What wind speed to lift a 3.5kg delta?

[Tony Kenny]

I know, it's not just about the wind speed, but also about the slope itself in how much lift it's generating from that wind. But it got you here didn't it?

So, is there a rough guide I could use to determine what kind of conditions are going to be suitable to lift 3.5kg of delta wing model in a pure slope soaring environment, without having to throttle up the motor?

thanks in advance

Tony

[Andy Meade]

Would need the wing area Tony, to work out the cubic loading

[Tony Kenny]

Good point... I'll go measure it this evening and then try to remember trig from my school days a quarter century ago

[Andy Meade]

Here is the website that I usually plug figures into.

It says :

Gliders	-	under 4
	Trainers	-	6 to 7
	Aerobatic	-	9 to 10
	Scale	-	12 to 13
	Racers	-	15 and over

But, as how that transfer to slope lift...well, it's all a bit of a fudge I believe! I've got a PSS machine that ends up at WCL 16, yet flies well in a 30mph SW on the Orme, which gives plentiful smooth lift. I'd double the WCL to get required minimum wind speeds on a "good" (I know, it's a bit subjective, sorry) hill.

Edited By Andy Meade on 21/09/2017 15:55:45

[Dave Bran]

Yes, well................ there are FAR more factors in play here than wing loading.

Example: I had two flying wings, one slope, one power. Getting into Dynamic Soaring (I wish, where I live!!) I built a second slope machine. These were identical as far as size, planform, and section, ALL starting out as 1 metre span FlyingWings Slipstream foam cores. (which has an excellent section FAR better than Zagi/SI, etc!)

Soarer one was built as light as practical and came out around 300g with 800mAh 2S flight battery.

Power one was built with extra spars, lots of reinforcing around the motor, a large Inrunner that I had rewound and balanced, a large heavy motor mount to carry the power, and ran on a 2200mAh 3S. Can't tell you weight but it was a heavy lump, I'd guess over 1kg nearer 1.5kg.

Soarer two was built with two extra spars, double taping, etc and without ballast came out at over 800g.

Soarer 2 was maidened on a day when there was little or no breeze evident. I was passing a slope I knew well and thought I'd give it a chuck to check CG and trim in a down slope hop or two. Quite shocked when on the first tentative lob it climbed out and away and proceeded to be capable of slow but stable circuits!!

SO, a few weeks later at the same slope I tried the power one as well, only took the prop off, There was some wind, a low 10mph or so, and it needed great care to get it away, but once away and nursed up "on the step" it flew nearly as well as the other two.

What did alter was the flying speed, heavier = faster, but not the lift capability to anything like as much as I expected. Quite an eye opener!

SO, far too many factors, thermal lift (yes, at a slope). slope lift, wind speed/stability, wing section and area, weight, drag, pilot ability, etc, etc, etc. IMO calculation is utterly pointless, go and throw!!

[Andy Meade]

Up to a certain extent, yes I'd agree. But there's nothing wrong with putting a bit of intelligence and thought behind it.

Imagine just "going and throwing" my 38lb A10 off into a 10mph breeze?

No. Me neither. Getting a good idea of the likely operable wind speed, then choose your day. Then no one is endangering what could be a lot of hours of workshop work.

[Tony Kenny]

The wing itself is approx 108cm at the base, 82cm long, so an area of 4438 cmsq approx.

That website give the wing loading of 7.9g/dm2

[Andy Meade]

Yes, about 26 oz sq ft - not all that really, and WCL at just under 12. > 25mph on the edge (especially if you do take it to the Orme), and you should be good to go imho. I see on the other thread that you are considering getting to the Orme next month - hope to see you there! There is a good grass lower shelf if you get in trouble.

[Mark Kettle 1]

As Andy said in £'s shillings and pence:

4.77 sq ft at 123 oz = 25.7 oz a sq ft

[Erfolg]

There was a time, perhaps there still is when every modeller knew that Lift is proportional to velocity squared.

A often quoted relationship was L=0.5*(Roe)*v^2*S*CL.

On the basis that the relationship is correct, your models are identical, it can be seen that you do not have to increase speed much to increase the lifting force. This is relevant as the only relevant differences is the weights. perhaps some drag.

Changing the drag changes the angle of decent. It can be shown that by playing with the various equations that this argument is true. Perhaps not a intuitive conclusion. As every lecturer would say "gliding is down hill all the way".

[Erfolg]

I have just been working on a model, one of those tasks where your mind drifts a little.

A handy little exercise would be re-arrange the formula and plug in your own parameters for the model.

v = (2L/roe*S*CL)0.5^2 as the lift = weight everything else is equal, doing the calc on the three models will reveal the differences in velocities. Roe is pretty standard in the UK @14.7 ibs /in^2 or 101.3 pascals, choose any standard CL for your wing section, and bobs your uncle.

[Former Member]

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[Erfolg]

You can discuss the issues which keep a model up on a slope, yet you can get a good handle on the relative speeds needed to fly level.

In the past I have done calcs on the ballast needed to maintain the same trim, in windy conditions. From memory, to get that 5mph or so increase in airspeed, needed a big chunk of weight, over the CG. The reason I no longer know, is that I used to keep my flight box, with note pad permanently in the car boot. Unfortunately, that car was stolen some 35 years ago (in Liverpool whilst visiting my daughter at Uni). The car was found, the flight box was not.

[Tony Kenny]

All good stuff and lots of food for thought, thank you.

Whilst I'm at it, this is going to be too big to launch by myself and at my nearest slope I'm often alone. Would it affect much if I screw a handle to the bottom of the delta wing so i can one hand launch?

It does have a fixed undercarriage for normal runway work but it's not fitted in such a way that it can be removed.

[Former Member]

[This posting has been removed]