Typical Sink Rates for Thermal Gliders

[AZ_Astro]

Would anyone have a reference to a table of sink rates for various gliders? Or some information based on experience?

I noted the following from the web:

"It has been possible to measure a vulture's rate of sink by flying in aircraft in close formation with them. Turkey Vultures have a minimum sink rate of 2 feet per second, while Black Vultures have a minimum rate of 2.6 feet per second. Black Vultures, therefore, need stronger thermals than Turkey Vultures, which helps to explain why they are restricted to the southern United States while Turkey Vultures can penetrate the relatively cool climes of southern Canada."

Here on the EZone, Tic reported the following a year or so ago:

"Get the CG a little rearward of recommended.. It will glide all day long.. A forward CG really kills the glide on the bandit. Probably on anything else too."

"Just start going back 1mm at a time. soon you'll find the sweetspot and flights will be much longer.. everyone wants to see the highspeed flybys, but I actually enjoy getting my bandit WAY up there and going for duration.. I typically get 20min. flights on eight cp-1700's.. (B40-8S, 13-8 aeronaut cam carbon) with no thermal or ridge help.. I just bring out the recliner and put a coke in the cup holder, climb up to 1,000' at least, cut the motor and try for min. sink. Soon, the peanut gallery starts chanting for a highspeed pass. I usually oblidge, but lately it's been upsetting the "safety" officer. With some thermal help, I can stay up for an hour."

1000 ft / 20 min is 1000 ft/1200 sec = .8333 ft / sec sink rate

And this is on his Bandit which is not exactly a thermal glider.

I was just wondering about recorded sink rates and what I should be able to expect from my Bandit, but also what kind of sink rates have been recorded by high-performance thermal models.

Thanks.

[Ollie]

There are three main factors that affect minimum sinking speed. The sinking speed is inversely proportional to the square root of the coefficient of lift. The second is wing loading whose square root is proportional to sinking speed. Induced drag is more than half the total drag at the minimum sinking speed and the induced drag is proportional to the square of the coefficient of lift and inversely proportional to the aspect ratio.

Very lightly loaded thermal soarers like the Allegro Lite have minimum sinking speeds less than 0.9 feet per second. More typical 2-meter gliders have sinking speeds from about 1.1 to 1.4 feet per second. Three meter thermal soarers usually have minimum sinking speeds somewhat lower than 2-meter soarers because of their higher aspect ratios.

See:
http://my.athenet.net/~atkron95/pcsoar.htm
PC Soar is freeware that you can download and use to calculate the minimum sinking speeds of most model sailplanes that you might be interested in.

[Jack Hyde]

"Fundamentals of Sailplane Design" by Fred Thomas, p 168, shows a table listing measured minimum sink rates for 50 full scale sailplanes in actual flight. Sink rates of 1.3 to 2.3 feet per second are shown. They went to some trouble to get it right.

I wonder how good those sink rates for buzzards are. How do you get a buzzard to fly at min sink if it isn't in lift. I guess they measured the climb rate and thevertical velocity of the air the bird was flying in and did some subtraction. I have read buzzards weigh about 5 lbs and have a 5' span. Can you imagine the lousy performance of a TD sailplane with a 5' span and 5 lbs? The birds stay cored in the thermals. A good example of TD flying is 90% pilot.
By the way I have read articles where Thomas and Martin Simons both state that sink rate is a weak function of cg position. If you get longer flights with an aft cg its because the plane signals lift better and is more responsive to controls allowing more you to better keep the plane in lift and out of sink.

[Sparky Paul]

Several years ago I placed an altimeter (Avocet bicycle computer) in an electric and flew it with different wings. Ran the motor for a minute and then glided.
The average sink rate was about 200 feet per MINUTE :), except for the Gentle Lady wing, which averaged 100 feet per MINUTE :).
The GL wing didn't get as high as the Challenger and Eclipse wings, but it took longer to get down.
Flying GL wings today I get similar results.
(OS 15 on the front)
.
Gets close to OOS in 30 seconds! :)
.
The altimeter was active only when the motor was running, BTW.

[Botterii]

hey AZ,

what slopes do you fly in the Phoenix/Tucson area? or is it mostly flat land thermaling?

thanks!

[AZ_Astro]

FWIW:

I haven't done any slope flying here but I understand there are a number of places in the state that are excellent. There are a few places in the Phoenix area, usually in city or state parks, that would also serve, I expect, but there aren't too many people who fly slope here.

Thermal flying, on the other hand, is widespread. We get significant thermal activity during the mid-day, just about everywhere. Thermaling in the flats, primarily.

[AZ_Astro]

By the way, thanks to everyone for responding.

I should amend my computations for the Bandit above. When Tic mentioned climbing to 1000 feet and getting a 20 minute glide, I suspect he is able to do that several times. Using 3 climbouts as a conservative estimate, that would change the Bandit sink rate to 3 x .8333 = 2.5 ft / sec. I suspect it is closer to 5 feet/sec.

Jack Hyde:

Interesting about the vultures. They soar in the thermals and glide from thermal to thermal, changing their wing shape when they go from one to the other. It does seem like the measurements were likely taken while the vultures were 'gliding' point to point, instead of soaring in gentle circles. But - the info is lacking so who knows.

Part of my reasons for asking is this, my next construction.

[CactusJackSlade]

AZ_Astro,

Is this going to be a glider... motorized, what? Interesting project, I'll be watching.

One thing aI have noticed with "bird shaped" flyers is that there seems to always be the difficulty of getting enough weight far enough forward to get the CG correct.

The RC Gull by birdworks is an example, they had to sweep the wings back a bit or they would have had to lengthen the head/body forward... or put a tone of lead (birdshot??!) in it's beak.

I did my own "bird shaped" flyer recently, here is the thread with photos:

http://www.rcgroups.com/forums/showthread.php?s=&threadid=176171

Mine looks much more like a plane than bird compared to the shape you are attempting, I think a friend of mine did an RC golden eagle one time, I wish he would post some pic's and info!

Good luck, keep us posted!

CJS

[AZ_Astro]

Thanks for asking.

I recently completed a 2 oz tow glider which a buddy of mine pulled up to altitude with a Slowstick. The glider was free flight and has since flown away!

Anyhow, I wanted to try to replicate that success with a bird-like model. Except that this time around I plan to add elevator and aileron control. I would do rudder except that it would ruin the bird "profile" in the air, so I guess I'll do ailerons.

It's going to be a thermal floater, and the anticipated AUW is about 5 oz. We'll see. Wingspan is 47".

Here are a few more pics. I haven't completed the head, which I'll probably carve out of foam or balsa. Today I finished the glide testing and it glides very well! I can confirm that the cg is very far forward.

[AZ_Astro]

This also will be a towed glider, by a brushless Slowstick.

[AZ_Astro]

One servo for ailerons using torque rods.

[AZ_Astro]

The other servo for elevator / nose tow ring release.

[AZ_Astro]

The wing is undercambered, and I have sanded down the trailing edge at the top.

In the picture above, you can see the bolt I taped to the nose to get the cg right during my glide tests. That entire nose triangle will go away when I put on the head.

Bluecor fan-fold construction. It's only taken two days to build this!

Anyhow, I'm looking forward to having a buddy tow me up and seeing how long I can float around up there. On a good thermal day, it should stay up with ease. I'm guessing it will have a 2 ft/sec sink rate, but I'll try to get a more exact readout during the first few flight tests.

Great fun!

[davidleitch]

Motocalc calculates minimum sink rates as part of its output. For my electricfied 2.5 m Organic minimum sink is calculated at 120-130 ft per minute. I suspect my actual sink rate is a bit higher than that. That's with a wing loading of around 11/12 oz sq ft.

As Ollie has pointed higher wing loadings hurt sink rates because of the greater bank angles and consequent drag in turns. Minimum sink doesn't usually allow for losses due to turns. However my model gliders spend half their flying time at least in turns.