing it tip over.

Consider a balanced board on a fulcrum.
Push down on one end, the other end will rise up.
Pull up on one end, the other end goes down............
That's what happened to my plane, since the motor was pointing up,

Hope this helps?

Don

Jim Casey
07-08-2008, 04:33 AM
OK, I'm gonna be a persnickety purist here. Upthrust and downthrust mean the same thing whether you're talking about pushers or tractor mounted engines. If you have a pod-mounted engine you will in all likelihood need upthrust. That means the thrust vector is inclined upward from the chord line of the wing . The thrust vector (by golly) SHOULD point toward the front of the airplane, unless you're taking off backwards. The front of the crankshaft is higher than the rear of the crankshaft, regardless of where the prop is mounted. Positive thrust angle: Lifts nose.

Also, this blows downward on the elevator with increased power, keeping the plane from nosing down.

If the motor is mounded very far aft (Northstar) you don't want a positive thrust angle as it will lift the rear of the plane. Note how the engines of the space shuttle are directed so the thrust vector is through the CG of the shuttle. It seems to fly pretty straight, regardless.

Murocflyer
07-08-2008, 10:08 AM
Jim,

Can you define up thrust and down thrust by the relationship of the nose of the motor?

Thanks,

Frank

donjiskra
07-08-2008, 01:11 PM
Jim,
I respectfully submit this photo from the HELMOdels and an explantion from another poster, in link Frank mentioned.

My motor is mounted the same way, but the rotation direction reversed, (no need to use a pusher prop).

I think the photo and explanation defines the angle of the motor.
Hope this helps?
Don

Jim Casey
07-09-2008, 12:40 AM
Wow!, that's some really radical downthrust.

With the motor up on a pod, It gets leverage to push the nose down, and it's generally fruitful to point the thrustline UP to counter that effect.
http://www.aranha.de/tia/gfx/C6-SBA_01.jpg
You can see upthrust supplied liberally with this Lake Renegade Pusher.

It's all a question of balance and how far the thrustline is from the center of drag, center of mass, and also fore-and-aft placement of the prop.

donjiskra
07-09-2008, 12:30 PM
Jim,
You are right, "it's all a matter of balance".

In your photo of the "Lake Renegade Pusher", a floatplane, it does have upthrust. The center of gravity balance is much lower because it has a "boat" hull.

A plane with floats below the fuselage, such as my "Sky Arrow", has a much higher balance point.

The upthrust of the SA literaly made it trip over itself because the floats are way out in front of the motor. The motor location, at the rear of the wings, was pushing the plane up.
The forward motion and "rotation" made the plane nose in and tip over. The CG point, ahead of the motor, caused the plane to "rotate" at that point.

I will see what happens with downthrust, (motor pointing down).

Thanks for your replies,
Don

Murocflyer
07-12-2008, 11:42 PM
Don,

Any luck with some new flights?

Frank

donjiskra
07-13-2008, 02:11 AM
Frank,
Just made a water taxi test, seems like the downthrust will work, but the pond is too small for flight. Our next float fly is scheduled for this Friday.
Till then, "Happy Flying",
Don

Jim Casey
07-13-2008, 04:44 PM
Good luck with it, Don!
The right thrust angle is the one that makes it fly the way it should.

donjiskra
07-13-2008, 08:03 PM
Thanks Jim,
I just discovered that when I took off the floats to strengthen the cross bracing and install the water rudder at the center of the rear cross member, I had the whole float assembly was aft, which changed the planes flotation. I think that contributed to the nosing over also.
I've since moved the floats foreward and with the downthrust angle of the motor lift off will take place once again. Let you and Frank now after our Friday's float fly. The water rudder performed flawlessly during the taxi tests.
Don

BTW, I'm involved in converting a glow PBY to electric, What a neat experience.

Angler-Hi
07-13-2008, 08:17 PM
"BTW, I'm involved in converting a glow PBY to electric, What a neat experience"

I wouldn't mind seeing how she turns out friend. Good luck with that. I'm actually saving up for the PBY from GreatPlanes:
http://www3.towerhobbies.com/cgi-bin/wti0001p?&I=LXRLP8

donjiskra
07-13-2008, 08:50 PM
Angler-Hi,
Here's a thread on my PBY conversion that might help you. The original owner said he "scratch built it", so I don't know how it compares to the GP version.
Don
http://www.rcgroups.com/forums/showthread.php?t=863937#post10138292

Angler-Hi
07-13-2008, 09:49 PM
Wow, that's a big bird Don! I'm looking forward to the finished product! Can't wait to get that flight report too. Good luck friend.

Mike

donjiskra
07-13-2008, 10:07 PM
Thanks Mike,
It's the biggest one I've worked on and my second conversion.
Lot's of fun figuring this one out.
I'm taking it slow so it will be awhile before it gets in the water.
I'm looking forward to that day, she's a beauty!!!
Don

Angler-Hi
07-13-2008, 10:54 PM
Well good things come to those who wait:D

donjiskra
07-19-2008, 02:29 PM
Attempted flight yesterday with motor downthrust (rear of motor elevated 1/4"), 4s 2100 pack (9.5 ounces)., flaps deployed. elevator full up. Plane balance CG is set and plane floats level in water.

1st attempt: Taxis nicely until more is applied then front of floats submerge. Motor downthrust test revealed no difference.
2nd attempt: Shifted pack rearward 2", float tips still dig in.

3rd atempt: Replaced the 4s pack with 3s (4.5 ounces), previously used with successful flights. Plane stayed level, no problem with the floats digging in but motor had much less power, could not ROW. I'm going to replace the SK400XT motor with an ATLAS 2909/26 and stay with the 3S.

Jim Casey
07-22-2008, 09:53 PM
Don, Have you tried adding strakes to the nose of the floats for more lift?(also known as spray deflectors)

donjiskra
07-22-2008, 11:01 PM
Jim,
Haven't considered doing that, never gave it a thought as the plane had not ever had this problem before. Don't know if the "plowing in" was caused by the weight of the 4s battery or the increase of power.

I'm in the process of replacing the SK400XT motor with an Atlas 2909/16 with the 3s pack. Seems that at my last attempt when I replaced the 4s with a 3s the plane did not plow in the front of the floats, and definetly had less power. So much less that it never lifted off. even with the flaps deployed. It would move along forever, suffering from what I determined as "lack of power".

Thank's for your suggestion.
Don

donjiskra
08-12-2008, 01:45 PM
I went to EAA's "AirVenture" and saw this "Challenger" pusher with floats.
The floats are positioned just like my "Sky Arrow" except the wing incidence and motor thrust angle is at a much greater angle. I enlarged my elevator to match it.
Also I moved the push rod on the control horn to the first position, closest to the elevator, (for more throw). This should help keep the tail down and nose up. Next flight test this Thursday.
Don

Jim Casey
08-13-2008, 02:45 AM
I have been thrashed for using full-size airplanes as examples, but I remain convinced that the basic physics are scaleable. One thing that does not scale well is airfoils. Remember that a scale or small airfoil will stall more easily than a full-size foil. It's probably not wise to use quite as much incidence as shown in the full-size example shown. However you may wish to inspect the upthrust shown in your photo....

donjiskra
08-13-2008, 09:37 PM
Thanks Jim,
I agree with your comments entirely.
The incidence on the "Challenger" is unbelievable, I was astounded.
Reckon the aerodynamicist had a reason and I reckon it works for that bird.
There was no one present that could explain it to me, too bad!!!
The upthrust of the engine made sense however, and the elevator area made me think of full scale and model scale realities.
I do know that Hacker's elevator design is insufficient and needed to be enlarged. Hopefully my modifiation of flaps and a larger elevator will work this Friday. I'll post the answer.
Thank's for your comments,
Don

donjiskra
08-16-2008, 02:05 PM
Problem resolved !!!

I realized that the elevator was simply not sufficient enough for the propwash to push the tail down. Just as a tail-dragger on grass needs to eliminate nosing over.

I was at EAA's "Airventure" and took a photo of a Sky Arrow's stab & elevator. Reduced the photo down to my SA's size and discovered what I needed to do. The photo shows the difference. Yesterday successful ROW's took place with the modified elevator.

Don