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! 

 

WELCOME TO JETEX.ORG

Jetex.org was set up in 2004 and, with its active forum, quickly became a comprehensive online resource for free flight jet modellers all over the world.

The original Jetex.org site covered the historical and technical aspects of small rocket motors used in model aircraft, hydroplanes and cars, of which the British post-WWII Jetex motors and modern Rapiers are the best known.

The original Jetex.org site is well and happy, but in retirement.  It can still be accessed at: archivesite.jetex.org.

Building on the old website's success, the present Jetex.org site includes lots of new information, a model gallery, links to other relevant sites and a growing online store.

The active forum discusses everything from how to build and fly small model jets, to reports of meetings and collecting vintage motors. 

We welcome contributions from fellow modellers, rocket scientists, historians and micro rocket enthusiasts.

We hope you enjoy looking around the site.

Do join the Jetex Forum or contact us if you have any questions or would like to contribute.

And please visit our About Us page – why we're here, how we got here, who's behind it, and, most importantly, how you can help Jetex.org become even better!