Derek's scratch built "Specter"

[Derek]

Hey guys! I think the flying weather is finally here! WHooHOO!!!

I've given this a fair bit of thought and I'm confused (imagine that, lol). If you do a google search for "Ibcrazy Specter", you should see a very cool little twin boom pusher plane. I've loved this plane since I first saw it. Naturally, me being the "scratch build" type...I built one based on pictures that I saw on the internet. It turned out real well. I did maiden it last year. When the throttle is applied, the plane noses up violently. Shut the throttle off and it does glide very very well. So..I'm sure that I had the cg correct but I'm also sure that my thrust angle is wrong. If the plane noses up, do I need to have up or down angle on the motor? Keep in mind, this is a twin boom pusher. The motor and prop are basically in the middle of the plane.

[Vipertech13]

Well now I have to think! I know the "Bixler" has the motor mounted on an angle to correct that problem and the motor is mounted behind and above the main wing. the top of the motor is forward to make a down thrust angle. The pictures I see of the specter it looks like the motor is still above the wing but just slightly so I would go with the top of the motor forward and if your motor is below the wing I'd say bottom of motor forward. That is my understanding any way. It has to do with where the CG verses the center of thrust meet and if the thrust is behind the cg it makes it act tail heavy under power and pitch up. The motor tries to rotate the plane around the CG. It's all sci-ency stuff that makes my head hurt.

[pvolcko]

Hmm. The horizontal stabilizer complicates things in my mind a bit. The air flow from the prop will be passing along that directly. Normally I'd expect angling to prop up to result in the tail to go down and for the plane to nose up. But with the stabilizer in the flow, I'd expect that to create a low pressure zone on the top of the stabilizer, resulting in the tail creating lift and causing a nose down tendency.

So I guess, counterintuitively, I'm saying angle the prop up a bit. Hopefully the natural nose up tendency will be cancelled by the stabilizer lift effect.

Other option would be to go neutral and setup a throttle -> elevator mix to try to counter the nose up/down tendency.

[Derek]

Gerald, i have to agree with you here...all this sci-ency stuff hurts my brain too, lol.

Paul...i wont mind setting up a throttle/elevator mix once i get closer to "smoother." My motor is directly inline, and centered with the fuselage. therefore, the motor is below the wing. I did consider the Bixler, as Gerald mentioned. It has a down-thrust angle but that motor is actually above the wing. I took a piece of 1/8" aluminum, 5/8" wide and 2 5/8" long, and I drilled two holes in it that line up with the motor mount screws. I put that piece of aluminum under the bottom two motor mount screws and that gave me a bit of down thrust. I think I'll just try the plane like it is now and I'll toss it off the large hill at my flying field. I'll toss the plane to do a glide test, just to be sure that I do have the cg correct. Then, I'll go back to the top of the hill...toss the plane again, as if to do a glide test, and then slowly add throttle and see what happens. If the down thrust angle helps, then I'll know that I'm on the right track. If the down thrust forces more "violent nose up" tendencies, then I'll know that I need to put that piece of aluminum on the top of the motor mount and that would result in some "up thrust" angle.

Trial and error, I guess, lol. Because the motor is "inline" with the fuselage (and below the wing) I can see where "up thrust" would be needed, therefore pushing the nose down. But, I can also see where "down thrust" would be needed, therefore pushing the whole fuselage down. Perhaps I'm just over thinking the whole thing...which would be no surprise at all, lol.

[Tony]

My brain is hurting just reading all this.... Wow....

[Derek]

ha ha ha, poor Tony!

[Tony]

ha ha ha, poor Tony!

I thought planes were easy to setup and fly... .?

[Derek]

ha ha ha, poor Tony!

I thought planes were easy to setup and fly... .?

Most of them are lol. Sometimes I over think things and then I'm confused lol

[Stambo]

ha ha ha, poor Tony!

I thought planes were easy to setup and fly... .?

Most of them are lol. Sometimes I over think things and then I'm confused lol

Derek overthink things?
I don't believe it.

[Vipertech13]

In comparison to helicopters or multi rotors, planes are easy, but there are issues to consider such as Derek has with this plane. Many planes if you look at them closely you can see the motors are cocked to the right and pointed downward to reduce a condition called torque roll. I did not fully understand how it worked, being a car guy I thought it was just the power of the engine/motor trying to rotate the plane behind it! NO! it is actually caused by the prop wash. The air being twisted by the prop pushes down on the left wing and up on the right, causing a left pull, which is why on take off you need right rudder to keep it straight. When you get up to speed and lift off the affect is less and it evens out. There is always the CG battle, right size motor/prop combination, weight issues, landing gear position, too far back and nose over on the landing (sucks). Anyway that's just some of the plane pilots issues.

[Graham Lawrie]

Great description Gerald:)

Like all flying things the further you go from the ground the better it gets and the less interference from ground effect:) Then prop wash, thermals, wind direction and speed, aerodynamics of wings acting on air current, coanda effect,vortex ring........

[Tony]

Too complex for my old brain to take in....