ballistic turbulence

[Ben Longden]

Okay, its not really an ICAO term, but one that pretty accurately describes the thermals encountered at Shepparton in the stinking hot weather the last few days.

 

The sky has typically been crystal clear, blue sky and not a cloud in sight.

 

My question is... is there any way to figure out the locations of the rising thermals bearing in mind there are no clouds, and lots of open, clear land.

 

Ben

 

 

[Guest pelorus32]

Hi Ben,

 

I've always found it very reliable to fly through them, then you know where they are:cool:

 

This is a great topic for the glider guys - Captain where are you?

 

From my gliding time a long time ago I think that there are two things at play - differential heating and the drift of the rising air downwind.

 

Classic example at Shepparton is when on final for 18. As you come across the Broken river you always get a bit of a burble - I think that's probably from a combo of things. Probably some mechanical turbulence when it's blowing a bit plus trees as I recall don't tend to generate thermals whereas bitumen, hard dry ground and tin roofs do I think. So when landing on 18 you have thermal activity upwind of the Broken so you may get thermals drifting downwind and appearing as if they come off the Broken when in fact they were generated upwind.

 

Another truism is that if you are in lift you are soon going to fly into sink.

 

Come on Captain, lob into this and put me to shame:;)3:

 

Whatever the process I hate it when you have everything nicely trimmed for 60 knots and about 300 fpm descent and next thing you know you have 6-700 fpm up and the speed all over the shop. How can a bloke fly a stabilised approach under those conditions? Drive it in, drive it in...

 

Anyway how come you get to fly on week days?

 

Regards

 

Mike

 

 

[Guest pelorus32]

Also this is an interesting read:

 

http://www.raa.asn.au/groundschool/umodule21.html

 

Regards

 

Mike

 

 

[Guest Juliette Lima]

Hi all,

 

It is not uncommon in any given area of a local strip, for some thermals to be in much the same location for years on end...... the only variation being wind direction, or strength of upward movement depending on heat. Simply put, they emanate from the same ground position for whatever reason triggers the activity. How often do you see soaring birds in much the same location time and again, even in a township or builtup area?

 

These thermals (usually within a ten mile radius or more of a strip or airport) are known as 'House Thermals' by glider pilots and are often sought after life savers when other activity is not readily available. I've had many five hour flights (one of the milestones in gliding) returning to a known thermal activity spot or 'House Thermal' to gain some height before heading off again in search of more delightful thermals.

 

There are about four in the immediate area of Cessnock in the Hunter Valley (doubtless stacks more) where I fly. One that works consistiently is about 5 miles North of the strip and unfortunately a little too close to be 'playing' with.

 

One however is a about 14 miles NW and is a corker....always working and quite often with wedgetail eagles soaring....It is great to reduce power, add some flap and gain a few thousand feet, even if considerably less efficient than a glider.

 

The point of all of this is that it pays to pinpoint 'house' or local thermals if flying regularly from a strip....you can either play with them, or avoid them, except if regularly on downwind !

 

Enjoy

 

JL

 

 

[Guest Juliette Lima]

Hi again,

 

I remember a wag at a soaring club I belonged to convincing many of us that he had special glasses that could pick out thermals....problem was he kept on winning comps !

 

JL

 

 

[Guest Fred Bear]

lambadaman will also know a lot about this topic. I know that when in the Bantam you can smell the sheep and cow dung when you have entered a huge thermal.Same goes for the Drifter!

 

 

[Ross]

One of the things that happen with thermal activity is the difference during the day. In the morning a shallow water area or a dark area of soil or rock or trees may be quite cool relative to other areas and therefore act as a sink area as it soaks up energy from the sun(both for aircraft and air). Other lighter coloured areas may start giving off weak thermals earlier and therefore may not get as hot as the darker areas that are soaking up more energy instead of giving it up to rising or passing air.

 

Later in the day as this area is now hotter than the surrounding area it may trigger more thermals (rising air and aircraft) as the surrounding ground has already given up a lot of its heat. Some of these areas like the very shallow small lake near the Brobenah airfield at Leeton may have still have weak thermals for some time after sunset.

 

Sometimes a car or a tractor working a paddock may be enough to trigger a thermal. If you have a recording temperature trace on an airfield where there is good show of cumulus clouds you will see a noticeable dip in temperature of maybe one degree or so at approximate five minute intervals as thermal activity passes the recorder. If you know the local dewpoint temperature you can calculate the height of the cloudbase. At Leeton usually during the summer you would expect the cloud base AMSL to increase during the day by a least 1,000 feet as the water content of the air is gradually reduced by evaporation from the ground and plants and transported elsewhere by the prevailing air mass which is usually moving by at least 5 km per hour.

 

Though the velocity in that mass can change dramatically depending on where you are in relation to the thermal activity or other local influences. On the ground it feels as though the air mass has stopped moving and then speeds up again perhaps to double prevailing air mass velocity as each thermal passes the observer.

 

Of course all these rising air columns are also associated with just as much sinking air so if you want to use them like a sail plane pilot you have to learn how to stay centred in that rising column of air which may have a lean on it and also have different rates of lift.

 

Inversion layers in the atmosphere may cap the height a thermal can go thus preventing it reaching a height where the dewpoint temperature would make the moisture condense in the air thus showing it up as the base of a cloud, usually cumulus. Watch a cumulus cloud continuously and you will usually see them form and collapse in as little as five or ten minutes.

 

One of the dangers of flying into cumulus clouds aside from the lack of IFR capability and a very short life expectancy is the fact that as water vapour condenses into visible droplets making a cloud visible it gives up a lot of energy. This extra energy can cause aircraft flying near the base of a cloud to be sucked up into the cloud despite deploying dive brakes or cutting the throttle to idle or pushing the stick forward up to VNE and attempting to dive out the bottom.

 

A possible escape from this situation is to spin the aircraft providing it can be safely done and recovered from. A spin that develops into a spiral dive will probably remove the wings or destroy the aircraft as flutter speed is reached.

 

I vaguely remember from the pilot handling notes of the Blanik that its actual vertical airspeed down when in a stable spin was something like about 80 knots - or about 8,000 feet per minute! I once spun one from about 5,000 feet in clear air down to about 2,000 feet on purpose - it didn't take long and I think my instructor on the ground nearly had a heart attack.

 

Safe flying

 

Regards

 

 

[pylon500]

I never thought of myself as the worlds greatest glider pilot, so when I was supposed to be studying the ground from the air to look for thermol triggers, I was also using another method of finding them...

 

If you fly in a straight line long enough, you usually hit one!

 

Arthur.

 

 

[Yenn]

No sheep up here but rather than smell we can hear the Bull**** at the airstrip. Seriously though does a spin develop into a spiral dive.

 

I've done a bit of spinning and never felt worried by it, but on instruments once I got into a spiral dive and it was frightening, OK when the instructor flipped up my goggles and it taught me never to go into cloud and when I hear the expert RAAus pilots saying they can fly in cloud I get a bit stroppy.

 

 

[Ben Longden]

The good guys and gals at CASA have a math equation for this;

 

VMC into IMC = CD

 

Where VMC is visual Met conditions, into a flight that is instrument rated equals certain death.

 

Or

 

As a training vid they have for VMC / VFR pilots without any instrument training who decide to fly into cloud, you have "178 seconds to live".

 

One accident site they investigated and did during a CASA training seminar at Shepparton was a flight such as this, that ended up with the aircraft decending, inverted at 4000 feet per minute, from an altitude of 2000ft.

 

The blunt message was if you ever contemplate IMC conditions, then make sure you are instrument rated.

 

Ben

 

 

[facthunter]

Spins & spirals.

 

To Yenn's serious question, Can a spin develop into a spiral dive?

 

I suppose it can but there is no reason why it would.. Some aircraft spin in a steep nose down attitude. In a spin, the airspeed does not increase much above the NORMAL 1G stallspeed (depending on the direction of spin & the location of the pitot tube, minor effect).

 

Depending on the spin characteristics of the particular aircraft, and considering the steep nose down mentioned already, it is possible that more than 1G can be experienced. The aircraft remains stalled, but at a higher speed, Like a flick roll, but the axis of the roll is vertically down rather than horizontal. The roll is caused by one wing being & remaining stalled more than the other.

 

Remaining in a spin will not of itself stress the aircraft significantly, (unless you hit the ground)& generally the rate of descent in the type of aircraft we fly is not that high. ( Rarely above 6000 fpm)

 

The recovery (if possible, and this is a complete unknown, so don't try it in a non certified aircraft) may impose much higher loads,& I suppose if carried out incorrectly, could result in a spiral. (I can't see why it's particularly likely, however.)

 

A spiral dive results from a situation similar to when the nose drops toward the lower wing in a steep turn,For whatever reason,( Usually a badly applied rudder) & an attempt to lift it by pulling on back stick results in tightening the turn, Pulling more G, Airspeed keeps on increasing, & if recovery is not effected by rolling the wings level & removing power very quickly the aircraft will be destroyed by excess speed& dynamic loading. Any rate of descent up to the terminal velocity of the falling bits, can be achieved. A spiral is a pretty serious matter, Generally terminal with a non-instrument pilot in IMC. N.....

 

 

[Guest pelorus32]

As I understand it some aircraft are reluctant to remain in a spin and tend to unstall and end up in a nose down situation that can then turn into a spiral.

 

Some aircraft will remain in a spin quite comfortably until recovered but others require severe encouragement to enter a spin and things like stick back and outspin aileron to hold them into the spin. Release that and you have an aircraft that is nevertheless nose down and with roll and yaw happening but not stalled.

 

Others may have more detailed info of the spin characteristics of particular aircraft.

 

Regards

 

Mike

 

 

[facthunter]

Spins

 

To respond to pelorus32 & for others who may be interested,and acknowledging that a great deal can be written on this subject.

 

The only way to get first hand experience of spinning ,responsibly & safely, is to do it in the correct type of aircraft,after a full explanation of what is involved including how particular aircraft design features, in general affect the ability of a particular type to be able to spin, and be able to recover from a spin & what particular recovery technique is appropriate for that aeroplane. This should be done under the auspices of a competent training organisation.

 

At risk of generalising, I will say that no 2 types spin the same. They are all different. Some aircraft will not spin, Or certainly will not "settle " into a spin. The extreme end of coaxing is when you maintain height with power off till you have full back stick, the A/C is able to stall fully(ie enough UP elevator & tail heavy enough), at which point FULL rudder is applied. Full back stick is maintained, and sometimes full out of spin aileron helps to achieve a cleaner entry (less buffeting, DHC-1) and may help the A/C to stay in a steady spin. Most modern light aircraft, (Don't try it ,they are not certified for it) will drop a wing , go a bit (or a lot) nose down and unstall at that point , and will respond to normal control inputs at that stage. These aircraft are deliberately designed that way for the market they serve. However if you REDESIGN the aircraft BY LOADING IT TAILHEAVY, all this changes and you can create a monster. Nev....

 

 

[Guest pelorus32]

Hi Nev,

 

I have done quite a lot of stalling, incipient spins and multi turn spins. All of the incipient spin and spin stuff in one aircraft type - a Blahnik glider. I learned to fly gliders as a school kid/young adult many years ago. At that time stall/spin accidents were very prevalent in the glider world and so a great deal of time was spent on getting us to understand the process of stall, incipient spin, spin, spin recovery.

 

It was great fun and has ingrained a "first learned" set of stall behaviours that are instinctive. The first time I was put through a stall with wing drop in the Tecnam I recovered the wing with my feet, before I had thought about it. Ingrained behaviours.

 

Obviously in RAA type aircraft all we do is simple stall work - power off/power on, turning/straight ahead etc.

 

The point I was trying to make however was about the question "can a spin decay into a spiral?". Without being able to reel off the specific aircraft types I certainly believe that the answer is yes.

 

Regards

 

Mike

 

 

[slartibartfast]

Me too Mike. I used to love spinning the Blanik.

 

Loops were my favourite, although there's a lot to be said for an elegant chandelle.

 

Ross

 

 

[facthunter]

Spirals etc

 

Yes Mike , Your background stands you in good stead & if you can be as a result of training and your knowledge, able to resist the INSTINCTIVE things that can occurr with control movements, you are the better pilot for it. A lot of readers of these threads won't have our experiences, & quite possibly don't need them,(I am not sure). Glider training, probably essential, as you fly near the stall,have large wingspans & manoeuver in turbulence. Most of the refined & even some of the most basic aircraft we fly are pretty benign in their flight characteristics. The wing drop at stall, & stall demonstrations are frequently ,not very convincing. I am not sure that our approach to the training in this area is as across the problem as it might be. We are supposed to give the student the best chance of survival in the situations that can / will occur, yet we accept techniques for stall recovery where hundreds of feet will be lost. In many situations ,that height is not available. I am very aware of the need to be responsible when putting opinions/facts out there, that on the subject of spinning/spirals & the like one has to be very careful not to encourage dangerous behaviour, and that the material is relevant. I don't always find this easy, so I try to be thorough. Sometimes I feel I am stating the obvious, but maybe it's not obvious to all . You can't take all day to make your point either, so I'll end now. N......