What is a typical sink rate for a 2M glider?

[Simon Chaddock]

With my Hobby King altimeter it is possible to calculate an average sink rate.

A full size high performance glider like the Schleicher ASH 25 can achieve a minimum sink rate of 1.4 ft/sec. How well does a 2M RC glider compare?

This is a plot from my foam ST330.

Rather blustery so not ideal conditions for flying close to the stall but the average sink rate was 3.68 ft/sec.

I know at small sizes the viscosity of air works against you but I did not expect the min sink to be that much worse than full size.

I did another flight with the 2m super light Depron Dig Dragon pusher but without a folding prop.

This gave a slightly better 3.2 ft/sec average minimum sink but still along way off a full size performance.

The question is could a 2m span glider ever achieve a minimum sink approaching that of a full size machine?

[Pete B]

That's interesting, Simon, as I was going to do the same with my EG flight from yesterday - but modding, building and gardening have conspired against me so far! Just need to do the maths and upload the graph, etc - should be ready by March!

Pete

[Pete B]

Here's the graph from my Easyglider flight yesterday. Conditions were clear blue sky but with a nip in the air and a mild, blustery breeze. I didn't set out to go for min sink - some flap on the EG helps there but I didn't use any - so this is possibly fairly representative of a 'no lift' flight:

Crunching some numbers, the 3 consecutive descents worked out at 4.3, 4.79 and 3.79 ft/sec, so say an average 4ft/sec. I'm sure that could be improved if I try hard enough!

It's given me a thought. With warmer weather approaching (here at least) we could perhaps let this thread run through the summer and have some informal vying for best min sink figures amongst the altimeter users?

Pete

[Simon Chaddock]

Pete

You don't know how relived I am that you are getting similar sort of figures!

As a lone flyer I was beginning to think my planes were rubbish!

Yes indeed let the thread run - I would love to see what other people can achieve. Of course 'min sink' is not the whole story as at times you do need 'speed' penetration at which my low sink rate glider is particularly poor!

[PatMc]

Simon, here's a record of some flight durations made from (mainly) 30 sec power climbs from launch with the max achieved height measured using an Eagle Tree altimeter.

I was trying to aproximate the 30 sec, 300m max altitude power run rules for my own satisfaction without going to the expense of buying a dedicated limiter unit.
Some of the flights recorded benefited from thermal assist, some were in sinking air, most were in fairly neutral air. I also experimented with flying upwind at shallow angles before climbing sharply which accounts for most of the relatively low altitudes achieved by Igor. The lower altitudes for RelaxE are because its a heavier model with much lower power/weight ratio.
Because I was trying to limit the power run time I was glancing at the Tx screen from about 20 secs & occasionaly losing sight of the fast climbing model this sometimes resulted in stalling problems with height loss at the start of the glide phase.

RelaxE is 2.6m that was primarily intended for my Flycam, Igor is a 2m bitsa. (Dithcworld readerth will underthand the name'th origin)

I reckon the still air sink rate of Igor to be about 2ft/sec.

[PatMc]

Typo alert - it should be - 30 sec, 200 mtrs max altitude.

[Simon Chaddock]

PatMc

That is very impressive and clearly the sink rate is at least in the same order as full size, although I can atleast match your climb rates!

How much does your Igor weigh?

[PatMc]

Hi Simon, Igor weighs 950g power is 250W.

Actually I think I was being pessimistic on the sink rate, I'll have a closer analysis of the figures later.

Our sink rates should probably be better than most full size. They may have much better lift drag ratios at but they are also traveling much faster when they are at min sink. The sink ratio hinges on the combination of l/d & airspeed.

This is Igor - old Protech Unlimited fus/tail & a long forgotten 6ft wing that had been languishing in my loft for years.

Edited By PatMc on 18/02/2013 10:57:28

[Simon Chaddock]

I too thought a models should atleast match full size but infact the very latest high performance types do actually have some of the best min sink figures ever obtained.

The best modern glider I have found is 1.4ft/sec which better than even the best of the pre war 'floaters' that had only half the wing loading and flew at half the speed.

Just for interest I stuck the altimeter in my 48" Endurance. At only 14.5 oz (411g) and flies on just 9W it is firmly a 'floater'.

In the smooth air above 600ft it managed 2.8ft/sec.

The previous flight from 863ft to ground it averaged 3.2ft/sec.

I suspect what I am proving is that what works well as a 'minimum power to fly' plane does not necessarily result in a particularly low sink rate!

I shall have to try it with a folding prop.

Edited By Simon Chaddock on 18/02/2013 17:48:10

[PatMc]

Simon, I used to regularly fly thermal models from150mtr bungee & hand tows. My 100s model weighed 76oz unballasted & BARCS league size ones a little more but lower wing loading & able to take more ballast if necessary.
During a day with little sign of thermal & no strong wind to contend with, if a model landed in much less than 5 mins I would have been blaming patches of sink, a mistake on my part or I'd be making plans to re-trim it from scratch. In effect, this means that we expected the sink speed to be around 1.65 fps or less.
That was with designs from the 1980s & 90s. With todays all moulded & cleaner built up models I'd expect better min sink to be easily achievable.

I'm hoping that HK stock the FD Altitude Permit soon as it seems to do the same as the altimeter plus it will cut power at a predetermined altitude/time. If they don't I may just get an altimeter to play with.
My only concern is the accuracy of both these devices. If the altimeter is recording optimistic altitudes then this would make your sink rates seem worse than they really are.

BTW min power for flight would be acheived at best L/D trim not min sink trim.

[Simon Chaddock]

PatMc

The Altitude Permit looks like quite a gadget but how much is it?

I think you will find the minimum power to fly is the minimum sink.

Rate of doing work (against gravity!) is the mass times its vertical velocity. Min sink is the lowest vertical velocity condition so in level flight gives the best speed for duration.

Best L/D gives the greatest distance for height so in level flight gives the most economical speed to fly for range.

Of course motor and prop efficiency can alter the optimum speeds.

[PatMc]

Simon,
I'm hoping that HK will market the AP as there's more chance if it being cheaper from them than any other retailer/distributer. IMO the existing alternatives are a little too expensive for non-competition use. Fingers crossed anyway.

Consider that in level flight : lift = weight & thrust = drag.
Weight & therefore lift must be constant at every velocity.
It follows that drag must be at it's lowest when an aircraft is trimmed at the best L/D ratio in level flight.
But drag = thrust, ergo at the best L/D ratio thrust must also be at the lowest level that will maintain level flight.

Actually the best L/D only gives the greatest distance when there is no wind. Flying faster gives greater range into a wind & vice versa flying with the wind.

[Erfolg]

Although the sink rate is important, possibly as relevant, if not mor soe, is the glide ratio, which would drop out if a GPS output is available.

Watching 2m glass ships, it is apparent there sink rate is very low, compared to my traditional build models. Where they really surpass my models, they can and do cross the sky with far less loss in height.

I have a Flair Sunrise, whose sink rate can be very low, in absolutely still air, yet as a thermal model is all but useless, other than that one still day.

[Simon Chaddock]

PatMc

Consuider the energy the glider is using as it flies from one point to another. If the speed is the same at the beginning and end then only the potential energy has changed. As the mass is the same then the height is has dropped is the energy used.

It drops less per second at min sink speed than it does at best L/D speed and thus is using less energy.

It is all defined by the planes Polar Curve.

I fitted a folding prop to the Endurance this morning. 7.5x4.8 rather than the fixed 7.8x4.3 it normally uses. Definately took the edge of the climb rate but as you might expect it did improve the minimum sink.

Took two goes to get it trimmed exactly right (the folder is quite a bit heavier and being a pusher it is well behind the CofG) but it did achieve a sink rate of exactly 2ft/sec over a reasonable period which is gratifying.

None of these particular planes are much good as gliders, except in the most benign conditions, as they come down like bricks if flown at any speed but then they do have a long power duration to make up for it.

[Erfolg]

Simon

I have a 3m Algebra and a 120" open glider that will stay up for incredibly long times using large Lipos, and driving to dots in the sky under power. I now, never fly them, as it is not a challenge.

Now launching to 200m and eking out the time, hunting for lift, using it, that takes skill and a good all round model. It is the former that I totally lack and the money for the second.

[PatMc]

Posted by Simon Chaddock on 19/02/2013 17:50:32:

PatMc

Consuider the energy the glider is using as it flies from one point to another. If the speed is the same at the beginning and end then only the potential energy has changed. As the mass is the same then the height is has dropped is the energy used.

It drops less per second at min sink speed than it does at best L/D speed and thus is using less energy.

It is all defined by the planes Polar Curve.

Simon,

I don't understand what point you are making here.

[Tony Smith 7]

I agree with Simon, minimum power occurs at minimum sink speed, not at best L/D. Drag is lower at max L/D but because the speed is higher then power required/consumed is higher.

[Erfolg]

Do we really care?

What use is this peice of knowledge?

Other than you could be going backwards for longer, if at all breezy, before disappering down wind.

[Tony Smith 7]

Posted by Erfolg on 20/02/2013 12:25:30:

Do we really care?

What use is this peice of knowledge?

Other than you could be going backwards for longer, if at all breezy, before disappering down wind.

It depends on how methodical and serious your gliding is. Understanding the difference between minimum sink and max glide, and how they each interact with lift, sink, G and head or tail wind is pretty fundamental stuff. It can make the difference between going down or staying up. And yes going backwards is sometimes the most efficient course, particularly in wave where the lift is stationary, unlike thermal.

[PatMc]

Posted by Tony Smith 7 on 20/02/2013 12:16:24:

I agree with Simon, minimum power occurs at minimum sink speed, not at best L/D. Drag is lower at max L/D but because the speed is higher then power required/consumed is higher.

Tony, I'm afraid that's not logical.

Thrust = drag
So if drag is lower thrust & therefore power required must be lower.

[Erfolg]

Tony

In this instance I was referring to Simon using power, as i understand it, to maximise duration. From my experience, seeking minimum sink rate it most often and not, very productive, if seeking to maximise air time, other than very still, dead air. In all other circumstances the ability to get out of sinking air, or join rising air (marked by a bird or others) is far more productive.

I personally would prefer to know the best glide ratio trim setting, particularly if in a competition.

Most of us can judge the trim for minimum sink rate, it is the trim, just about when the model is about to stall. Max, AoA.

All Simon actually knows, is the sink rate for a particular model. Most modellers know that a clean, very light model is the starting point. Now with control of the trailing edge via the crow braking arrangements, it is a question of increasing camber to hang about, reflexing to move forward efficiently.

If all you want to do is hang about, just build a bag of sticks with a undercambered (high camber wing) and either go backwards in a wind, or come down like a brick, if trying to stay still, relative to the ground.

[Tony Smith 7]

Posted by PatMc on 20/02/2013 13:52:18:

Posted by Tony Smith 7 on 20/02/2013 12:16:24:

I agree with Simon, minimum power occurs at minimum sink speed, not at best L/D. Drag is lower at max L/D but because the speed is higher then power required/consumed is higher.

Tony, I'm afraid that's not logical.

Thrust = drag
So if drag is lower thrust & therefore power required must be lower.

With respect, power = force x speed. It's just like with an engine where max HP isn't necessarily at the same rpm as max torque.

[Erfolg]

If the discussion is about minimum drag from the airfoil. It certainly does not occur at the best L/D, the minimum drag occurs at a lot lower AoA.

The tip losses will also be lower than at the best L/D ratio, which the traditional graphs obviously cannot show.

Balanced against this, the drag from the body will also increase, as velocity increases.

I still wonder why it matters? There has to be a reason why anyone is interested!

[PatMc]

Tony,
in level flight drag & thrust are always equal. When an aircraft is trimmed from best L/D ratio to lowest sink speed the parasitic drag reduces but the induced drag increases. The combined drag components are at their lowest at best L/D. But lift = weight in both trims thus L is a fixed parameter in level flight. Therefore although the forward speed is higher at best L/D the power needed to overcome total drag is less. In fact it is at the lowest required for any trim condition to sustain flight.

The analogy you give doesn't fit with the level flight conditions.
The thrust of a prop is analogous to the lift of an aircraft & in the pitch to an aircraft's trimmed angle of attack. In this case it's the thrust (lift) that's changing whilst the pich (AofA) remains the same.

[PatMc]

Erfolg the discussion isn't about minimum induced (airfoil) drag it's about minimum gross drag versus the fixed measure of lift.

It matters because Simon is interested in the getting the longest duration flight time for the minimum of power as a personal achievement

i.e. it's the sort of thing hobbies are about.