Thermals...

[Tomo]

Got a Question:

 

Why when you fly into a thermal does your airspeed increase? If your trimmed S/L.

 

 

[Tomo]

Well seems tooi_dunno

 

 

[The Wolf]

If there is a slight gust, i dont think the thermal wont be going directly up. So it would be pushing you forward aswell. The hot air rising would seek to go to the lowest pressure point, so could pressure variations at different hights mean that the lowest pressure point isn't straight above the source of the hot air, and such wont go directly up but diagonally and cause the hot air to be going forward (relative to your plane) aswell?

 

Also, isnt there drag produced by lift, so if air is producing the lift instead of the wing, are you reducing induced drag by the wing?

 

Just my reasoning :thumb_up:

 

 

[Guest check-in]

You are trimmed straight and LEVEL. To stay at altitude you push forward in the lift generated by the thermal, so the aeroplane 'thinks' you are going downhill.

 

If you want a faster ride - and the opportunity presents itself - try flying level in the lift generated by a wind at right angles to a range of hills, on the windward side of course. Along the beaches in a sea breeze, always fly just to seaward of the dunes. Traffic permitting of course, because some coastal routes have recommended out and back tracks.

 

And if appropriately qualified and in a suitably-equipped aircraft, fly a few feet above the cloud tops - again traffic permitting - for a few more knots or a bit less power. A less comfortable but faster ride can be had just at the base of cumulus - just be careful not to exceed your aircraft's structural capabilities because it can be rough there, and again unless you are IFR there are limits on how close you can legally be to cloud (though how one accurately measures it I have yet to learn.....).

 

In something like a Skyfox it is possible to ridge-soar almost the full length of Moreton and Stradbroke Islands in the right conditions, using not much more than idle power.

 

Just be aware that strong winter winds over major features such as the Snowy Mountains are quite capable of generating lee turbulence that has broken aircraft, so whatever you do never get to leeward of large lumps if there are strong winds at altitude.

 

 

[Tomo]

Thanks guys,

 

I understand the 'down hill' idea, but I was curious the other day when coming home from St George, we where S/L at 5500ft 100 IAS, when we hit a thermal and the ASI went to about 120-125kts, the aircraft rocked a bit, but nothing serious, and we hardly went up at all. I didn't push forward on the stick, just wanted to see what it'd do, we exited out the other side possibly a 50-100ft higher, and our speed dropped back to 100 IAS. Getting higher I can understand, but why the increase in speed, so dramatically?

 

That's what made me wonder, If I was in the glider or drifter and wanted to climb, I'd 'use' the thermal and pull back on the stick, either hanging a sharp turn around it or just continuing on. I'd never given it much though until now.

 

What we can gather, is similar to what Mr Wolf said, the forward motion (of the aircraft), plus the upward motion of the air, causes a front on angle of relative air flow, thus increasing the ASI or relative airflow around the aircraft. Thus an increase in 'actual' airspeed for a moment.

 

i_dunno

 

 

[Kiwi]

A thermal is just rising air, I have been in thermals that are so strong that to maintain a constant height (Amberley was active) I pulled the throttle to idle, pushed the stick forward and managed 70 kts in a 503 drifter.

 

Don’t forget stick =speed, throttle = height lost or gained (oop’s wrong tread)

 

kiwi

 

 

[Tomo]

A thermal is just rising air, I have been in thermals that are so strong that to maintain a constant height (Amberley was active) I pulled the throttle to idle, pushed the stick forward and managed 70 kts in a 503 drifter.Don’t forget stick =speed, throttle = height lost or gained (oop’s wrong tread)

Yes, I've had a that a few times too in the 503 Drifter, coming back from the Oakey flyin comes to mind.

 

Hang on, you push the stick forward to descend? and pull the power back so you don't go to fast...!!

 

 

[Guest eland2705]

Thermals...hmmmmmm, To quote a well known kiddies song "these are my favorite things". :big_grin:

 

As someone else said, all this is a column of hot air rising. Hot air and cold have different densities/pressures, and this affects your ASI as it is driven by the difference between what is in your Pitot Tube and the Static return. Chances your TAS has no changed, well not much anyway.

 

Another thing to remember, a thermal spins, your relative airflow could increase or decrease if the spin is towards you or away from you.

 

We try and glide into a thermal against the rotation, so the relative airflow over the wings is higher, therefore more lift.:big_grin:

 

Hope that helped. (Come fly with us sometime, leave that noisy engine behind and have fun - just teasing!)

 

Regards

 

 

[Tomo]

Ok, thanks... though it sounds like no one has actually sat down and worked it out fully - it just happens sometimes, and that's as far as it's got?

 

Don't worry Eland, I do gliding as well, so know exactly what your on about!

 

 

[Guest eland2705]

CFICare,

 

You never know until you get into it.

 

It's not like the water running out the bath, always clockwise in the southern hemisphere (or is that anti clockwise, never remember). Sometimes you'll feel the "bump" under your tail and go right, cause that wing went up first and your surprised there's "nothing" there. If your alone in the thermal you turn around and go the other way, suddenly there is 5-7 knots up on the vario.

 

A good clue as to which way is to watch the Wedge tails. Even though they don't have instruments, they still get it right 99% of the times. Besides, nothing nicer than siting under a 5' Wedgie watching how effortlessly he flies, so much grace and elegance.

 

(Why am I sitting in the office, I should be out there flying!)

 

Regards

 

 

[shags_j]

Knew all the glider pilots would come out of the woodwork for this one ;)

 

 

[eastmeg2]

Hi Tomo,

 

Reading your post #6, it sounds more like you encountered a headwind gust rather than a thermal and it was the extra airspeed that provided the extra lift rather than any rising air. Seeing your ASI spike like yours did is the usual giveaway for the gust rather than lift scenario. I'm a bit suprised that none of the glider pilots have pointed this out yet since they aim for total energy management.

 

Then there are other tricky things to try to work out, like whether the turbulence you're experiencing is due to thermals, terrain or wind sheer.

 

Cheers,

 

Glen

 

 

[Tomo]

Yes, thanks Glen it could have been that... Wouldn't be terrain though, pretty flat out here. And I'm pretty certain it was a thermal though, because we were under a big Cu at the time.

 

It's not the only time it's happened either, and glider pilots would agree, when you enter a thermal, your airspeed rises. Why? I'm pretty curious now, but I guess there could be so many variables its hard to say what it could be really. i_dunno

 

 

[Bidgee]

Well seems tooi_dunno

What did your ground speed do?

 

 

[Guest eland2705]

CFIcare,

 

I have looked for some "scientific" publication that mentions this rotation of thermals. Not with a great deal of luck.

 

I have attached the relevant chapter from the GFA (Gliding Federation of Australia) Gliding Manual that deals with thermals and thermaling. Again no mention of the rotation. (If you have problems getting it, PM me, I can send it to you by e mail.)

 

From experience I can say that there is very definitely a "right way" and a "wrong way" to fly in a thermal. As said before, if you join a thermal, say, in a left hand turn, you know that there is lift there (all the signs say that it's there) but you find it very scratchy, turn around and fly it in a right hand turn. Suddenly there is a marked improvement in the quality of the lift, and the aircraft flies much more positively. I have watched eagles fly into a thermal, switch direction, and "get away". I figure that they know more about it that we do, so if it's good enough for him, then who am I to argue? I bow humbly to his superior knowledge and I am greatful to be allowed to share the sky with him.

 

The only thing that I can say about that is the relative airflow over the wings is better, therefore producing more lift. Much like the difference between landing downwind as opposed to landing into wind.

 

With regards.

 

10.pdf

 

10.pdf

 

10.pdf

[Al B]

Sorry Steph, that 'water down the drain' thing is a myth - it could go either way. Please see snopes.com: Coriolis Force Effect on Drains

 

 

[Guest rocketdriver]

With regard to direction of rotation, its my understanding that neither bath plugs nor willy willies are big enough for the coriolis effect to have an effect, as it were, leaving the rotation direction of thermals to be random and caused by local geometries and local wind conditions at the point of origin. I read somewhere that you need a "fetch" of around 100 miles for coriolis to kick in, so cyclones are about the smallest wind systems that have a predictable overall direction of circulation.

 

 

With regard to a speed change, as you enter a thermal, this is quite likely, especially if you are entering towards the top of the thermal. Remembering that the thermal is like a doughnut with up going air in the core (the hole of the doughnut), and sink around the outside, there is an area towards the top of the thermal where the air is flowing horizontally out, and at the bottom where it is flowing in towards the core. If you encounter a thermal, you are obviously to some extent flying towards the core, even if obliquely, and so must encounter one of these two horizontal winds, but which one?

 

Most likely the out flowing air. Why? Because if you encounter the inflowing air, you are towards the bottom of the doughnut and may miss the lift altogether, and so not even notice the thermal .... whilst if you encounter the out flowing air you are entering the active lift part of the thermal and are much more likely to notice it!

 

Regards RD

 

 

[Tomo]

Wouldn't it depend on which way the wind is blowing? If it was going to rotate at all?

 

Whirly winds and thermals are a huge difference, so no use even comparing them.

 

Thermals are Hot air rising from the ground as it heats up, by the sun. Hot air rises up until the air equalizes, or becomes the same temperature as the surrounding air, it will stop at that, and often create Cumulous clouds, the hot air rising will push the cold air down, and it will obviously go down the edge of the hot stuff, as per RD's donut description.

 

That's the stuff we can understand.

 

Now wonder what happens if the wind is blowing, sometime it blows all the thermals in a line type effect, often referred to by glide/soaring pilots, as "cloud streets".

 

Rotating thermals, Mmm, Eland obviously can feel something different in the change of direction - if it isn't rotation, I wonder what it could be? There must be some difference, because he wouldn't say it.

 

Then we should consider if it is actually a normal thermal or turbulence, caused with the surface. As the surface air tumbles over obstacles, the air in the lower levels is in constant vertical motion. Within the surface layer, all of the air is either rising or sinking. The rising air is expanding while the sinking air is compressing. If the turbulent layer becomes deep enough, adiabatic cooling brings the rising air to dew point and cloud forms. As the air begins to sink again, adiabatic warming causes the cloud to evaporate once more. By this process cloud is being continually formed in the rising air and dissipated in the sinking air.

 

The cloud layer, which is stratocumulus, forms between the condensation level and the top of the turbulent layer. The turbulence is enhanced by convection during the day, so the stratocumulus cloud often dissipates at sunset. This type of cloud can often be seen forming over the land along the coast, while the sky over the sea remains almost cloud free, because of the reduced friction.

 

Now, if you think about it, that type of lift would be enhanced by going one direction or the other, wouldn't it? Doesn't mean the rising air is rotating like a symmetrical shaft point up, but rather a rolling effect over the top. So flying into the wave (so to speak) would be more efficient, than going with it. If I'm making sense?

 

Ok, so I've gotten carried away again, but we have to consider a few things. :big_grin:

 

 

[Exadios]

I agree that thermals often rotate. I think that a left hand turn into the thermal is more likely to work.That is a thermal is more likely to rotate in a counter clockwise direction as view from above.

 

As somebody has already said, look for eagles.

 

 

[Benjamin Pitt]

Okay here is my theory as to why your airspeed "bumps" as you enter a thermal.

 

We can all agree that a thermal can easily have air rising at 15 knot's or so.

 

Rising air hits the surface's of the aircraft at a different angle of attack than before. But only one surface has an big effect on the attitude of the aircraft.

 

The upward push effect on the horizontal stabiliser gives a nose down attitude allowing the pitot tube to accurately read the air speed over the aircrafts surface from the uprising air flow.

 

It follows that when leaving a thermal the nose pitches up as the tail is hit with air flow going down, although again sometimes the airspeed indicator "bump" seems to be a drop? Perhaps the pitot tube does not register the downdraft, but simply the effect of losing the original updraft?

 

Anyone else have similar thoughts?

 

I have not noticed this effect in a helicopter which have much smaller rear horizontal stabilisers.

 

 

[Yenn]

Would not the first effect of flying into a thermal, be a raising of the nose caused by the wings encountering the rising air, quickly followed by a levelling a the tail gets into the rising air, and of course the opposite when leaving.

 

I have seen this effect when sailing a yacht into a sharply defined tidal stream, but I dont have the quick reflexes to see it in the air.

 

 

[Benjamin Pitt]

No, because the lift produced by the wings is acting on the CofG. The tail has a huge arm acting on the CofG.

 

 

[Benjamin Pitt]

BP...u amaze me

Why? Let me know if I'm way out of the ballpark :thumb_up:

 

 

[Ballpoint 246niner]

Work with me on this.. Horizontal speed is defined as a vector parallel to direction of travel- right? A thermal introduces a vertical component of movement( energy) that when it acts on the aircraft at a tangent to the original vector will have a sum component that adds to that vector known as resultant- OK?

 

so any vertical vector that works in the direction of Lift (even roughly will add to the forward vector (resultant), increasing the forward vector( airspeed). This is conversely true of sink which detracts from the lift vector- therby reducing airspeed.

 

Macready worked all this out with Polar curves and came up with formula's that gave "speed to fly" based on Best L/D vs lift/sink and min sink vs time( Macready ring)- Glider pilots use it all the time although most have it all compuerised now!

 

So tomo, you're right- if we do nothing else and encounter rising air- there is a minor(but noticable) increase in airspeed- think of it as vertical thrust, you will also hear and feel the increase in velocity( sum of vertical+fwd). It's how glider pilots, and Hangies "feel" lift and core through "seat of the pants"

 

Now let's finish off on the sink side- fly into increasing sink, descent rate goes up- do nothing with stick aircraft slows down- reaction is to pull stick back to stop aircraft sinking-WRONG! you can now see where all the ab initio problems start! Of course we all know power is our descent rate control- in gliders you soon learn to get it right- stick fwd- speed up- protect airspeed- get through sink faster. Hope I haven't bored you yet- love discussing practical physics, particularly with flying!

 

 

[Ballpoint 246niner]

Eland, you're absolutely right- read Reichmann, ( glider pilots) or any other noted glider authors like Welch etc and they back up the direction/rotation thing. Essentially thermals are all variations of a upward flowing vortex, with "donut" like internal rotations, and varying vertical/horizontal flows through their cross- section.

 

I spend over 30 years gliding and hang gliding and there is definately a "sweet" direction for any thermal, and it varies all the time. And yes as everyone has noted, only the birds get it right most of the time, but you can definately feel it when wrong, by constantly getting "thrown out' of the core, or constantly recentering to maintain best climb- oh it's all good fun- we even had the Tecnam going up on low pwr a few months ago. Returning to gliding is going to be my golf- but a lot more fun- miss it terribly.