Wind affecting handling ?

[john haz]

Hi All,

 

As usual, i have done a bit more flying & have questions, I like to understand as much as I can

 

This past week I have been out in light winds & getting more & more confident with the planes.   A month ago when it did something unexpected I was in an instant panic & trying to "recover" from the "problem",   now it's more a case of "oh well, do as you want & I will sort you out"  ...  seems to be working ok so far

 

My question this week is wind related (pardon me),  

 

When flying into the wind the model seems to climb & when downwind seems to lose altitude.     The mustang seems worst affected by this & I am wondering if it's a COG issue, or just something they do to a certain extent  ?

 

Am I right in thinking that if the COG is too far back (tail heavy) the model will be far more responsive & unstable  /   too far forward will increase the stall speed ?

 

 I'm not too concerned about any of the above, I am learning to handle it ok, it's just me wanting to understand why !           (found out last night that heading into win & flipping it round tight i could hold it at 90 degrees to the ground in a nice banking turn, loads of fun   )

[Stefan Hafner]

Hi John

 

The change in altitude is due to the change in airspeed,  as when you are flying upwind, say the wind is 10 mph and the plane is flying at a speed of 40, that means that the airspeed the plane is seeing is  50mph.  When yopu are flying downwind you can subtract the windspeed so the airspeed of the plane is actually 30 mph. 

 

As the airflow over the wing is then lower on the downwind leg, if you leave the plane at the same angle of attack  the lift generated by the wing will be less, as the lift is a function of the velocity squared this will be quite noticeable.

 

I don't think its a CoG issue if the plane is flying ok in less wind, but yes, you are correct in saying a tail heavy plane is more responsive than a nose heavy one.

 

Hope this helps

[Myron Beaumont]

Stefan

Don't confuse ground (as perceived)speed with airspeed.If you have anything like what you might call a lifting aerofoil wing section then it will "balloon" the faster it goes  ie more throttle !

You don't get this with a symmetrical section so much assuming that the engine thrust line is in line with the mean chord line (datum?) ie LE to TE  & the tail feathers the same .

Edited By Myron Beaumont on 01/10/2010 19:45:51

[Biggles' Elder Brother - Moderator]

Myron's right Stefan, you have to distinguish between airspeed and ground speed. A correctly trimed model will not change its airspeed when going into wind or downwind. To the model everything appears unchanged. All that changes is the ground speed.

 

Let's suppose the wind is at 10mph and we are traveling into wind at a ground speed of 30mph. Then our airspeed will be 40mph.

 

Now we turn downwind. If we leave the throttle alone what will happen is that the thrust will stay constant and so the airspeed will stay exactly the same at 40mph - because that's how fast the prop can pull the model through the air at that throttle setting. But the air is moving in the same direction as the model now at 10mph - so the ground speed will accelerate to 50mph.

 

Reviewing this the model thinks its travelling at constant airspeed, 40 mph - nothing changes. But to us, on the ground, its crawling along at 30mph into wind and then whizzing along at 50mph downwind. And this is just fine!

 

But if you try to balance this out - try to make the ground speed the same in both directions (potentially a big mistake!) then odd things can start to happen. Let's suppose we decide to hold the ground speed at a constant 40mph. To do this we will have to throttle up when going into wind and then throttle down going down wind. Our airspeed will now vary, being 50mph into wind and 30mph downwind for a constant ground speed of 40mph. Now while this might look good from the ground - because the ground speed is constant - its really potentially bad news from the model's point of view. The amount of lift generated, as Stefan says, at 30mph airspeed is much less than that generated at 50mph airspeed. So the model will climb going into wind and then decend going downwind - in the worst case it could even stall going downwind because the decreased airspeed might be below the stall speed of the model.

 

While it might sometimes "look" a bit odd - believe me if your model flies happy into the wind, then (provided you make your turns slow enough to allow the model to "keep up" - particularly when turning downwind) it will fly perfectly happily downwind and you should leave the throttle alone! This is by far the best plan.

 

To try to answer John's original question, the climbing and decending is probably happening because you are turning so quickly. When you very rapidly transistion from downwind into wind you will still be carrying the high ground speed of the downwind leg (at least at first). This will give you "excess" airspeed and result in an initial "balooning" as you come round into wind. Similarly, when you turn abruptly downwind you are initially "too slow" in terms of airspeed - because you are still carrying the slower into wind ground speed - and so you are temporarly low on airspeed and the model falls. Both these conditions will only last a short time immediately after a sharp and abrupt turn - the model will sort out the decrepency quite quickly left to its own devices. The best way to avoid this is to avoid sudden turns into or downwind. This is quite likely to the explaination in your case - given the description of your "bank n yank" turns! There is absolutely nothing "wrong" doing these - its fun, looks good and we all do it now and then. Just don't be surprised if it makes your model gain and lose high going into and downwind.

 

Some models with flat bottomed wing profiles have a tendancy to baloon when turning into wind anyway - basically their wings are just "too good" at generating lift! But they don't tend to correspondingly lose height when turning downwind - so that's less likely to be your problem.

 

Glad you are enjoying your flying - keep it up!

 

BEB

[Martin Harris - Moderator]

BEB, for once I'm afraid I don't agree 100% with your posting.  While I have absolutely no argument with the airspeed remaining constant irrespective of wind direction I'm quite convinced that ballooning is purely due to pilot input.

 

i.e. when turning from downwind to upwind the tendency is to tighten the turn, more bank, more elevator, with the result that most pilots leave a smidgeon of extra up elevator for a fraction longer as the wings are levelled, causing the zoom.

 

In countless full size thermalling turns in winds up to 35 - 40 knots (i.e. around stalling speed so going from a groundspeed of around 2 Vs to near 0)  I never needed to make any corrections coming into wind and the only place the wind made any difference was close to the ground where I needed to be aware that there was a danger of my eyeballs sensing relative movements interfering with normal procedure.

Edited By Martin Harris on 01/10/2010 22:54:44

[John Privett]

Martin - I agree with you.  I was "with" BEB until his paragraph "To try to answer..."

 

Once in the air, in a constant wind, there is no difference in the behaviour of any aircraft whether turning upwind, downwind, crosswind or whatever.  The plane is travelling in a moving "block" of air,  and all horizontal components of force act relative to that moving block of air.  To the observer (or r/c pilot) on the ground the flight pattern may look odd - speed varying,  turns being tighter or wider,  and it's "correcting" that that leads to the odd effects noted.

 

 

 

[Biggles' Elder Brother - Moderator]

Martin, I don't think we have a difference of opinion. I totally agree that in a "normal" turn there would be no balooning - unless the pilot puts it in. Because as you turn, the model gardually adjusts its speed to match the wind conditions - keeping it airspeed constant and so no zoom or fall as you say. But, as you know, models are capable of turning enormously tighter than any full size plane - even allowing for scale. The problem I'm talking about, of carrying excess/deficient airspeed into the begining of a leg, is caused by turning faster than the model can adapt its "ground speed". Full size aircraft (powered or glider) can't really do this.

 

I saw an extreme example of this at a model airshow a couple of years ago - no names, no pack drill . There was a stiff breeze blowing, the pilot was flying into wind - quite slow on ground speed but plenty of airspeed because of the high wind. He then decided to do something really dumb (showing off I suspect!), a really. really sharp 'bank and yank' snap turn 180 degrees into downwind. Turning the model in its own length. Result - the model simply fell out the sky - because its engine had not had an opportunity to accelerate its ground speed upto a rate it could mantain its airspeed. In a full size aircraft the trurn he made would have pulled the wings off and turned the pilot into a pulpy mass at the bottom of the aircraft - the model of course just did it - but it was one of the last things it did do!

 

John is enjoying his 90 bank turns that is what leads me to suspect that ths might be his probelm. 

 

BEB

[Brian Lambert]

Well as a matter of principle I keep out of these airspeed / ground speed debates, but just for BEB, a bit of advice I was given went something like this... when you find your self in a hole STOP DIGGING!

 

 

I always ask which part of the aircraft senses the speed over the ground?

Airspeed acts on all the external airframe!

Anyway where is Gemma when you need her?

 

As you were, Brian

[Martin Harris - Moderator]

Polyclarifier,

 

Your last point is particularly relevent to trying to explain the practical aspects of this old chestnut.

 

It's the turbulence / wind shadow effects that are the fly in the ointment to the  theoretical explanations.  These give rise to the false impression that the theory is flawed but the end result is that you need a margin of energy (airspeed) to counter these effects which certainly count in the real world but are outside the theoretical arguments.

 

I hesitate to argue with BEB as I'm aware of his qualifications but I really don't think that scale or the ability to pull more G influences the theory. What it would do is to dramatically increase the stalling speed and given sufficient elevator power I can quite believe that the model could flick out of a tight turn at high airspeed - I know my 1/12 scale combat models will do so at full throttle/max airspeed but I wouldn't want to be in one!

Edited By Martin Harris on 02/10/2010 00:56:33

[David perry 1]

Ha ha I love this one.  In theory the plane won't, in reality it often will.

 

It never does in a thermal turn of course, and they are slow.  I actually think BEB has a point, I have long thought this conundrum through.  What I know and what I see are different.  Sat in my full size planes I don't see the effect at all.

 

BEB, if you are right, that the model carries inertia, then the result would be as we see.  I think you are correct, at least in part (that is to say, your point is true and has some bearing).  IF BEB is incorrect, then Dynamic Soaring wouldn't work.  DS does work, ergo BEB has a point.

 

David 

[David Turner 5]

My models never "balloon".

 

I wonder why that is?

[Martin Harris - Moderator]

I did once come across some apparently learned writings where it was proved mathematically that the inertia is cancelled out and can be discounted.  I too have thought about the DS conundrum and I believe that it is because of the sharp edged transition from the wind shadow - not a situation encountered in free air but very much related to a model doing a slow climbing turn with a dying engine downwind of a typical boundary hedge, tree, barn etc. - and I think we've all witnessed the result!

[John Privett]

Martin, it can get confusing to get too hung-up about momentum or inertia. I'm sure you're right about the "learned writings" that prove that inertia can be discounted. 

 

After all,  what is the frame of reference that we are using to measure this inertia.  The "block" of moving air?  The ground?  But wait a minute,  the ground itself is hurtling along at about 1000mph.  Shouldn't we take that into account?  What about the fact that the earth itself is moving at some 67,000mph.  Shouldn't that make a difference too?

 

BEB,  when your model makes a sharp turn,  bear in mind that the forces causing it to accellerate (and I mean accellerate in the pure sense of the word,  not the everyday meaning of simply going faster) act about a point that is moving as the air moves.  Does that help with the "sudden turn" conundrum?

 

Polyclarifier (I like that version of the name!)  - I think the balloon analogy is a very useful one,  and of course turbulence and wind shear close the ground do help to "muck things up" a bit!

[john haz]

How does a newcomer sort the bones out of that lot !!     From where I am (early stages of learning about flight), there's an awful lot of good advice here, and some differing thoughts - every bit of which I've done my best to understand (well, the basics anyway).

 

I know there's a little debate going on here & hope i havn't opened a can of worms, but all that aside, i am a bit further forward & you guys have had some fun comparing notes

 

I was pretty sure it wasn't throttle response causing the climb & descend because i fly the P40 on 100% & that does suffer from it, albeit less than the mustang.     I don't tend to "yank & bank" most of the time, in fact other than this weeks "bravery" i have never done it before (intentionally), so I guess it must mostly be due to airspeed & the attack angle of the wing ?  

 

Does it matter ..... nope, not one bit !      I ask lots of questions because i like to learn & understand but if that's what they do, I will have to learn to prepare for it, accept it & react to it  - easy (ish) isn't it LOL.

 

Again, many thanks to all that have taken their time to reply, without help this would be such a hard hobby to learn & every time I post here I grasp just a little more.

 

Hope you are all having a good weekend flying  - i'm on heli's tomorrow afternoon for 3 or 4 hours (mate has the use of a small hall), so unless it's flat calm, no planes for me until monday / tuesday  

 

More soon ........  

[John Privett]

John - don't worry about opening the can of worms.  It's a discussion that has taken place many times before, and will, I'm sure, continue to re-surface in the various forums!

[Former Member]

[This posting has been removed]

[fly boy3]

HI all,on a relative lighter note,  a hot air balloon is moving north at 20kts.  the pilot decides to raise a flag,  which direction will the flag fly.  We all know the answer don't we?.  FB3

Edited By fly boy3 on 02/10/2010 18:54:25

[fly boy3]

Hi Phil, I should have said , which way will the flag hang.  Thanks FB3

[Marko]

hi john it wouldn't hurt to check your C of G anyway but also now these days are getting windy the planes will fly differently ,what l like to do on a windy day with my wot4 is

......fly into wind and pull up as if going into a large loop ,then once your at the top pointing slightly nose back ,shut the throttle and push forward just enough to hold it there and watch the wind blow your plane downwind  nose high then when you have gone far enough finish the loop back into wind it realy does look great

 

cheers .................mark

[fly boy3]

Hi Phil,  you are correct.  Thought it was a relative question due to the fact that the balloon and flag etc are flying together in a block of air.  The speed and direction are there just to confuse people.  Non aviation people normally come up with an answer that the flag will be fly ing at the opposite direction to the ballon direction.  Cheers  FB3

Edited By fly boy3 on 02/10/2010 21:10:25

[Biggles' Elder Brother - Moderator]

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

 

First of all to make nyself clear - I have no problem with the points expessed on airspeed remaining constant - that's true. Poly's balloon analogy, fine. Similarly the oft quoted "fish swimming in circles in a tank on a conveyor belt" analogy. All fine. I totally agree.

 

But...they only discuss realtive velocities at force equillibrium. I'm talking about the transistion where the forces are not in equilibrium. Let me try to explain what I mean.

 

Right, we have our model flying circuits on a breezy day, 10mph wind. We leave the throttle alone so our airspeed is a rock steady 30mph all the way round - into wind and downwind. Our ground speed however is obviously varying; 20mph on the upwind leg and 40mph on the downwind leg. Note we don't need to make these changes in ground speed - they happen for us automatically as the forces on the plane change and the plane adjusts to its different conditions and naturally adapts to keep its airspeed constant - mainataning the equilibrium between thrust and drag.

 

For example, when it comes off a downwind leg onto an upwind leg its higher ground speed is shed as initially the drag exceeds the thrust. Similarly when it turns from a upwind leg to downwind leg the drag lessens, there's less air resitance and so the model has a surfit of thrust and speeds up - thus keeping it airspeed constant relative to the moving air.

 

In my view the confusion arrives when folks confuse the calculation of realtive velocities (kinematics) and the forces at play (dynamics). The kinematics (the velocities) can be worked out relative to any frame of reference - but the simplest and most obvious is probably the air - hence the attraction of the "moving box of air" picture. And that;s fine for studying the velocities. Kinematically, relative to the air frame of reference, nothing exciting is happening. The model just stays at the same airspeed all the time. But the picture with the dynamics (the forces and how they change with time) is far from static. The forces on the model change as it turns into and out of wind. Newton tells us changing forces lead to changes in momentum (the first law of motion) and hence accelerations (the second law of motion). The bottom line is this model is accelerating from 20 to 40mph on every downwind turn and decelerating from 40 to 20 mph on every upwind turn. The model doesn't "appear" to go faster downwind - it is going faster.

 

These accelerations must be associated with an inertial force - these can't just be ignored. If you doubt that where do you think the g-forces on a pilot come from when a plane turns? The plane feels those forces just as much as the pilot.

 

If there are inertial forces then these accelerations are not instantaineous - if they were they would require literally infinite force which obviously isn't present! So the changes take a time to happen - maybe only a short time - but a time none the less.

 

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. In doing this it has to work against inertia - and the possibility exists that if we "turn on a sixpence" we don't allow the model that small time it needs to adapt.

 

Please remember - this is not (from my point of view at least) a debate about relative airspeed/groundspeed. That's kinematics - and quite naturally uses the aerodynamic frame of reference of the air in which the model flies. And I agree with all you say about that. This is about forces - and inertial frames of reference - and they are fixed and non-negotiable. It doesn't matter whether you are under water, on the ground, in the air or in outer space - like taxes they are one constant in the universe!

 

BEB

[Martin Harris - Moderator]

But if I walked down the aisle of a jumbo jet flying at 500 mph groundspeed and spun round to go back to my seat because I saw the queue for the toilet I don't think I would zoom at all - and what's happened to the inertial effect of going from 502 mph to -498 mph? I don't recall ever feeling any - and if I did I'd imagine my prospective visit to the little room would be superfluous!

 

I would contend that the forces you're referring to are all pilot induced as the flier on the ground attempts to make things look "right".

 

Perhaps I should have said that the various inertial forces in the calculation cancelled out?

Edited By Martin Harris on 02/10/2010 22:55:09

[Biggles' Elder Brother - Moderator]

It was there Martin - you felt the inertial forces involved in changing speed by 4 mph - just the same as you would if you spun round at home. That's the point!

 

The fact that it was from 502 to 498mph is irrelevant. Inertial force is linked to your change in velocity divided by the time that change took place over. It could be 0 to 4mph, -2 to 2 mph or (in the case of a space ship) 10,000 to 10,004 mph - inertially they are all the same and generate the same inertial force.

 

Newton's first law "A body stay at rest or moves at constant velocity unless acted on by a resultant force."

 

Newton's second law "A body experiencing a resultant force will accelerate"

 

Note in this case "accelerate" means "changes velocity" - that's any sort of change, could be speeding up, slowing down or changing direction. To inertia they are all the same!

 

BEB

[Former Member]

[This posting has been removed]

[Martin Harris - Moderator]

OK I agree with that (BEB - Eric's post popped in while I was replying) 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