Lippisch P13a

Been there, done that, got the T shirt.

As an ex full size glider pilot the control check is always "Full and free movement and in the correct sense".

Mainly because I now have a 12 mega pixel caera I thought I would take a close up picture of the tiny damaged gear from the now defunct 3.7 g servo.
 
I have two other such dud servos so I  suppose it is possible they have stripped a different gear.
Never tried to do any repairs with these servo but with a gear salvaged from a non working one.

For some reason there is no sound.. 
 

Ah yes... memories of hearing the servo and not noticing that the Z bend had almost worked loose and completing its divorce just about 1.5 errors above the ground....

The reason there is no sound is this camera requires it to be switched on in the settings menu!
Its a KodakC182 bought second hand very cheap off ebay.
 
One of the last 'pocket' digital designs they made before they went bankrupt in 2012.

Nothing to do with the Lippisch but Kodak is an interesting business case study.
A market leader in film based industry for many decades with150,000 employees and the majority of the world market. 
They actually invented the digital camera, were one of the first in the market and sold them quite successfully, however the directors could only see 'digital' as competition for their core 'film' business so spend money on that instead. By the time they fully realised the error they no longer had the resources to compete in the digital market with the likes of Fuji & Sony.    

Strangely calm and sunny today (Monday) so I flew the Lippish twice about a minute each time and No damage although the second flight was an "out landing" in the next field on the other side of a big drainage ditch! 
This time the battery was in its most forward position with several clicks of up trim added to compensate. 
As I feared it does have tricky characteristics in that it has virtually no pitch stability despite the best efforts of the gyro. Once it starts to pitch up it just keeps on pitching up. even full down doesn't stop it. Only after it stalls, at about 45 degrees AOA, does the nose drop and pitch control return.
It is of course possible that the CofG needs to go still further forward but with several degrees of up elevator on already at some point it will just run out of elevator power.

Anyway to actually fly with some sort of control and not break anything is a significant step forward and about time too!   

I flew the Lippisch again yesterday as the weather was so good.
First flight very hard to control so it ended with a nose plant. Apart from damaging the nose cone it also crushed the battery box a bit which moved the it about 5 mm forward.
As the damage did look too bad, although it was in fact quite severe internally, I tried again, The more forward CofG made all the difference an the Lippish flew two complete circuits. On the last "pass" with a bit more speed it went into a severe aileron oscillation. Closed the throttle and it all calmed down so I made a perfect slow glide landing.
Thought I was going to have to 'dial down' the aileron gyro a bit more but when I came to remove the battery there was the Lemon stab rx all crook and just retained by the servo leads.  Amazing it even flew!
Anyway yet another inlet nose printed and some quite major rebuilding required around the fuselage nose skin..
 
I thought I had videoed it all but on play back it only recorded walking to the first flight launch point and then stopped working. 

Re-installed the battery box, a bit further forward and slowly rebuilding the fuselage skin.
 
Slow because each bit of replacement plank has to be exactly fitted and glued in but the next bit can't be done until the glue has dried.
One of the penalties of doing a 'minimum area' repair.  
I will have to crack on as it could be calm again over the Easter weekend. .

Now all back together with a bit of filling and paint.
 
I do hope it will fly at least controllably again. I am not sure I could face yet another rebuild.   

I flew the Lippisch today very briefly to check the effect of reducing the aileron gain as on the previous flights it did a serious aileron wobble when flying fast.
Seems to have tamed it but the battery moved a bit rearward on the launch so it flew a bit tail heavy. Decided discretion was better part of valour so stopped whilst ahead.
Positive battery restraint next!

On another web site there was a question concerning the shape of an EDF nozzle. I was going to use the my Lippisch P13a with its oblong slot nozzle as an example.
Checking the area sums before going to print I noticed I had made a serious mistake. The nozzle I had made was 99 x18 mm = 1880 sq mm. To my horror this is 1.4 times the FSA of its 50 mm EDF. The nozzle should never be more than the FSA and ideally about 85% of it. No wonder the Lippisch had marginal thrust. 
So what to do about it?
The easiest solution will be to add carefully shaped "plugs" on each side of the nozzle to reduce the slot to 70mm x19mm giving an area equal to the FSA.
Of course if I rebuilt the complete rear end I could create a 200mm wide scale exhaust nozzle but it would only be 7mm deep.
It would look something like this.
 
In the meantime I will design suitable plugs for the existing 19mm deep slot and see if the static thrust is improved.
No chance of actually flying it for a bit thanks to storm Debbi.   
         

The as built oversize (99mmx19mm) slot exhaust nozzle.
 
The same but with a 15mm wide LW-PA  "plugs" on either side to reduce the nozzle area to just below the FSA of the 50mm fan.
 
Each "plug" is carefully shaped to match the shape of the original printed "fishtail" duct.

A video of the test bed run will follow.

I suspect the centre flow to be more effective than the split flow as there is less obstruction to the airflow.

Well that didn't work!
The thrust with the side plugs in was significantly less than with them out. It was noticeable that the fan ran faster with the plugs in suggesting the fan did not like the extra back pressure.
Fortunately I had only tacked glued the plugs in so easy to remove. It is now back to the previous layout.
I will have to check my sums again.
Oh well! Nothing ventured nothing gained.