TOPIC: New Models for 2021

Nice!
I have a small depron formed version for 30mm edf that I have not attempted as yet.
I'll look forward to this build!

Wow ambitious.
I once saw the real thing do a few fly bys at Farnborough, a fascinating aircraft which can be seen on display at the Southampton Hall of Aviation.

All done chaps. Ready for Old Warden in a few weeks! Will need a hand with this one Roger!

Very nice. I'd really like to get across to OW just to see this one fly.

Daniel's rate of building certainly puts the rest of us to shame. And it's not just the speed - it's also the subjects he chooses!

The SARO A1 is an ambitious model with an interesting history in that (a) it is the last model Bert Judge designed before being joined by Mike Ingrams and Peter Cock:


This team, with Joe Mansour then went on to develop the wonderful Tailored models.

(b) the SARO A1 kit is unique in the Jetex range in that it was powered by two Jetex 50s and an auto rudder. Daniel will of course be using (but he hasn't told us, tut, tut!) two TSP L-2 motors. These light reliably with fuses, which will take away some of the anxiety on the flying field.

(c) again, uniquely, the motors on the A1are behind the CG:



This poses a problem in that the CG will move forward in flight (the opposite of what one would want). As 'one shot' motors loose a greater proportion of their weight during ignition than the metal-bodied Jetex 50 this will make for an interesting flight pattern.

I wonder if Daniel has included the original's complicated auto rudder (Howard told me that the rudder was attached to a “paddle” between the motors, actuated by differential thrust, but he isn’t sure how it worked. More info, please Daniel!

The only SARO A1 I have seen fly (so far) was an EDF version :





Above: Anthony Druce's beautiful EDF SARO at Old Warden some years ago. It flew well, albeit without any smoke!

so I think we are for an interesting time on the flying field!

Well Roger this is a fascinating model.

Twin engined models, even ones with the thrust relatively closely spaced, are going to be interesting!

The aft CG on this design will, I presume, make for a pitch up under the initial thrust. But with low airspeed that's asking for a stall. Then, as the CG moves fwd, as the fuel is burnt, the lift moment arm reduces. Then, as the charges finish, and we want to glide, we need a predetermined nose down attitude.

Of course, this kind of situation is just begging for an electronic control system. Let's start with an acceleration sensor on the yaw axis to measure the effect of any differential thrust. (This is relatively mundane stuff. RC helicopters have long used a yaw axis accelerometer to help control the tail rotor.) Then, perhaps, another accelerometor to measure pitch rate and just limit it. A couple of minature servos controlling surfaces on the wings do the real-time trimming to keep things more or less level.

I'm assuming that any differential thrust would induce roll as a secondary effect of that yaw. So we would conteract that with an opposite aileron response. For pitch there's a number of possibilities. The simplest approach might be to measure the pitch rate and then wind in the opposite effect on the two control surfaces on the wings. (elevon style). But the system could get caught out if the pitch response to the control surfaces was too slow.

Perhaps an absolute pitch sensor would be better? The system would need to detect when the a/c was under power from acceleration in the fore/aft axis and switch between two control laws: 1) It would just limit the pitch up when under power to a maximum pitch angle and 2) choose the best pitch angle for a glide when it detected that the power run was finished.

MEMS gyroscopes, (micromechanical systems) are little chips that contain what are effectively multi-axis gyroscopes and accelerometers. Then all that's needed is a spot of software to sort out the angles and rates comming in from the MEMS and send appropriate demand values to the two servos! But this is by no means trivial.

It would be nice to come up with a generic system which could be tuned for different aircraft designs, that's still more difficult. But the real problem is the philosophical question, is it still free flight?

Roger Quips:

YES!!!
Andy Sephton and I on a long car journey discussed this, concluding that all these small electronic stability enhancing gizmos (to use a technical term) made for more realistic models (smaller tails, less dihedral) and safer flight patterns.

Bert Judge used a mechanical self-adjusting rudder on the SARO A. What's the difference if iwas electrical?

The essence of free flight is that you 'just let it go'.