Trimming thoughts, Jetex
I’ve had a gentle reminder from Roger lately that there is such a thing as the Jetex blog that I’d offered to write for! So, belatedly, here we go again.
Free flight stability has long been an interest of mine. And the way the behaviour of reaction powered models differs to that of propellor driven craft is quite interesting.
Now stability in roll is pretty much the same for Jetex (which is to say reaction powered aerodynamically lifted) type models as for propellor models. In the event of dropping a wing there is a correcting force generated because the dropped wing presents a greater lifting area than the wing on the risen side. Moreover, this inherent correction system works pretty much the same regardless of airspeed.
The consequences are the same for non-swept wings with dihedral, swept wings and deltas. There’s a moment in my video about 30 seconds in where we see a nice little roll disturbance and self correction on my Bill Dean designed Tendera powered delta-winged Cutlass.Terry's video
But things get a bit more complicated, and more interesting, when we start looking at pitch stability.
We can consider the model rotating in pitch around it’s centre of gravity. In order to achieve a steady glide we arrange things so that the up force of the wings is countered by a down force from the tail. This act being establishing the pitch trim. (I know one or two free flight models are trimmed with an up force from the tail but these types are not positively stable.)
If we change the airspeed the moment arm of the up force and the down force stay the same but the forces on those arms change and they do not do so proportionally. They are in balance, or trim, at just one airspeed. If we want to fly at a different airspeed we need to change the pitch trim to maintain level flight. Incidentally, the same procedure must be performed in full-size aircraft. In modern aircraft it is done automatically but in aircraft without automatic control systems the procedure of manually re-adjusting pitch trim for a change of airspeed is part of the process of piloting the aircraft.
Now rubber powered models tend to have, as it turns out, pretty much two flying speeds - under power and on the glide. A rubber motor, as it unwinds, has a pretty flat torque curve from fully wound to fully unwound. This means that the propellor will turn at pretty much a constant speed until the motor is unwound.
And a propellor turning at a steady speed tends to set the craft it is attached to to a fixed airspeed. If the plane flies faster the effective propellor pitch is reduced so the thrust is reduced. If the plane slows the propellor pitch is increased and so is the thrust. Thus, a steadily turning propellor is a fairly effective airspeed control system.
Thus, our rubber flier can first trim his glide using pitch trim and then trim his slightly faster power phase with a bit of off-centre-line thrust.
This is not the case with reaction power. The thrust from the motor is constant regardless of airspeed. So, the only possibility of the model establishing a steady airspeed is when the drag force has increased to the point where it balances out the thrust force. And this may never be achieved during the power run. The model may be accelerating throughout the power run.
With the model accelerating there is no possibility of that the up force from the wing is being balanced by the downforce from the tail for very long.
Thus Jetex fliers have had to find different techniques to manage pitch trim under power.
One possibility is to get the motor mounted forward of the CG position. This means, in fact, that the CG position changes throughout the power phase. Thus, the moment arm from the CG to the up force generated by the wing is initially increased which induces a nose down. This moment arm reduces as the fuel is used up and the CG moves aft.
With the motor forward of the CG position angling the motor down will also generate down thrust under power but in this case this is a steady down thrust amount based on the pitch down angle rather than a changing value as the moment arms change.
An alternative with none-scale type models is to have the motor mounted above the CG. This gives a nice additional pitch down under power but once more this tends to stay steady throughout the thrust phase.
I have to throw these 'tends to‘ caveats in because although our motors show pretty consistent performance - an initial spike of thrust, which is there to get everything burning thoroughly, followed by a pretty steady power phase in fact the thrust can change with air temperature and (with actual metal can Jetex motors) thrust can increase a little as the canister heats up.
Some of the profile models from Bill Dean also feature side mounted motors. These give a nice scale-like profile. The side mount induces a turn under power that can be offset by a control surface induced turn in the opposite direction for the glide. The power on turn tends to hold the nose down and the whole system worked out quite nicely with Jetex. But I don’t see it, and haven’t tried it myself yet, with Tendera.
What I have seen, and used with some success is the exhaust mounted trim tab.
With these I tend to mount the motor pretty much under the CG and then by trial and error tweak the thrust deflector in the exhaust. Made from soft aluminium such a tab is strong enough to get the job done and soft enough to be bent as one adjusts the amount of power on down trim.
The tab is not quite optimum as the airspeed induced pitch-up force gets more powerful with increased airspeed and can still exceed the fixed downforce from the tab but if one is lucky any looping will occur after a bit of height has been achieved.
The tab can also be used to induce a thrust induced turn, which will keep the nose down, and this can be offset by rudder/aileron to give a nice opposite direction turn for the glide.
So there, I think, we have it. I rather fancy that this is a revamp of an old Jet Reaction column from twenty years ago but if they can remake The Full Monty I can play the same game!
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2021 has seen the most widespread marketing of miniature, sustained thrust, solid fuel reaction motors since the days of Jet-X. And about time too, that was in 1986. For this we have to thank Piotr Tendera who has taken on significant expense and effort to get the CE mark for his Tendera motors. And while it might be argued that this has resulted, due to the mandated use of the wider green fuze, motors with a lower overall Specific Index. But, for myself the widespread availability of the CE marked motors is a price worth paying. Already we are seeing signs of interest from people who are outside the usual demographic of the vintage modeller. I expect to see some novel and interesting Tendera powered creations appear over the next months.
Additionally, I think, we are likely to see the most interesting developments using the larger of the motors in Piotr’s range. The L3 and L4 both offer some new possibilities and on these fronts I’ve been making (rather slow) progress.
Radio Control is of course one possibility. Indeed, one of my correspondents from here in Germany (Frank Schwellethin) has been flying R/C with the more usual short duration motors from the Estes range and he will now, I think, have a go with the Tendera type.
Frank sent me several videos of his R/C rocket gliders in flight and inspired I was moved to complete a Klima Me163 and fit it with an Tendera L4. (This is intended for a D3 model rocket motor.) I haven’t had the chance to fly it yet as I’ve been awaiting the delivery of a miniature receiver, a Lemon Stability + from the USA. This, as its name suggests, is not just a receiver but it also includes additional hardware to build in a modicum of pitch and roll stability.
Electronic stability is the second possibility and it has long been an interest of mine. Since I started hardware that weighed over 50 Kg can now effectively be replaced with modern electronics weighing just a few grams. Luckily for me, the principals, and the maths, remain the same.
Now R/C receivers such as the Lemon and the Spektrum SAFE range include the ability to re-establish hands-off level flight at almost no additional cost. And configurations such as quadcopter, that have no inherent stability, and even helicopters without the stabilising fly-bars are feasible. But what about fixed wing free flight and specifically scale, reaction powered flight?
Seeing a recent copy of the Aeromodeller I feared that once again I'd prevaricated too long. The byline, ‘Low wing FF stabiliser’ caught my eye. The article, by Steve Glass, sounded something like what I had in mind. In fact Steve is using an add on stabiliser, the A3S3, designed to work, downstream as it were, from a conventional RC system. Steve has added some circuitry to simulate the pulse stream from an R/C receiver and, it must be said, this is pretty much purely a wings levelling system. What I have in mind is something more specific for reaction power. Additionally, the A3S3 has a setup facility that must be installed on a PC or mobile phone. Not, I fancy, something that will find favour with everyone.
For me what is required is something that can be set up on the flying field. The user can set the glide angle manually and adjust the rate of ascent. The controller consists of a solid-state gyro accelerometer board, a micro controller, battery and two miniature servos.
One would go to the field with the model with controller installed, switch on and first sort out the power off glide with a used motor installed. When this is satisfactory, select (guess) an angle for power-on ascent. This could be done by holding the model at the desired pitch angle. This is saved in the controller with a button press.
The model is then loaded for powered flight and hand launched with the motor building up thrust.
The controller detects an X axis acceleration from the thrust which it can integrate to approximate airspeed. The appropriate pitch up is wound in on the elevator/elevons. (The board would need to be capable of managing elevator and ailerons or elevons). Perhaps a potentiometer is used to preset the power on flying speed. The controller would start to wind in pitch once this airspeed is achieved.
We should now see the model climbing to best altitude. The controller detects the end of the thrust phase and changes the pitch trim to that for best glide. We might also have a gentle turn trimmed in!
Sounds simple and maybe all this is justified as a means to achieve that perfect scale, no dihedral, rear mounted motor Lockheed U2.
But what class of model would this be? It’s not R/C, it’s not C/L and its not quite F/F either. I leave that philosophical question as an exercise for the reader.
Looking forward to doing some flying in 2022.
Hello from Terry and Tendera developments
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