Freesteel Blog » Slovenia trip complete

Slovenia trip complete

Thursday, April 12th, 2018 at 4:29 pm Written by:

Okay, so that last flying day at Meduno wasn’t very adventurous on the scale of the top pilots, but I was extremely pleased with it; I did just as well as anyone else in our xtc-paragliding (hang-gliding week) group and felt perfectly up with it.

Often you come down disappointed, and can watch everyone else from the landing field going higher and further and having more fun, and you’re down wholly because of your lack the skill and competence. But this wasn’t one of those days.


Here is the page of everyone’s tracklogs.

I was particularly happy with the part of the flight where I maintained my altitude over the flat lands at about 700m for 11 minutes before finally the air currents strengthened enough to carry me up. I had a sense of calm and flow rather than panic and disappointment this time.


It doesn’t look particularly low in the picture, but it felt like it.

I thought it was rising air from a pig farm I could see below and towards the dry river bed (because it smelled as such) but it couldn’t be as this as it was about 700m cross wind. I had consistently the wrong idea of the wind direction. It shows that even with totally mistaken ideas, I was still able to stay with the weakly rising air.

At one point I was passed high over a rifle range. The pops of the guns were like tap-taps on my breastbone.

I overflew the takeoff at the end of the day and took a photo of this cute pink training glider on the ramp beside the wood pile in the car park.

Then I tried to narrate part of my glide down to landing to the camera, which doesn’t work at all with my full face helmet.

One of the folks on the hill was SashaZ whose long blogpost about surfskis is what caused me to book my Tarifa trip with Becka.

Here are some other pics from previous days.

We had some long drives there and back in someone else’s car. Becka spent the whole time at SpeleoCamp caving, and so this shouldn’t count as a hang-gliding holiday.

Oh, I might as well put down my notion of the physics of flight here, while I have it worked out. It goes like this:

A heavier than air object with a mass of 100kg wishes to avoid accelerating downwards to the ground under a gravitational force amounting to 10 metres per second per second.

As each second that passes there is 100×10 = 1000 kg m/s of momentum that must be accounted for by blowing a volume air downwards at a speed k m/s.

Suppose the craft encloses a horizontal area a square metres within which it blows the air downwards at k m/s. In one second this would be ak cubic metres, which, with a density of about 1 kg per cubic metres, is ak kilograms, sent downwards with a momentum of ak2 kg m/s.

If the area a was circular, then you could cover it with a circular propellor like a helicopter, and maintain your altitude by blowing the air at sqrt(1000/a) metres per second downwards to counteract the gravity.

But imagine the shape of a is rectangular, and instead of a rotating blade, the blade moves horizontally on rails of length v and has a width w. This is somewhat like a wing with a span w flying at a velocity v.

My glider has a wingspan of the order of 10m, and an airspeed of 16 m/s, so the air needs to be blown downwards at a speed of sqrt(1000/(10*16)) = 10/4 = 2.5 m/s.

The kinetic energy embodied in this is 1/2 * mv2 = 0.5*160*2.5*2.5*2.5 = 1250 Joules/second.

If I weigh 100kg I can generate 1250 Joules from potential energy if I sink at 1.2 m/s — which is about the rate that my glider sinks on a steady glide.

This is a story of what needs to happen to the air to keep you up, not how it is done with aerofoils, vortices, induced drag or any stuff like that. And it also suggests that our lovely gliders have already hit certain limits of what they could physically achieve for their size and speed.

One way to get them to go up will be to add an electric motor to give you that extra to get off the ground, or to find a thermal when you’re going down.

That ad says they have 24 Ah in their 57.8V battery, which equates to 24*57.8*60*60=5Megajoules. This can maintain a horizontal flight for 27 minutes, which means it’s at the rate of 3000 Watts. That’s about a 50% conversion rate from the battery to powered energy, which is plausible.

It also gives a “max summit height” of 750m, which is a budget of 6660 Joules per metre. I need to give it 1000 Joules per metre in potential energy, so suppose my climb rate is k m/s then it will take me 750/k seconds to get up there, consuming 3000*750/k + 750*1000 = 5Megajoules which computes to a climb rate of 0.53 m/s over 23 minutes.

I can’t afford this stuff. I should be happy with the massive amount that I’ve already got.

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