Aerotow takeoff distance

[conflict alert]

Aerotow takeoff distance

can't be measured....relies on experience

[Doodlebug]

Hello David Roberts

Scary story, well told!

Reminds me of the time my first gliding-instructor decided that there must be wave-lift in 'them thar hills' (the first ridge of mountains in the Namib desert of the country today known as Namibia). That devine inspiration over with, we set about preparing the pilgrimage from Swakopmund, a little town on the coastal edge of the desert, to the ridge of mountains about a hundredandtwenty klicks into the desert: Two towcars, one a manual Toyota pickup of some description, the other a Ford Ranchero Automatic (the regular towcar.Yes, we learnt to glide off of car-launches in Africa!) and one tuck-wagon, a car dragging a trailer with food, beer, tents etc. The gliders were our beloved ASK13 and the SF34 (a little known Scheibe model) Tow-cables were around 800 metres. And off we went, gliders up there in the longest-lasting tows I've ever seen, two tows in a row with the tuck-wagon bringing up the rear, all hurtling along on a gravel-surface PUBLIC ROAD through the Namib desert! What with sudden serpentine bends in the roads forcing releases from time-to-time, and similar setbacks, we had a blast! Madness? Hey, we were kids back then, this was a quiet little piece of nowhere in Africa in the eighties, and the instructor was certifiable Wonderful guy! Rest in peace, Dieter..

In a nutshell, I started flying gliders at age fifteen. Later, when building hours towards my first commercial ticket, I tugged on a Super Cub because the club was keen on tug-pilots who knew what life is like on the other end of the cable. Only did tug for about a year, and all of that was off of a long, paved strip that was very close to sea level. (not in Namibia any more, sigh...)Temperatures were fairly low too, and much of the time there was wind. Needless to say a lot of the tows went straight to the nearest ridge, after the obligatory safety-circuit. What I'm getting at is that my tugging experience is limited and what towing I did was close to the optimum, performance-wise, due to the temperature, elevation, wind, etc.
Also, I have not been able to consistently keep at gliding and tugging seeing as I have been getting my day-job sorted out. After quite a few years I finally have the duty-roster that enables me to get back into 'real' flying, i.e. gliding. The search for aerotow takeoff distances arose because a group of glider-pilots is considering aerotow off of a hitherto untested strip, i.e. previously winch only. I have run into a lot of "Oh, about so far" or "You can't really get numbers" or "They've always tugged this kit off of this strip so XXX feet should be enough where you're at" and in spite of actually having tugged myself I couldn't give you exact figures either, but I figured there MUST be some nut out there who's gone and scientifically figured out all the variables. And it appears there is, witness the links so kindly provided earlier!

Thanks again for all the input provided!

Best regards to all, safe flying! (don't go launching off public roads! )

Doodlebug

[Dave Unwin]

I've done a bit of tugging. If they've previously winched off the field in question, it must surely be long enough to aerotow from (unless its a hilltop site, of course).

[Doodlebug]

Hi Dave,

I am now also convinced of that, thanks to some of the detail provided here.

Mind you, you wouldn't believe some of the postage-stamp sized fields I've seen the guys hurtle up out of using winches, in Germany. Crazy stuff!
Wouldn't try aerotow out of some of them unless utilizing JATO for the tug!

Regards,

Bug

[Dave Unwin]

Hi Bug, hilltop launches aside, a good rule of thumb is that if you can put out a thousand yards of cable you should get around a thousand feet AGL at the top of the launch (note this is very much a generalisation).
I have never seen a field that had a winch on that wasn't long enough to aerotow from, but have seen some aerotow sites that were far too short for a winch! Indeed, the club where I served as Tug Master for ten years was aerotow only for precisely that reason. Were the German clubs hiltop sites?

[Sedbergh]

Someone's obviously not tried the short winch run across the Long Mynd!

[Dave Unwin]

Sedbergh - note the qualifying note in both my previous posts re hilltop sites which, of course, the Long Mynd is.

[Doodlebug]

Hi Dave,

In fact, the German Club I was thinking of is quite the opposite, if memory serves! The field lies some 30 minutes by car to the north-west of Oberursel, which in turn lies 23 Km north of Frankfurt, in the Taunus range. The strip is very short, slopes considerably, and is surrounded by slightly higher ground. It is possible that the rising ground around it rules out aerotow, I suppose. I'd certainly have thought twice about even considering taking off from there in a powered light aircraft (but then I'm always a bit too worried about the damn thing quitting on me) The mob there were(are?) vintage-enthusiasts, not a fibreglass wing in sight. Probably not too fussed with overland and therefore quite happy with the limited opportunity for getting away before having to give up the fight and joining downwind. Not in any way ideal for gliding (Inbound traffic to Frankfurt rumbles overhead!) but then try getting permission for a new gliding strip in Germany. Or permission for anything at all, which is why this detribalised Kraut no longer lives there, but that's another story.

Allow me to tap your experience: what is the best performing taildragger tug, in your opinion? (Ruling out round engines)

Thanks,

Bug

[IFMU]

Towing simulataion

Bug,
I have thought about your problem, i.e. can I safely aerotow out of a particular strip with a particular tug. Here is what I came up with.
When I was getting a tow pilot checkout, we did some simulated tows with no glider on the back. We just used a lower RPM, and took off & climbed with less power. If you roll the super cub up to 2200 RPM with no glider, it may perform very much like it does running full out with a bird on the string. If you do an aerotow with the glider, observe your ground roll, height above the obstacle at the end of the field, and climb rate out of ground effect. Then pick some RPM that feels about right and see if you can duplicate the performance that you had towing, with no glider. Once you have a good RPM, fly over to your proposed field, short yourself out of 200' (or whatever is appropriate for your rope length) and give it a go. Of course, if you don't have enough margin, you pour the coal to it and climb out with full power. A bit safer than just trying it for real.
-- IFMU

PS on the "The Private Pilot Flying Album" thread I posted some soaring pictures. If you haven't found them already, look here:
http://www.pprune.org/forums/showpos...&postcount=423

and here:
http://www.pprune.org/forums/showpos...&postcount=410
and here:
http://www.pprune.org/forums/showpos...&postcount=412

if you are interested...

[Dave Unwin]

Hi Bug, I've towed with a number of different aircraft, including Rallyes, Robins, Super Cubs, Super Dimonas, Super Falkes and Austers. However, most of my 14,000-ish tows have been with Pawnees (mostly 235hp C models). If you're looking at a short strip (and I've done some field retrieves from a few 'interesting' fields) there is absolutely no substitute for horsepower. A 260hp D model Pawnee would be ideal, although a 235 C would prob be OK. As I'm sure you are well aware,there really are so many variables that determine the viability of a tow. For example, at Talgarth Airfield in the Black Mountains wind direction is fundamental, due to the close proximity of the mountains. If the ground is firm, the grass short and the wind a cold, fresh North-westerly I'll tow just about anything. If its hot, with little wind and long-ish wet grass I'm a lot more cautious about what I'll allow to be attached to the back of any aeroplane I'm flying. Throw in a strong south-easterly and I won't even consider it. BTW I'd be very careful about taking off with reduced power, as advised by IFMU. I've always understood that you should take off at full throttle, as this allows additional fuel to flow to the engine, which is used for cooling. According to Lycoming "harm to the engine can be caused by using less than take off power".

[IFMU]

Dave Unwin,
At full throttle there is indeed additional fuel that helps in cooling. High power = high heating and the need for better cooling. On aerotow our airspeeds are low, power is high, and we need it. If we take off with low power, not just a little low but a lot low like 2200 rpm, we have a lot less heating and we can get away with less cooling than we get with the throttle wide open.

What is bad if we pull the power back just a bit, because I think there is a valve in the carb that adds some extra fuel when at the wide open stop. I forget what this valve is called. Old automotive carbs had this too, on those I think it was called a power valve.

If you are out flying around on a sunday afternoon, can you climb at less than full power, i.e. a cruise climb? This is more akin to what I am proposing.

-- IFMU

[Dave Unwin]

Hi IFMU, the cruise-climb analogy is flawed. When climbing at cruise speed there is plenty of air flowing through the cowling to cool the air-cooled engine. I'm not an engineer, so I always defer to the POH and engine manual. If the manufacturer of the engine advises against taking off with less than full power, then I'll take their advice. After all, they built the thing!
Cheers, Dave

[BackPacker]

If you're worried about take-off lengths from an improvised or new field, I have seen two rules of the thumb in powered flying:
- You should have 50% of your flying speed at 1/3 of the runway
- You should have 70% of your flying speed at 1/2 of the runway

I have never had the need to apply them, but I wonder if they would work well in towing?

Obviously if you defer your decision to continue until you're halfway down the runway, at close to 70% of flying speed, and then decide to abort, you do have the challenge of bringing the whole combination to a safe stop.

[ProfChrisReed]

Backpacker wrote:

Obviously if you defer your decision to continue until you're halfway down the runway, at close to 70% of flying speed, and then decide to abort, you do have the challenge of bringing the whole combination to a safe stop.

Been there, on the back end not up front. There's nothing the tug pilot can do to bring the whole combination to a safe stop. By that time the glider is probably flying, in ground effect, and with a far better glide angle than the tug.

Get out of the way if you possibly can!

My tug didn't, so being 3 ft in the air I cunningly shaved the top off the standing crop alongside the runway with a wingtip, swung elegantly through 180 degrees and landed backwards with no damage to the glider, me, or my 75-year old trial lesson student. This is obviously a maneouvre I could repeat effortlessly, but I'd rather not.

Luck aside, I can tell you that I had time to work out that there was no way I'd even touch the wheel down, let alone stop, before hitting the tug, and no chance of flying over the top.

I'd say the best course of action for a marginal strip is for the tug to take off alone, and then, if the tug pilot thinks it can work and the glider pilot is convinced, go for it on the basis that at any point either can release and leave the other to sort it out. I always have a mental "bye bye rope" point, and fly the tow with my hand on the cable release.

[Doodlebug]

Hello All

IFMU whilst I do think Dave has a valid point maybe that's not such a bad idea, maybe just one 'flex-power' takeoff won't do too much harm. It would certainly give an idea of the distance needed.

Somehow I did think you'd say 'Pawnee', Dave. Must be an impressive tow!Now if only they weren't so expensive. Another thing that springs to mind is that you cannot do on-the-job-training with a new fellow, obviously. You were on about cruise-climb, reminded me of the old Maule they once had at Bitterwasser, Namibia. It had had a mod done to increase the efficiency of the oil-cooler. Apparently wasn't cooling well enough when labouring along in a tow. Just skipping through the earlier posts again I got stuck on something, by the way, your thoughts on winching, to be precise: If I read correctly you were stating that if 1000' of cable is run out, the glider will release at about 1000' AGL? Maybe I misunderstand where you're coming from, but seeing as the winch puts a large part of that cable on it's drum during the launch I believe that the glider will release substantially lower than at the 1000' used in the example? The old-fashioned car-launch will provide a much higher release-height in relation to the cable-length used seeing the entire length is available to the glider for the entire duration of the tow (well, of course there is sagging due to the weight of the cable). The downside is that you need a much, much longer level bit of ground for these kind of antics, of course.

Backpacker, thanks for the rules-of-thumb. The first thing that sprung to mind when reading your post was how deeply disinterested I'd be in an abort, especially in aerotowing. The Prof has put it beautifully!

All things considered, winching is probably the safest launch, wouldn't you all agree? It whips you away from the ground immediately, none of this dawdling along behind a car, cable-chute flinging gravel onto the canopy, wing-runner bursting a gut, while the car gradually gets going. Also safer than aerotow (though I'm probably sticking my neck out here) because the two aircraft also need to gradually build up speed, there IS the risk of mayhem near the ground, what with an inexperienced glider-pilot being able to bury the tugs' nose, aborts, engine-failures, etc. The down-sides are, of course, that you need experienced people to crew a winch, it IS more labour-intensive, even on a multi-drum, and that the release-heights are the lowest of the three, therefore 'getting away' is not a guarantee for the glider.
Anyway, I was just meandering, there. Still think we need to go with aerotow.

Take care Gents.

Bug

[Dave Unwin]

Hi Bug, I said a thousand YARDS of cable will give (approx) a thousand FEET AGL.
I don't have the stats to hand, but I think you'll find that aerotow is a lot safer than winching.
Cheers, Dave

[Sedbergh]

Dave's right on both counts there.
But after 1800 winch launches and 150 aerotows - it's the aerotows that make me nervous!

(It's exactly the opposite for them as trained on aerotows)

Never tried a car tow yet

[Dave Unwin]

I'm exactly the opposite Sedbergh - winching makes me nervous, as I've mostly aerotowed. Car tow is fun - I did some off of El Mirage dry lake. Have you bungeed? That's what I really fancy!
Cheers, Dave

[Sedbergh]

Did one bungee launch in a K13 some years back at the Long Mynd (where they still keep the giant elastic band available as far as I know)

About 40 knots wind blowing straight onto the ridge (that seemed to be the minimum windspeed for them to get the bungee rope out). The gliders were loaded in the lee of the hangar & then wheeled out onto the bungee point with a large crew clutching all available corners.

We sat there with the airbrakes pulled out & the wheelbrake hard on, with the mainwheel sitting in a concrete depression watching the bungee rope stretch itself over the edge of the ridge (the bungee team was marching off down the slope so we couldn't see them).

When the elastic stopped stretching, the brakes were banged in and we were off. There was no real sensation of forward movement, the glider just seemed to rise vertically (which it probably did more or less in that wind!) and very smoothly. Certainly no aircraft carrier steam catapult style of acceleration at all!

Having done a few fast & bouncy runs up & down the ridge it was time for someone else to have a go so we went round behind the ridge (not too far!).

Guy in back had said leave plenty of height in the circuit. I left way too much and despite howling gale & full airbrake we were not about to hit the ridge (it's quite short crossways!) Guy in back banged in the brakes & we went round for another go - definitely the one & only time I've ever done that in a glider!

(Apologies for being totally off thread folks!)

[Doodlebug]

That explains it then, Dave, sorry!

I suspect Sedbergh is onto something in that it's probably what you're used to..

I envy you gents the bungee-experience, sounds like fun!!

Spoke of Bitterwassers' Maule earlier. Not so, brain-fart. It was a Rallye. (Ralleye?)