Towrope tensions....Part II

On Mon, 15 Oct 2018 08:59:21 -0700, moshe.braner wrote:

Possible explanation and connection to this thread: 250 kg
divided by a lift-to-drag ratio of about 25 at that speed means a rope
tension, in level flight, of about 10 kg. With such a light glider the
rope tension was so weak that the air drag on the rope in the propwash
behind the towplane was stronger, keeping the front half of the rope
horizontal.

Yes, that's very close to my Libelle numbers. In a 6 kt climb the tow
tension is around 39 kg, but over 28kg of that is due to the work being
done by the towplane in lifting to glider at 600fpm. This leaves about 10
kg to be accounted for by the glider's drag. When I first did the
calculation I was surprised that hauling a glider uphill would exceed the
drag load by so much.

I normally winch launch, so pretty much all the aero towing I do each
year is a high tow to 3,500 in the club's Puchacz for annual spin checks,
which are part of the club's mandatory checks at the start of the thermal
season. I have flown low tow during these flights and quite like it, but
have never tried it in my Libelle. Must do so to see how big the bow gets.

As it happens I did aero tow my Libelle once this year, but as it was
just a quick trip to 2000 for a test flight, I didn't even think of going
low.

FWIW, last year the truly amateur TE probe mount in the fin failed (no
idea who installed it, but probably the GSA since they bought the glider
from Glasflugel). However, Glasfaser weren't happy with having both TE
probe and pitot on the fin. So I ended up moving the pitot from fin to
nose to keep Glasfaser happy. They hold the H.201 type certificate and it
says the pitot must be in the nose. I re-used the excellent fin pitot
installation and plumbing as the TE probe mount and pneumatics. The aero
tow was to check that the ASI reading for known values such as stall
speed was still the same (it was) and that the TE probe was working
correctly (it was).


--
Martin | martin at
Gregorie | gregorie dot org

Well, I said ‘during tow’ thinking that would cover beginning to end. I need to determine the attachment strength for a tow hook on a towplane.
What might rope tension be from initial roll in grass (thinking start will be highest). I thought maybe testing was done years back and a chart or data existed.
Years ago back when I was teaching at Schweizer, the topic came up and less than 300#, towing the 2-32, 3 adults, behind a 180 Supercub was agreed by Paul and a couple of engineer types. I took it as gospel, but have wondered over the years how close they were.
Now I am really curious.
My previous thread got off track into formulas, winch launching, and pitch angles.
The info provided so far seems encouraging. I thought it might be higher.

R

On Monday, October 15, 2018 at 2:33:35 PM UTC-4, Retting wrote:
Well, I said ‘during tow’ thinking that would cover beginning to end. I need to determine the attachment strength for a tow hook on a towplane.
What might rope tension be from initial roll in grass (thinking start will be highest). I thought maybe testing was done years back and a chart or data existed.
Years ago back when I was teaching at Schweizer, the topic came up and less than 300#, towing the 2-32, 3 adults, behind a 180 Supercub was agreed by Paul and a couple of engineer types. I took it as gospel, but have wondered over the years how close they were.
Now I am really curious.
My previous thread got off track into formulas, winch launching, and pitch angles.
The info provided so far seems encouraging. I thought it might be higher.

R

The tow rope (or weak link) are supposed to be able to withstand between 0.8 and 2 times the weight of the glider. What's the point of ensuring that if the attachment to the towplane is built for less than that, e.g., 300 pounds? Why not build that tow hook to handle 2x the flying weight of the heaviest glider you will tow?

At 18:33 15 October 2018, Retting wrote:
Well, I said =E2=80=98during tow=E2=80=99 thinking that would cover
beginni=
ng to end. I need to determine the attachment strength for a tow hook on
a
=
towplane.

Try AC43.13-2B Chapter 8 rather than do your own guessing

Well, I'm not a mechanical engineer, but...

It seems to me you should be more interested in the breaking strength of
the highest weak link you could imagine using and then add something for
the wife and kids.Â* If you design your tow plane mount to a 300 pound
limit, you'll likely be leaving the mount, release, and rope behind the
first time a glider pilot gets sloppy and gives you more than a gentle tug.

On 10/15/2018 12:33 PM, Retting wrote:
Well, I said ‘during tow’ thinking that would cover beginning to end. I need to determine the attachment strength for a tow hook on a towplane.
What might rope tension be from initial roll in grass (thinking start will be highest). I thought maybe testing was done years back and a chart or data existed.
Years ago back when I was teaching at Schweizer, the topic came up and less than 300#, towing the 2-32, 3 adults, behind a 180 Supercub was agreed by Paul and a couple of engineer types. I took it as gospel, but have wondered over the years how close they were.
Now I am really curious.
My previous thread got off track into formulas, winch launching, and pitch angles.
The info provided so far seems encouraging. I thought it might be higher.

R

--
Dan, 5J

Thank you everyone.....SF provided me with the engineering data I was looking for and is the winner of the 100 Shilling award.
Cheers,
R