Gliders > Slope Soaring

Kostez

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Fred:
Hi Brian,

Looking forward to it  :clap:  :D

Brian:
Hi John/Fred,
I'm just sticking down a few thoughts/figures for reference.
My understanding of a 'good' VTPR model is that it should be light, whatever that is ???.
The French Kostez's are 7.5/8 kg.
On the RCG thread they said they were 60g/dm\2.
My Voltij (all glass-heavy'ish) is 46g/dm\2 @ 1.65kg for a 2m.It flies good but needs some air.
The Limande is approx 40g/dm\2.
It is likely that there is a threshold wing loading where these planes go from being good (agile/lively/efficient with good energy retention) to being not so.
I have no idea where that threshold might be.
We have to be careful about adding weight for strength as the weight can become the source of the problem, eg for landing.
At an impact the energy involved is a multiple of the weight and the speed  -  ironically the heavier the wing loading the faster the landing/impact (normally/as a rule).

I am going to aim for 45 to 50g/dm\2.

Brian

Fred:
Hi Brian,

Wing loading is only one thing to be taken into account for a VTPR capable machine (for info, the Brittany machines are below 40g/dm2).
Static moment, profile etc etc is as important, and that's the difference between an aerobatic glider, and a VTPR glider.

Brian:
Hi Fred,
I am just trying to explore this subject a little bit more - for me and for other pilots that have an interest.

By static moment do you mean tail plane position relative to wing cord - ie; tail moment ??. If so do they have longer than normal or shorter than normal gliders.

So the Kostez at 60g/dm\2 plus is 'outside' the normal wing loading for VTPR. I know it is big and that weight does not scale up pro-rata with size.

Brian

Fred:
Hi Brian,

Not to sure how to translate that, but that would be the fuselage moment (by itself), basically where in the fuselage the most lift is generated. not to be compared with were the stability is most generated (usually at the CofG of course)
Position is critical if you want a glider that perform well on knife edge (without much, or in theory without rudder input), and is about 15 to 25% from fuselage nose (just look at all the proper vtpr glider fuselages), the tail of the fuselage generating almost no lift (your fuselage tail can be a stick, that will not change much the knife edge performances)

Then, you need to select a good profile that will work at the correct Re number you are looking for, but most specificly for VTPR, a profile that will work well in no air speed (able to do a roll with no airspeed for exemple). You can have a look at the TP series, and of course, the good old SBs. All these profiles work with a certain size of ailerons etc etc

Anyway, all that is only computer theory, you will never match the speadsheets performances of a profile etc with a build, even Boeing don't pretend that, but the goal is to get as close as possible to the "perfect" recipient. As Dassault says, if it looks good, it will fly right!  :D

On another note, I think lots of experts are talking about VTPR gliders lately (seems to have been discovered only a year or two ago...) but most, if not all the gliders they choose, compare etc, are barely VTPR gliders in the true sense of the term.
But they are great aerobatic machines. VTPR is another league, another way of flying altogether. I flew / tested tons of aerobatic gliders, but flying a glider truly designed for VTPR, is totally a different world by just the way the machine fly and react to your inputs.
Almost all aerobatic gliders can do a roll at wingspan altitude from the slope... But a handfull will do it at zero speed! I think this is the simpliest way to describe these machines.

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