flat turns in a thermal - RC Groups
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Apr 13, 2001, 03:08 PM
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maciek's Avatar

flat turns in a thermal

If a glider has ailerons, are they used to keep the wings level when circling in a thermal? How can you keep turns tight and flat with just a rudder?

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Apr 13, 2001, 04:55 PM
son of a pilot
FishPilot's Avatar
Flat turns are draggy and cause spins. Stand that glider on the wingtip and keep the turns coordinated.

Apr 17, 2001, 04:58 PM
Registered User
Flat turns let you fly slower, a wing flying straight and level is most efficent. You cant get tight fast turns flat, that is for standing on a wing tip, this is not as efficent as you have to fly faster than min sink to keep the ship up. At 45 deg you have only half the effective wing giving lift the other half is pulling you around. If the thermal is big than a flat turn at thermal speed is great. As for the drag issue it is a mute point. Flatter is better.

You need a "little" bank angle to get your lift vectors right. Use the rudder to guide the turn not to force it.

I have never had a machine spin in on a flat turn.

Fire proof suit on.

[This message has been edited by dhrider (edited 04-17-2001).]
Apr 17, 2001, 06:58 PM
son of a pilot
FishPilot's Avatar
Ok, I give up. What is a flat turn? Any coordinated turn requires a banked wing.

"How can you keep turns tight and flat with just a rudder?" -- this will cause a spin for sure.

Apr 17, 2001, 10:29 PM
Never stop learning!
Yaw + Stall = SPIN
I have not conducted a scientific study on this, but I believe that the amount of rudder required to turn "flat" will raise the total drag above that of the same aircraft in a coordinated turn of the same diameter. a grossly un-coordinated turn(flat) at minimum sink speed would be close to a good stall/spin situation. Also remember, a flat turn would require opposite aileron on most aircraft, thus lowering the efficiency of the carefully chosen airfoil!!! As a side note, there was an experiment done a few years back, in full scale sailplanes, that proved(with hard data) that a sailplane in a slight(very slight)"slipping" turn was more efficient(only a couple of percent or less)...this is the opposite of a "flat turn"!!! I don't remember all of the details, will have to find the article. Sorry for the rambling....hope this helps a little...
Apr 17, 2001, 11:01 PM
..teach monkeys to fly..
Marten's Avatar
agpilot has it more or less correct.
dhrider is flying on borrowed time, unless he stays high in wide thermals.

Recommended reading is Derek Piggot's "Understanding Gliding" ISBN o 7136 5568 2

Its not the rudder that allows you to keep the turn flat. It creates yaw, which dihedral, apparent sweep-back of the inside wing, fuselage blanking of the inside wing near the root, and the relative decrease of airspeed over the inside wing, will combine to induce early stall of that inside wing. So, add a bit of up aileron to lift the inside wing. You are now in uncoordinated flight, your drag is huge, and if you are close to the ground in a full size glider, participating a very near death experience.

Apr 17, 2001, 11:07 PM
Never stop learning!
agpilot agrees with Marten 100%!!
Apr 17, 2001, 11:20 PM
Registered User
A flat turn is uncoodinated and essentially you are sideslippng without maintaining directional control. Big increase in drag. Sideslipping is usually done to lose height on a high approach in aircraft without flaps or air brakes. Not very efficient at all.
Apr 17, 2001, 11:37 PM
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R. Carver's Avatar
I don't think that when someone suggests keeping the turns "nice and flat" that they mean keeping the wings perfectly level. I think we can all agree that you should always have some bank to the turn. We can also probably agree that when working a thermal you should be flying nice and slow, which would require less banking than if you were flying around at cruising speed. You shouldn't HAVE to stand a plane on it's wingtip..If you do you should probably slow it down, kick in a little TE camber and "flatten" the turn a bit, which makes for more wing for the thermal to lift. You would still need a small amount of bank to keep things tidy.
Apr 18, 2001, 09:03 AM
..teach monkeys to fly..
Marten's Avatar
If you _are_ in a big thermal, you will still be better off finding the core and banking around in a tighter, coordinated turn. At the speeds that gliders operate, and it applies to full scale as well as R/C, you only have to stay a bit above the stall to stay around minimum sink. Even at steep angles of bank, the stall speed doesn't increase that much, nor does the speed you have to fly for minimum sink, but you have to stay coordinated. If you get draggy, your speed decays very fast.

Perhaps "flattening" the turn should be redescribed as "opening" the turn.

Apr 18, 2001, 09:56 AM
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Ben Diss's Avatar
Flattening the turn, should be thought of as slowing the plane to minimum sink speed. Widening the turn often won't work because the thermal core is too narrow.

Couple thoughts:

1) I've spun a glider by using too much rudder and flying uncoordinated. If this happens to you while flying your neutrally balanced EuroMoldie about 20 feet high in a very light thermal, you'll never, ever do it again. Lesson: fly coordinated.

2) A 60 degree level bank doubles stall speed. This is bad. The plane is inefficient and ready to drop out of the thermal. Lesson: fly slow.

Apr 18, 2001, 02:34 PM
Registered User
I have stated from the begining that some bank angle is needed to help pull the machine around.
Yaw angles of 10 deg or less has a minimum effect (still has some effect). Keeping the turn flat has the effect of exposing more wing area to the available lift.

My thoughts on the subject.
1 If you want a flat turn
a. go slow
b. Slight banking
c. use 10 deg or less yaw
d. Keep a close eye on your plane
e. Follow the 10 deg rule
2 If you want a banked turn
a. fly faster
b. be coordinated
c. keep a close eye on your ship

All turns induce drag!!

A freind of mine is building a NSP Pulsar XL. I have one too. If he gets it built some time this summer,(you know who you are )I will hit the same thermal. If he is banked than I will go flat. Let you know what happens.

I suspect that the thermal will dictate the turn.

[This message has been edited by dhrider (edited 04-18-2001).]
Apr 24, 2001, 06:14 PM
Registered User
Originally posted by agpilot24:
Yaw + Stall = SPIN
I snipped some of that quote out of there, look further up for the whole thing.

Yaw+Stall=Spin, definitely, but I think the point here is that you can do a flat turn if you don't stall. I used to fly a Hangar9 Cub in flat turns all of the time...flew it cross-controlled. Y'all can discuss all ya want, but I seen it happen, so I'se a believer.

Of course, I wouldn't think of doing that at low altitude/airspeed in the real ones I fly..it's asking for a spin. But remember, the second part of the equation is also required: STALL. If you don't stall it, you won't spin it.

Also, in the real thing...if you fly THAT cross-controlled, you'll polish the seat of your pants as you slide around in the seat. :P

Apr 24, 2001, 06:51 PM
Registered User
I am a new member but here are a couple of points:

The optimum flight condition in a thermal is minimum sink speed (if your goal is to gain altitude) - this speed varies depending on the minimum bank angle necesary to stay fully in the thermal. The goal being minimum bank and fully coordinated flight at the best lift to drag speed at that bank angle. Anything less than fully coordinated flight adds drag. More bank than necessary to stay in the thermal's sweet spot effectively adds weight which increases induced drag.

The bank angle does not effect the lift that the thermal provides. If you are completely in the thermal, all the air surrounding the plane is moving upwards. The plane is moved upwards at the same rate by the thermal regardless of the bank angle. As above, too much bank angle increases the sink rate of the plane relative to the thermal lift rate.

[This message has been edited by steve262 (edited 04-24-2001).]
Apr 24, 2001, 07:25 PM
Registered User
I always laugh at model reviews which tout how the model will do nice, flat thermal turns. For a coordinated turn, bank angle is purely a function of turn radius and speed. Think of it as suspending a plane from a string and watching it do circles. So--you get "flat" from any plane, as long as speed is slow and radius is great!