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A tiny wing - the Bachem Ba349 Natter

#21
You have so much side area and so little rudder area to control it. Get a good gust of wind, slow down and you'll have no way to recover though I would think you'd have gotten something in that dive down.

Have you considered gimbaling the motor and using that as steering control? I know, more weight and complexity. You could also go with full flying controls but I know on my light stuff, if there isn't any prop blast going over the control surface at low airspeed, I don't have much control.
 

quorneng

Active member
#22
I too thought it was the CofG but at 20% it should be ok and in a low power glide it behaves reasonably, if anything rather nose heavy, running out of elevator authority and the nose dropping long before the sort of attitude it reached in the video.

The pitch up seemed to be related to power/thrust and to get worse as the flying speed reduced.
Obviously as a pusher it can't be the direct prop wash but could it be due to the prop inflow in some way?

As a by product of the 'rudderon' servo in the nose the CofG is now at 17% chord.
RBCofG.JPG
 

earthsciteach

Moderator
Moderator
#23
It could be cg if the wing position (center of lift) is not correct. Well, then that's a compounded problem. Keep in mind that the full scale aircraft was made for high speed.
 
#24
Perhaps the drag of the large top vert. stab. and horiz. stab. is creating enough drag to create a pitch up moment? I was thinking this because the horiz. stab. is almost the same size as the wing. Also, it probably is just the picture, but it looks as if the motor is turned slightly up with reference to the wing.

Another possibility is that the body is creating some lift through the redirection of air.

Also, the loss in elevator control was probably because of the wing and the T-tail
250px-Deep_Stall.png
(From Wikipedia)
 
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#25
That picture that g828 posted seems to explain the characteristic beautifully. It looks about right for the strong pitch up. Inwash on a prop will have negligible effect on your control surfaces - it was an outright stall. The thrust line could well be the first thing you might want to try fixing. If you can find yourself a decent hill or a high spot to biff it off and work on trimming it to glide hands off then start working on the thrust line. If you know you have a great power off glide then the default 'bugger it's all going wrong' is a return to predictable control, unless of course you've already ended up in the 'deep stall' condition.
 

quorneng

Active member
#27
Thanks for all the comments and advice.
All the surface are a symmetrical section and are set at zero to the fuselage datum.

I recall seeing a diagram of the inflow to a prop when static. It has a strong inward radial pattern extending forward only about a prop diameter.
It occurred to me that with the pipe axis at the fuselage centre line the tailplane is set towards the top of the prop's arc.
The prop inward radial flow would thus cause the tailplane to exert a down force that would be at a maximum when static, reducing as the speed built up or as the power was reduced.

If this is the case it would certainly agree with the observed flight characteristics - a 'nose heavy' glide requiring significant up elevator to maintain a constant glide yet a strong pitch up under power at slow speed despite full down elevator.

If the prop was on the axis of the tailplane the prop inward radial flow would have no effect on the tailplane, however on the Natter this would require a major rework of the tail assembly.

It would be relatively easy to raise the motor to the top of the fuselage and thus get it close to the tailplane.
Highprop.JPG
The blanking effect of the fuselage would also raise the axis of the net effective thrust line a bit further as well.

A bizarre solution for a rather bizarre plane!
 

quorneng

Active member
#28
I had never heard of the prop inflow having much effect on a plane although a significant radius inlet is essential in a ducted fan for maximum static thrust.

The result exceeded all my expectations. Not the easiest thing to fly, I very nearly crashed it in the video! As a result I kept fairly high until I gained a bit of confidence later in the video.
[video=vimeo;37768227]https://vimeo.com/37768227[/video]
Nevertheless it certainly flies and even glides quite nicely.

Now for true scale it just needs a vertical catapult launch tower!
 
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quorneng

Active member
#35
So did I!
It all started as it was going over head banking to the right. I moved the stick the wrong way!!!
It ended almost inverted!

With such tiny wings at speed the roll rate is pretty high so by the time I reacted and got it back the right way up it was pointing at the ground. In desperation I cut the power to reduce the impact but at the last minute it responded and lifted the nose so I slammed on full power and climbed away.

My hands were physically shaking which is why I flew high for a minute or two to calm down!

I am too old for that sort of excitement! :eek:
 
#36
unique...

I had never heard of the prop inflow having much effect on a plane although a significant radius inlet is essential in a ducted fan for maximum static thrust.

The result exceeded all my expectations. Not the easiest thing to fly, I very nearly crashed it in the video! As a result I kept fairly high until I gained a bit of confidence later in the video.
Nevertheless it certainly flies and even glides quite nicely.

Now for true scale it just needs a vertical catapult launch tower!
very nice work man, for a second i thought you lost it...i bet its not easy to fly, even it is dailed in perfectly....good job, not to mention a very cool model, never seen that one before, not even scratch build, very cool :applause:.