Wind affecting handling ?

[Biggles' Elder Brother - Moderator]

Oh, just re-read you post Martin.

 

No you hae an error there, when walking back down the ailse you were not doing

 -498pmh ground speed! When you walked up the ailse you were doing +502mph, when you turned round and walked downyou were doing +498mph (not minus!). Assuming you walked at 2mph!

 

Then my post above applies.

 

BEB

 

PS This is kinematics again!

Edited By Biggles' Elder Brother on 02/10/2010 23:00:04

[Martin Harris - Moderator]

Ah, but which way do you define walking up the aisle!

[Biggles' Elder Brother - Moderator]

Posted by Martin Harris on 02/10/2010 22:57:16:

 but the point is that the motion of the medium I'm travelling in is irrelevent, whether in an aluminium tube or a block of moving air.

Edited By Martin Harris on 02/10/2010 22:59:01

 True - and in both of them you are subject to the same inertial forces - and they're the same as you would feel on the ground. If you fall over you accelerate to the floor of the aeroplane and hit it just as hard - no matter how fast the plane is going!

 

BEB

Edited By Biggles' Elder Brother on 02/10/2010 23:05:22

[Martin Harris - Moderator]

I'm sure we're in general agreement and debating a very fine point here - it's just that I can't accept that the relationship with the ground has ANY physical effect (other than from ground induced turbulence) on a model in flight.

 

This statement:

We don't notice these effects 99.99% of the time because the model has quite low inertia and has the small interval of time it needs to adapt to the new conditions - to adjust its ground speed so as to keep its airspeed constant.

 

...just doesn't make sense to my empirical interpretation of the subject.

Edited By Martin Harris on 02/10/2010 23:15:37

[Biggles' Elder Brother - Moderator]

Posted by Eric Bray on 02/10/2010 22:56:07:

All these high-falutin' arguments - the model doesn't know if it is windy, or a flat calm. Regarding the balloon comparison - I once saw a chap flying an r/c model off an aircraft carrier, when I was 'in', We were batting along at about 25mph. Did the model know this? Of course not, but it carried on flying around anyway.

What is a flat calm? - As I used somewhere else - 

 

 The world is hurtling through space at 1670 kilometres an hour, every hour, every day, - WHOOSH! Keep up!

 

So in a flat calm, the atmosphere is moving that fast, to stay in the same place. Does the model know this?

 

 

Don't disagree with this at all - but, with respect Eric, its not the point I'm talking about!

 

BEB

[Biggles' Elder Brother - Moderator]

Posted by Martin Harris on 02/10/2010 23:14:08:

I'm sure we're in general agreement and debating a very fine point here - it's just that I can't accept that the relationship with the ground has ANY physical effect (other than from ground induced turbulence) on a model in flight.

 

This statement:

We don't notice these effects 99.99% of the time because the model has quite low inertia and has the small interval of time it needs to adapt to the new conditions - to adjust its ground speed so as to keep its airspeed constant.

 

...just doesn't make sense to my empirical interpretation of the subject.

Edited By Martin Harris on 02/10/2010 23:15:37

 I agree its a fine point - maybe even in practice an academic one. But consider this...

 

If a model had a stall speed of 30mph and I flew it under two different conditions:

 

1. at 31mph ground speed downwind with a tail wind speed of 10mph - you would agree it would fall out of the air?

 

2. If however I flew it at 31mph ground speed (the same) into a 10mph head wind - everything is fine - yes?

 

Now imagine that I instantaneously switched from condition 2 to condition 1 - remember instantaineously. What would happen? Well I say it would fall - unless it increases its ground speed.

 

The reason the ground is important in this case is it defines our inertial frame of reference - because it is by a country mile the earth is our nearest large gravitational body!

 

BEB

[Martin Harris - Moderator]

But you can't possibly do that.  To do so would give an infinite G load and apart from the wings falling off it would stall because of that irrespective of any wind.

 

I'd guess that gravity originating from the centre of mass of the earth is theoretically relevent but any difference due to a change of position of the model would be infinitessably small?

[Biggles' Elder Brother - Moderator]

Martin, I think we are just going to have to agree to differ on this one

 

BEB

[Martin Harris - Moderator]

Agreed - my problem is I've never been one to meekly accept things at face value. I remember arguing with a maths lecturer over 1/0 equalling infinity until he got quite excited - I see the argument and accept that it's very very close to being correct but simple maths tells me that if it were so, then infinity times nothing equals 1 and I know that can't be right! I'd guess that sometimes we need to accept something as near enough...

 

I'll bet John had no idea what he'd start when he posed his question! What's interesting is that everyone seems fairly comfortable with the basic principles of downwind/upwind turns which has certainly not been the case in past debates.

[Myron Beaumont]

Being as I was the first to give my penn'orth having read all the comments ,may I suggest something ?

If you put in the same movements to your sticks turning from downwind to upwind in exactly the same way as upwind to downwind you'll soon see the effect of wind relative to the ground .Bearing in mind the flight path achieved ,it will give you a good idea as to what to expect to happen from the ground point of view thus enabling you to make a smooth transition from one direction to another (if that's what you want of course ) in various wind strengths . You have to "feel" for the A/C for yourself if you know what I mean .

To summarise --The same stick inputs whatever direction the turn & that includes cross wind .

[Tony Smith 7]

Posted by Biggles' Elder Brother on 02/10/2010 22:30:11:

Guys with the greatest of respect you cannot ignore the inertial forces! Inertia never "cancels out".

...

 

 Inertia is concerned only with changes in velocity, not the absolute values.  The inertial forces are exactly the same accelerating from 10-30mph as from say -10 to +10.   So the model flying at a constant 30kn and executing a 180deg turn has to change its velocity by 60kn.   In a 10kn headwind that change is from +20 to -40kn (=60).  With a 10kn tailwind it ground speed changes from +40 to -20kn (=60 again).  Neither is different from the flat calm where +30 changes to -30, again a 60kn difference.

 

As one with  a lot of airtime in hang gliders I know without any shadow of doubt that an upwind turn is no different from a downwind one, assuming level flight and constant wind speed.    Anyone who thinks they experience any difference is being hoodwinked by reference to the ground either as a modeller whose natural reference is the ground, or even in some cases full size pilots who have made the same mistake.

 

[Biggles' Elder Brother - Moderator]

Oh boy - I think I'm lossing the will to live

 

Once and for all - Tony I completely agree with absolutely everyword you say. Not one word do I disagree with. Just as I stated that I agreed with Poly, and Myron, and John and Eric - but,,,,that's not the point I'm making!!!!

 

I've already said that to the model "nothing exciting is happening" - it just flies around at constant airspeed. Agreed,...er how can I put this,,,er.... I know - I agree!

 

But the fact still remains that turning downwind you have accelerated - you have changed momentum, you have acquired more knetic energy. That acceleration will be opposed by your inertia. My point is - a model (probably not a full size aircraft) but a smaller very agile model can change its direction before it has been able to achieve this acceleration. That's what leads to the potential probelm. It can end up facing downwind momentarially short on airspeed.

 

Final suggestion - try it. Take your most agile powered model, fly into wind just above the stall speed. Then whip it round as sharp as you can into the downwind direction and see what happens! Only do make sure you do it at altitude and be ready to execute a stall recovery! Not every model will be able to turn fast enough( many models will accelerate up to the necessary speed as fast you can trun them) - but some will be able to turn fast enough to catch you out, especially if the wind speed is fairly high.

 

BEB

[Martin Harris - Moderator]

...and then do the same experiment from downwind.  I think it will exhibit the exact same result. I'm not able to follow your logic this time but I don't claim a deep mathematical understanding of the subject so I'm happy to wait and see the results.

 

To me, the kinetic energy in relation to a point on the ground changes - i.e if the model hits me at 30 knots airspeed flying into a 29.9 knot headwind, I'd feel a lot happier than if it's just done it downwind at 59.9 knots in relation to my bonce...but as we all mostly seem to agree, in free air there is no effect from windspeed on a model's flight characteristics.

 

Interesting discussion, though.

[Biggles' Elder Brother - Moderator]

Hi Martin,

 

I think I've thought of another way of trying to explain what I mean! It is a tricky idea and is easily confused with the relative airspeed argument - I accept that.

 

So, let's forget models, forget full size, forget airspeed, even forget the wind!

 

Imagine you're walking along pavement at a steady speed - doesn't matter what speed! Then suddenly you spin round on your heel 180 degrees. What happens? Answer in all likelihood you teetter over backwards. Why? Becuase you still have momentum in the direction you were travelling in - you've changed direction faster than you have changed velocity. You can do this becuase your polar moment of inertia about a vertical axis through your body is smaller than your bulk linear inertia - i.e. you can turn faster than you can stop!

 

It would be exactly the same if you were walking along the corridor of a moving train. If you spin round 180 degrees whilst walking you'll fall backwards - at exactly the same rate as you would if you were just walking along a stationary pavement. The fact that you are in a moving frame of reference makes no difference to the inertia.

 

So it is for the model - wind or no wind (i.e. with, or without, a moving frame of reference) this still holds. Its still the same. If you can change direction faster than you can change velocity you have a problem - under any conditions! But I suppose what I'm saying is its more likely to catch you out and have "consequences" if there is a wind.

 

I accept its an extreme pont - and the turn involved would have to very sharp and sudden indeed to get the effect - but that's were we came in - with John's bank n yank in the wind. 

 

BEB

[Dan Hedges]

BEB is right, if you are going downwind at a high groundspeed / low airspeed and turn into wind the groundspeed will not come off instantly, you will see the model baloon up as it's airspeed suddenly increases unless you correct it, this will only happen in a tight turn.

 

Also you will see the opposite turning upwind to downwind. Infact someone posted a vid on here where their aircraft stalled, reversed it's turn and crashed turning downwind due, I think,  to the same issue.

 

You will see this on full size aircraft as well, but only advanced pilots flying aerobatic aircraft will see it as they can turn fast enough, and they are ready for it!

 

[Myron Beaumont]

Judging by all the responses on this thread ,I can assume that the commenwealth games are extremely boring -building up to peeps that just run & jump at enormous cost to all .Imagine the Nats with a thousand or more drummers beating out a rythm before accomplishing something with nothing to do with money / economics etc interesting (think I'm off thread already) Back to the thread ,I do believe in what I've said & nobody has disputed it so far..

If you were flying a model in a 10 mph headwind and were sat in a car doing 10 mph backwards then you'd fly it slightly differently  .'nuff said .Get the message ?

[Dan Hedges]

Are the commonwealth games on then??????

[Myron Beaumont]

Dan

Nice one ! Mind you ,if you like B-----t  there's all you need for a couple of weeks . Bring back Golf I say - The only thing to spoil a good walk !

No ' What makes you think I'm synical ?

The other day ,someone said to me -I know what's wrong with the NHS ---It's 'olby city .!

In actual fact, what he did say I found out later was ----Obiesity

Edited By Myron Beaumont on 03/10/2010 16:27:48

[Myron Beaumont]

Phil

You'd have to alter the A of A and it might go further than chucking it downwind .Imagine that you have a 50 mph or so tail wind -then the javelin would drop to the ground ?

Like you say   Sod it !  My brain hurts (again)

Edited By Myron Beaumont on 03/10/2010 17:23:00

[Tony Smith 7]

Posted by Biggles' Elder Brother on 03/10/2010 13:36:31:

...

But the fact still remains that turning downwind you have accelerated - you have changed momentum, you have acquired more knetic energy. That acceleration will be opposed by your inertia. My point is - a model (probably not a full size aircraft) but a smaller very agile model can change its direction before it has been able to achieve this acceleration. That's what leads to the potential probelm. It can end up facing downwind momentarially short on airspeed.

. ..

BEB

 I'm afraid what I said still stands ... a model flying downwind has absolutely no more or less kinetic energy or inertia or anything, unless you bring it into contact with the ground.  Relative to the air it is exactly the same.  In all these comments I assume constant wind speed.  

 

I can even think back to watching a friend's glider which lost power and therefore control.  It was a stable model and simply circled smoothly all the was back till  it hit the hill - certainly no surge up on the upwind turn or drop on the downwind and in that case of course there was no pilot to be confused between ground speed and airspeed!

 

Maybe if you really can't believe this, try and get a flight in a full size aircraft or glider and see first hand from where your frame of reference is the same as the aircraft's.   Once you've been there and actually done it then it will be clear.

[Myron Beaumont]

Tony

As an ex-hanglider pilot / Rogallo home built then Sunspot, I know what you mean

I flew off the cornish coast  both sides Perranporth ,St Agnes,,& Gorran Haven ,Rhossilli bay .(in "Whales "-one for Timbo) .When you're up there you get a sort of understanding of where the ground is & where you want to be don't you !

Myron .

I bet by now that a lot of peeps are thinking "What hasn't he done ? " Well I have . BUT I've made no money -So what ?

[Biggles' Elder Brother - Moderator]

Poly - you've got it!

 

Tony - you haven't!

 

Tony everytime you say "The glider just circled..etc." you miss my point. I agree - in a steady smooth turn this effect wont happen - because as Phil says the prop (or in the case of the glider - gravity) has time to allow the model to adapt to its new conditions!

 

Re KInetic energy - so it only exists when I hit the ground - that's an interesting idea . I was always believed that energy can't be created or destroyed - only converted. So if this kinetic energy only comes into being at the moment of impact with the ground where's it been hiding then?

 

If your in space ship - zero gravity - a million miles from earth and you fire the rocket engines and your speed goes from 25,000mph to 25,500mph are you seriously saying there is no increase in kinetic energy until you hit a planet? If so - where did the energy in the rocket fuel go?

 

I agree with you that in any sort of "normal" flying this effect wont happen - the model's inertia is small enough that it can always balance its thrust/drag forces in a relatiely short time by adjusting its absolute speed. But in very violent manueours it may not be able to do this quickly enough and then we will see an effect of either excess airspeed or deficient air speed.

 

You you are very unlikely to have experienced this in the full size - you can't turn fast enough. Except as Dan says possibly some very agressive aerobatic pilots.

 

Last point, think about the 3D flier's transistion into a prop hang. He has to transistion quickly and quite violently - because he's going to go through the stall alpha then well beyond it and he is relying on the model's inertia to keep him airborne unitil prop thrust kicks in. If inertia wasn't carried through - and if he couldn't convert his kinetic energy in the air he woukn't be able to "buy" the fraction of a second he needs to do this. But we all know he can

 

BEB

Edited By Biggles' Elder Brother on 03/10/2010 18:16:45

[Martin Harris - Moderator]

BEB, you'll need to excuse me if I'm confusing my energy types - physics lessons were a long time ago, but isn't the kinetic (doing) energy being used to oppose the wind speed resulting in zero potential energy left to whack my head if a model is doing the same airspeed as the windspeed opposing it?

 

Yes, of course you can stall the model by turning too tightly with the required amount of elevator but the case I think you're looking at is more that of a flat ruddered turn, reversing the direction the model is pointing in virtually instantaneously but I don't think this is actually possible - would there be enough inertia to carry the tail through the relative airflow past 90 degrees of rotation?

 

When I fall over rotating quickly (in or out of the train) it's due to my foot being in contact with the ground so I can't relate to that analogy.

 

Dan, I think you may be under a misapprehension over the ballooning issue - have a look at some of BEB's earlier posts in which we all seem to agree on in that the effect is pilot induced.

Edited By Martin Harris on 03/10/2010 19:00:20

[Tony Smith 7]

Its funny that only models have a problem with this, in spite of comments from people who've actually done it and in spite of all the aircraft completing their journeys every day without "ballooning" when their turn is towards the wind or dropping when its away from it.

 

I think its because the modeller has one foot firmly in each camp - his eyes fixed on the model and its behaviour, but all the time that visual reference is tempered by the fact that you're watching from the ground. I think maybe its a misapprehension that's felt by people who learned to fly models first, and that's why others like myself and Myron don't suffer from it.  

[Tony Smith 7]

Regarding kinetic energy, I don't think that's an absolute concept.  Instead I think it is relevant only when you want to change it - ie how much energy can be released or is consumed by a change in velocity.   If you think about it, if it was absolute then we'd all have so much kinetic energy from the earth's rotation and movement that a few hundred knots either way would make no difference.

 

Remember that velocity has a direction as well as a magnitude.