Bachem Ba349 Natter

[quorneng]

The principle of large (relatively) and light seemed to work remarkably well on my Depron Skyray so I wondered if it could be taken further.
The rocket powered Natter was in effect a manned missile built by the Germans right at the end of WW11.
   
Launched vertically it had tiny wings, just 12' span, and with no ailerons just elevons. Tested but the only manned rocket flight killed the pilot.

I tiled printed the above 3 view to give a plan with a 26" span or just over 1/5 scale! Not having access to a suitable rocket motor it would have a pusher prop at the back.
   
The fuselage was going to be enormous, 6" wide, 8" deep and 40" long. With just about 1sqft of wing area the total weight would have to kept to 12oz to maintain a reasonable 'park flyer' wing loading.
Each wing is very simple (just like the full size) a spar, 5 ribs of a symmetrical section with top and bottom skins.
   
Completed each wing half weighs just 1/2oz (14g).
So far the easy bit. That huge fuselage would be harder.

[quorneng]

The fuselage was so big I decided to build in three sections with each section built vertically.
   
Very wobbly to start with but adding the planks equally each side prevents a banana build. This method also allow the complete skin to be fixed on without removing it from the building board.
   
Completed each section is amazingly rigid and light.
The wings were added at this stage.
   
The first trial assembly showed just how big the fuselage was compared to the wings.
   
The bare airframe so far weighs just 3oz so still on course for the 12oz all up target.

[quorneng]

The tail section with the tailplane, fin and servos.
   
The 1500kV Blue Wonder motor is fitted right at the back.
   
The cables run with the tail and centre sections joined.
   
To say there is plenty of room is a bit of an understatement!
You can only just about see the radio and ESC in the nose!
   
Complete and painted.
   
No cockpit glazing as it is based on one of the unmanned prototypes.
Initial flight testing did not go too well as it behaved severe tail heavy despite the CofG at 25%.

[quorneng]

The first solution to the tail heaviness was to move the 1000mAh battery right up to the nose.
   
This moved the CofG to about 22% and improved things a bit but what puzzled me was the nose up pitch only occurred with power on when flying slowly. At speed and when gliding it was perfectly ok.
It did not expect a thrust line issue as the wing, tailplane and motor were all set parallel to the fuselage datum.
In addition elevon power was very marginal resisting the motor torque at the launch, requiring full right aileron just to keep straight until the speed had built up.
After a few test flights the inevitable happened but it does show the nature of the 'pich up' problem.



Quite a major rebuild.
   
   
As the Natter has both up and lower rudders a novel approach was adopted to improve the roll response - rudderons.
Using a closed loop cable system, with the lower rudder cables crossed over, driven by a single servo in the nose connected in parallel with the aileron channel.



The power on nose up pitch was almost certainly a result of the inward radial airflow to the prop in static or slow speeds resulting in downward airflow over the tailplane.
To help reduce this effect the motor was raised to bring it nearer to the axis of the tailplane.
Finally a 1500mAh battery brought the Cof G to just 17%.
   
Whilst it still is not the easiest thing to handle it now flies really quite well and capable of simple aerobatics. It even has a reasonable glide!



It certainly looks impressive and the 1500mAh battery gives it quite a respectable endurance (about 15mins) which given it tricky characteristics is plenty long enough for me.