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Author Topic: What is Radio Control (RC) Soaring?  (Read 749 times)
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Konrad
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« on: September 29, 2017, 12:33:40 PM »

What is Radio Control (RC) Soaring?

I ask this question as I see a lot of confusion as to what is a glider elsewhere on the web.

I think the problem starts with the intuitive notion that a glider is a light weight ship that “floats” on a bubble of rising air. This floating idea might be a sufficient explanation coming from a Free Flight perspective or even the entry level RC model like the Goldberg Gentle Lady.

But once one starts to move out of the entry level models to higher performance ships. The idea of floating on air rapidly becomes obsolete and/or down right wrong!

An RC glider needs to fly through the air. This means it must move forward to get air to flow over the wings. Per the lift equation this is key to understanding how a glider works, air must flow over the wings! The idea that a glider floats on the upward moving air is true only for trash bags. To properly fly an RC glider the model must be moving forward (falling), not floating, through the air to generate lift as a result of air flowing over the wings. The upward moving air of a thermal is allowing the glider to fall through the air, and not meet the ground.

Once we understand that a glider is always falling relative to the air around it to generate the forward velocity need for the wings to generate lift. We then come face to face with the nemesis of all heavier than air flight DRAG!  It is the sum of all the drag that directly defines the performance of the aircraft not weight!  Now weight does have a direct effect on drag as it forces the wing to have to work at a higher angle of attack. This actually might not be a bad thing as all airfoils have a relationship between lift and drag. At some proper weight (wing loading) the amount of lift from an airfoil (wing) can be optimized for the lowest drag.  This can be seen on the L/D plots of airfoils. This is why properly ballasted and heavier gliders easily out perform the old light gas bag models like the Goldberg Gentle Lady.  

In my youth it took me a while to grasp this, that the enemy of a glider is drag. I knew that the profile drag of an object when up as a function of the cube of the speed. I was puzzled that the good sailplane pilots were adding weight in the form of fairings, great surface finishes and even ballast, when thermal glider flew at low speeds. But these lead sled, glass slipper were always finishing ahead of me and my Wanders and Windfrees! Once I learned that the energy I had to generate lift was limited to the kinetic energy of the falling glider and that this energy was spent overcoming drag, my flight times and placement in contests rose.

This again became very evident when I was racing FAI F3D Pylon. In the late 80’s I raced FAI F3D Pylon which were/are small and heavy 6.5cc powered glow engine planes capable of over 300KPH. For planes going this fast drag reduction is intuitively obvious. What I found amusing is that at the end of the 10 lap heat we would kill the engine and coast to altitude. We would glide with our heavy racers waiting for the slower guys to finish and allow those that had declared a MayDay to land. It was fun to watch and be part of the three heavy pylon racers thermalling at the end of the runway while the fourth was landing. This really was only possible because these pylon racers were low drag ships.

So to me RC Soaring is all about drag reduction and energy management.

What is your take on RC Soaring?

All the best,
Konrad
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lincoln
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« Reply #1 on: September 30, 2017, 01:39:52 AM »

Most of what you say seems quite reasonable.

However, for a given angle of attack, the profile and parasitic drag goes up approximately as the square of the airspeed, not the cube. The power required goes up as the cube. However, if you're using ballast to go faster, you've got more power to use. The sink rate goes up with the square root of the wing loading. However, Reynolds numbers change with more speed, making airfoils more efficient, so a heavy model will have a bit less sink rate than you might expect. The increased weight, though, increases the turning radius.

Adding weight doesn't necessarily mean increased angle of attack. You can just increase speed instead, as discussed above.

I vaguely recall that if you've got extensive laminar flow, the drag may not go precisely with the square of the airspeed. But I'm fuzzy on just how it does go. Looking at the Wikipedia article on the thickness of the boundary layer, it appears the thickness of a laminar one on a flat plate, all else being equal, is proportional to 1/sqrt u0     where u0 is the "free stream velocity" away from the plate. The drag is proportional to the "shear rate", so as the thickness of the layer gets less, the shear rate goes up and the drag goes up. Since the shear rate is raised by a thinner boundary layer, I think maybe the pertinent exponent is 1.5, rather than 2 as is the case with turbulent flow. Feel free to show the error in this.
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Konrad
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« Reply #2 on: September 30, 2017, 10:54:09 AM »

Good discussion, thank you.
I apologize for confusing drag and power.  P = ½ρCAv^3. I was concentrating of the concept of energy and its management.
https://www.grc.nasa.gov/www/k-12/airplane/drageq.html
https://en.wikipedia.org/wiki/Drag_equation

Much of what you discuss can be used to explain why bigger often flies better.

As a general rule while setting up my gliders I’ll gladly gain weight to reduce drag. This is counter intuitive.  Now for the most part the weight gain has to reduce drag to be of any benefit. The one exception is to add weight (ballast) to gain energy to penetrate the wind (cover ground).

All the best,
Konrad
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Yak 52
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« Reply #3 on: October 03, 2017, 10:53:55 AM »

So to me RC Soaring is all about drag reduction and energy management.

What is your take on RC Soaring?

Obviously RC soaring varies in it's objective depending on the competition format, but essentially a good glider has to do two things well:

1. Maximise the climb achieved in a thermal.
2. Get from one thermal to another with minimum height loss.

Unfortunately improving one of these aspects tends to harm the other, so some compromises are needed.

Maximising thermal performance is essentially about reducing sink rate to a minimum (although tight turning and good handling are important.) This is done by reducing wing loading and also by improving the drag at high lift coefficients and slower speed. Generally this means more camber in the airfoil. Free flight models and 'gas bags' are good at this bit.

However a model optimised purely for good thermalling will not have the speed and penetration to get through sink to the next thermal or return from downwind. Any model can go fast by diving but to go fast without much loss of height, ie good penetration, is another matter.

To get good penetration you need to increase the wing loading (ballast) and reduce the drag at higher speed and lower lift coefficients. This means less camber. You can see that this is mutually exclusive from the above!

So sailplane designers have to optimise for a balance between the two. These days full span ailerons and camber changing mixes are common so the pilot has a 'speed mode' and 'thermal mode' available.

Careful airfoil design is also a factor because thinner airfoils tend to penetrate better. As structures have got stiffer, thinner wings have become the norm. In previous times the only way to get good penetration from thicker wings was to ballast them up excessively. Nowadays light models penetrate extremely well due to better design. Some older airfoils had poor performance at low Reynolds numbers and so reducing weight just slowed them to the point where they got draggy.

For these reasons there can be the perception that weight is 'a good thing' in RC gliders. But rather than weight being a good thing per se, it's better to understand it as part of an over all strategy and ballast according to the conditions. In no wind conditions with light lift a 'gas bag' may do very well indeed. However in strong lift and high winds the heavy, slippery model will get you from thermal to thermal leaving the light model floundering downwind and landing out.


A further consideration for glider design is the launch method. Obviously a DLG has a need for low drag at high launch speed, whereas a winch launch model needs super strong spars and so on.

So everything is a compromise but an understanding of the task requirements is key to making the right compromises Smiley


Jon
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Konrad
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« Reply #4 on: October 03, 2017, 07:33:19 PM »

...
A further consideration for glider design is the launch method. Obviously a DLG has a need for low drag at high launch speed, whereas a winch launch model needs super strong spars and so on.

So everything is a compromise but an understanding of the task requirements is key to making the right compromises Smiley
Jon
So true! Engineering is the true art of compromise. HLG (Javelin, in my day) bring all the constraints of RC soaring into sharp painful focus.

Once we understand that a glider is always falling relative to the air around it to generate the forward velocity need for the wings to generate lift. We then come face to face with the nemesis of all heavier than air flight DRAG!  It is the sum of all the drag that directly defines the performance of the aircraft not weight!  Now weight does have a direct effect on drag as it forces the wing to have to work at a higher angle of attack. This actually might not be a bad thing, as all airfoils have a relationship between lift and drag. At some proper weight (wing loading) the amount of lift from an airfoil (wing) can be optimized for the lowest drag.  This can be seen on the L/D plots of airfoils. This is why properly ballasted and heavier gliders easily out perform the old light gas bag models like the Goldberg Gentle Lady. 

So it is still true, it is all about drag reduction and energy management. The notion that RC Soaring (thermal hunting) is solely focused on upward rising air is false. It is in fact more of finding this elusive column of rising air, rather than parking on top of a prairie dog fart. This means being able to cover ground, move efficiently.

All the best,
Konrad
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Yak 52
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« Reply #5 on: October 04, 2017, 06:05:20 AM »

The notion that RC Soaring (thermal hunting) is solely focused on upward rising air is false. It is in fact more of finding this elusive column of rising air, rather than parking on top of a prairie dog fart. This means being able to cover ground, move efficiently.

Well it's a bit of both. You need to get around and search plenty of sky, but equally you need to thermal well when you find lift. The trick is optimising this balance between the two for the particular competition/task/conditions/wind strength.

The ideal is a very light weight structure that permits a good airfoil and thinner wing design but with plenty of ballast capacity for windy days.


So it is still true, it is all about drag reduction and energy management.

Essentially yes. But more specifically it's about reducing drag in the two different regimes:

- For thermalling, improving minimum sink by reducing drag at the speed for best 'power factor' (CL3/CD2 or CL1.5/CD)

- For penetration, reducing drag at some speed rather higher than best L/D.


Again, which of these you prioritise is the designer's choice, depending on what kind of model you are after. When you look at some of the latest F5J models (electric launched gliders) like the Ultima, you will see that they are incredibly light but very slippery too!
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« Reply #6 on: October 04, 2017, 11:10:13 AM »

In my mind there is nothing prettier than an A-2 bouncing around in a thermal.  40 years ago I built several RC glider designs in the hope of being able to observe this beauty while standing in one place.  Much to my dismay they flew nothing like an A-2 so I quit with that offshoot of the hobby.  Now RC equipment has gotten small and light enough to get to where I was wanting to go but my enthusiasm has evaporated.
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Konrad
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« Reply #7 on: October 04, 2017, 11:23:21 AM »

Yak52,
I think we are in full agreement weight is not the enemy, well not directly.

Adding weight in the form of fairings, covers and hatches actually benefit the glider’s over all performance if the drag actually is lowered.
So does the added weight for streamlining, such as the use of Rotary Drive System (RDS) vs push rods and control horns.
https://alofthobbies.com/rds-coupler.html
http://www.charlesriverrc.org/articles/supergee/SG2/sg2_rds.pdf

Adding weight to have strong spars to allow for vey thin, low drag airfoils. (Weak spars need thick airfoils to provide the needed stiffness). Adding weight in the form of epoxy composites to obtain razor sharp trailing edges. Both of these add weight and drastically lower the drag of the sailplane.

Adding weight if it reduces drag is to be done as it will improve the performance. If you can reduce drag and not add weight so much the better. But that usually take advancements in materials. Such as moving from balsa and spruce to glass and carbon composite matrices.

Now you obviously can add too much weight but that is covered with statement “added weight if it lowers the drag”. And that is easily seen in the L/D plots as the added Coefficient of lift has a corresponding increase in drag once you are out of the sweet spot (drag bucket). And here is where weight takes it toll on a glider’s or any aircraft’s performance.

What I’m trying to get across to the new RC glider drivers is learn to get your glider to move through the air.  And the best way to do that is to cut down on the drag even if than means adding weight.
I’ve seen guys remove servo covers thinking the weight loss was helping them ride thermals! I’ve even seen guys leave out the shear webs to “save weight”, thinking that they add little strength. Shocked

All the best,
Konrad
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Konrad
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« Reply #8 on: October 04, 2017, 11:28:10 AM »

FAIF2D
I too am often enamored watching dust devils carry all sorts of light trash into the air. And that has a nice zen aspect. But for RC flight where I want to be in "control".  The light draggy ships are a true frustration, actually the opposite of a zen experience.  Angry

All the best,
Konrad
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« Reply #9 on: October 04, 2017, 11:42:14 PM »

It's refreshing to come here and read some intelligent discussion. I spend too much time on RCGroups where the chatter is always about tip stalling and the mythical downwind turn. I have nothing to add here as I'm really just a hack with some gliders that I'm passionate about but I just wanted to thank you guys for conversing on a little higher level than what I'm used to. Cheers
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