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

[Erfolg]

Patmac

The problem with this question, is how do you know you have achieved your goal?

At an abstract level, the maximum duration will still occur, when trickling in sufficient power, to maintain level flight almost at the stall.

Like much in life, there is a big but, in many wind speeds the lightly loaded bag of nothing, will be going backwards, relative to its operator. To remain stationary (relative to the ground), its airspeed will need to be increased, from now on its drag is increasing disproportionately. A airspeed will be reached where extra weight to maintain an advantageous balance between AoA and drag from the tailplane etc.

It is the sort of problem that can be investigated with a spread sheet, plugging whatever data is known, or assumed (at various values) to determine what matters. Many basic text books have all the basic relationships, some mags in the past, had gathered together bits of relevant, or best available information, altogether.

Other than in a massive hanger, with controlled conditions, I really do doubt that any definitive answer can be shown.

I do think that is why all the record categories and competition classes are preferred by most people. You know that you have broken a record and can show it, or that on a particular day some one won, or some one else achieved a relative goal etc.

Edited By Erfolg on 20/02/2013 17:02:57

[Tony Smith 7]

Posted by PatMc on 20/02/2013 15:40:25:

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.

Let's work a theoretical example. You may argue that no real glider would have this performance, but never the less it will show that minimum power does not always equate to minimum drag. I'm going to approximate one kgF as 9.8N, feel free to rework with an actual aircraft's performance and with more exact factors.

Lets assume weight of 1kg, minimum sink 1m/s at airspeed 10m/s which equals L/D or 10:1. Best glide of 13:1 at 16m/s which equals a sink rate of 1.2m/s

Drag at min sink = 0.1kgF = 0.98N. At 10m/s that equals 9.8W

Drag at best L/D = 0.075kgF = 0.735N. At 16m/s that equals 11.76W

Power is force x speed, by definition. If different forces occur at different speeds then you can't derive power without accounting for the speed difference.

Even if you just look at the loss of height and therefore loss of potential energy you see exactly the same. One kg mass descending at 1m/s is lossing less energy (9.8W) than one descending at 1.2m/s (11.76W)

[Simon Chaddock]

The difference in the power to fly between min sink speed and best L/D speed can be as much as 15% so if you are trying to fly for maximum duration, which is was what I was trying to do, flying at the right speed can be very significant.

PatMc

Power to fly is proportional to the drag times the speed squared.

In terms of energy at best L/D speed the lower drag is more than offset by the extra speed required to achieve it.

This explains it.

[Tony Smith 7]

Posted by Simon Chaddock on 20/02/2013 17:09:18:

Power to fly is proportional to the drag times the speed squared.

In terms of energy at best L/D speed the lower drag is more than offset by the extra speed required to achieve it.

This explains it.

I don't think so. From first principles power is energy per second, one J per second = 1W. One J is equal to (among other thinks) the energy to move a force of one N through one metre. So one N moved at 1m/s = 1 W, or moved at 2m/s = 2W. Speed's not squared, it's a direct relationship, the product of force and speed. Or force x distance / time comes to the same thing.

What you may be thinking of is a frequent relationship where the resistance increases with speed, so at higher speed you're multiplying a greater force with a greater speed, and in those cases power increases disproportionately.

[PatMc]

Posted by Tony Smith 7 on 20/02/2013 16:51:47:

Let's work a theoretical example. You may argue that no real glider would have this performance, but never the less it will show that minimum power does not always equate to minimum drag. I'm going to approximate one kgF as 9.8N, feel free to rework with an actual aircraft's performance and with more exact factors.

Lets assume weight of 1kg, minimum sink 1m/s at airspeed 10m/s which equals L/D or 10:1. Best glide of 13:1 at 16m/s which equals a sink rate of 1.2m/s

Drag at min sink = 0.1kgF = 0.98N. At 10m/s that equals 9.8W

Drag at best L/D = 0.075kgF = 0.735N. At 16m/s that equals 11.76W

Power is force x speed, by definition. If different forces occur at different speeds then you can't derive power without accounting for the speed difference.

Even if you just look at the loss of height and therefore loss of potential energy you see exactly the same. One kg mass descending at 1m/s is lossing less energy (9.8W) than one descending at 1.2m/s (11.76W)

Tony, your example doesn't work because you've randomly assigned specific L/D ratios at specific speeds. You may as well consider them to be different aircraft.

The theoretical aircraft is in level flight therefore there is no loss of potential energy due to descent.

All energy being expended is due to drag. Partly to it's induced drag & partly to it's parasitic or form drag.

[PatMc]

Posted by Simon Chaddock on 20/02/2013 17:09:18:

The difference in the power to fly between min sink speed and best L/D speed can be as much as 15% so if you are trying to fly for maximum duration, which is was what I was trying to do, flying at the right speed can be very significant.

PatMc

Power to fly is proportional to the drag times the speed squared.

In terms of energy at best L/D speed the lower drag is more than offset by the extra speed required to achieve it.

This explains it.

Simon, where do you get the 15% from ?

Power is not proportional to drag times speed squared. It's proportional to thrust.

Thrust = drag - at any speed.
Lift at any speed to maintain level flight = weight therefore is constant throughout.
Glide angle as a ratio = L/D.
Therefore since lift is a fixed at any speed you must agree that this ratio is always highest when drag is at it's minimum.
Logically this can only the best glide angle.

You must remember that this is total drag not form drag or induced drag alone and that this takes into account the model's speed.
At lower speed than that for best L/D form drag reduces & induced drag increases both exponentially. At higher speed the reverse is true.
If both drag curves are plotted graphicaly against speed because they are the curves are exponential they must cross at the lowest combined total.

[PatMc]

Posted by Simon Chaddock on 20/02/2013 17:09:18:

This explains it.

Actually all it does is explains the obvious regarding duration & distance travelled for a glider at differing L/D ratios.
It doesn't help in explaining which example would need the lowest power to maintain level flight, which is what we are discussing.

[Erfolg]

I assume this is where Simon gets his V^2, although it is the drag force.

 

The real trouble is this is for a body which is aligned to the direction of travel. A model, is a lot more complex, with respect to where and how drag is generated, as the orientation of the form changes with AoA, as a reference.

I still think it is a waste of time, a light as possible model, with a airfoil drawn around a old shoe, going as slow as possible, in still air, will stay up for ever.

The same model, in a breeze, will be in the North Sea (German Sea prior to 1916 apparently), faster than we can resolve the issues before us.

I just do not belief you can get anything approaching reliable results to be worth debating about.

What all competition modellers know, a good mouldie, will trounce a bag of sticks 199 out very 200 launches, however measured. No amount of calculation and discussion will change the result, even with me flying.

Edited By Erfolg on 20/02/2013 20:09:53

[Tom Wright 2]

Posted by Erfolg on 20/02/2013 20:09:3.

I still think it is a waste of time, a light as possible model, with a airfoil drawn around a old shoe, going as slow as possible, in still air, will stay up for ever.

I just do not belief you can get anything approaching reliable results to be worth debating about.

If Simon is truly looking for maximum duration in still air then in my experience Erfolg's statement say's it all.

I have a very light 5 M span Sailplane that will out perform an AVA in still air , but bring on the wind and the AVA wins hands down .

The part of the discussion were standard formula is used can only produce meaningful results when a wing section ,wing plan form, aspect ratio, wing loading and wind speed are quantified, along with the parasite drag at lowest sink speed.

Tom.

[Erfolg]

There are two Avas and a Bubble Dancer in the club, quite remarkable machines. Remarkable in their construction, even more remarkable when flying.

They are so beautiful, that I covert any one of them, yet I know in my hands, would be soon reduced a poor shadow of their former glory.

I do agree Tom. I just cannot see how any truly definitive answers are possible, far to many "ifs" & "buts". If flying to obtain results, different day, different results.

Edited By Erfolg on 20/02/2013 21:11:32

[Simon Chaddock]

To be fair all I originally asked was a typical sink rate for a 2m glider.

The best minimum sink rate appears to be a bit below 2ft/sec.

My interest was how did my very light 'endurance' airframes compare. As it turns out not too bad but obviously an improvement is possible so long as doing so does not significantly increase the airframe weight and hence the power to fly.

An endurance plane will share many of the characteristics of a glider but the need to maintain a good L/D ratio over a wide speed range is not one of them.

[Tom Wright 2]

Simon .

I have flown my own glider designs over a period of 50 years and have found that a design solely dedicated to max duration in light or near zero wind conditions will achieve very close to 1ft per sec sink , but catch a sniff of lift and all of a sudden VNE becomes a very significant design factor .

A friend of mine who took an interest in this type of model succeeded in coming up with a very light design, lost the model as it passed through 3000ft with all attempts to lose height fruitless . I now restrict my floaters to a max height of 200 ft and promptly fly out of any lift before things get interesting .

Some say what's the point of such models but the joy of ultra slow flight is a thing I have always been interested in .And I suspect it has a certain appeal to you to . .

Tom.

[Tony Smith 7]

Tony, your example doesn't work because you've randomly assigned specific L/D ratios at specific speeds. You may as well consider them to be different aircraft.

Feel free to publish your calculations using real aircraft performance. Every glider I have ever heard of, or flown, has a higher airspeed at their best L/D than at minimum sink.

Edited By Tony Smith 7 on 21/02/2013 08:36:39

[Tony Smith 7]

Posted by PatMc on 20/02/2013 19:45:04:

Power is not proportional to drag times speed squared. It's proportional to thrust.

Thrust = drag - at any speed.
Lift at any speed to maintain level flight = weight therefore is constant throughout.

Apologies if I've cropped off too much of your message, I was trying to retain only the bits where you explain how power is derived, and with the greatest respect I do not agree with your analysis. You assert that power related only to force, and not to the speed at which that force is moved. I'm afraid that is simply wrong.

You're correct in some of your other assertions, yes in level flight or when ascending or descending at a constant speed then lift = weight = constant (assume not turning either). Yes thrust must equal drag to maintain a constant airspeed, and yes drag will differ at different speeds (again in 1G flight). It's only the effect of speed on the final power that you are missing.

[Erfolg]

I am not sure that Simon is an engineer and may not be familiar with some of the meaning of terms.

That power is the rate of doing work.

[PatMc]

Posted by Tony Smith 7 on 21/02/2013 08:36:19:

Feel free to publish your calculations using real aircraft performance. Every glider I have ever heard of, or flown, has a higher airspeed at their best L/D than at minimum sink.

Edited By Tony Smith 7 on 21/02/2013 08:36:39

I don't have any real aircraft performance figures, do you ? They are not necessary when the point can be demonstrated without them using a formula as I have done. If you have any real life figures they can be put into the formula & will confirm my point.

I have never disagree that the best L/D is always achieved at a higher airspeed than min sink, in fact min sink occurs just above stall speed. The point I made is that the lowest total drag is always at best L/D, because the airspeed is higher doesn't mean that the drag is always higher.

[Tony Smith 7]

I'm sorry I must have missed the relevant formula in your posts, but even reviewing them I don't see where you have any expression that derives power as it's final output. I stand by my view that power is equal to force x speed, and therefore my previous theoretical calculation correctly illustrates the principle.

I challenge anyone to demonstrate that pushing a given force at say 100mph uses the same amount of power as pushing the same force at 1mph!

Regarding real figures, I can provide estimates from my last hang glider, as follows. Estimates again, but the comparision holds for any example where best L/D is faster than min sink, and where best L/D has a higher sink rate than min sink (the latter being kind of obvious).

All up weight 118kg, min sink 130ft/min @ 20 mph = 771W, best L/D 17:1 @ 30mph = 912W

[Simon Chaddock]

Surely everybody has to agree that with the same start conditions a glider at minimum sink will stay up longer than at best L/D.

If it stays up longer then its rate of doing work (power to fly) is also less.

It needs no formula to show this just an accurate measurement of air speed and sink rate.

In modern full size gliders this is done experimentally by the manufacturer over its full speed range and the results plotted as the gliders Polar Curve. The best L/D speed can be determined from this graph.

Typically the difference between the min sink rate and best L/D sink rate is about 15%.

In Tony S hang glider figures above it is 18%.

 

Edited By Simon Chaddock on 21/02/2013 13:04:28

[Sandy Colquhoun]

Just a thought on this subject- sink rate on fullsize sailplanes is measured while flying in a straight line. When you turn them they drop much faster, as models do(unless you`ve found a thermal) We can`t usually let a model continue in a straight line, and if you look at the altimeter readouts at the beginning of this thread you`ll see a continuous series of drop & levelling out. I`d guess the majority of the drops coincide with turns.

You may find the performance improves if the model is trimmed to fly in a wide circle, as per the free flight endurance models of my distant youth, to minimise the disruptive influence of stick waggling at the transmitter.

I`ll put this theory to the test when the weather warms up.

Cheers

[PatMc]

Tony & Simon, we are not considering a glider in this point of discussion & never have been !

We are discussing -

Posted by Simon Chaddock on 18/02/2013 17:46:53:

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!

Simon the formula in question is contained here & elswhere in one form or another throught my posts -

Posted by PatMc on 19/02/2013 00:21:57:

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.

[Tony Smith 7]

Posted by Simon Chaddock on 21/02/2013 13:03:00:

Surely everybody has to agree that with the same start conditions a glider at minimum sink will stay up longer than at best L/D.

If it stays up longer then its rate of doing work (power to fly) is also less.

It needs no formula to show this just an accurate measurement of air speed and sink rate.

I agree, it's self-evident. I think we might be straying into another one of these areas where aeromodellers inhabit a different parallel universe with different laws of physics and thermodynamics.

[Tony Smith 7]

Posted by PatMc on 21/02/2013 16:22:09:

Tony & Simon, we are not considering a glider in this point of discussion & never have been !

The powers that I estimated are those needed to maintain level flight, which I think what you are trying to derive. If you don't agree, then could you indicate which of my figures you dispute, is the drag figures for the two speeds, or the derivation of power from Force x Speed?

You say lift=weight and that is constant, in 1G level flight I agree

You say that drag is lowest at best L/D and I agree with that as well

Where we differ is that you assert that power required relates to drag only, and not to speed.

You are concerned that I am referring to gliding flight and I will accept a very small difference due to the glide angle, where in gliding flight the drag/thrust vector is angled down, and therefore not exactly at right angles to lift/weight. That would make my estimates slightly wrong for gliding flight, but correct for level flight.

[PatMc]

Tony, I’m trying to format this in such a way that it’s presented and answered in an order that makes sense.
So hope you don’t mind that I’ve edited it to suit.

Posted by Tony Smith 7 on 21/02/2013 11:54:38:

Regarding real figures, I can provide estimates from my last hang glider, as follows. Estimates again, but the comparision holds for any example where best L/D is faster than min sink, and where best L/D has a higher sink rate than min sink (the latter being kind of obvious).

All up weight 118kg, min sink 130ft/min @ 20 mph = 771W, best L/D 17:1 @ 30mph = 912W

Posted by Tony Smith 7 on 22/02/2013 08:41:53:

The powers that I estimated are those needed to maintain level flight, which I think what you are trying to derive. If you don't agree, then could you indicate which of my figures you dispute, is the drag figures for the two speeds, or the derivation of power from Force x Speed?

I don’t disagree with your figures as being the rate of energy being expended during descent. But they are the vertical component only.

If the hang glider descended by 500ft in a straight line trimmed for min sink it would travel 6770ft horizontally, the same descent at best L/D trim would give 8500ft horizontal travel.
Both would have used the same total energy.
The extra distance at best L/D is because the energy had been expended more efficiently – less energy has been needed to counter total drag.
At the lower sink rate L/D = 13.54 : 1
Therefore at min sink total D = 118/13.54 kg = 8.7kg = 19.2 lb.
At best glide angle total D = 118/17kg = 6.9kg = 15.3 lb.

To maintain level flight an opposite vertical energy would need to be derived from horizontal thrust moving the aerofoil at sufficient speed to create vertical lift.
In each case the thrust has to equal the respective drag to sustain level flight.

[PatMc]

Posted by Tony Smith 7 on 22/02/2013 08:41:53:

You say lift=weight and that is constant, in 1G level flight I agree

You say that drag is lowest at best L/D and I agree with that as well

Where we differ is that you assert that power required relates to drag only, and not to speed.

Drag is already a function of speed. Speed doesn’t have to be considered separately it’s already in the equation.

At any fixed weight AoA will determine the speed of an aircraft and the magnitude of drag. When thrust and drag are equal the result is 1G flight.
You agree that drag is lowest at best L/D therefore it’s only logical that you agree that this is the lowest thrust for 1G flight.

Or do you dispute that thrust = drag in 1G flight ?

[PatMc]

Posted by Simon Chaddock on 21/02/2013 13:03:00:

Surely everybody has to agree that with the same start conditions a glider at minimum sink will stay up longer than at best L/D.

If it stays up longer then its rate of doing work (power to fly) is also less.

Yes it will stay up longer.
Yes the rate of work is lower but the total work done will be equal and will have been expended less efficiently due to the higher drag.

However this has nothing to do with determining the minimum power required to sustain flight.