Towrope tensions....Part II

On Monday, October 15, 2018 at 11:59:23 AM UTC-4, wrote:
On Monday, October 15, 2018 at 10:20:53 AM UTC-4, Retting wrote:
Looking for towrope tensions information applied during a typical AERO tow. Singles and twins up to 600kg/ 1500#.
Perhaps testing was done back in the 60’s before Al Gore invented the internet. Prefer some numbers over methods of achieving.
The Uganda Information Award is still in play .... this thread only .
Credible Best Wag will be considered.
Civil disagreement with counter points encourage.
Numbers boys numbers.

R

Retting, you didn't make it clear whether you want the rope forces in the air, or in ground roll, or both.

Some months ago I landed my Russia (total flying weight about 250 kg) at another airport and got an aeroretrieve. Thought I'd try the low tow position for a change (always done high tow before). Once I got stabilized below the towplane's wake, the rope shape was very different from what I expected: it went horizontally away from the towplane, then down to the nose of my glider at a steep angle. When my speed or position changed a bit, the rope sometimes looped back over the nose, trying to kiss my yaw string. I did not like that at all, so went back to high tow. 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.

If the loop was as described you almost certainly were flying the low tow too low.
I point out to my students tat if the rope goes up when you release from low tow you are too low.
FWIW
UH

On Monday, October 15, 2018 at 12:04:33 PM UTC-7, Retting wrote:
Agree, but if I can get support for a reasonable starting point, a consensus, then I can work it from there.
R

The answer depends on the strength of the weak links you intend to use. In fact, consistent with most tow release STCs, you should have a placard in the towplane with the weak link limit listed (our does). Typically the installation should be good for 150% of the design load (listed weak link strength). The design load for the installation needs to be considered for all possible orientations. If the rope/weak link is stronger than the installation, the release may depart the aircraft instead of breaking the weak link.

As noted previously:

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

1.5 to 2 x's the weak link strength for the attachment point. Someone at Tost could tell you the shear strength of their mounting kit bolts. It depends on what you want to fail first. Pull apart the mount or part of the tail still attached to the mount.

SF

4,000 lbs or 50% of the yield strength of something you dont want pulled apart.
SF

We are almost there.......RR threw out a nugget of using 1600# towrope which is a good established practice. That will be my start point. Then SF tacked on the ‘shear’ strength of the tow hook attachment system, which is where I am at now. I am using 125,000 psi tensile strength bolts which , if I remember my engineering from years pass using the 60% rule, equates to approx. 75,000 psi shear strength.
So, the tow hook/tailwheel assembly is attach to a 1� rod with two bolts which in turn attach to the aircraft fitted assembly. Does the tailwheel assembly system strength exceed the breaking strength of the rope, and by how much?
I do not know how to convert psi to pull strength (pounds).
R

On Monday, October 15, 2018 at 5:38:33 PM UTC-4, wrote:
On Monday, October 15, 2018 at 11:59:23 AM UTC-4, wrote:
Some months ago I landed my Russia (total flying weight about 250 kg) at another airport and got an aeroretrieve. Thought I'd try the low tow position for a change (always done high tow before). Once I got stabilized below the towplane's wake, the rope shape was very different from what I expected: it went horizontally away from the towplane, then down to the nose of my glider at a steep angle. When my speed or position changed a bit, the rope sometimes looped back over the nose, trying to kiss my yaw string. I did not like that at all, so went back to high tow. 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.

If the loop was as described you almost certainly were flying the low tow too low.
I point out to my students tat if the rope goes up when you release from low tow you are too low.
FWIW
UH

I was just below where I could feel the towplane's wake.

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

Retting- If you got reasonably decent advice from RAS, you should count your lucky stars and be grateful. Don't be a cheap *******. Send SF 200 Ugandan shillings, or really look like an early Microsoft investor an send a Zimbabwean $100,000,000,000 bill to the guy!

https://www.ebay.com/itm/Zimbabwe-10...S!-1:rk:2:pf:0

Lucky? I knew someone would arise to the challenge.
Cheap? How many people you know have 100 Uganda Shillings.
A fair trade.

R