Wing loading at different wind speeds question

[sbxcflyer]

A very respected soaring fellow asked about the calculation and conversion of wing loading. It brought a question to my mind about how to determine the correct wing loading for different wind speeds. I also wonder how to determine the wind speed at different altitudes. Does anyone have a rule of thumb for loading in the lead based on ground or advertised or perceived wind speed.
Thanks

[Ollie]

Sailplanes polars:
http://home.att.net/~jdburch/polar.htm

Then apply PC Polars with your RC sailplane:
http://my.athenet.net/~atkron95/pcsoar.htm

The performance of an airplane depends on the density of the air in which it is flying. It depends on temperature, altitude, atmosphere pressure, etc.
http://www.digitaldutch.com/atmoscalc/

[rogerflies]

I don't see any relationship between wind speed and wing loading. The plane flies in the air at airspeed. Each airfoil has a "best" airspeed for a particular wing loading.

If you need more "penetration" on a windy day, you simply need more airspeed. You can get it by adding ballast, but you'll use more potential energy (read: altitude) to make the plane go fast.

To go fast without using more potential energy, you need to reduce parasitic drag to a minimum. That means getting rid of mismatches on the surface that cause disturbances in the airflow, having fairings around pushrods, or anything else that makes the air flow around the plane with the least distrubance. IOW, a "clean" plane.

You can also go faster by reducing the wing loading, since that reduces the "induced drag", which results from producing lift. A heavier plane has more induced drag because it must fly at a higher angle of attack to produce the required lift. A plane flying straight down has no induced drag, because the wing doesn't need to produce any lift at all. Until you want to pull out of the dive, of course.

If you want to "penetrate" on a windy day, just put in a little down elevator when you need to go faster. You'll give up a little altitude, of course. But you'll only be giving up altitude when you want to go faster, not all the time you're flying.

Remember, heavy planes fly fast because they must. Light planes fly fast because they can.

Roger

[jbeanelliott]

The following website has a good explanation with graphics of how ballast is useful when flying in a headwind:
http://home.att.net/~jdburch/polar.htm

Excerpt from above:
A different polar curve exists for the same glider at every gross weight. As the weight increases the polar moves down and to the right and becomes a little flatter, but retains approximately the same shape. Both airspeed and sink rate increase for equivalent points on the curves, but their ratio remains the same. Both heavy and light glider achieve the same best glide ratio, as shown here, but the heavy glider does so at a higher speed. This is the reason for carrying ballast to improve glide performance when the thermals are strong enough, even though the climb rate suffers.

[rogerflies]

"...ballast is useful when flying in a headwind:" really should read "ballast is useful ONLY when flying INTO a headwind:".

The plane will continue to fly at the higher airspeed AND higher groundspeed when you turn crosswind or downwind. This offsets any advantage gained from adding ballast to increase penetration. You'll be squandering the potential energy you've got, and making it more difficult to get more.

I can see adding ballast to get the plane to fly at the airspeed where its airfoil has the best performance, but adding ballast beyond that is counterproductive.

Use the elevator to increase airspeed when you need to. If the plane is built clean, you can probably get back a good part of the altitude you'll lose with a gentle climb at the end of the speed run, and you'll still be able to slow down to a speed comfortable for working lift again.

"I also wonder how to determine the wind speed at different altitudes." I think you could find this type information in a book on weather written for pilots. You could also look at aviation weather charts which give the wind speeds at various altitudes and on the ground. A Google search on "wind speed altitude" will probably turn up some good stuff.

Roger

[OVSS Boss]

All have stated good points, and I am going to go to these sites to see what they have to say. But Roger hit the nail on the head, adding ballast to optimal performance, i.e. you move where you need to go when you need to, is the goal, not going "fast" in TD, F3B is different but that is another talk. From that point on, adding weight, you are just burning rubber so to speak instead of constructively moving effecient.

Second thing I see, the research is good, but what are your practical observations Dave? Wind at altitude is always there, even when it is nearly calm on the ground. Do not make the mistake to out think what you are dealing with. Most modern TD ships are very clean, dramatically clean really, ballast is not needed in most circumstanses we fly in. Ships such as the Pikes, Fazer, Tragis and others can easily handle winds up to 15-20 mph without ballsat. Remember that day last summer it was blowing a gale in FTW and I had that kick arse contest with the Superior? That was no ballast, yes, I might have loaded up if I could have, but by mistake had none with me. I learned that just like a Fazer, it was doable and if you flew smart with a plan (like getting to the tree line with altitude to spare) you exploited the opportunities given you by nature and your skill.

Do your research, but remember to not let it cloud just what is there, you do not need to go fast, just fast enough.

Marc

[Sparky Paul]

I've noticed a relationship between wingloading and wind speed on the slope.. a slippery plane will fly in much higher winds than a draggy plane with the same loading.

[David Taylor]

Roger, youre forgetting the effects of reynolds numbers....with a light glider when you use down elevator to get more airspeed you are moving farther away from the best glide airspeed and your parasitic drag goes up considerably. When you add ballast the entire polar for that glider moves to the right (faster), stall speed, minimum sink, best glide, and all speeds are now faster.... and because you are flying faster your reynolds numbers go up and the glider is flying more efficiently at a faster best glide speed which cancels most of the parasitic drag.
Ballast is your friend with todays modern ships if you know how to use it.
There is VERY little difference in the sink rate of the glider with ballast but it will travel a lot farther and faster with ballast.
The only real down side to ballast is that when thermalling your min sink speed is also higher so you will have to thermal faster with a bigger circle.... so in light lift you might have some problems (IE windy days with light lift) and have to decide which you want more, penetration or thermalling.
If you don't believe me just check out any modern full scale sailplanes polar, you lose a VERY SMALL amount in your overall sink rate but get huge returns in speed, in fact most all modern gliders get their advertised best glide ( L/D ) when fully ballasted.
So in summary, ballast is useful in all conditions except when thermal's are small and light (but if their very far apart then you may want to ballast up some so you can cover more sky looking for them).

Dave T.

[rogerflies]

Parasitic drag increases considerably with speed, period. It doesn't matter if the speed is obtained by ballasting or changing the pitch. Nothing "cancels" parasitic drag. That's why it's so important to build clean ships.

Every plane is designed to perform best at a certain weight, and that's the weight you should fly it at under ALL conditions. The plane cannot tell if it is flying in a moving mass of air (windy day) or a stationary mass of air (calm day).

"When you add ballast the entire polar for that glider moves to the right (faster), stall speed,.." The last thing I want is a higher stall speed. :)

Roger

[David Taylor]

"It doesn't matter if the speed is obtained by ballasting or changing the pitch."

Sorry Roger, you need to study more......changes in pitch, changes the point at which you are flying at on your gliders polar and there is only one best glide speed (pitch angle) for your glider, or you can increase your best glide speed with ballast (go faster with same sink rate). By the way your best glide speed changes depending on whether you are in bouyant (slower) or sinking (faster) air and headwind (faster) VS tailwind (slower) among many other factors, but im not going to get into that now.


"Every plane is designed to perform best at a certain weight, and that's the weight you should fly it at under ALL conditions. "

NOT!, try saying this to any full scale sailplane pilot that races or just fly's X/C and see what he says about his 500-1500 pounds of ballast on board!.

As for a higher stall speed, with a little practice you will be able to tell when your glider is getting mushy, slow to respond and ready to stall.
The difference in landing speed in calm conditions with my gliders (full flaps) is about 5-6 MPH unballasted to 6-8 MPH ballasted, no biggie, but seeing as how I usually ballast in wind I can land ballasted at 3-4 no problem into the wind.

Basically if you want to go faster efficiently add ballast and if you want to go faster inefficiently push the nose down.

Dave T.

[David Taylor]

A very respected soaring fellow asked about the calculation and conversion of wing loading. It brought a question to my mind about how to determine the correct wing loading for different wind speeds. I also wonder how to determine the wind speed at different altitudes. Does anyone have a rule of thumb for loading in the lead based on ground or advertised or perceived wind speed.
Thanks

sbxcflyer, you are going to have to do a little experimentation with your glider to find whats best for different wind conditions, you can use a calculator and figure out what the wing loading is for each slug of lead you put in your glider, if you know the weight of each slug, but you are going to have to fly your glider in windy conditions and see what amount of ballast you liked for those conditions (wind, temp., air pressure), write these down in a log and the next time you encounter similar conditions you can check your gliders chart for those conditions, ballast it as before and it should fly the same as you remember.

As for wind speeds at different altitudes, not only can wind speed change but the direction can change too, even a 180 digree change is possible!
You can check avation sites to see the changes in velocity and direction but even these are basically educated guesses.
Your best bet would be to do some searches and reading on speed(s) to fly.
If you ballast properly for the conditions and use speeds to fly properly you will be doing better than most. :D

Cheers, Dave T.

[dephela]

I don't ballast for wind but I do fly heavy sailplanes.
The ONLY disadvantage is that I can't fly light lift as well as the lighter planes.

Like DT said.

Dennis

[sbxcflyer]

Wow
Thanks for all the input. Many wonderfull responses.

[Miami Mike]

Many intelligent and authoritative-sounding, yet mutually contradictory, responses. I guess we'll have to keep watching this thread to figure out who's right and who's wrong.

[David Taylor]

OK,

I have seen this posted several times before but it gives a very good explanation of basic speed to fly theory and down at the very bottom it shows a ballasted and unballasted polar for a SGS-126 glider, not a very high performance glider by todays standards but even it still improves its glide with ballasting.
http://home.att.net/~jdburch/polar.htm
Also,
Heres the flight manual for a DG-1000 high performance 2 place sailplane and the polars, with different wing loadings (ballasted) for its two different wingspans on pages 62 and 63, once you get to them rotate the page clockwise 90 digrees (one of the adobe tools on top) so you can look at the polar properly.
http://www.dg-download.de/Manuals/flugh-1000s-bs-e.pdf
Looking at page 62 the 20 meter wing spans polar it's easy to see that in the unballasted configuration its minimum sink is around .5 meters per second and fully ballasted it only goes up to just over .6 meters per second.
So your climbing ability slows SLIGHTLY.
Now if you look at the 170 KPH line you can see that the unballasted glider achieves this speed (by pushing the nose down) at a sink rate of around 2.5 meters per second and the ballasted version achieves this speed with a sink rate of around 1.5 meters per second.
Obviously its much better to ballast up when you want to go fast instead of pushing the nose down to go fast, and it works the same for our models too.

Any other questions?........NO?
Good, this ends todays lesson in aerodynamics and unpowered flight :D :D
D.T.

[Miami Mike]

You can also go faster by reducing the wing loading, since that reduces the "induced drag", which results from producing lift. A heavier plane has more induced drag because it must fly at a higher angle of attack to produce the required lift.I get the impression that Roger is talking about powered planes here, not gliders. Otherwise I can't make any sense out of this.

[rogerflies]

Thanks. What a great reference!! That really makes it clear.

The very last part is interesting. Adding ballast does, in fact, allow you to go fast with a lower sink rate than you would have by changing pitch. You give up the ability to work light lift and your stall speed is going to be higher, but life's full of compromises.

Roger

[wingsnapper]

I always see folks saying you can't thermal heavy planes in light lift.. as if thermals actually have to "lift" our airplanes. It's not altogether true. If you are flying through air that is going up faster than your glide angle is decending then you'll go up too. I'd rather have my Paragon (5-6oz/sq ft) flying in the same air as my Gambler or Chrysalis (3.5-4oz/sq ft) because the Paragon will cover much more air per ft of decent. Sometimes simply being big means everything.
JS

[David Taylor]

One other note of interest regarding ballast, if your LZ is downwind of a treeline and your seeing people have a hard time landing their gliders, add some ballast and your heavier glider will cut through the turbulence better and allow you to make a more accurate landing.

D.T.

[BMatthews]

SBX', one little bit that may have been missed if you didn't read between the lines is that there really isn't any set rules or charts for this stuff in practical terms. It's very design specific as the comparisons of the two full sized gliders shows.

Your best bet is to add ballast of varying amounts and fly the model to get a feel for what each amount does to the best speeds and how it alters the speed range. In effect the ballast addition will train you, the pilot, in the only way we can be trained.... through practice. Without instrumentation we need to rely on our visual senses and at the end of a flight our stopwatch to decide if we done good or not. On light wind days take the ballast out but if you have any wind at all try playing with various amounts and put in lots of flights to really learn what works best for you and to get yourself trained to use the ballast to as close to optimum as you can manage. In particular just how much down trim is needed to really punch up the speed and what it does to your glide slope at the various speeds. When you know how well it works then you are ready for the contest flying.

.... and as with all sailplane flying the elevator trim control is your constant companion.....

[histarter]

I always see folks saying you can't thermal heavy planes in light lift.. as if thermals actually have to "lift" our airplanes. It's not altogether true. If you are flying through air that is going up faster than your glide angle is decending then you'll go up too. I'd rather have my Paragon (5-6oz/sq ft) flying in the same air as my Gambler or Chrysalis (3.5-4oz/sq ft) because the Paragon will cover much more air per ft of decent. Sometimes simply being big means everything.
JS

Great point. You clarified my position. Airspeed (correct for demand) is everything. :cool:
Size itself widens speed range - actually permitting slower flight by ease of control (softer stall). Airspeed alone dictates ability to handle small thermals in light air - and the larger machine spiralling at the same bank angle as the smaller will sink less due to higher LD, thus has the advantage. :D

[Miami Mike]

I always see folks saying you can't thermal heavy planes in light lift.. as if thermals actually have to "lift" our airplanes. It's not altogether true. If you are flying through air that is going up faster than your glide angle is decending then you'll go up too.No, that's not correct. It's not your glide angle that matters here, it's your sink rate. Your glide angle doesn't mean a thing when you're trying to rise within a thermal. A heavier sailplane might have the same glide angle that it would have if it was lighter, but it will have a greater sink rate and will need a stronger thermal to gain altitude.

Glide angle is the ratio of altitude lost to horizonal distance covered. Sink rate is the ratio of altitude lost to time elapsed. A sailplane's glide angle come into play, for example, when it needs to make it home from a distance without the aid of lift.

[BMatthews]

Semantics Mike. He said "faster than your glide angle is decending" which could be taken to mean the sink rate if you consider the whole phrase. At least that's how I took it.

The idea is right even if the wording is a little cloudy.

[Miami Mike]

No, it's not just "semantics". Glide angle and sink rate are two very different concepts. I've seen scraps of paper lifted in thermals. A scrap of paper has a terrible glide ratio - virtually zero - but its low sink rate allows it to be lifted.

The important point is that adding weight to a sailplane has advantages, but it won't help the plane rise in a thermal. In fact it will do the exact opposite.

If you want to argue semantics, I should have written that glide angle is a function of the ratio of altitude lost to horizonal distance covered.

[histarter]

No, it's not just "semantics". Glide angle and sink rate are two very different concepts. I've seen scraps of paper lifted in thermals. A scrap of paper has a terrible glide ratio - virtually zero - but its low sink rate allows it to be lifted.

The important point is that adding weight to a sailplane has advantages, but it won't help the plane rise in a thermal. In fact it will do the exact opposite.

If you want to argue semantics, I should have written that glide angle is a function of the ratio of altitude lost to horizonal distance covered.
Obviously sink speed goes up with weight, and your thinking is correct for a free flight. However, why are you discarding a pilot skill in your comments? Airspeed because of weight, improves a pilot's control while improving L/D for straight flight (running for new lifting areas). A skilled aggresive pilot will beat a floating competitor, and a scrap of paper that drifts onto obscurity - the majority of the time!! I would throw away some potential time for improved soaring dynamics - compromise is (usually) wisdom!:p

[Miami Mike]

Yes, I said that adding weight to a sailplane has advantages. I was very careful to stipulate that but apparently you missed it anyway, even though you quoted it. I specifically stated that I was referring to a sailplane's ability to rise in a thermal.

[jbeanelliott]

This thread may be confusing because the original question on wing loading did NOT specify the type of soaring being done. I assumed that since the
the name of the original poster had SBXC as part of his name that the poster was interested in cross country soaring. Some of the posts so far may apply equally to all soaring types. However, for cross country soaring where speed is a consideration on a course which has both headwind and downwind components the optimum wing loading would for most cases probably be the heaviest possible that would keep the glider under the 5 Kg FAI maximum weight limiit used at typical XC contest like Montague. I believe almost all of teams in the Montague XC contest add enough ballast to bring their all-up weight up to the 5 Kg limit.

A good article by Joe Wurts related to this subject was in the 1987 National Sailplane Symposium Proceedings sponsored by the Madison Area Radio Control Society (MARCS). Joe Wurt's article was titled "Cross Country and Slope Racing Design and Strategy". In this 9 page article Joe Wurts talks about such things as aspect ratio, wing loading, wing chord, speeds to fly, etcetera and discusses the implications of his computer simulations.

Here is an interesting excerpt from Joe Wurts' article:

Running through the aspect ratios, it is kind of surprising that on an average day aspect ratio is not that important. An aspect ratio of 9 on a 300 foot per minute day gives you a time of maybe 51 minutes whereas an aspect ratio of 18 gives you a time of 48 minutes. There is a clue there. You people with AstroJeffs are competitive with the higher aspect ratio planes if you have the SAME WING LOADING. On a strong day there is actually a small benefit to going to lower aspect ratios, but not a big benefit. One thing that is assumed in this graph is constant WING LOADING. If you start running to a high aspect ratio with a higher WING LOADING, you are going to be more efficient, but the big problem is that your average chord gets so small that you can't see it very high. You are not going to thermal as high so you are limiting yourself on distance between thermals. From what I have seen, the average chord is a much better indicator of seeing the airplane than the wingspan. A large wingspan with high aspect ratio is not going to be seen that well. In fact the plane that I flew this year (1987) had an aspect ratio of only 13 because I wanted to get higher on the start. I think it turned out pretty well because I was getting 500 to 1000 feet higher than those with aspect ratios of 16 or 17. <snip>

[sbxcflyer]

I like to fly in contests because it is fun. I have tried to fly in gusty very windy conditions (25-35mph ground speed wind) with out much luck. Being a car guy the analogy I remember is that above 45 mph a car starts to use gas
to compensate for drag with a curve that is exponential. The polars show that there are limits also.
However I have seen some amazing pilots fly in the same air (you know who they are) and do very well most of the time. This is the air I would like to fly well in.
Anyway thanks for all the help

[BMatthews]

Storm wind flying can be a LOT of fun but taxing on the pilot as well. I've related the story once or twice before about the day when I dynamic soared in the low level turbulence on a flat field. By dynaic soaring I don't mean it as is commonly done now but rather I crabbed back and forth across the wind and whenever the windward wing tried to lift I would turn hard into the wind and pull slighty up to use the sudden gust energy to gain altitude. This kept me in the air for a good 20 minutes with altitudes from 10 to 100 feet. It was great fun and I heartily recomend it to folks.

But what had started as a promising "normal" day had turned into an oncoming storm. I have to admit that despite having enjoyed flying in this weather I have passed up many a day that promised more of the same just because I knew the rain would likely be falling by the time I could get to the field and set up. But when you're already there and it suddenly turns to garbage it's nice to have a model that'll do the job.

For this sort of flying you really need a good aileron model. The one I flew that day is shown below. It's light and theramls easily but could punch out nicely on demand. In a weak moment I sold it to a buddy with the thought that I'd build a better one. One day......

[histarter]

I also wonder how to determine the wind speed at different altitudes. Does anyone have a rule of thumb for loading in the lead based on ground or advertised or perceived wind speed.
Thanks
I follow my full size experience to create my 'rule of thumb'.
Statistically (gathering data for 2000 ft AGL wind speeds for a couple of months) it seemed that at Addison Field TX the average increase of wind over ground speed value (sensor @ 20 ft AGL) was slightly over 1.8X (Ave.), and wind direction was shifted aprox. 15 degrees typically. :D

So, if I intended to use this altitude band, I would simply double the reported wind speed value to help figure ballast for XC. The reported value is good enough for flying at less than 800 ft AGL (normality). This system seemed to work for me regardless of where I went in the USA, and is one of the reasons I created altitude bands that I felt controlled designs. 400 ft AGL and short coupled floaters rule, 800 ft and Dr. Drela's designs rule, 2000 ft and Dr. Selig's designs rule. KISS :eek:

[histarter]

I don't see any relationship between wind speed and wing loading. The plane flies in the air at airspeed. Each airfoil has a "best" airspeed for a particular wing loading. 1
If you need more "penetration" on a windy day, you simply need more airspeed. You can get it by adding ballast, but you'll use more potential energy (read: altitude) to make the plane go fast.

To go fast without using more potential energy, you need to reduce parasitic drag to a minimum. That means getting rid of mismatches on the surface that cause disturbances in the airflow, having fairings around pushrods, or anything else that makes the air flow around the plane with the least distrubance. IOW, a "clean" plane. 2

You can also go faster by reducing the wing loading, since that reduces the "induced drag", which results from producing lift. A heavier plane has more induced drag because it must fly at a higher angle of attack to produce the required lift. A plane flying straight down has no induced drag, because the wing doesn't need to produce any lift at all. Until you want to pull out of the dive, of course. 3

If you want to "penetrate" on a windy day, just put in a little down elevator when you need to go faster. You'll give up a little altitude, of course. But you'll only be giving up altitude when you want to go faster, not all the time you're flying. 4

Remember, heavy planes fly fast because they must. Light planes fly fast because they can.

Roger
1. Right on the money!
2. & 3. Totally incorrect! Drag on a sailplane has very little to do with airspeed. Airspeed, for a given alpha is dependent primarily on mass. Drags greatest effect is on sink - because the velocity down the LD slope must be fast enough to lift the programming mass - with the 90 degree offset drag force slowly adding vectorily as the slope increases, until the drag at vertical replaces the lift to maintain regulated dive velocity.
4. Yes, speed range by delta alpha is much greater than that of delta mass (that is logritmic). Mass only sets the program for the alpha speed range - like music identifies the first note (do) - calling it a Key.

[rogerflies]

I'm going to go back to my original statement: Each airfoil has a "best" airspeed for a particular wing loading.

If a sailplane is more effecient at a higher speed with ballast, it's solely because that's the airspeed and wing loading it was designed to be most efficient at. Increasing the speed by lowering the nose, decreasing the angle of attack, and unloading the wing is less efficient because the induced drag of the elevator/horizontal stabilizer working to raise the tail is more than the induced drag of the wing from the higher loading.

The sailplane would probably go faster more efficiently by shifting the CG forward when extra speed was desired. Does anyone have some polars for a weight-shift glider with low drag and an efficient airfoil?

Operating the plane at a lower wing loading increases the usable speed range and lowers the sink rate, making it possible to work light lift more easily. Adding ballast brings the plane to it's most efficient configuration for going fast, but you give up some flexibility. Either choice is valid for a particular situation.

Sailplanes are designed to have a minimum amount of parasitic drag, and successful designs do have low drag. However, that drag there still obeys the laws of aerodynamics, and it increases as the square of the increase in airspeed. You don't get a free pass from drag just because you're flying a sailplane.

"Drags greatest effect is on sink." Absolutely, because the only energy available to overcome the increased drag of going faster is the potential energy of altitude.

Roger

[rscarawa]

Point number two is true to a point. You can only clean up the plane so much. THen it is up to the airfoil to do the rest of the work and stay effiecient.

On point 1, I am siding with Roger and Histarter. Point 3 is not correct as Histarter stated. Speed is increased by increasing wing loading. That is why you ballast sailplanes for better wind penetration. The fullscale guys do it too. There is insignificant efficiency change with more weight. It could be argued that at a certain speed, more weight reduced effieciency, but people add more weight to go faster which cancels out the loss.

My 1.8 cents worth.

[histarter]

I'm going to go back to my original statement: Each airfoil has a "best" airspeed for a particular wing loading.

If a sailplane is more effecient at a higher speed with ballast, it's solely because that's the airspeed and wing loading it was designed to be most efficient at. Increasing the speed by lowering the nose, decreasing the angle of attack, and unloading the wing is less efficient because the induced drag of the elevator/horizontal stabilizer working to raise the tail is more than the induced drag of the wing from the higher loading.

The sailplane would probably go faster more efficiently by shifting the CG forward when extra speed was desired. Does anyone have some polars for a weight-shift glider with low drag and an efficient airfoil?

Operating the plane at a lower wing loading increases the usable speed range and lowers the sink rate, making it possible to work light lift more easily. Adding ballast brings the plane to it's most efficient configuration for going fast, but you give up some flexibility. Either choice is valid for a particular situation.

Sailplanes are designed to have a minimum amount of parasitic drag, and successful designs do have low drag. However, that drag there still obeys the laws of aerodynamics, and it increases as the square of the increase in airspeed. You don't get a free pass from drag just because you're flying a sailplane.

"Drag's greatest effect is on sink." Absolutely, because the only energy available to overcome the increased drag of going faster is the potential energy of altitude.

Roger
Very interesting. Is there a democratic majority that thinks like this??

I made a statement in my book that Airline pilots should have at least 10 hours per year spent flying light planes, or models - because manual flight for them is dangerous when a operational program that is followed for years by the numbers fail; creating some of the air disasters we have seen. :eek:

Please don't take this as some hostile attempt; I am just totally curious how your unique phylosophy was developed. Alpha is an older term for projected angle of attack referenced to the 'relitive wind', and is unnecessary when CL/CD information is available [Note: CL/CD = LD].

Close values for flight:
6 degrees alpha TYPICALLY = max LD for the majority of profiles in use today.
Using the profile's CL generated at that Max LD value is good for referencing airspeed changes i.e. 2X total aircraft weight increases airspeed (@ max LD) almost 40% (yes, there is a small correctional value by re). Now this ratio stays basically constant for all the CL changes that are controlled by alpha. Zapping along at CL 0.1 is pretty close to the limit, and is about 1/5 the lift available of the typical profile employed, meaning the airspeed ratio is about 2.25 faster than referenced LD velocity, thus with ballast added the machine is now moving at 2.25 X 1.4, or 3 times original unloaded LD velocity. Specific profiles will display different lift quantums (CL) for a 6 degree alpha, however reaction by ratio through changes in alpha to establish CL would be the same! Unless the profile is tripped - but that is a different story.

[davidleitch]

I fly electric gliders that typically end up at a 30-50% weight premium to unballasted unpowered equivalents. Heavy planes by flying faster certainly can do just as well in thermals as lighter planes.

However a heavier plane puts a correspondingly heavy load on the pilot. The pilot has to be able to find the lift otherwise you will be back on the ground sooner. Its kind of like handing Jimi Hendrix's guitar to someone in the audience it just doesn't sound the same.

At a recent F3J competition I attended there was a definite feeling amongst some of the pilots that they didn't want a model that was "too" light. A certain degree of penetration is useful provided you know where you need to penetrate to.

I can say that having a 3 metre 100 oz glider moving around in good lift is about the best experience I get in rc modelling.

[BMatthews]

...At a recent F3J competition I attended there was a definite feeling amongst some of the pilots that they didn't want a model that was "too" light. A certain degree of penetration is useful provided you know where you need to penetrate to......

This has been exactly my finding as well.

I've got a wing and tail set with two fuselages. One is a lightweight glider fuselage that results in a 32 oz 2 meter model. The other is an electric power fuselage that results in a 54 oz model (old school motor and batteries). The heavier version in our typical light to moderate winds is by FAR the better fuselage to mount the wings onto. I do give up some low speed ability and I suppose the sink rate is a touch higher but this is way more than compensated by the far wider useable range of speeds available to me and the wonderful ability to run around more and test a lot larger portion of sky for lift. The light lift damage is minimal and far less than the 45% risein weight would suggest. But I have to admit that if it was much heavier then it would get pretty doggy.

Contrary to what you say about speed range on the models that I have played with ballast on the results were that WITH the ballast the speed range was wider than at the light weight for reasonable glide settings.

[histarter]

This has been exactly my finding as well. Contrary to what you say about speed range on the models that I have played with ballast on the results were that WITH the ballast the speed range was wider than at the light weight for reasonable glide settings.

We are in totally agreement. Stationary grounded pilot sees speed range relitive to his zero velocity. Basically "musical DO velocity" is shifted by mass increase by ballast (2X as per my original statement). This means 'center LD velocity' has been shifted 40%, thus range in velocity observed for different alpha settings would be much wider! I believe you got it my friend!! :D

So far nobody hit on my 5X lift quantum statement - that is very concervative. :rolleyes: Drela's potential is like 7.5X, while Selig's approach ( and other profile magicians) can press 9X - meaning the XC aspect of soaring is currently in their hands. :D

[histarter]

As for wind speeds at different altitudes, not only can wind speed change but the direction can change too, even a 180 digree change is possible!
You can check avation sites to see the changes in velocity and direction but even these are basically educated guesses.
Your best bet would be to do some searches and reading on speed(s) to fly.
If you ballast properly for the conditions and use speeds to fly properly you will be doing better than most. :D

Cheers, Dave T.
I missed this great comment.
I took data from TX Addison Airport for a couple of months to see an Average shift of wind at 2000 ft AGL of 15 degrees, and a windspeed 1.8 times the 20 ft AGL reading (that is advertised). I have used this as a "Rule of thumb" for all my soaring with great success. :D

[rscarawa]

We are in totally agreement. Stationary grounded pilot sees speed range relitive to his zero velocity. Basically "musical DO velocity" is shifted by mass increase by ballast (2X as per my original statement). This means 'center LD velocity' has been shifted 40%, thus range in velocity observed for different alpha settings would be much wider! I believe you got it my friend!!


One thing that needs to be said though is that by increasing the weigh by 2x you do not increase your flying speed by 2x. If you increase your load by 2x, you might see the polar curve velocity shift of 40% or so.


Scot

[sbxcflyer]

I have learned alot from this discussion. I would like to reword my question a little. With out ballast I like to, when turning flare into the wind at the start of the turn, fatten out on the downwind portion, and flare a bit into the wind on the final third of the turn, flattening the plane as it completes the turn. I also like to use the camber slider to improve my perception of the efficiency of the turn. ( + camber into the wind and - camber downwind)
However when the wind is higher I can not get the plane to flatten and fly the themal turn repeatedly. The downwind portion gets longer as does the upwind portion so flattening the plane becomes more important to efficiency.
Is there a rule of thumb to get a 70 ounce plane that flies great at 10-15 mph to fly the same at 20-25 mph.
I remember seeing JW at the Nationals changing the ballast in his plane between flights. I was not even using ballast.
Perhaps this is a style question. I have used 6 ounces, 8 ounces and 12 ounces for different wind speeds.
Please let me know what you use.
Thanks

[Miami Mike]

You'll probably be better off if you realize that windspeed and direction mean nothing to a plane in flight. Your plane flies relative to the airmass that surrounds it, whether that mass is moving or not, and there's no reason to change your plane's attitude at any point in the course of a turn just because of its direction relative to the wind.

As a fellow club member once commented, "wind is the navigator's problem, not the pilot's."

[Don Stackhouse]

The other thing that needs to be considered is that not only does induced drag increase when you add ballast, but also your minimum turning radius increases. If you can stay high, where the thermals tend to be nice and wide, that isn't a problem. However, if you have a bit of bad luck finding lift and end up at low altitude, the combination of higher induced drag and sink rate, due to the extra weight, and also the wider turning radius, also due to the extra weight, makes it difficult to climb in lift if you find it, and also difficult to circle within it. Instead, you will find yourself sinking around the outside of the thermal, or flying big circles through it, poking in and out of the core but never being able to center in it.

This is less of an issue for XC, where the strategy is generally to stay high, but on other classes, especially the smaller ones, it can be a problem.

Wing loading controls penetration, but span loading (or actually the square of span loading) controls induced drag and thermalling ability. The way to get both high wing loading for good penetration and high speed, and low span loading for good thermalling ability, is to go to high aspect ratios combined with airfoils designed to work well at those aspect ratios. In general, we've found that it's possible to get excellent performance over a wider range of airspeeds with very high aspect ratios, such as with our Spectre series of sailplanes. It also requires less ballast to achieve a given change in wing loading, so you can go fast without sacrificing as much thermalling ability.

However, there are other considerations, such as visibility. Chord, particularly root chord, is the major factor there. By optimising airfoils for lowest drag and for lower lift coefficients, you can still get excellent performance over a reasonably wide speed range with an easy-to-see low aspect ratio wing.

On our Chrysalis series, which is the choice of a lot of beginners as well as sport fliers, this approach is more appropriate. Yes, I could squeeze more performance out of it by going to a higher aspect ratio, such as some of its competitors do, but it would not be as well suited for its typical customers. By designing the wing to be efficient at that low aspect ratio, the performance at all airspeeds is still quite respectable. For this reason in particular, if I was designing another member of the Chrysalis family, I would still choose to stay with moderately low aspect ratios.

The other option is to go to a high aspect ratio, but in a very large model, so the root chord is still large enough for good visibility. It's also possible to play with the planform, using tailored airfoils along the span to allow a wider than normal root chord while still keeping the aspect ratio high, while still providing good handling and tip stall resistance. It makes the airfoil design work especially tricky, but can be done, as we demonstrated with our Spectre 120.

Had we not left the competition TD market (purely a business decision; competition TD sailplanes are a great way to make a reputation, but a terrible way to make a living, and a good way to run a small company into the ground if you stay in that market too long), I had the beginnings of an even bigger Spectre in the works, which would allow for better visibility while still providing better performance at all airspeeds than its contemporaries.

I do sometimes regret not getting to run that program to its completion, we had only begun to scratch the surface of what could be done with the concepts in the Spectre series.

As usual, there are a number of conflicting requirements in any airplane design, and the key is in understanding, prioritizing, and managing the tradeoffs involved to get the best overall combination for its intended customers. In the end, the "best" approach depends on the particular airplane, and what you intend to do with it.

[Don Stackhouse]

...I would like to reword my question a little. ... I like to, when turning flare into the wind at the start of the turn, fatten out on the downwind portion, and flare a bit into the wind on the final third of the turn, flattening the plane as it completes the turn. I also like to use the camber slider to improve my perception of the efficiency of the turn. ( + camber into the wind and - camber downwind)
However when the wind is higher I can not get the plane to flatten and fly the themal turn repeatedly. ...

This is a variation of the old "downwind turn" myth.

You should let the model drift with the turn. By altering the bank angle and airspeed, you are shifting the position of the turn with each circle, and flying right out of the lift as you do so.

Thermals drift downwind as they rise. Your model should be allowed to drift with them. Concentrate on keeping your bank angle and pitch attitude constant, and leave the flaps alone, other than setting them for thermalling when you first enter the thermal. Let the plane drift with the wind. The circles you see from the ground will not be round, but if you keep bank angle and airspeed constant, they will be perfectly round with respect to the air, and to the thermal contained within that air.

[Miami Mike]

Don, I suspect that there may be a case where there's some possible benefit to altering your airspeed at different points in a thermal turn. In your post #42, you mentioned thermals that are too small for you to circle within them. The alternative is to follow a circle that brings you in and out of the thermal, and it occurs to me that in that case, you might benefit from lowering your flaps or increasing your pitch to slow down while inside the lift, and decreasing your pitch or raising your flaps to speed up while outside of the lift.

I got this idea while watching a buzzard circling. His path looked something like this:

http://www.vvsss.com/ezone/circling.gif




.

[histarter]

Don, I suspect that there may be a case where there's some possible benefit to altering your airspeed at different points in a thermal turn. In your post #42, you mentioned thermals that are too small for you to circle within them. The alternative is to follow a circle that brings you in and out of the thermal, and it occurs to me that in that case, you might benefit from lowering your flaps or increasing your pitch to slow down while inside the lift, and decreasing your pitch or raising your flaps to speed up while outside of the lift.

I got this idea while watching a buzzard circling. His path looked something like this:

http://www.vvsss.com/ezone/circling.gif




.
Looks a bit like dynamic soaring - if the breeze is comming from the right and the buzzard is attempting to stay ahead of a tree line - that is to the left - in order to catch thermals that are snaping off. When you work out the tansential formulah for Bank Angle (BA), you will find that spiral diameter is proportional to delta mass. 2X mass equals 2X diameter for the same BA. [while delta velocity is increased by 1.4X] :)

For successful low launching, BA is a more critical factor than concern for absolute sink rate - which translates to employment of flaps that increase the lift factor - making the mass factor less effective, i.e. to lower the airspeed. Since it is the lift factor that controls airspeed, the drag of plus 6 degrees of flap on a now distorted profile, has less importance.

Example: A 6 mtr / sec. 3 mtr Mirage (with its original profile) yields about 18 LD - and it will beat the heck (in duration with lift) out of a 11 mtr per sec. 3 mtr Ellipse with LD about 21 (RG-15) when both are launched below 320 ft AGL, because of spiral diameter; however, when both are launched above 600 ft AGL, the overall superiority of the Ellipse is very pronounced! [This experiment was attempted under varying wind and weather, by 4 different pilots in 1993 in Dallas Texas] :)

[histarter]

Is there a rule of thumb to get a 70 ounce plane that flies great at 10-15 mph to fly the same at 20-25 mph.
I remember seeing JW at the Nationals changing the ballast in his plane between flights. I was not even using ballast.
Perhaps this is a style question. I have used 6 ounces, 8 ounces and 12 ounces for different wind speeds.
Please let me know what you use.
Thanks
70 ounce sailplane Hmm, With my Paragon (58 oz) my load steps are 16 oz, 20 oz, 32 oz. (in cassette cartridges). :D 10 to 15 mph - I usually carry 16 oz.

On my 2 lb Shuttle 2 mtr. I use 8 oz, 16 oz, 24 oz (Note: Shuttle reMark was flown in RES comp at 56 oz with winds above 25 mph very effectively). :eek:
At 15 mph I am carrying 8 oz - the same for my GL (that has less wing area, but a higher lift profile).

[Jim Bacus]

Is there a rule of thumb to get a 70 ounce plane that flies great at 10-15 mph to fly the same at 20-25 mph.
I remember seeing JW at the Nationals changing the ballast in his plane between flights. I was not even using ballast.
Perhaps this is a style question. I have used 6 ounces, 8 ounces and 12 ounces for different wind speeds.
Please let me know what you use.
Thanks

Dave,
JW was walking out to the flight line with two pipes for his ICON, the first one is about 10.5 ounces, and the second one is about 9 ounces. I too was watching him make his decisions and load up. I use one or two pipes in my ICON as well when it gets windy. Your step size is too small, you are barely changing the wing loading with the ballast steps you are taking. Think about it a bit... :D

[Miami Mike]

Looks a bit like dynamic soaring - if the breeze is comming (coming) from the right and the buzzard is attempting to stay ahead of a tree line - that is to the left - in order to catch thermals that are snaping (snapping?) off.It was a relatively calm day and there was no tree line involved. I believe the bird was simply flying in and out of a small bubble of rising air, slowing down to pass through it and speeding up while outside of it.

[histarter]

It was a relatively calm day and there was no tree line involved. I believe the bird was simply flying in and out of a small bubble of rising air, slowing down to pass through it and speeding up while outside of it.
Sounds good Mike. :)

Remembering that birds are food oriented; I flew competition against red tailed hawks with my Mirage, that I identifed as a 2 hawker (climbed in lift twice as fast) however I later realized that the birds were at a large disadvantage - the lesser their 'ballast', the more aggresive their hunt for snacks!! :eek:

[rscarawa]

I wonder if the bubble was too small to thermal in comfortably, so the bird just decided to take straight passes thru it?