Influence of drag on climb rate

[davidleitch]

Guys

I have two electric gliders identical in all respects except that one glider has 10% more wing area (via a wider chord) than the other.

These gliders climb vertically under 7 cells. The gliders are 2.5 metre wing span with areas of 700-800 sq inches. MH32 profile. Organic 2.5 and 2.5 special if relevant.

I would like to know what percentage slow down in the climb rate will occur as a result of higher drag (induced I guess a profile should be the same) as a result of the higher wing area.

Can anyone tell me the answer or suggest a formula. The present climb rate is around around 3000 ft per minute (50 ft per second) if that is relevant.

Eventually I will get an altimeter to measure the difference but I am interested in the theory for the time being. Actually the bigger winged plane is 2.5% more all up weight, but I assume that climb rate fall proportionately to the weight increase, hopefully more than matched by a lower sink rate.

[BMatthews]

If it's TRULY climbing vertically then your wing's lift coefficient is 0 since no lift is required so the only drag you have is frontal area and wetted surface drag. Those are minimal though at the low speed you'd be climbing at. Weight would be the primary hold back. The excess thrust after the weight is countered would then be used to counter the frontal and wetted drag. Speed would be stable once that reached equilibrium.

Realistically if it's a more reasonable 45 degree'ish climb then the airfoil is providing some lift and enters into the mix. Depending on the lift coefficient and related drag coefficient they may both be close to identical. It depends on where on the lift drag curve the section is operating. Just because the nose is high that does not mean it's lifting its heart out. If the speed in the climb is a bit quick the wing may be operating at a lower lift coefficient than you think it would. This explains why some models climb quicker if you hold the nose down so they can speed up compared to hanging on the edge of the stall or otherwise keeping it in the slow speed, high lift and high drag region.

So to properley answer the question you'd need to provide much more and accurate info. Climb angle to within 5'ish degrees, speed while in the climb, model weight, airfoil chord and type for both models and wing and perhaps something I may have forggoten. Then you need someone to plug it all into the equations and decide what weight the wing is actually seeing in a steep climb like that. I'm a bit rusty so I only know what you need and not exactly what to do with it once you have it. Only then can you determine what difference if any there is.

[davidleitch]

It definitely does climb at between 80-90 degrees. These gliders are used in Australian 7 cell glider competitions. The rule is there is a 300 second flight task with 1 point deducted for each second of motor run plus a landing bonus. To win the competition in still air motor run times need to be under 10 seconds. As many motor runs as required are allowed. This means that the competition is generally determined by landing accuracy and then motor run times. A one second difference in motor run time can be meaningful.

The question for me, since I own both gliders, is whether the slower climb rate that the wider chord version must have due to its higher weight (2.5 % and higher drag) will be more than compensated for by the slower sink rate and easier landing.

The models are thrown from rest in the initial climb out, so they need to accelerate to max climb rate quickly.

The weight of the models is around 1.5 kg (53-54 oz). The aifoil chord is MH32 for both models. Speed while in the climb ranges from zero at launch to about 3200 ft per minute after say 3 seconds.

I have well over 100 flights on each glider, but I haven't used the wide chord one for 6 months, and am trying to work out whether its worth the effort of going back.

[BMatthews]

Oh, well in that case the weight is the only real issue for the climb.

I'm just guessing here as I have not seen either fly but if they were coming down from exactly the same height would the larger one sink slower? If so it may be worth using it for calmer days and use the other with it's higher cruise speed potential for the other days. Just a thought. Really you or your close buddies are the only ones that can tell if the loss of climb is worth the possible better glide.

[davidleitch]

My assumption is that the 8% lower wing loading on the 10% larger glider would result in a lower average sink rate, either because it can signal lift better via flying more slowly or just because the lower wing loading would lead to lower minimum sink. I agree that's debatable assumption.

Thanks for the thoughts. Its likely that pilot skill or in reality the lack of it is the true limiting factor here.

[portablevcb]

I assume from what you said the wingspans are the same and only the chord (and weight) have changed.

The weight difference will be the major factor (as Bruce said), however, from seeing the speeds some of these things have on climb there might be a small drag increase from the increased wetted area and what little lift the wing is generating. I suspect neither aerodynamic effect is significant.

Which does lead to pilot skills. You will probably have more drag created by control movements during the climb than from any other aerodynamic effect on the wing. ;)

charlie

[Sparky Paul]

I agree with Charlie.. any deviation from trimmed will add drag.. the tip vortexes off the control surfaces can be substantial.
If it were possible to do a standard climb without any control inputs, and compare the flight times with a similar climb with them, it might be possible to determine a difference, but I expect any such would be small.

[tim hooper]

OK so I'm a dunce, but how does the angle of climb have any effect on the 'lifting' effect of the wing?

Even in a vertical climb, won't the relative 'lift' generated try to push the model over onto its back, meaning that the wing which produces the most 'lift' will need more drag-inducing down elevator trim (or downthrust) to counteract that tendency?

tim

[davidleitch]

Tim

I too am a dunce and wondered about that. In practice it doesn't seem to. I hear what you guys say about fiddling with the trim as little as possible during the climb. In practice that is quite hard as the plane has to be thrown hard to give maximum initial speed and then a vertical climb right above your head is difficult to manage. And the climb is fairly short 0-10 seconds. So corrections will be in place for a singificant percentage of the climb.

What is obvious, particularly in retrospect, is if you have to make a major aileron correction in the climb you lose a lot of height. So that is something to focus on.

Well based on this discussion I won't let the drag from the bigger wing influence my choice. As it happens I prefer flying the smaller glider, for some reason I like the slightly higher airspeed and the flaps don't catch the grass on landing as easily. I suspect my comp scores might improve though if I switched to the bigger glider because average landing would be better.

Thanks for the input gentlemen, its appreciated as always.

dave

[BMatthews]

...Even in a vertical climb, won't the relative 'lift' generated try to push the model over onto its back, meaning that the wing which produces the most 'lift' will need more drag-inducing down elevator trim (or downthrust) to counteract that tendency?

tim

The effect is minimized if the model has been set close to neutrrally stable using the dive test or some other means. But you're right, it's still there. But the limited airspeed compared to level flight and a tiny bit of down trim will make up for it with minimal drag penalty.

A very interesting set of test flights would be to get one of those small wrist watch recording alitimeters and put it into the model. Then do some vertical climbs with a flaps or spoilers out dive back to the launch point. Follow a few of those with a shallower angle climb of perhaps 50 to 60 degrees where the speed is higher. Same instant decent.

I can't help but wonder if the model would climb higher with a shallower angle so the flying speed is higher and the wing can help out. The issue would be to see if the vertical component of the angled climb is faster than the fully vertical climb.

To optimize the test some prop selection work may be required to obtain the best speed in the shallower climb.

[davidleitch]

Bruce we have done some of those tests with a zlog altimeter. In this class competition with the geared motors, high diameter props we use, a vertical climb has been shown to produce the lowest motor runs. I run a plettenberg 7 cell fb5 motor with gp2200s, 16x13 prop at 115 amps. As mentioned competitive climb rates are always in excess of 2500 ft per minutes and can be as high as 3500.

I have only used a borrowed zlog on one occasion but its on the shopping list for the next couple months. They can now be plugged into a palm pilot or equivalent and climb graphs shown at the field. A big help in prop selection, climb angle choice and also in showing sink rates (thermals).

[BMatthews]

....16x13 prop at 115 amps......

:eek:.... I am not worthy to comment further..... :eek:



:D At that current I would imagine straight up IS the shortest way. :D

[vintage1]

I did some calculations on almost exactly this.

In general a glider wih - say - a 1:10 sink rate - is experienceing drag to the extent of 0.1 of its weight in the glide.

IIRC the induced drag: profile drag is of similar orders at 'best glide speed' so if you point it straight uup and (listen carefully Tim) adjust the angle of attack to give zero lift, and thus minimise induced drag) you might expect drag at this speed to be somewhat lower - e.g. maybe 0.05 of the weight.

So the power to go straight up at glide speed is only 5% drag related - the rest simply relates to the model weight. since the thrust is largely overcoming that..and this will remain true for vertical velocities up to maybe two times glide speed...at whih teh profile drag will start to incraese very sharply.

If we take an average sort of glide speed as 15mph and rotate the model to go straight up, the gain in potential energy is equivalent to 33W/lb, wheras that due to the drag losses is 3W/lb approximately at C/L = 0.1. In practice it will be even less dure to reduction in induced drag.

Thats a climb rate of 1320 fpm.

If you double up to an upwards speed of 30 mph (2640 fpm) its still only 66W/lb for the climb, and maybe 5-10W/lb for the drag.

At best only 10%-15% of the total, and with a slippery glider, less than that.

With motor, oix and propellor efficiences all at about 80% each for a decent setup. thats about 51% overall efficiency, meaning an input power of approximately 70W/lb for 15mph straight up, and about 150W/lb for 30mph straight up.

Very achievable figures.

Also gives a clue as to how to prop teh model: You need e.g. for a 2600 fpm climb to go to
- 150W/lb
- thrust equalling model weight at 30mph forward speed.

So a pitch speed of at least 2-3 times the glide speed - 15mph - is needed - say 50mph pitch speed estimated.

[davidleitch]

Vintage1.

I haven't checked the calculations. They sound reasonable. In a race 10-15% advantage can be very significant. These contests are won and lost by small amounts.

If you consider a 1 metre v a 3 metre glider it seems intuitively obvious that the 3 metre glider will have significantly more drag. It will also have a lot more lift but that is largely irrelevant in a vertical climb.

I am using a 2.5 metre glider when most competitors are in the 1.8-2.2 metre range. Thus I am already at a drag penalty. Just was trying to work out the additional penalty from the 10% wider chorde Organic 2.5 sp. I think the consensus is the penalty is so small that I shouldn't let it drive the decision.

[vintage1]

Aha. I see where you are coming from.

What you really need to do is to work out the overall profile in terms of sink rate drag and climb speed.

There has to be an optimum pint where slightly more drag in climb is balanced out by lower sink rate.

Competiyon sailplanes are simply something I have never done.

However, waht looks extremelyt relevant is high power to weight ratio, and efficiency all round, as well as low airframe weight.

So I'd expect to see LIPOS, Hackers and carbon fibre in the top models, as well as very clean aerodynamics...?

[Salto]

However, waht looks extremelyt relevant is high power to weight ratio, and efficiency all round, as well as low airframe weight.

So I'd expect to see LIPOS, Hackers and carbon fibre in the top models, as well as very clean aerodynamics...?

Ah yes V1, but this popular 7 cell class in Australia only allows 7 Nicad or NiMH cells. What stops me getting involved in this class is the absolute requirement to have the very latest motor/ESC/pack in the model. Without this, the time penalty in motor run time will mean you are just not comptetive. What started off as a class intended to limit cost, has ended up requiring a lot of dollars.

Graham.

[BMatthews]

Yeah, that happens a LOT. The only way to limit costs and KEEP them limited would be to specify a watts limit and have each model stuff it's nose into some form of on site dynamometer just prior to flying. Otherwise it doesn't matter what you do folks will find a way around the limits.

I suppose another way would be to have the contest management supply a limit fuse on the day of the meet and it must be checked before each flight to be sure that the current limit and cell count is not being exceeded.

Or you can rely on the good intentions of human nature. That's fine for most but sadly it only takes one now and then.

[davidleitch]

Ah yes V1, but this popular 7 cell class in Australia only allows 7 Nicad or NiMH cells. What stops me getting involved in this class is the absolute requirement to have the very latest motor/ESC/pack in the model. Without this, the time penalty in motor run time will mean you are just not comptetive. What started off as a class intended to limit cost, has ended up requiring a lot of dollars.

Graham.

Graham and others:

That is not at all correct. Every 7 cell contest I have been to over the past two years has been won by a different pilot with a different plane and motor combo. If your flying is good enough you can place in the top 3 with a combo as simple as an Omega 1.8 + Mega 16/15/5 direct drive. I've seen it done. The main requirment is learning to land well, particularly in the wind and to identify lift. After a while you will want better equipment and it makes the difference between 1st and 3rd but no more than that. I have excellent equipment and practice pretty hard. I still struggle to get in the top 10 because I don't fly well enough. A Hacker B40 6L and 7xgp2400 and controller is around A$500. Nothing more is needed power wise to come first. Better batteries come along from time to time. The airframe needs to be only a simple 2 metre rudder elevator design perhaps with a landing aid (flap,spoiler).

dave

[Salto]

I knew that would get a bite out of Dave - I should have kept my mouth shut!

Dave, 7 cell contests I've seen have indeed revolved around the landing scores, a point here, a point there. There's obviously alot of skill, and most likely some luck involved in getting good landing scores. But surely, if you can achieve a motor run consistently a few seconds less than another competitor, then with equal landing skill you are on average going to come out ahead?

I don't have any data to back up this assumption, but I've heard many competitors talking about their expensive equipment, which is what put me off getting involved.

Out of interest, do you have any info on what set-up is winning the contests. It would be interesting to see if there is any correlation between placings and equipment cost. I strongly suspect that there is.

Graham.

[davidleitch]

As I say every contest is won by a different setup. Probably more than half are won by self designed planes around 1.8 -2.2 metre span powered by Hacker b40 6L or 5L + 4.4 gearing and a 14x10 or 15x10 prop using traditionally cp1700 batteries however now gp2200 is better.

As the batteries get better more motor and wing span possibilities open up.

The difference between a Hacker B40 70 amp setup and a Plettenberg 115 amp setup is less important than landing skill.

As in all sports you will find that the better pilots end up getting themselves good equipment. 7 cell contests are an excellent way to improve flying skills, meet people, and give you a goal to aim for. Highly recommended particularly if you are thinking of going to the Easter meet where they will be the major item on the agenda.

If you want to get into an area where equipment really does matter try F5B.

Dave

[pocket rocket]

hello David

have you got the altimeter yet ???

I've had a RAM from www.soaringcircuits.com for a few weeks now. lots of useful info available about plane's performance under power and glide.

in fact too much info !!!!! need some sort of analysis program other than my brain.

on my 12-cell High Aspect the best climb rate was from a 16x16, but it couldn't get enough speed from a hand launch to stop the prop stalling.

how do you measure angle of climb ?? by guess or from an innocent bystander's opinion ??

3000 fpm sounds good but it's only 50 fps which is only 34 mph.

I've seem 5000 fpm, reading from the slope of the graph from the RAM printout, as an absolute best climb rate but I don't know how to estimate the climb angle accurately.

my guess is about 60 degrees from horizontal.

and the way to win is the thermalling ability, launch into lift and stay there. look at the F5J Extreme results, some of us did 10 minutes from 6 sec motor run (not 7-cell though)

why don't you use a discus launch ?? you'll get to 150' with no motor run and it's only a 5 minute flight.

Philip

[davidleitch]

hello David

have you got the altimeter yet ???



No. But I am going to order today. I need to get a Palm to do analysis at the field as well. So the total budget has to be managed.

on my 12-cell High Aspect the best climb rate was from a 16x16, but it couldn't get enough speed from a hand launch to stop the prop stalling.

how do you measure angle of climb ?? by guess or from an innocent bystander's opinion ??


Guess. It ends up pretty much over my head and looks vertical.

3000 fpm sounds good but it's only 50 fps which is only 34 mph.

Rember only 7 cells



and the way to win is the thermalling ability, launch into lift and stay there. look at the F5J Extreme results, some of us did 10 minutes from 6 sec motor run (not 7-cell though)

Agreed. And in comps people sometimes record 1 second motor run. Nevertheless a comp is held over an entire day, and its the still air performance that is of relevance to this discussion.

why don't you use a discus launch ?? you'll get to 150' with no motor run and it's only a 5 minute flight.

Never thought of that. Motor has to run for at least one second. Just don't feel like launching my 1.5 kg organic via discus.

[davidleitch]

Please to report:
1 Acquired a lolo2. Not the cheapest but an excellent piece of kit.
2 Climb rate with the standard Organic 2.5 (chord 190 mm) was around 2300-2500 fpm. Unfortunately there is imprecision on when the climb starts and finishes.
3 Climb rate with the Organic 2.5 sp (chord 210 mm) was around 2200-2400 fpm

I managed a 9 second motor run for a 5 minute flight before 7:00 am on an overcast Sydney morning on the 2.5SP. The best I have managed with the standard is around 8 seconds and that was at 8:00 am. All in all it doesn't appear the wider chord does create any more significant induced drag.

The wider chord Organic is slower in the air, is more agile in turns and its big flap really will stop it for landings. The next problem is learning not to over use the flap when there is any wind, but that's another story.