Retrospectively Positioning a Tow Release
By Chris Williams
Where should the aero-tow release be on your scale glider. Easy, I hear you mutter, right up the front. Ah, but in this case there’s a snag, you didn’t fit a release mechanism during the construction period. So, the front of the glider is full of those goodies so important to steady flight, ballast, batteries etc. – leaving precious little room for the release gear.
This is one of those subjects which, like locating the CG, is passed down from generation to generation in the form of aeronautical folk lore, father to son, so to speak
“Now listen son, I’ve been meaning to have a chat with you for some time now, and now that you’re shaving your legs, I reckon the time has come. I expect you’ve been wondering how gliders get to be born into the sky the way they are and all and I’m afraid that I have to tell you, that all that birds and bees stuff,…well, it don’t rightly ‘happen like that..:’
Sometimes I wonder if my childhood was entirely normal, but, to get back to the subject, what we need to look at are the basic factors concerning this situation.
These are, in no particular order, Moment Arms, Centre of Gravity and the Rigging Angle between the wing and tailplane (Decalage). I’m sure we can all remember from our days with lightweight soarers and bungees, that in this situation the towhook would normally be situated around I5 degrees or so forward of a line drawn vertically downwards from the Centre of Gravity. Moving the hook further forward would result in a flatter and more stable launch – further back would give a higher release at the expense of yaw stability. Although an aerotow is a different proposition, the principles are remarkably similar. In this case, perhaps we can draw a parallel with the thrust angle of the engine in a powered model. You may remember that on a high wing model with the engine below the wing itself,I you usually need some down thrust to counteract the pull of the engine being below the wing. If the engine is mounted on a pylon above the wing the opposite may apply, and if the engine is in line with the wing, it could often apply that you need no up or down thrust at all.
If you simply substitute the towline from the tug for the engine, you can see that the same thing applies. If your tow release were on a pylon above the wing, then there would be a force acting upon your sailplane to make it fly nose down. If you have a deep fuselage like a Rhonbussard for instance, and the release is in the bottom of the fuselage, you would get a tendency for the model to fly nose up. If you don’t believe the foregoing, then try looking at ridiculous extremes, which often admirably serve to illustrate an arcane point.
lmagine a 2 metre pylon on top of your wing with a release fitted to it. Yes, that’s right, the model would never get off the ground due to the long moment arm. The best place for the release, therefore, is right in the front, which is usually (but not always) where it is placed on the full-size.
But what if you can’t do this, for the previously stated reasons! You may have a choice of location somewhere between 1/4 and 1/2 way back between the nose and the LE of the wing. For a mid-wing configuration the moment arm is pretty small. As this is still well forward of the CG, I would have thought this a reasonably safe bet, and would forecast that maybe a small amount of down trim might be needed ~ to compensate during the aerotow. Once again we must look at extremes to illuminate ~ what is happening.
Imagine if the release point was behind the CG, say at the wing TE. Presumably the nose would rise up until the model was being dragged along tail-first and tug pilot was wondering about the change of CG in his trousers. What about if the release was on the CG itself! Well, I wouldn’t fancy it – personally, but it would be a longer, and far more interesting ride than the previous one that’s for sure.
From this we can deduce that for stable flight during tow, we need the release point as far forward as possible, if for no other reason than that the line tension will act as a damper towards any untoward pitch oscillation.
The other factor to consider is the decalage situation. If your model is set up with a fair degree of positive pitch stability, then the bigger and faster the tug, the more the nose will try to rise and more down elevator will be required to keep a good tow position behind the tug. You may think, if you’ve never aerotowed before, that all these factors may make for a confusing situation when you walk up to the flight line for the first time. Fear not, your model will either fly with no elevator trim input from you at all, or if it won’t, the amount required will be obvious and easily applied.
I wouldn’t want to be beaten around the head with the physics, but I would guess that a release position anywhere in the front third of the nose (from the wing LE to the tip of the nose) will give you a trouble free launch.