Flap Setting in a Thermal?

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Lenticular
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Flap Setting in a Thermal?

Post by Lenticular » Tue Jun 15, 2021 7:56 pm

Hi,

What is a good flap setting in a thermal?

In both the JS1 and Diana-2 I find I'm using maximum flap for greatest stability when flying at min sink. It just alleviates the tendency for it to reach the stall and prevents the aircraft accelerating wildly, but it appears not to adversely affect the ability to climb.

As soon as I come out of the thermal I'm back to flap 3 (JS1) or flap 8 (Diana).

Anyone else find similar?
G-ZULU /// LTN
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wickid
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Re: Flap Setting in a Thermal?

Post by wickid » Tue Jun 15, 2021 8:07 pm

Several planes have landing flaps that can be used in thermals as well. For example in the Arcus manual it is explicitly stated that this is a possibility.

On these planes most of the time flaps L has a very narrow speed range were they are more efficient than the flap setting just below it. This speedrange also depends on your bank angle. So if you are able to stay in that area of the polar it should improve climb.

Other planes like the ASG29 or the EB29 have dedicated landingflaps which put the plane in a high drag configuration. On those flaps L is not suitable for thermalling.
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Lenticular
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Re: Flap Setting in a Thermal?

Post by Lenticular » Sat Jun 19, 2021 5:59 am

Hi,

Thanks! I did some more research, and broadly had the right idea.

I just seem in Condor to find there is no such thing as "too much flap". I'm fairly careful with flap usage, extending them when slow, and cleaning up early during acceleration (but not below stall speed, obviously).

The JS-1-21 doesn't like landing flap in a thermal, but that seems to be expected.

Reading about the Diana2 flaps, it seems some pilots of the real aircraft stick strictly to the recommended flap/speeds as suggested by the manufacturer, but using these in Condor feels...not quite right.

If you take an aircraft without ballast, the flap settings feel too much/too much drag, and sometimes not one but two notches of flap less helps significantly.

I find in the Diana, at best L/D I'm using +8, and to go any faster +3 or less (>60 kts).
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wickid
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Re: Flap Setting in a Thermal?

Post by wickid » Sat Jun 19, 2021 8:36 am

The recommended flap speeds on the rings in Condor are for full water/ballast*. If you want the speeds for an empty glider the formula is the squareroot of the old weight over the new one. It differs quite a lot on the Diana as it has a very broad weight range. I think about 20 kph off the top of my head.

* ASW20 is set for 0 ballast as it is used in clubclass. There is a scale at the bottom (top for imperial) that shows what weight/wingloading the ring is set at. The area of the scale aligned with the 0 on the ASI is what weight/wingloading the ring is valid for. In real life you can rotate the ring to your actual wingloading. Hopefully one day in Condor too.
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Lenticular
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Re: Flap Setting in a Thermal?

Post by Lenticular » Sat Jun 19, 2021 11:28 am

Yes, I saw that.

You are correct - approximately 10 kts/20 kph difference.

I am in the process of editing the rings so I can "select" the ring suitable for the ballast loaded.

You gave me an idea actually! :D
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Kestrel_BY
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Re: Flap Setting in a Thermal?

Post by Kestrel_BY » Wed Jun 23, 2021 10:19 am

Basically the "spirit" behind flaps is the following :

- +1/+2 : Thermaling. +1 generally allows for a slightly better climb BUT requires you to fly at a higher IAS and with a smaller bank angle, so this is only profitable in wide thermals, while +2 decreases the stall speed much more, allowing you to narrow your turn and therefore generally puts you much closer to the thermals core, but at the cost of a (very) slightly increased sink rate.

- 0 : Max L/D Speed (you generally don't cruise at that speed for it's too slow, unless thermals are really weak or you're crossing a very wide "blue" area).

- -1/-2 : Transition, adjust depending on the speed.

Note that on the JS1 for example, you have the following flaps correspondance :
5 = +2
4 = +1
3 = 0
2 = -1
1 = -2

Based on this, you have to "adapt" to your glider. Some gliders do thermal well in landing flaps (even though you will slightly increase your sink rate), some won't. On some gliders you can even use the landing flap to do a quick, very narrow turn when necessary and then go back to +2 (the Ventus is very good at that game for example, while it's an absolute no-no in schleicher gliders because of how their Landing flaps work). Aditionnally some gliders have much more negative flap settings (the ventus has S and S1, the ASG 29 has 1/2/3 as negative, 4 as max L/D, and 5/6 for thermaling).

I haven't tested the 21m much but on the JS1-18, I'd recommand thermalling somewhere between 110 and 120 km/h in flaps 5 when fully ballasted, with the use of the Landing flaps only when necessary (for example when using the Heintz Huth (I hope I wrote it properly) method). If you're not confident in fully ballasting your 18 meters gliders, they all fly very well and gently between 50 and 55 kg/m² while keeping a fairly decent polar at that wing loading.

Also here is a rule of thumb I like to give about flaps :
When you're increasing your speed, it's better to keep your stick where it is and move the flaps forward, while when you're decreasing your speed, pull the stick back and only get flaps out when you reach the appropriate speed (to avoid dissipating your energy too fast).
Indeed sometimes you do pull full flaps at once when entering a thermal you didn't expect to find, but that's an exception, not the rule.

I hope this all makes sense, feel free to ask me to be more precise if necessary.
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