Aggregate A9 (A4b)

Garry
Yes it was a bit chilly but EDF flights tend to be quite short, total flight time 7 minutes, total time on the field about 15 minutes and it is only 3 minutes drive from my house.

The more I think about it the more I worry that vanes in the EDF exhaust will not be adequate unless they are very large.
As it appears that simple elevon bank and yank control is quite adequate for normal flight it seems logical to use the now redundant rudder servos to actually 'vector' the EDF itself on a gimbal mount. This will provide powerful forces to keep the A4b vertical although roll control would have to rely on the aerodynamic forces from the elevons.

Next will have to be a vectored EDF control test rig!

Just before I start an EDF gimbal mount rig I needed to work out what sort of space there would be inside the tail to allow the EDF to pivot.
To allow the 70 mm enough space the A4b would end up quite a bit larger.

In fact just about 50% bigger!
With over twice the wing area there should be no problem lifting the much heavier 70 mm EDF and a 2000 mAh 4S which is about the minimum capacity battery that could be expected to handle the 50A+ vertical take off power. That will be over a horse power held by just 2mm Depron. :woohoo:
The object of the test rig will not be to test whether vectoring the EDF can actually 'balance' anything but simply to check that the gimbal mount itself is strong enough and that the servos can physically control it at full power.
Only if that proves successful will the decision be made whether to proceed with 'vectoring' or 'vanes'.

Having looked again at the problems of building a gimbal mount that is strong enough to handle the full 70 mm EDF thrust and the relatively small angles it would have to be controlled over I am rather strongly brought back to the 'vanes in exhaust' solution.

I suspect its inherent mechanical simplicity will end up being significantly lighter and unlike a vectored EDF will provide 'heading hold' in the roll axis as well.
The real issue will be that until the gyro can balance the A4b satisfactorily it is hard to imagine any crash at lift off would ever be 'minor'! :woohoo:

Flew the A4b this morning in clear blue calm conditions.
Still 'uncomfortable' to fly accurately so the video is heavily edited but no damage apart from the wet and mud! Gyro was on for the whole flight.

I still believe its handling can be improved but the frustrating bit is it glides very nicely - stable and docile!

It still looks very touchy on elevators with the power on.
I may be way off - but perhaps when the fan is 'sucking' it is pulling the air away from the tail-plane, and destabilizing the model, because, as you put, the glide is great.

Eric
You may well be right. The difference between power on and off is very noticeable.
Although the four inlets have equal area above and below the tailplane it is possible that when there is a positive angle of attack (up elevator) the airflow into the EDF could be disturbed. Most likely the disturbance would be to the upper half of the fan thus 'up' elevator might create a thrust nose up pitch as well!
Only a hypothesis - no way of knowing for sure.

If you fancy experimenting - try putting a flow-straightener vane in the upper ducts, and see if it makes any difference.

Frosty but calm this morning sothe A4b had another outing this time with the 1/3 oz nose weight back on just to confirm its 'twitchyness' was not due to a rearward CofG.
As before the nose weight had very little effect - other than to reduce the 'up' elevator authority!
This suggests the big difference in the way it handles between power on and off has something to do with the EDF rather than the CofG.
If the EDF was adjustable I would be tempted to add a bit of up thrust!.

Deflector plate in eflux tube maybe

I was thinking of just that sort of solution - the equivalent of 'down thrust' in an EDF.

Calm(ish) this morning. Its not really the wind speed that is a concern (the A4b flies plenty fast enough) but it is very sensitive to turbulence which makes diagnosing anything difficult.

A good flight, mostly with the gyro off. I think it might actually be easier to fly without it!

With its huge inertia fore and aft its control harmonisation is odd. Sluggish in pitch but sensitive (although not particularly quick) in roll. I also suspect there is some pitch/roll coupling thrown in as well.
With only pitch and roll stabilised by the gyro (there is no rudder) it seems to struggle in anything but straight and level and even then it does not feel particularly 'solid' in the air.
This could explain why it is difficult to be confident for a low pass - although I am getting better.

With the gyro off I simply cant detect the small flight anomalies so it just continues to fly inaccurately. As a result it 'feels' more docile even if it does not quite go where you think it should! :woohoo: .

Calm again this morning and no quite so bitter - flew without gloves!
Set all the gryo gains to 50% and put 30% expo on the elevator.
The first half of the 7 minute flight had the gyro on although the video is edited to a reasonable length.

Out again this morning. Cold, a bit of a breeze but smooth.
Tried 30% expo on the elevators.
Works well in normal flight but... when pulling a tight manoeuvre needing rather more stick it is rather easy to actually put on full control deflection. With the A4b the drag builds up very rapidly as the AoA increases so I found I tended to pull a tighter manoeuvre than I intended and thus slowed down more than I wanted.
No problem except when you try to pull out at speed rather close to the ground!

Very nearly made but the fin touched.

Which of course forced the nose down.

The ground was very soft (there was a good bit more soil and grass stuck to it when I picked it up!) which limited the damage in what was a relatively high speed impact.
To be fair I am not sure it would have pulled out any better without the expo but you got to blame something!

Glad it WAS soft for you Q. On Langar runway it would have been destroyed :sick: :sick:

Garry

Sometimes a hard surface is not that bad.
The soft mud this morning kept the damage local to the point of impact with virtually no slide. On tarmac it might have bounced with more surface damage.
I piled in the Depron Natter at Ashbourne a couple of years ago and it must have slide for 20 yds scraping away the Depron but nothing too severe structurally.
Anyway the A4b is all mended now.

A bit of filling, some white emulsion and it will be ready to go.

All back together, filled and painted.

Flew this morning, just under 24 hours after the crash! Two flights. For the second I took the expo off the elevator which I found preferable.
I did video it but when I came to download it the one and only video file on the SD card was empty!
May not stay calm long enough to have another go.