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

[Erfolg]

I am still at a loss to understand as to what you think you can prove, even less predict that your objective has been achieved. Without trying to be offensive, the goal is woolly.

I have noticed a number of relationships being banded about, in my opinion in a non structured manner. Plucked out of the air (or possibly a text book) and seen as the touchstone to define an outcome.

I really am not trying to upset anyone, but I think a few things need to be recognised, as to why you will never have a repeatable answer, or be able to say something is proved.

The first is that the data for airfoil sections, assumes that the wing is infinitely long.

Perhaps more relevant, at high AoA, the wing tip losses are at the highest, reducing with AOA.

Also important is the relationship between wing and fuselage, normally set so that least drag is generated at the flying speed chosen for operational needs, which is unlikely to be best L/D. There is a consequence at all other speeds, the cross sectional area will apparently change, with the alignment to fluid flow and the Cd will also change in value.

We all know in the real world, air is not in a steady state condition. Its behaviour is complex, to the extent, the smaller the packet considered, the less predictable it is. Although we may consider it is blowing say from the west, in reality its direction is changing to a degree in all axis.

The more you look at the detail of the problem, the more details you will find.

Again as an ex-engineer, I was always aware that many (that should be all) relationships used by me were approximations. It was up to me to decided if they were adequate for my needs. Typical was the "A" level maths problems mentioned earlier. It is apparent that they did not replicate the real world, as bicycles, do not coast, from hill top to hill top. Missing were simple things like friction, then there is drag all sapping energy. If peddling, the power input is not even constant over one peddle stroke, where wheels as a consequence, accelerate and decelerate with this power input and the real model builds, until you say, yeas, that replicates the real world problem. A sure as eggs are eggs, we are still nowhere near a good model. I have omitted frame flexing, torsion with power etc.

What I find most frustrating, what ever you may record,cannot be predicted, the variables are to many. Everything about an observation will be meaningless.

If we were good mathematicians, we would be sitting down with our differential equations, Laplace transforms, and seeking to model outcomes.

As an engineer, I ask myself, how will I know, I have succeeded. The answer I get back, there is no answer that I could justify, as demonstrating that my supposition is correct.

If I look at commercial or competition aircraft, it becomes easier to justify a design. It will be capable of making so many flights, point to point in a day, using so much fuel etc. Competition gliders also come with a predictable glide ratio at a certain airspeed and it is doing better than other sailplanes in a competition regime, which demonstrates the claim.

Edited By Erfolg on 23/02/2013 21:37:12

Edited By Erfolg on 23/02/2013 21:37:42

[PatMc]

Elfolg, you are no stranger to introducing esoteric topics. I find it strange that because your opinion is that this topic has no practical value you deride it. If you're not interested in it then ignore it. Without being offensive, I certainly prefer to ignore many of your posts for this reason.

You mention that the goal is woolly, that you don't seem to understand what it is seems to me to be a shortcoming in your thinking not the discussion that we are having.
The bulk of your post demonstrates that either you don't understand or don't appreciate what is being discussed.

 

Edited By PatMc on 23/02/2013 22:20:43

[Simon Chaddock]

If I may draw your attention to the title of the original post.

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

I am not trying to calculate anything from first principles - it is almost impossible - but simply to compare.

The only calculation I perform is on the experimental results I have obtained.

[PatMc]

Simon, with a programmable Tx & some of the measuring equipment that's available quite cheaply today it should be feasible to make meaningful measurements in order to optimise the performance of an E-glider to one's own requirements without much difficulty.

BTW you do remember that it was you who broadened the topic's beyond the OP title, don't you ?

[Tom Wright 2]

Simon your OP has certainly created some interesting comments on the basic theory required to ascertain, or should I say predict the factors that influence performance.

How ever If I may say the OP question cannot be answered without further qualification that would go something like this ......

What is the typical min sink rate for a 2M glider ? When the wing loading is......The section is.....the wetted area is......and the AR is.....And assuming the wind speed is constant at .......And the air is neutral in terms of lift and sink .

Maybe this is another version of Erfolg's comments?

Tom.

.

[Tony Smith 7]

Posted by PatMc on 23/02/2013 18:42:27:

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

No, I think our only difference is that you believe power to be proprotion to force only, therefore power to overcome drag is determined only by the magnitude of such drag and independent of the speed at which that drag applies.

Your assertion that energy lost in descent is not related to the energy needed to maintain height merely confuses the discussion, as well as being wrong, and distracts from the main difference.

Power equates to force x speed, and this has been proven and relied on in engineering probably for a couple of hundred years. I am surprised that once it is distilled down to this you do not agree. I think it's now more a matter of dogma than physics or engineering.

One last time, do you REALLY believe that it takes the same amount of power to propel a force of 1kg at one mph as would be taken propelling a force of 1kg at 100mph?

[Tony Smith 7]

Or to put another way, what do you calculate the power would be to maintain level flight for each of the conditions I quoted?

(1) 20 mph L/D 13.5:1

(2) 30mph L/D is 17:1

All up weight 118kg in both cases. I offered my figures which you dismiss. What are yours?

[PatMc]

Tony, I will open a new thread in order that we can continue & hopefully conclude this particular discussion without causing problems to the OP topic. In order to maintain some continuity I'll use quotes from existing posts in this thread.

I won't open the thread until I have some spare time which might be this evening or later in the week.

[Erfolg]

Patmac

There is a reason that I do not think there is any one answer, the range of values for sink rate of 2m gliders, just from observation is very wide.

If you go back to the days when bungee launching was prevalent, the difference in time between one launch and the next was significant. Now I do not remember with any precision typical times, only vaguely that about 2 minutes to about 3 minutes, without contacting definite lift. The launch height had many variables, dependant on bungee in use, the wind strength and how effective your launch technique was on any one launch. There after, how many turns you made, the local conditions of pockets of air etc.

The other issue, is that gliders were not set up for minimum sink, you went looking for lift. Sink rate was traded for distance.

I maintain, that the variables surrounding the parameters of the flight of a electric glider are so many and variable in there nature that any prediction based on just staying up are not practical. The main reason being that many models designed for a high L/D ratio, will be going backwards, and out of sight on most days.

A calculated design based on defining duration, needs a lot more factors in the calculation than are being considered, if it is to produce a result which is in the same ballpark as reality.

It should be realised that many of the early calculations undertaken by students are to introduce concepts and how they play out in the real world. Later the mathematical models become far more complex as more factors and the inter relationships are considered, so as to make realistic assessments.

How viable it would be to determine sink rates with telemetry, is just as fraught, on a similar set of variables. Unless that very long hanger can be found and used, under controlled conditions.

The problem then becomes, now I know a sink rate, can I use it in the outdoors to predict anything, particularly if other issues are ignored.

[Mike Rolls]

Very interesting thread - I was intrigued by some of the sink rates quoted as an illustration - perhaps - of piloting techniques? The lowest sink rates I have come across are in the F/F F1A class (a/2 to we old fogies) where good models achieve around 1 foot per second in dead air.

Mike

[Mike Rolls]

Oh - I should have mentioned - wing loading for these models is 4.5 to 4.7 ozs per square foot.

Mike

[Erfolg]

Our club is a 50/50 type club, in that approx 50% are FF and the rest RC. Although we do not fly together anymore, as our fields are just not big enough for FF. I do remember what it was like when we did fly together.

Essentially, the models would circle in a packet of air, drifting down wind with an essentially constant airspeed. With the modeller following on behind (& they were much younger then), clambering over fences, jumping ditches.

The penny has just dropped with respect to Tony's argument, he is probably correct. Dependant on the airfoil, at least in principle, the most efficient speed (conceptually) is probably not at the best L/D point, but a little lower AoA. At some point where there is optimal drag, relative to lift. Where the drag bucket is pretty constant, other than the turning points.

At a theoretical level, a lot of factors would need considering, under a range of operating conditions. I think Tony's argument is along the lines. In level flight, weight = lift necessary to maintain height. At high angles of attack, such as at max L/D the drag is at a higher value, than when flying a little faster, hence less energy is required is required in this condition. If tip losses are also factored in, this induced drag also reduces, compared to max AoA. Against this the drag force from the body increases with speed.

At a practical level, i still cannot see how you can verify for any one flight or point in a flight, you have optimised the trim to minimise energy requirements. I think this is where competition flying comes from, a verification, that on a particular day, someone and their model was better than others.

Edited By Erfolg on 27/02/2013 21:27:29

[Mike Rolls]

We always used to trim by reducing tailplane incidence until just at the stall - packing up the tail TE, then removing the last piece of packing or possible and/or tightening the glide circle.

Mike

[PatMc]

I decided to make one more post on the min sink versus best L/D sub topic rather than bother with a fresh post. I hope no one minds.

Posted by Tony Smith 7 on 24/02/2013 09:40:59:

Posted by PatMc on 23/02/2013 18:42:27:

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

No, I think our only difference is that you believe power to be proprotion to force only, therefore power to overcome drag is determined only by the magnitude of such drag and independent of the speed at which that drag applies.

Tony,
You are quite right.

Sorry, I see where I went wrong in assuming that thrust (to overcome drag) alone was the parameter proportional to required power.
I should have twigged as I’ve pointed out when people have quoted static thrust that it isn’t a useful guide to the power of a motor/prop combo in flight.

However, I didn’t dismiss your figures. I’ve already said that I don’t have any measurements of my own but, as I posted previously, I used yours to calculate both total drag figures. I thought that since drag was dependant on speed it (drag) didn't have to be considered any further & didn't appreciate it's significance to calculate power.

By a coincidence we were at an inlaws family do the other day where I was chatting to a distant relative of my wife who had come up with his daughter from Bristol area for the occasion. He’s nearly 90 used to be a full size gliding instructor, at one time worked for De Haviland & lectured on aerodynamics at a college (I think in Bristol). I’ve met him a few times previously & he always asks about my models, I think to get away from the usual family gossip.

Anyway I mentioned this thread & he was immediately interested. He told me that he’d once been involved with designs for man powered flight & that getting the best results with the minimum power was obviously paramount.

I quoted your figures to him & he confirmed where I’d been going wrong but also pointed out that a weight shift hang glider isn’t representative because the two trims are with different cg positions which accounts for a 50% increase in speed from min sink to best L/D whereas it’s normally between 18% – 25% in conventional gliders using elevators.

He gave me a bit of a lecture on aerodynamics which was interesting but a lot was over my head. The upshot (As I understand it) is that for flight with min power at best L/D it should be at a speed close to best min sink speed by careful airfoil selection combined with low form drag design also the difference in ratios of the two L/D should be as high as possible. Otherwise min power needed would be at min sink trim & would be used less efficiently.

He also emphasised that the above only applied when camber changing flaps were not used on the wings.

[PatMc]

Back on track - I decided to get one of the HK Altimeters so ordered it last Sunday. It arrived on Tuesday & I tried it out in my EasyGlider on Thursday with the results from 4 launches shown below.

It was a dull, grey day with virtually no wind or any sign of lift. The inconsistency is mainly because I was trying to time the power run to 30sec with a seperate countdown timer that wasn't very well fixed in place.
I later had 3 flights with Igor taking readings using the Eagle Tree max alt recorded altimeter & jotting down the results.

 

Edited By PatMc on 01/03/2013 20:37:26

[PatMc]

Forgot to mention I got an email ad from HK that included this Altitude Limiter which I would have prefered over the simple altimeter. Hopefuly they might soon become available via the UK warehouse. One of these should make it easier to achieve more consistent power runs without having to keep glancing at or listening for a timer & also it would allow concentration on getting the power to glide transition right without unwanted distraction.

[Erfolg]

Patmac

Something similar is used by the competition flyers in our club. Which I understand cuts the motor at 200m and with a 30 sec max motor run. It seems freely available in competition circles and a specific requirement.

[PatMc]

Erflog, I know of a couple of alt/time limiters that have been available for some time but they are relatively costly just to be used for a bit of fun & personal satisfaction. IIRC one available in UK costs about £80 the other is a bit less but from the US & usually hit for duty & tax.

[Erfolg]

Ptmac. briefly looking at the graphs you have posted, the slopes are far more consistent than i would have guessed or believed possible. Makes you think, the conclusions, i am not all sure, but interesting and somewhat surprised.

[Peter Griffiths 1]

I reckon the sink rate for my Odyssey 2 metre esoarer to be around 2 ft/sec. This figure is derived from data provided by an onboard altimeter from rc-electronics.org (available from esoaringgadgets.co.uk). The altimeter device I have only records height and time, not speed or distance travelled, hence its performance metric is sink rate. To calculate glide angle or glide ratio would necessitate flights being measured along a fixed course on the ground, such as a triangle between pylons, or, more accurately, by an onboard GPS module by which to determine distance travelled from motor cut-off to landing.

The problem about obtaining reliable test figures for glide performance is the difficulty of controlling atmospheric conditions. Clearly, lift and sink should be avoided, but other variables such as wind speed, humidity, temperature and launch height have not been standardized when measuring model characteristics. The best one can do is pick a time of day at a flat site when the air is judged as stable. Also, the way the model is flown affects how it will descend. During a flying test, control inputs should be minimal as moving aileron, flap, rudder and elevator surfaces away from their neutral positions creates drag. Although a straight line flight test over level ground would be ideal in order to establish glide performance, practically, we have to opt for launch and landing points to be the same and to fly using lazy circuits as much as vision will allow.

The graph below shows my Odyssey 2M tested in such a manner on a summer’s evening close to sunset when the air was calm and dry and thermal activity had ceased. I set the height limiter switch to 650 ft (200 metres) in keeping with esoaring competition rules. The logger showed the throttle switching off at 652 ft with a climb time of 29 secs. The glide duration was 5 mins 49 secs. I left the glider’s controls largely alone and did not use flap or spoiler on landing. The logger gave the average sink rate over the flight as 1.84 ft/sec. Dividing height loss by duration (i.e. 652 ft/349 secs) gives a similar value of 1.87 ft/sec. The perfect theoretical sink-rate graph is a straight line from start to finish of the glide and what I obtained came close. A graph showing deviations indicates zooming or diving caused by turbulent air or pilot input and must be discarded for the purposes of testing the efficiency of the model.

A previous posting questioned why maximizing the glide ratio matters. In glider design, this is the holy grail and what has driven improvements in design of full-sized and model gliders over the decades! In duration or distance competitions, all other factors being equal, the glider with the highest distance to descent ratio, i.e. the lowest sink rate, will be the winner. A glider is powered by the the steady force of gravity and is always sinking in the surrounding air, even in ascending wave or thermal. As gravity cannot be altered, all the glider designer can do to is experiment with weight, aerofoils, streamlining and materials in a continuing quest to improve lift and reduce drag.

[PatMc]

Posted by Peter Griffiths 1 on 11/08/2013 11:14:35:

A previous posting questioned why maximizing the glide ratio matters. In glider design, this is the holy grail and what has driven improvements in design of full-sized and model gliders over the decades! In duration or distance competitions, all other factors being equal, the glider with the highest distance to descent ratio, i.e. the lowest sink rate, will be the winner.

Hi Peter, welcome to the forum.

The figures for your Odyssey are interesting but the graph isn't displayed. You need to create an album then upload your image(s) in that before inserting them in a post or (I think) you can do it from one of the photo sharing sites - Photobucket, Flickr etc

Re the above quote : I may be wrong but think that in competition flying distance to descent rate (best L:D ratio) is more important in full size gliding than in models because the former generally have distance targets as the goal. In model gliding most of our comps are about, or have, a time duration element. The best L:D ratio does not give the lowest sink rate.

[andyh]

Posted by Peter Griffiths 1 on 11/08/2013 11:14:35:

all other factors being equal, the glider with the highest distance to descent ratio, i.e. the lowest sink rate, will be the winner

highest distance to descent ratio is best-glide, not min-sink

even then, that's not the full story, depending on the thermal strength & wind speed on the day, the best L/D at the appropriate *speed* is the critical factor. you could have the best L/D in the world at, say, 30mph, but if your polar drops off sharply as speed increases, then you're not going to do too well flying into a 30mph headwind. google "speed to fly" & "mccready rings"

Andy

(Hangglider/Paraglider Pilot)

[Erfolg]

I do not know if your figures are exceptional, although it is easy to observe that glass moulded models have a far superior to traditional models.

In part it is the ability to cover large distances, for little height loss, it is also the ability to loiter again with low sink rates. The basic design and accuracy of manufacture and the standard of fits etc., is superior to the best traditional models. This design incorporates the ability to drop the trailing edge, reflex the section, use crow barking and of course a really good Tx, to make it all possible.

Then there is the magic ingredient, a modeller that knows his model, what each twitch means, and how to respond. A modeller that can consistently fly the slot out. They also hit the spot every time, as this is where the contest can easily be won and lost.

As to your model,we need data from other modern gliders under the same conditions, to have any view.

[Peter Griffiths 1]

 

Here is the graph from my previous post (assuming I've fathomed the upload procedure).

 

<img src = "http://s1290.photobucket.com/user/pvg47/media/Odysseysink-ratetest_zps1ec46b4d.jpg.html">

 

 

 

Edited By Peter Griffiths 1 on 13/08/2013 20:48:19

[Peter Griffiths 1]

Here's my third attempt at posting the graph (it may show a web link rather than an image):

http://s1290.photobucket.com/user/pvg47/media/Odysseysink-ratetest_zps1ec46b4d.jpg.html