Aero tow line characteristics

[Grasshopper]

Started aero tows of small gliders with my Mentor.

Other folk seem to use 1.5mm strimmer line and don't bother with tug releases, weak links etc. which seems foolhardy to me.

 

I have fitted a tug release but know that in practice I will not want to bother dropping the line between tows but only if there is a lock-up between tug and tugged which is potentially destructive to both 'planes.

 

Problem is what happens if during landing the tow line snags on a rock or similar which will result in a substantial accident! I want to insert a weak link to allow the line to part at the tug fixing - but how strong does this have to be to prevent breaks during towing and yet part in the above case?

 

30 - 40 Kg fishing cast has been suggested.

 

 

[Klippy]

Hi Grasshopper, when I was flying full size gliders we found that the actual pull when off the ground was very low, less than 50kg! I would go for a weak link of not more than 1kg. I feel that any more will result in serious damage to your Mentor, how does it fly by the way, I have one still in its box that I must get around to!

[Grasshopper]

 

 Tony,

 

yes I had thought of this sort of breaking strain as the Mentor will not take much more.

Problem - the gliders are small and have no release so the pilots worry about having the line left attached should the weak link break. The tow is taped to the underside and peeled off by zooming the glider above the tug. (this is in France!) I need to discuss with the others I think.

 As for the Mentor - great 'plane, stable and ideal for towing. I have a 3S 5000 LiPO (Far East import) and that gives me 12 or so tows plus some zooming about on the return trips. We fly off the beach and it handles the pebbles etc. no problem.

 

In your gliding days did you ever 'fly' a Slingsby T.38 Grasshopper (see my little photo). We had one in the playing fields and each Friday put it together and had a couple of hours of bungy hops - great fun, and probably too dangerous for todays PC world?

[Klippy]

That takes me back 49 years, we had something very similar at my school, I 'flew' it at the annual sports day. Out here in South Africa we don't have too much trouble with PC, except in Politics of course, Our polititians seem to take themselves Very seriously!. We just have to remember that there's no safety net!

[Former Member]

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[Grasshopper]

Yes Eric, all true but these gliders are of the pencil thin carbon fuselage type with receiver, 2 servos, battery jammed into the fus and 2 more servos out on the wings. There is just no room in there for a release let alone another servo or mechanism. Also as  Brit living in France (where of course I fly) I have to put up with what the locals treat as OK if you know what I mean. 

I have just finished and tested today a release for my Mentor so I can at least throw free an errant glider which refuses to unstick from me so all that remains now is to negotiate, in French,  the breaking strain of my weak link so I have another safety 'fuse' between tugged and tug as they all seem to use 1.5mm blue strimmer line here which must have a breaking strain of many tens of KG.

Flying my Easy Glider is - as it says - easy compared with this.

[Former Member]

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[Klippy]

You can actually do a calculation, first approximate the glide angle, say 1:20, then divide the AUW of the glider by 20, this is very approximately the pull on the line. If you treble that number it's going to be more than enough, even if the glider is far out of position. Two things spring to mind, 1/ in full size, gliders with nose hooks were always much easier to fly behind the Tug. 2/ In very turbulent conditions, going to 'low tow' position (it's much lower than you think) gives you a very easy tow, but you must warn the Tuggy, he needs a lot of down trim! Whoops! Just opened the box on my Mentor to discover that you can't low tow whithout ripping the tail off! Scratch 2/ then, it's a very funny place to put a tow hook!

[Grasshopper]

Phil, Tony,

 

thanks for this feedback I think we are beginning to get a clearer picture here.

 

As regards the Mentor tow hook position, this may seem funny - but it results in a very neutral tow load. That is, when the glider drops the tow, the trim of the Mentor does not alter.

 

There is an alternative theory to this of course and that is that the load of the glider as a towed object is much less than the load of towing 20 metres or so of 1.5mm Nylon strimmer line. I could experiment I suppose by trying the effect on trim of dropping the tow line with no glider attached?

[andy harrold]

I do not know the Mentors tow position but I guess it is on top of the main wing above the C of P.

As models are weak at the rear this is where towlines tend to be positioned.

low towing is a NO NO. the glider must stay high and keep above the tailplane. make sure the line is over the top before you commence take off.very exciting if you dont!

At my club we have done a fair few tows this summer/ autumn with an electric Rainbow, with a 2826 motor on 4s and with a Boomerang trainer with an Irvine 53 for when it is windy!

as for the line, we use kite string, 30m i think.and no weak link.

each model has a release servo and top tip. keep your finger ready.  when it goes wrong, it goes very wrong, very quickly.!

 

However , with a bit of practice and teamwork you will be smiling from ear to ear as you succeed in a high tow.

GOOD LUCK.

andy

 

 

[Klippy]

Thanks Andy, I have a Mentor waiting in the shed, I think I will be tempted to put a length of CF down the fuselage to give added strength to the tail hook. You are correct in your guess about the Mentor tow hook, I will put the servo in its designed position but run a snake down to the tail.

[andy harrold]

Tony, if your towline eminates from your tail any out of line pull forces , up down, left right from the glider  will be tranferred to the tug. a case of the tail wagging the dog! you cannot fly 100% behind the tug like you can in a real glider.

[Klippy]

Hi Andy, perhaps I'm re-inventing the wheel here, I'm an ex full size glider pilot and tuggy, and from experience the glider can get quite far out of position before causing the tug any real discomfort.............except high! That pulls the tugs tail up and is VERY difficult to correct, usually necessitating a change of underware for the tuggy! It just seems so wrong somehow to tow from a point just aft of the CG on the wing. We found that keeping the glider wings level with the tugs'  was the most important thing, as long as the glider was trimmed neutral.

[Grasshopper]

As I started this thread I thought I can throw my (same) penny worths in again.

 

I suspect from the responses that I may be among the few who have actually towed with the Mentor and although the Mentor tow position is right behind the wing - I can only say again that this does give a very neutral feel to the tug pilot - me!

 

We are talking here about 2+ Kg of tug with 350-400 Watts and small gliders by the way, probably weighing 300 grams maximum. In typical French fashion (I live in Brittany) the gliders are flown with some panache and often appear not to be connected to the tug in any way but we have yet to have anything serious go wrong.

 

Watch this space ?

 

 

(By the way, I have added 1mm carbon struts around the tailplane to guard against the tow line getting tangled in the tail control  surfaces and I think this is essential.)

 

[gliderpilot01]

Grasshopper - if you run a wire from the glider rudder servo output arm forward, so it moves backward and forward inside the fuselage behind a small hole, you will be able to use the rudder as a release system. Just put the loop from the towline through the hole and catch it with the wire.

 

You will find that you do not need the rudder for the aerotow, and by applying a swift push of full left (or right) rudder you will release off tow.

 

I use this system on my MiddlePhase and it works just fine.

 

Like Tony, I have done a great deal of fullsize aerotowing ( at the tail end of the tow rope ) but with models the glider will be out of position pretty much all of the time. By towing close to the c of g on the towplane, the glider can be well out of position without causing an upset.

As Andy pointed out, unlike fullsize, the glider needs to be kept ABOVE the towplane at all times.

 

Hope this helps

 

Chris

Edited By gliderpilot01 on 18/11/2009 22:10:01

Edited By gliderpilot01 on 18/11/2009 22:10:57

Edited By gliderpilot01 on 18/11/2009 22:17:54

[Klippy]

Hi Grasshopper, having read your thoughts, and after all, having had the experience is the thing, I'm going to go with the Mentor system. Your great idea of using CF struts at the tail is obviously the way to go. The other thing that I realised is that different people have different ideas about distance. Therefore, in all probability, the only time the two aircraft are going to be in for/aft line is when they are going away, or coming towards you. I'm looking forward to getting my Mentor in the air and trying these things out.

[Grasshopper]

Hi Chris,

 

nice idea - especially if like me you had already  glued in place the towing servo blank piece (#5)!

 

I ran a piece of control cable inner with some braided stainless fishing wire from the towing point down into the big open area under the canopy and dropped a servo there. I used some 2x10 mm carbon as a towing post with a hole for the tow loop to pass through. The braided inner cable was glued into a small length of tubing which acted as the tow 'bar'. Works a treat.

 

And Tony yes, in my limited experience the two 'models are rarely in line and often far from it! I'll be interested to hear how you get on and how the model experience compares with the real thing.

[David Davis]

Hi Guys

 

I'm building a Senior Telemaster, 8ft or 2.4metre wingspan trainer. Plan to build it with the earlier inset-aileron, so-called "Barn Door" wing. While I'm at it I'll also fit a tow release and an extra hard-point on the underside to allow for the fitting of floats.

 

Will keep you posted,

 

Happy New Year to Everyone.