Myth busting! I MIGHT STALL IF I TURN DOWNWIND!

[Guest rolfeja]

Sorry guys I just cant let this go. The reason being I know someone that has stalled when turning onto base with a crosswind blowing. Ie turning base was actually turning so they then had a tail wind. and from my own flying i know that when you turn downwind the controls will feel sloppy or if you turn into the wind you will feel slightly more airspeed. It might only be slight and only for a few seconds until the aircraft decelerates but one the less I think it is something that cant be ignored. if there are any control line model fliers here. If so you will know that when you fly your model in a wind it will climb when it flies into the wind and decend when flying away from it. This is because when it first turns into the headwind it still has the extra energy that it gained while flying downwind. because of the momentum of the aircraft it will take an instant to use this energy. if there is any pilots here that used to fly the old skycraft scouts maybe they can share some light on what they experienced when they turned downwind. Im sure flying around at treetop height just above the stalling speed they will be very aware of how much performance they lost when turning down wind.

 

 

[Guest rolfeja]

just to add to the above it is easy to see that an aircraft does indeed have more energy when flying downwind and this can be shown by the landing distance take when landing with a tailwind compare to a headwind. the landing with a tailwind will take longer to stop because of the extra energy

 

 

[Guest pelorus32]

thats suppose to say a plane pulling out from a dive to a tailwind

Hi Rolfeja,

 

you are needlessly confusing yourself. Please forget about pulling out of dives, steep climbing turns and all other extreme evolutions.

 

The easiest way to understand this is to work through it from still air, to moving air but with the aircraft flown in a sedate fashion.

 

As for the more extreme evolutions - I can stall an aircraft pulling out of a steep dive in still air - turning down wind is an irrelevance - it's just a matter of exceeding the critical angle of attack.

 

Regards

 

Mike

 

 

[vk3auu]

What is confusing the issue is Newton's First Law of Motion.

 

" Every object in a state of uniform motion tends to remain in that state of motion unless an external force is applied to it.

 

 

 

To change direction, you have to use energy, in this case, if you are going from upwind to downwind, the energy needs to be somewhat higher. In fact, to go from 80 knots indicated downwind to 80 knots upwind having started with a tailwind of 20 knots, you need to input about 25 percent less energy than if it was the other way around. So, if you were flying at the point of stall before you made the turn, you would theoretically stall if you turned in the other direction without inputting that extra bit of energy.

 

 

 

Even without wind, you still would need to input some extra energy to execute a 180 degree turn. This was why the early Royal Flying Corps pilots were instructed to lose height in a turn, because with the low powered engines, insufficient extra power was available to turn without stalling. The extra energy was obtained by losing a bit of altitude, thus converting some potential energy into kinetic energy.

 

 

 

However, in the case quoted above, a stall is unlikely, because, hopefully Ultralights will be flying well above the stall, unless of course he is flying a 503 powered Thuster two up.

 

 

 

So, beware the downwind turn.

 

 

 

David

 

 

 

[Ben Longden]

Beware the angle of attack, you mean....

 

Time for us all to dig out the BAK and have a good re-read methinks. its all in there.... among the basics..

 

Ben

 

 

[Flyer]

if there are any control line model fliers here. If so you will know that when you fly your model in a wind it will climb when it flies into the wind and decend when flying away from it. This is because when it first turns into the headwind it still has the extra energy that it gained while flying downwind.

The reason you notice this as a control line flier is that your feet are on the ground and that is your reference point. You have an air mass moving around you so your model will behave as you describe.

 

we have seen many RC models bingled because people turned downwind "too slow" and stalled.

 

What has actually happened is that they have taken the reference point from the ground and tried to relate this to the speed of the model.

 

Just as a matter of interest Rolfeja, at what speed does an aircraft stall ?...this is not a trick question....

 

regards

 

Phil

 

 

[Guest rolfeja]

well an aircraft can stall at any speed. It will stall when the angle of attack exceeds the critical angle and the airflow over the top of the wing can no longer follow the shape of the wing and will break away becoming turbulent. you will then loose nearly all of your lift. but in my argument I am assuming that you are maintaining a constant power setting and a constant angle of attack through out the turn. the reason the control line plane will climb when turning into the wind is because for a given angle of attack and a given airspeed the wing will produce a given amount of lift. if we increase the airspeed or the angle of attack the wing will produce more lift as well as more drag. When the control line plane flies into the wind it will climb initially because as it turns into the wind it will pick up airspeed and with out reducing the angle of attack the wing will produce more lift making the plane climb. that is until the extra drag created by the lift over comes the inertia of the plane bringing it back to the speed in which it will maintain with the given power setting.

 

Jamie

 

 

[Flyer]

Correct on the stall and correct with the control line plane. Now unglue your feet from the ground and float with the wind while flying that control line plane . Does the plane now fly into the wind and away from the wind?

 

No.

 

Why? Because you are floating along at the same speed as the wind hence no wind movement just like the fish in the bowl in the train and the hot air balloon.

 

Regards

 

Phil

 

 

[Guest TOSGcentral]

//Leaps up and down in anguish!//

 

 

Rolf mate, you really must do something about your situational awareness and perceptions!

 

 

Of course the control line model will gain energy into wind and lose it down wind! Exactly the same as the lines will slacken in the upwind quadrant as the model is blown in towards you and tighten to maximum when transversing the downwind quadrant of the circle.

 

 

This is because the model is not free flying in an airmass that happens to be moving across the ground and the model is being taken with. The model is very firmly anchored to the ground by your feet and the connecting lines!

 

 

You personally feel the wind because your fixed position reacts against the wind force and you can therefore feel the wind pressure. Try flying in a balloon! Even in a howling gale it is dead calm as the balloon is moving with the airmass that it is flying in.

 

 

However, to give you some alleviation there is apparently something in what you have to say generally about turning into and out of wind!

 

 

Quite a few years ago I became embroiled in a ‘Readers Letters’ debate in Pacific Flyer on this subject. Some egg head took me on and came up with a string of equations and physics to prove that some inertia dynamics do actually having a bearing on airspeed.

 

 

The maths were beyond me! I just teach people to fly and fix aircraft and have grave difficulty counting beyond six beers (but there may be other reasons for that!). So my stance was fly with me and demonstrate it in terms that I can show and teach a student – and – it has some kind of relevance on the safety of the student and their control of the aircraft in the sort of flying that we do – as in 40 years of flying instruction it is not something that I have noticed IF there is no airmass energy changes going around me.

 

 

You could perhaps profitably study the RAAus Theory part of their web site written by John Brandon. John goes into depth about visual perceptions of turning into and out of wind via reference to ground features. Perception can be a powerful thing my friend and totally eclipse what is actually happening in favour of what you believe you see happening.

 

 

Another example is parallax! The aircraft may appear to be pointing in anywhere other than the direction it is travelling. That is not in the flying training syllabus so students get to spend a lot of their hard earned cash until they work it out for themselves and not really know what they have done other than quieten the oaf in the right hand seat!

 

 

Aye

 

 

Tony

 

 

[Guest rolfeja]

ureka!!! I understand now and I agree thanks. I am a jump pilot in tas and I often get to fly high where the winds are up to 60 knots so next time i get a chance i will give it a try in the cessna just to reinforce it for myself. thanks for being patient guys :) hmm think i can smell some humble pie better go eat some hehe

 

Jamie

 

 

[Guest rolfeja]

TOSG central yeah i understand I was not worried about the ground perceptions i just had diffculty getting my head around that the inertia the plane has going downwind wont be converted into airspeed when it turns up wind. I know it has more inertia because it takes longer to stop flying down wind but i guess it doesnt convert it into airspeed because as you are making the turn the plane is constantly getting pushed with the wind and accelerating at the exact same speed as the wind so that as it turns with the wind behind it it has already caught up witht he wind in effect. Does that make sence?

 

 

[Flyer]

:thumb_up:

 

 

[Guest TOSGcentral]

Not really Jamie - you are still trying to over-complicate something essentially simple. As far as the aircraft is concerned there is NO wind.

 

What is of much more interest to me is your perceptions of landings and inertia - so a few words on that for the benefit of student readers (if I have read you wrong then please correct me).

 

Firstly, the aircraft certainly has inertia and this is relevant in terms of the ground via both airspeed and groundspeed.

 

To reach a status of being stationary relative to the ground (when landing) then the aircraft's ground speed has a huge bearing on what is going on.

 

If there is zero wind and the aircraft's landing speed is, say, 40 knots then its ground speed will be 40 knots and a certain amount of runway distance is required to come to rest.

 

If the aircraft is landing into a wind of 20 knots then the airspeed relative to the airmass is still 40 knots but the groundspeed is now 20 knots (groundspeed directly into wind is airspeed plus or minus windspeed depending which way you are going) so the aircraft touches down at 20 knots and requires half the distance (which is why we land into wind when we can).

 

If you land downwind in a 20 knot windspeed then the groundspeed is 60 knots (windspeed plus airspeed) and you are going to need the devil of a lot of runway to land on!

 

Tony

 

 

[Guest rolfeja]

yep so wouldnt it be correct to say the aircraft has more inertia when landing downwind? isnt that why it takes longer to stop

 

 

[Guest rolfeja]

in other words doesnt somehting travelling at 60 knots have more inertia then something travelling at 20 knots?

 

 

[Guest TOSGcentral]

You are correct Jamie but it depends not just how you define inertia but how you use that definition and in what context.

 

Certainly in recreational flying instruction there is a screaming need to express things, which may have a complex physics base, in very simple, everyday terms, that people who are possibly not blessed with a great deal of education or even significant brain power, can understand and apply and thus protect themselves - because such people can and do become highly competent and safe pilots.

 

It is very simple to say that something travelling at 60 knots is going to take longer to stop than if it were travelling at 20 knots. Do we need anything more than that as a base to rest airspeed/groundspeed understanding on in the landing scenario?

 

However, as I suspect you are itching to do, it is not really valid to extend the inertia argument into free flight in a stable airmass.

 

Tony

 

 

[hihosland]

in other words doesnt somehting travelling at 60 knots have more inertia then something travelling at 20 knots?

An interesting question.

 

As I understand it an aircraft flying at 10 feet and 40 kts has the same inertia regardless of whether it has a 20kt head wind or 20 kt tail wind.

 

However take away the 10feet and the same aircraft is now trundling along the ground at 60kts in one case and 20kts in the other obviously a significant difference in inertia.

 

So where is the extra inertia going-to or coming-from? or have I missed (yet again) something here?

 

Davidh

 

PS

 

Please don't mention conveyor belts

 

 

[Guest rolfeja]

hmmm what about if 2 aircraft were flyin head on. one of them flying into the wind and the other flying with the wind. if they were to collide head on which one would have the most force? would one of them push the other aircraft back?

 

 

[Guest pelorus32]

hmmm what about if 2 aircraft were flyin head on. one of them flying into the wind and the other flying with the wind. if they were to collide head on which one would have the most force? would one of them push the other aircraft back?

Oh no...I thought we'd got there....throw myself to floor, curl into a foetal position, clasp head in both arms and rock slowly whilst moaning softly :confused:

 

Firstly at flying speed it probably wouldn't matter as both sets of occupants would be on the way to their makers.

 

However again I think that we have a frame of reference problem in your question. They are both travelling at their airspeed (lets assume 100 knots each) in the airmass. That they are both travelling with the airmass across the ground (say 20 knots to the north) matters not a jot. The reason is that they are both subject to the same "translation" with the airmass so the relative speeds are 100 knots each and a head on closing speed of 200 knots.

 

Regards

 

Mike

 

 

[Guest TOSGcentral]

Perception Brain Teasers

 

OK this is my (slightly off topic) brain teaser given such a good opportunity!

 

 

How can a car travelling at 100kph be stationary at the same time?

 

 

A car is travelling down a perfectly horizontal road at 100 kph. The car has a perfectly vertical windscreen relative to the road surface and direction of travel.

 

 

Coming the other way on an exactly reciprocal heading is a fly travelling also at 100 kph (This is a high speed fly with a go faster stripe!)

 

 

The fly collides in the centre of the windscreen and (obviously) gives way. Its remains begin going back down a reciprocal to its original track.

 

 

OK for something to change path by exactly 180 degrees it has to first stop to change direction. If the fly was stationary at the point of reversal then what it was in contact with (the windscreen and by definition the rest of the car) had to also be stationary. Yet the car is travelling at 100 kph!

 

 

Why is this so?

 

 

Warning there is a bit of physics in this and also perceptions – I have had many people who cannot refute the logic but refuse to accept it.

 

 

Another quick one on perceptions:

 

 

We have a perfectly spheroid planet. The upper hemisphere is entirely land mass up to exactly the equator. The lower half is all water.

 

 

Is the northern hemisphere an island (land totally surrounded by water) or is the lower hemisphere a lake (Water entirely surrounded by land)?

 

 

T.

 

 

[Guest pelorus32]

An interesting question.As I understand it an aircraft flying at 10 feet and 40 kts has the same inertia regardless of whether it has a 20kt head wind or 20 kt tail wind.

 

However take away the 10feet and the same aircraft is now trundling along the ground at 60kts in one case and 20kts in the other obviously a significant difference in inertia.

 

So where is the extra inertia going-to or coming-from? or have I missed (yet again) something here?

 

Davidh

 

PS

 

Please don't mention conveyor belts

No conveyor belts. It's all about frames of reference. Don't go switching from one to the other - stick with either the airmass or the ground don't switch:

 

Landing aircraft IAS 60 knots. Headwind 20 knots therefore GS 40 knots. At 1 foot altitude the IAS is 60 knots at 1 foot the GS is still 40 knots. At the point where all three wheels touch down the IAS is 60 knots and the GS is 40 knots. At the point where the GS slows to 0 knots the "IAS" is still 20 knots. (Engage Beta Range) At the point where the GS is minus 20 knots the "IAS" is 0 knots. Nothing "goes away" nothing is lost. This example assumes a steady headwind with no gradient or anything.

 

However in terms of "how hard will we hit something on the ground" it's only the GS that matters. I remember reading a NZ air accident report about 30 years ago where the primary cause of the accident (a CFIT) was put down as "excessive proximity to an immovable object". The sad part was they weren't trying to be humorous.

 

Regards

 

Mike

 

 

[poteroo]

Tony mentioned the illusions due to wind, and although these are more a low level flight problem - there is a relevance for all pilots. When pilots make turns downwind at lower levels, the apparent slip illusion causes them to apply extra left rudder to 'prevent' that apparent slip. That causes a nose drop toward the lower,(L), wing, which they then prevent by back pressure on elevator. If power isn't increased, then airspeed will decrease - often quite rapidly. Although we always teach adding power in these turns, if it is applied sharply, the slipstream effect rolls the aircraft further left - more obvious at low speeds. No especially complex physics there.

 

cheers,

 

 

[poteroo]

Correction: should read extra left or right rudder, but left the most overused.

 

cheers,

 

 

[vk3auu]

I explained the physics of the situation a few posts back. The same theory also applies to climbing turns for instance when climbing after take off and turning crosswind or from crosswind to downwind. You probably already are operating near full power so there is no opportunity to increase power in the climbing turn, so you must push the stick forward and reduce the angle of attack and rate of climb, otherwise you might well stall if you turn too sharply.

 

For instance, as a fairly rough approximation, my CH701 with me and half fuel will climb straight ahead at 50 knots and 1600 feet per minute. If I try to do a 2 G climbing turn, I must reduce my rate of climb to around 700 FPM to maintain 50 knots and that is in still air. It gets worse in a head wind, so best not to try it.

 

David

 

 

[John Brandon]

There is some confusion in the use of terms in this thread so perhaps a couple of definitions might assist.

 

Inertia is the property of a body resisting any change in motion, or continuing in the same state of rest or state of motion relative to the Earth's cg. The mass of a body is a measure of its inertia i.e. its resistance to being accelerated or decelerated by an applied force increases with mass; a heavier aircraft has more inertia than a lighter one, so is more resistant to random displacement forces — atmospheric turbulence.

 

As long as an aircraft's mass remains unchanged so will its inertia whether it is at rest or moving i.e. any motion [speed] or pulling g has absolutely no effect on an aircraft's inertia.

 

Momentum on the other hand is mass [inertia if you like] multiplied by speed and for an aircraft in flight the speed is the true airspeed. Momentum is the property that allows you to trade airspeed for altitude, following engine failure for example.

 

I think someone stated that an aircraft cruising at 45 knots into a 45 knot headwind would have zero kinetic energy, that is not correct. Kinetic energy relates to mass multiplied by speed squared and the speed for an aircraft in normal flight is the true airspeed, so any airspeed above Vs1 can still be swapped for gravitational potential energy even if the aircraft was moving backwards relative to the ground.

 

Groundspeed is only used in the kinetic energy equation when you want to calculate the work to be done [i.e. the kinetic energy expended] to bring the aircraft to a halt on the ground.

 

cheers

 

John Brandon