Wind direction, ground speed vs airspeed

[Viper Pilot]

As I understand it . . . .

If your plane has a stall speed of 15 mph and . . . .

1. . . .You are flying into a wind of 10 mph. Your ground speed is 5 mph and your airspeed is 25 mph.

You plane is just fine, no stall because your airspeed is above the stall speed.

2. . . .You are flying with the 10 mph wind behind you. Your groundspeed is 25 mph but your airspeed is 5 mph.

Why doesn't the plane stall??

[jkettu]

1... If you are flying into a wind of 10 mph and your ground speed is 5 mph, then your airspeed is 15 mph.

2... If you are flying with the 10 mph wind behind you and your groundspeed is 25 mph, then your airspeed is 15 mph.

[Viper Pilot]

1 . . . . Airspeed + Windspeed = 15 mph + 10 mph = 25 mph

2 . . . . Airspeed - Windspeed = 15 mph - 10 mph = 5 mph

[jkettu]

Ok, so you fly at constant airspeed of 15 mph and your stall speed is 14.99 mph. So you always fly at just above stall speed and are doing just fine.

1.. your groundspeed is 25 mph.

2.. your groundspeed is 5 mph.

Groundspeeds have nothing to do with stall.

[gouch]

If the airspeed is 5mph and it stalls at 15 mph, you ARE stalled! It isn't flying, but blowing away....until it hits the ground. ;)


As I understand it . . . .

If your plane has a stall speed of 15 mph and . . . .

1. . . .You are flying into a wind of 10 mph. Your ground speed is 5 mph and your airspeed is 25 mph.

You plane is just fine, no stall because your airspeed is above the stall speed.

2. . . .You are flying with the 10 mph wind behind you. Your groundspeed is 25 mph but your airspeed is 5 mph.

Why doesn't the plane stall??



I don't really understand your original question. In #1 you say it's flying at 25 mph, why is the airspeed now 5 mph when you fly downwind?? assuming nothing has changed with the crafts airspeed, why is it now 20 mph slower???
If you haven't changed the airspeed then on downwind it's still doing the upwind airspeed, only the aircrafts groundspeed has increased to the ground based wind speed.

As jkettu said, it has nothing to do with groundspeed. Take up slope flying, that'll learn ya :D

If you are a speck of dust in a 10 mph wind, there is no wind ;)

Cheers
Paul

[D-Rock]

try to think how fast the airplane is moving through the "body" of air, or how fast the air is traveling over the wing.

D-Rock

[DRETW]

This post is now totally, factually, and indisputably correct.

[HELModels]

Crypto downwind turn thread.

[agpilot24]

notable exception; If you go from 1 to 2. ( quick 180 degree turn ) your plane must accelerate 20 mph relative to the ground very quickly or it will stall ,


AHHHHHHHHHHHHHHHHHHHHHHHHH!!!!!!!!!!!!!!!(runs out of room with his hands over his ears) ;)

[HarryC]

1) Forget windspeed & groundspeed, think only of your aircrafts speed relative to the air around it,

2) notable exception; If you go from 1 to 2. ( quick 180 degree turn ) your plane must accelerate 20 mph relative to the ground very quickly or it will stall

3) Just remember, the aircrafts flying speed dosent change due to the windspeed.

Oh well, 2 out of 3 ain't bad, as Meat Loaf said.
Statement 1 and 3 are correct. Statement 2 totally contradicts statement 1 and 3. Statement 2 is wrong, a plane circling in a level turn will not change its airspeed. The existence of a wind is irrelevant, as statements 1 and 3 correctly said.

H

[biber]

Imagine a boat on a river.
The water in the river is moving constantly in one direction
and the boat is moving in the water.
It has a speed relative to the water and a different speed relative to the bank.

Does that Gedankenexperiment help a little?

biber

[Aio_1]

What Harry said!

[DRETW]

Post deleted account confused people really shouldn't try to help confused people....Leads to confusion!.?.

[Aio_1]

....The plane should see 15 (airspeed ) all the time but its groundspeed must Rapidly go from 5 to 25 grownspeed to maintain your 15 airspeed. Thats close to 20 mph groundspeed acceleration isn't it? Actually, the same can occur in gusty headwind conditions. When the wind suddenly drops from 15 to 5 the aircraft must equally, suddenly accelerate 10 (groundspeed) to maintain 15 airspeed or it will stall.
There is no equivalence between the to cases.
A gust in the direction of flight causes a reduction in airspeed due to the slower acceleration of the plane because of it's greater inertia. The plane may stall or just drop the nose a little until it returns to its trimmed flying speed.
A turn does not involve a change airspeed nor does it involve any magical accelerations to account for the change in groundspeed. Every turn involves acceleration by definition (as you'll have experienced flying full scale planes). Whether in still air or a constant wind the acceleration for a given rate of turn is the same and will fully account for any variaton in groundspeed. The groundspeed itself is however completely irrelevent as far as the flight mechanics are concerned.

Also there is no such thing as 20mph acceleration. Acceleration is the rate of change of velocity NOT the total change in an arbitrary speed. It may seem I'm being picky but it can lead to massive confusion if the term is misunderstood. This thread (http://www.rcgroups.com/forums/showthread.php?t=117986)from around page 18 contains the same material being discussed here plus some more detail and may be of interest but as you'll see these same errors caused massive problems.

...I fly electrics now and this is probably more relevant to trainers and low slow scale planes as opposed to higher powered stuff but if you reverse direction fast enough it will become apparent.
If the wind is constant the only problems that will arise will be due to pilot error, specifically misjudgement of airspeed. There is no REAL problem as far as the aeroplane is concerned. There is no problem in this respect for any plane fast or slow when the plane is slow the pilot is more likely to make errors since the windspeed is a larger proporion of the airspeed.
The only actual problems with regards the flight mechanics are when the wind is not constant. There are 3 ways this is usually encountered.
1. Gusts
2. Wind gradient
3. Turbulence
Gusts I've already covered. Wind gradient is due to the earth having a boundary layer just like anything else moving through a fluid. Therefore windspeed reduces approaching the ground. If you change altitude quickly in otherwise smooth air this may be noticable.
Turbulence can be due to geographical features like trees, buildings and ridges or due to thermal activity. It is where there are variations in air velocity at a small scale. This is more of a problem for lightweight and small planes as they have less inertia to resist both changes in speed and orientation.

I used to fly full scale ( Cessna 150, Piper 140) and have seen this first hand. It's easy to recognize. The VSI indicates down, the ground comes up, and you can suddenly re-taste whatever you last ate!

I stand by my post as correct.
I'm not sure what you've experienced but I don't accept your explaination as correct. Can you give a detailed description of what you experienced and I'll see if I can explain it?

Aidan

[HarryC]

The plane should see 15 (airspeed ) all the time but its groundspeed must Rapidly go from 5 to 25 grownspeed to maintain your 15 airspeed. Thats close to 20 mph groundspeed acceleration isn't it?
Yes, but what has groundspeed got to do with it? Nothing! Apart from navigation, everything an aircraft does is with respect to the air, not some spot on the ground. The plane has absolutely no knowledge of the surface. The kinetic energies relating to flight manoeuvres are those related to its airspeed, not its ground speed. If the plane flies over a flowing river so that its speed over the surface has suddenly changed, does its airspeed suddenly change? Of course not, a plane in flight knows the square root of zero about the surface.

I too fly full size both glider and power and have spent hours and hours circling gliders in the wind, etc etc. Aio_1 is right, if you observed any effect it was either a temporary gust or pilot induced.

H

[DRETW]

Okay, I concede. I will delete my posts to avoid further confusing anyone. My sincere apologizes.
Also to you Sir Issac for the misquote.

Thanks Aidan, and HarryC
ted

[HarryC]

Here's another example to demonstrate how you must relate everything a plane does to the air it flies in, and how the ground and groundspeed are irrelevant to the aircraft's performance and flight energy levels.

A glider pilot wants to do a stall turn so he accelerates to the entry speed of say 100kts. But he happens to be flying into a 100kt wind. His Air Speed Indicator says 100kts so he believes he is ready to pull up. But a model flier standing on the ground, with a ground based view of flying sees the glider has no ground speed. No ground speed means no kinetic energy and hence when the pilot pulls up the glider can not climb because it has no kinetic energy to convert into potential energy - if it isn't moving, how can it have any kinetic energy?

So, what happens when the pilot pulls back on the stick? Either he or the model pilot on the ground is going to get a shock! Does the glider rotate its nose up but not climb as the model pilot says, making the on-board pilot very puzzled how he had 100kts of speed and yet it didn't climb one inch? Or is the model pilot on the ground amazed at how the glider suddenly accelerates from rest and shoots vertically upwards (and backwards if he went air vertically) yet it has no huge engine or rocket motor capable of propelling it vertically from a standing start?

The answer of course is that the glider shoots vertically upwards and gains the height that 100kts of speed should give it. The pilot in the glider is not surprised, after all he didn't do it from a standing start, he did it from 100kts of speed. By having 100kts of speed and pulling back on the stick, the wing created a huge amount of excess lift over weight and that caused the vertical acceleration. The plane flies with respect to its airspeed, its kinetic energies when doing manoeuvres are related to its airspeed and no other speed. The model pilot standing on the ground is amazed to see it but that's because he mistakenly relates everything in flight to himself. It's a very common, perfectly natural mistake to make, virtually every model pilot finds it very hard to believe that an aircraft doesn't give two hoots about the ground and argues against it. The plane must have kinetic energy of its ground speed, they say. It took me years to half-heartedly believe that the physics of a plane is only relative to air and not the ground, but it took quite a bit of full size experience until I really believed and came to understand it.

The only time a plane has kinetic energy relative to the ground is when the plane comes into contact with the ground, in which case it isn't flying any more!

H

[DRETW]

Thank you again, HarryC
With the sailplane analogy, it really is easier to understand. I now have a real strange look on my face and an overpowering wish to say "but, but", but I have no valid objection.
One very hard part of my learning to fly full scale was learning to keep my nose down in the landing pattern. I suppose that is where my idea came from and I just never questioned it.
Thanks again for your effort to help me understand, it truly was/is appreciated.
ted

[jkettu]

Glad to see you sorted it out. This comes kind of late, but if you want one more good analogy, there is this article. (http://www.rcgroups.com/links/index.php?id=3555)

[HarryC]

And just to make it more bizarre, it is indicated air speed that matters to the handling, but true airspeed that matters to the dynamics of motion. Indicated air speed is what the air speed indicator says, which is not the same as true speed through the air. An ASI is subject to position errors so the real indicated air speed is amended to be called the rectified air speed or corrected air speed depending which side of the Atlantic you live! But these errors are of the magnitude of a few knots so for the moment we will take IAS to be the same as RAS/CAS.

But when aircraft climb high into thin air, they encounter fewer air molecules. This is air that is less dense, which is a different thing to air pressure. Fewer molecules means less drag, less lift, less power, less thrust etc. It means fewer molecules hitting the piston in the ASI's pitot probe, therefore the ASI shows a speed lower than you are actually hitting the air molecules because it was designed to work with sea level density of air hitting it. As a plane climbs, if it keeps a steady speed of actually hitting the air which is its True Air Speed TAS, the ASI will steadily reduce and show a lower and lower speed.

All the handling comes from ASI not TAS. The amount of lift the wing can generate is related to how much air it can affect and in less dense air, it can only affect fewer molecules.


H

[adam_one]

I guess the glider uses its own weight in order to keep the airspeed above the stall speed and may behave differently.
For instance, a powered plane flying north with airspeed of 30 knots against 30-knots headwind has 0 ground speed.
If you turn 90 deg. left (west), the plane's airspeed is still 30 knots but it's now drifting 30 knots to the south resulting in 42 knots to the west-south-west.
If the plane keeps turning south (180 deg.), the drift due to the wind is still 30 knots, the ground speed becomes 30+30 = 60 knots, while the airspeed still is 30 knots.

A pilot on the ground will see the ground speed and not the airspeed and since the plane seems to move much faster flying downwind, the pilot may instinctively slow down the plane (in case of landing approach) below the stall speed.
This results in a pilot-induced stall due to the optical illusion of the plane's high ground speed.

[Aio_1]

I guess the glider uses its own weight in order to keep the airspeed above the stall speed and may behave differently...
I'm not sure what you mean. There's no difference I can think of between a glider or powered plane with respect to airspeed, groundspeed or wind. You trim a powered plane at a given throttle setting to maintain a given airspeed (or more accurately IAS!) just like you trim a glider. There's no fundamental difference in the rules which govern their flight.

Aidan

[adam_one]

I mean if you turn a glider downwind and wish to keep the same airspeed as it had upwind, you may need to lose altitude…

[HarryC]

Adam, assuming the normal condition where a glider is losing altitude all the time to maintain its speed, if it then flies in air circles its airspeed and rate of descent do not change. There is no indication in the cockpit that a wind exists, unless the pilot can see the ground and see that the circle is shifting position.

If it is flown in circles over the ground rather than through the air, once agin airspeed and nominally its rate of descent do not change but the pilot has to bank more and bank less in order to compensate for the wind effect to keep the glider circling one point on the ground. Then the rate of descent will change but simply because of the change in bank angle and not because of the wind acting on the glider.

Model pilots do genuinely experience their model losing height as they turn downwind and gaining height as they turn into wind but sadly it is due entirely to model pilot error and not to any laws of flight. When a model turns downwind it appears to get blown back especially at the 90 degree point in the turn and the model pilot relaxes his pull back on the elevator, this allows the nose to drop and the model starts to lose height. When turning back into wind the model pilot sees the model apparently travelling over the ground the opposite way and is tempted to pull back harder to stop the apparent motion, thus by pulling back too hard on the stick he causes it to climb. So the loss of height when turning downwind is all caused by the model pilot misreading the motion of the model.

H.

[Aio_1]

Adam,

No, that's incorrect. Upwind and downwind have no effect on the plane, just the observer. If you trim a glider in perfectly calm weather and performa turn and then provide all the same control inputs in a steady wind the glider will experience all the same accelerations and have the same airspeeds at all times during the flight. Only the groundspeed will vary and that has no effect on the plane. It's only relevent to the observer. The airspeed will only change if the windspeed is varying such as with gusts or turbulence.

Aidan

[Aio_1]

If it is flown in circles over the ground rather than through the air, once agin airspeed and nominally its rate of descent do not change but the pilot has to bank more and bank less in order to compensate for the wind effect to keep the glider circling one point on the ground. Then the rate of descent will change but simply because of the change in bank angle and not because of the wind acting on the glider.
I think this confuses things a bit since you are not really "flying" circles in this case. You are flying a distorted curve which provides a circular groundtrack but the rate of turn and accelerations experienced vary at throughout the turn.

Aidan

[gouch]

I think what dretw said had merit. WAIT, before you drill me about it :D

Nothing he said in his first post was wrong, was it?

Here it is again



1) Forget windspeed & groundspeed, think only of your aircrafts speed relative to the air around it,

2) notable exception; If you go from 1 to 2. ( quick 180 degree turn ) your plane must accelerate 20 mph relative to the ground very quickly or it will stall

3) Just remember, the aircrafts flying speed dosent change due to the windspeed.




Harry C said straight after:

Oh well, 2 out of 3 ain't bad, as Meat Loaf said

Statement 1 and 3 are correct. Statement 2 totally contradicts statement 1 and 3. Statement 2 is wrong, a plane circling in a level turn will not change its airspeed. The existence of a wind is irrelevant, as statements 1 and 3 correctly said.

Who mentioned Airspeed in quote 2 from dretw? Not him. he said relative to GROUND. Harry, maybe it's because you see the confusing downwind thoery all the time here, but you read the wrong thing there fella :)

Just a few weeks back I saw a guy flying in wind, stall his plane on a downwind run. He did so because he obv. was trying to maintain a constant groundspeed (throttled back and held height) and stalled because of it.

My take on what Dretw was saying is that because we are flying FROM THE GROUND, you DO need to see an increase in groundspeed when you fly downwind compared to an upwind leg. Esp if you are flying close to stall speed in the first place.

Don't forget, we are flying FROM the ground, not the plane (if you read the forum name RCgroups). We know the plane dosen't care what the ground is doing, nor which way the wind is going, but the ground based RC pilot does need to allow the planes groundspeed to increase on a downwind leg.

Let me have it :D

[HarryC]

Gouch, the use of the phrase "notable exception" and dretw subsequent posts indicate he was thinking that the plane had to experience an acceleration otherwise its speed would drop below stall speed.

You might hope that
"We know the plane dosen't care what the ground is doing, nor which way the wind is going, but the ground based RC pilot does need to allow the planes groundspeed to increase on a downwind leg."
but sadly a lot of model fliers don't hence the common misunderstandings about losing and gaining heights and airspeed during a turn. The ground based model pilot often doesn't believe that on board the plane the people are not feeling it speeding up and slowing down. Heck, there are plenty model pilots who see a plane crabbing in a sidewind and will tell you the pilot must be holding on rudder all the time to compensate for the side wind!

H

[gouch]

Fair enough, I didn't take the "notable exeption" bit in.
Also, I didn't read his further posts as they are mostly deleted, and was only refering to the original.

But aside from that furfy, it was correct, no?

And, yes, I too have heard a few RC guys talking about their plane crabbing in sidewind. I actually questioned a guy why he was cutting down his fin on a homebuilt glider one day, he said because it was nosing into the wind too much when going across the slope. That didn't sound right for some reason, but he told me it did the trick!

One more question after seeing a solo PC-9 roulette doing some VERY fast aerobatics above work today for our regional show day, Yeeha :D : Man it was great to see!!! came right overhead our shop at no more than 1k ft at what sounded like full noise, pulled vertical and rolled probably 5 times before
cranking the rudder and rolling vertical down 3 times and pulling out REALLY tight. After just recently going up in a Pitts special, I now have a very special understanding of the amount of G he was pulling, knowing the PC-9 can leave the pitts in it's wake when it comes to G ability and speed. Sorry for the rant, it was just awsome to watch, we don't get to see much of this stuff normally, on to the question :D

When in an aerobatic comp, does the pilot have to compensate his manouvers for the wind so the judges see a true vertical climb for example that isn't moving downwind? Or when doing a loop for example, does the pilot have to do an "oval" loop so it appears from the ground that it is a true circular loop? If so, that is VERY impressive to hear.

[DRETW]

Glad to see you sorted it out. This comes kind of late, but if you want one more good analogy, there is this article. (http://www.rcgroups.com/links/index.php?id=3555)

Ohboyohboyohboy,
Please, let me start by saying that I really appreciate all ya'lls help. I am not trying to argue for arguments sake, I'm only trying to get this concept properly situated in my thick, sunbaked, Texas skull.
The above article, paragraph two, says just what I originally thought/said, or maybe more precisely, thought I said.
Is that guy correct?
Back to the glider diving at 100 into a 100 wind, yes it would zoom upward even though it has no groundspeed, but (finally found a valid "but", HarryC :D ), if i were standing in a field holding afore mentioned glider and facing the same 100 wind, when I released it would it not also zoom upward? And would this not be the mass's resistance to accelerarion that would cause the zoom? In either instance from the ground the aircraft would seem to accelerate backwards, correct?
I think I understand basic aerodynamics, and I think I've got a pretty good grasp of Neuton's laws. Am I just mixing them in the wrong amounts?
One last, and most probable cause of my/this confusion/misunderstanding, could be my right hand first finger, it has to do the work of all 10 digits when it comes to typing and consequently, it tires easily :o , too often, I fear, before I've properly stated my thoughts.

ted

[capncrunch]

When in an aerobatic comp, does the pilot have to compensate his manouvers for the wind so the judges see a true vertical climb for example that isn't moving downwind? Or when doing a loop for example, does the pilot have to do an "oval" loop so it appears from the ground that it is a true circular loop? If so, that is VERY impressive to hear.

RC pilots do, yes.

-barrett

[gouch]

But what about full size? So the judges "see" a true loop from the ground?

[HarryC]

Yes, same for r/c and full size. Aerobatic judges are looking for perfect shapes from their point of view, so the pilot has to compensate for the wind. This is far easier for a model pilot than full-size because, for example, the model pilot can see the shape of the loop just as the judges can see it and can control accordingly, but the chap in the full-size can't see the shape.

H

[HarryC]

Back to the glider diving at 100 into a 100 wind, yes it would zoom upward even though it has no groundspeed, but (finally found a valid "but", HarryC :D ), if i were standing in a field holding afore mentioned glider and facing the same 100 wind, when I released it would it not also zoom upward? And would this not be the mass's resistance to accelerarion that would cause the zoom? In either instance from the ground the aircraft would seem to accelerate backwards, correct?
What it will do will depend on its attitude and elevator trim when you release it. Imagine the attitude and trim that it would have if it was in flight at a steady 100kts. If you put it into that attitude and trim and release it it will have 100kts airspeed, it will stay fixed over the ground and descend at the correct rate until it meets the ground. If you release it nose high then it will zoom upwards and backwards, if you release it nose low it will accelerate forwards and rate of descent will increase.

H

[adam_one]

What it will do will depend on its attitude and elevator trim when you release it. Imagine the attitude and trim that it would have if it was in flight at a steady 100kts. If you put it into that attitude and trim and release it it will have 100kts airspeed, it will stay fixed over the ground and descend at the correct rate until it meets the ground. If you release it nose high then it will zoom upwards and backwards, if you release it nose low it will accelerate forwards and rate of descent will increase.

H
So, it means that in certain circumstances the glider may increase altitude headwind and decrease it downwind?

[DRETW]

Whatabout that Navy pilots article? Paragraph two.

[Ollie]

The air is invisible most of the time. The air moves in velosity with wind, gusts, shear, etc. but you don't know about air speed or direction with out measuring them. The problem is with people thinking about ground speed rather than air speed. The aircraft behave relative to airspeed not ground speed. The problem is in your mind and thinking about it. The problem is assumed "fact" not true measured fact. Aircraft fly relative to air and crash relative to ground.

[adam_one]

One thing is for certain, the planes crash faster when the winds are stronger.
Is there any objection?
:D

[HarryC]

So, it means that in certain circumstances the glider may increase altitude headwind and decrease it downwind?
only if you pull back or push forward on the stick, in which case it is pilot induced and not wind induced.

H

[adam_one]

only if you pull back or push forward on the stick, in which case it is pilot induced and not wind induced.

H
I understand that Harry, but my point is:
The powered plane doesn't need to lose altitude to maintain the airspeed, the glider does, right?
So if you manage to keep the glider at the same altitude into headwind I guess you can't keep the same altitude when turning downwind.

[capncrunch]

Whatabout that Navy pilots article? Paragraph two.


It was sarcastic, read the WHOLE THING.

from wikipedia (http://en.wikipedia.org/wiki/Tongue-in-cheek) "The term tongue-in-cheek refers to a style of humour in which things are said only half seriously, or in a subtly mocking way. To say something in a tongue-in-cheek way is to speak with irony."

-b

[Aio_1]

I understand that Harry, but my point is:
The powered plane doesn't need to lose altitude to maintain the airspeed, the glider does, right?
So if you manage to keep the glider at the same altitude into headwind I guess you can't keep the same altitude when turning downwind.
You're still thinking there's some difference between upwind and downwind! There isn't. At least not in constant wind which is what we're discussing.
All that matters is the flying speed of the plane.
A glider doesn't get any extra energy depending on the direction it's flying (i.e. upwind, downwind or crosswind) and neither does a powered plane. Of course a glider must always descend relative to the air around it in order to maintain airspeed but since we're talking about horizontal wind I don't see where the confusion comes from. Even in the case of a thermal or slope where air moves upwards there's still no difference depending on the direction you fly as you still have to decend relative to the air around you.

There is no difference between the effect (or lack effect) of wind on a glider and a powered plane.

Aidan

[adam_one]

Aio_1,
I mean there's a difference between a glider and a powered plane with respect to what makes them move forward and hence a difference in factors that may affect their airspeed.

Could you please address the following assumption:
If you manage to keep the glider at the same altitude into headwind I guess you can't keep the same altitude when turning downwind.

[DRETW]

It was sarcastic, read the WHOLE THING.

from wikipedia (http://en.wikipedia.org/wiki/Tongue-in-cheek) "The term tongue-in-cheek refers to a style of humour in which things are said only half seriously, or in a subtly mocking way. To say something in a tongue-in-cheek way is to speak with irony."

-b

Please,
Lets wait until next week when I misstate " tongue-in-cheek"!
ted

[Ollie]

Myths are made of assumed "facts." Your eyes fool you when you can't see air.

[adam_one]

Myths are made of assumed "facts." Your eyes fool you when you can't see air.You sound like a priest Ollie...
We can't see the air but we can feel it.
:)

[Aio_1]

Aio_1,
I mean there's a difference between a glider and a powered plane with respect to what makes them move forward and hence a difference in factors that may affect their airspeed.

Could you please address the following assumption:
If you manage to keep the glider at the same altitude into headwind I guess you can't keep the same altitude when turning downwind.
I don't understand the assumption because what you've said already seems to contradict it. We're agreed I think that a glider must decend relative to the air around it to maintain airspeed but you're saying we should assume the plane maintains altitude in a headwind. Either there's some massive confusion or the "assumption" is incomplete.

So if you want me to address the assumption you'll need to clarify what it means.

Aidan

[Ollie]

adam,
"We can't see the air but we can feel it."

You can feel the wind around your body but you can't measure the air movement around the plane unless you have measuring instruments near or on the plane.

[Sparky Paul]

Joe Wurts can see wind.
But no one else! :)

[adam_one]

I don't understand the assumption because what you've said already seems to contradict it. We're agreed I think that a glider must decend relative to the air around it to maintain airspeed but you're saying we should assume the plane maintains altitude in a headwind. Either there's some massive confusion or the "assumption" is incomplete.

So if you want me to address the assumption you'll need to clarify what it means.

AidanOk, I agree that the glider may have to descend relative to the air around it to maintain or increase the airspeed, but what happens if the air is moving towards the glider instead?
It may even gain altitude as long as it is not thrown backwards losing airspeed.
Let's assume that the headwind is just strong enough to hold the glider still in the air, with no change in altitude and zero ground speed. Impossible?
Then what happens if you turn downwind?
I guess the glider has to dive in order to gain airspeed, whereas the powered plane doesn't.


You can feel the wind around your body but you can't measure the air movement around the plane unless you have measuring instruments near or on the plane.I know that Ollie, I was kidding... :)
that's why the RC pilot may be more easily fooled by the model's ground speed than the pilot in a full-size plane.

[capncrunch]

Joe Wurts can see wind.
But no one else! :)


well, chuck norris.

-b

[adam_one]

Joe Wurts can see wind.
But no one else! :) If my memory serves me right Chuck Yeager claimed he heard and saw the wind when crossing the sound barrier…
:)

[Aio_1]

Ok, I agree that the glider may have to descend relative to the air around it to maintain or increase the airspeed, but what happens if the air is moving towards the glider instead?
It may even gain altitude as long as it is not thrown backwards losing airspeed.
Let's assume that the headwind is just strong enough to hold the glider still in the air, with no change in altitude and zero ground speed. Impossible?
Then what happens if you turn downwind?
I guess the glider has to dive in order to gain airspeed, whereas the powered plane doesn't.
That's roughly what I thought you were saying. It's completely the wrong way to look at it.

Airspeed is airspeed and it doesn't matter if you look at it as the air moving around the plane or the plane moving through the air. It's all about relative speeds.

You say "as long as it is not thrown backwards losing airspeed" as though it were a minor thing but it is pretty much the most important concept. Flying and drag cannot be separated.

When you say:
I agree that the glider may have to descend relative to the air around it to maintain or increase the airspeed, but what happens if the air is moving towards the glider instead?
You are really saying that although the plane has to descend to maintain airspeed perhaps sometimes it doesn't need to. It's contradictory.
I think your intuition is thinking that groundspeed is important and then trying to add airspeed into the picture. In reality groundspeed is only incidental and you only notice it because you're on the ground.

Aidan

[adam_one]

Airspeed is airspeed and it doesn't matter if you look at it as the air moving around the plane or the plane moving through the air. It's all about relative speeds.
Yes I know, that's my point.
The plane needs air flowing over its wings with a certain speed in order to create lift. That's what we call airspeed.
The airspeed is often created when the glider is descending but if you have strong headwind your descending may be minimal and may even stop or turn into a climb (no need to be a thermal).
And then you may turn downwind and dive to gain airspeed in the opposite direction and inertia that may allow you to turn back upwind and climb again.

Anyway, my initial argument was that with a powered plane you don't need to dive to keep the airspeed when turning downwind. That's why I say they've different behaviours.

And when I say airspeed I do not mean ground speed, I know the difference.

[biber]

The airspeed is often created when the glider is descending but if you have strong headwind your descending may be minimal and may even stop or turn into a climb (no need to be a thermal). No, that's not true, believe me.
I have quite some experience in fullsize gliding and there are definately no differences in model gliding for that matter either.
The only way to get energy out of a steady wind is a rigid link to the ground, like a cable on a winch launch, or like on a kite.
Let the kite rope go off and it starts to sink immediately except for the presence of rising air masses aka lift.

biber

[adam_one]

biber,
I understand that a too strong headwind would push the glider backwards reducing the airspeed, but I think it should be a certain optimal headwind force which gives the lowest sink ratio, or is only thermals and updrafts that prevent the glider from keeping sinking?

[biber]

I understand that a too strong headwind would push the glider backwards reducing the airspeed, but I think it should be a certain optimal headwind force which gives the lowest sink ratio, The amount of headwind has absolutely no (!) influence on the sink rate of a glider.

If the model/aircraft is trimmed to maintain a certain airspeed, it will maintain this airspeed regardless of the direction it is pointing.

Only bank will rise this airspeed of equilibrum a bit, since it's really the alpha, that is beeing trimmed
and the corresponding airspeed depends on the accelleration (perpendicular to the direction of flight),
wich again depends on the bank.
That is for quasi static flight conditions.
This airspeed is, what has an influence on the sink rate.
You can let it do random turns happily until it hits the ground
and it will still maintain the trimmed alpha and corresponding airspeed.
But all that doesn't have anything to do with wind.
Note, that I didn't involve the ground at all for that.

If you want to know the groundspeed, simply add the airspeed vector to the vector of the moving air mass.
Note, that wind directions are given as the direction from where it's coming, while we want to add just the opposite, the direction it is going.
That means, you would have to take the inverted common wind direction
and add that vector to the airspeed vector to get the groundspeed vector. or is only thermals and updrafts that prevent the glider from keeping sinking?That is perfectly correct (except for dynamic soaring).

biber

[adam_one]

The amount of headwind has absolutely no (!) influence on the sink rate of a glider.

If the model/aircraft is trimmed to maintain a certain airspeed, it will maintain this airspeed regardless of the direction it is pointing.
biberOk biber, but I thought it just applied most to free flight condition without pilot intervention.
I know that a glider has an ideal airspeed that gives the best L/D ratio and lowest sink rate.
The pilot may try to maintain that ideal airspeed if the goal is flying the longest distance, but if one wants to stay aloft as long as possible there may be other strategies such as dynamic soaring DS.
But since you say you've flown full-size gliders I have to revise my thoughts somewhat... :)
Thanks for sharing.

[biber]

The whole sinking polar depends on airspeed, so the minimum sink is at one certain speed.
Whether it is fullsize, RC-model, or a free flight glider doesn't count a thing for that point.
Any stable aircraft/model does maintain the trimmed airspeed all by itself and is supposed to do so.
Otherwise it would be a lot work to keep in the air.

DS shouldn't be taken into account, because it involves dynamic, wich complicates things a lot.

To the pilot intervention thing I'd like to state, that most problems in flight are initiated by it. ;)

That applies in an extreme way to model airplanes.

biber

[adam_one]

To the pilot intervention thing I'd like to state, that most problems in flight are initiated by it. ;)

That applies in an extreme way to model airplanes.

biberYeah, but we also want to control that thing, don't we? ;)
I've seen flight shows where full-size gliders did several loops just before landing, I guess it was due to the pilot intervention, but it was awesome... :)

[biber]

Some intervention is needed sometimes, that's right. ;)

And some of that can be very well entertaining, I have a glider acro licence myself... ;)
(Yes, over here there is one required for having that kind of fun)

But OTOH one has to know, when it's better not to interfere...

biber

[Ollie]

biber, Aio1, HarryC, Capncruch, Sparky Paul and others tried to help Adam over his myth. Adam can't help it? Forget about Adam's until he opens his mind to true knowlage.

[adam_one]

biber, Aio1, HarryC, Capncruch, Sparky Paul and others tried to help Adam over his myth. Adam can't help it? Forget about Adam's until he opens his mind to true knowlage.Thanks Ollie, that's comforting.
I guess I've already accepted Biber's explanation on one of my previous posts with thanks, and btw I reserve the right to know more today than I knew yesterday... :)

[gouch]

To the pilot intervention thing I'd like to state, that most problems in flight are initiated by it.

and this

But OTOH one has to know, when it's better not to interfere...

biber

After reading that, I have to mention a story when I went in a full size glider for the first time. My instructor Eddie, told me to fly to "that hill over there", "nice and straight please". So me being a total newby (still am, only had a few Le$$ons so far) I concentrated SOO hard on keeping that damn glider straight and level. Correcting every bump and wiggle. Eddie then tells me to stop playing with the damn stick and wouldn't you know it, we aimed straight at that hill as if we had an auto pilot. :D He then went on to tell me when I drive a car, I don't keep turning the wheel left and right non stop to go in a straight line, do I? Of course not....well not in my new car anyway :D


Ollie, give Adam some slack here :) You can tell him to just believe what he is being told, but it will take him to understand it before he believes it. The information is there, he just needs to see it, that's all.
The one who believes what he reads purely because he is being told to, will never understand why he believes what he does. (does that make any sense? )

Stick with it Adam, it will make sense soon :)

In the mean time, buy a slope soarer, or better yet make a cheapy and go fly off a slope. It taught me so much about all this stuff. Assuming you have a hill to fly from of course?

Cheers
Paul

[adam_one]

The one who believes what he reads purely because he is being told to, will never understand why he believes what he does.
Cheers
PaulI like that one... :)
And believe me, I've no trouble in understanding the difference between airspeed and ground speed...
Well, I guess the most controversy here was caused because I based some of my assumptions on dynamic soaring where the glider actually gains altitude flying upwind, then sinks downwind and may keep flying in that fashion as long as the wind lasts… but I realised that's another story...

http://i58.photobucket.com/albums/g264/bumpa2/dynamic_soar.gif
http://www.geocities.com/soaringbythebay/dsoar.htm

[Aio_1]

Yes I know, that's my point.
The plane needs air flowing through its wings with a certain speed in order to create lift. That's what we call airspeed.
The airspeed is often created when the glider is descending but if you have strong headwind your descending may be minimal and may even stop or turn into a climb (no need to be a thermal).
And then you may turn downwind and dive to gain airspeed in the opposite direction and inertia that may allow you to turn back upwind and climb again.

Anyway, my initial argument was that with a powered plane you don't need to dive to keep the airspeed when turning downwind. That's why I say they've different behaviours.

And when I say airspeed I do not mean ground speed, I know the difference.
I'm not sure how you're perceiving the airspeed, inertia, acceleration, etc. The way you are describing that a plane somehow holds station and rises in wind is completely and utterly wrong but it's difficult to explain why without getting quite involved given that my previous posts didn't hit the mark. It's actually very simple in one respect but that doesn't mean it's easy to explain or to realise.

I think essentially where you are going wrong is that you're assigning some sense of movement and inertia to the plane that does not and cannot exist.

If we assume that the body of air you are flying in is all moving at the same speed (i.e. a constant wind) then a plane flying in that body of air acts no differently than a plane flying in calm air. From the ground it will appear to speed up or slow down depending on the direction it's pointing but that's simply because the air is moving relative to the ground. The airspeed and all of the aerodynamics will be exactly the same whether in calm air or constant wind. There is no link between the ground and the plane that makes the plane tend to stay still relative to the ground (your comments sounded to me like you were thinking there is). So the plane flies exactly the same but the air it's flying in moves past the ground so it looks different from the ground.

Let me put it to you like this. If you are on an airliner sitting on the runway and you drop a piece of paper it will just fall vertically onto the floor of the plane and stay there until you pick it up. If you instead wait until you have taken off and are cruising at a constant speed of say 500kts over the Atlantic and you drop the same piece of paper it will still fall vertically and sit there on the floor until you pick it up. As a passenger in the plane the only way you know you're moving over the surface of the earth is when the plane accelerates (this includes speeding up, slowing down or turning). When the plane is flying in a straight line at a constant speed everything operates the same as if you were completely stationary so cups don't fall off tables and people can get up and walk around.
In the very same way the plane flyig in a wind doesn't have any connection with groundspeed. It's operating in a body of air moving at a constant speed over the ground just like the environment within an airliner. Now if the wind is not constant and it swirls and gusts then things are not so simple but that's easily understood once you understand the situation for constant wind.

Aidan

P.S.
Just saw your DS post.
The DS situation is similar to what happens if wind is gusty but in DS you use the ridge to dictate how and where the plane encounters the gusts. In my explanation above I didn't really deal with acceleration or inertia and these are what explain how DS works. If you get to grips with the constant wind scenario it will then be easy to explain exactly what happens in DS.

[adam_one]

Aio_1
I do really appreciate your effort but as I told previously I understand the difference between airspeed and ground speed.
You may check out what I wrote on my post #21 (http://www.rcgroups.com/forums/showpost.php?p=5530199&postcount=21) just disregard the first paragraph (it's the source of controversy which has been clarified for me now).
Cheers
:)

[gouch]

The last paragraph you refer to does make sense, but it's the stuff between that and now that is causing some confusion here

The airspeed is often created when the glider is descending but if you have strong headwind your descending may be minimal and may even stop or turn into a climb (no need to be a thermal).

I may be wrong, but that tells me you haven't really "clicked" yet. A glider will always decend in the air that it is flying in. There is, as you said earlier an optimal configuration to minimise this decent, but it is always going down in it's surrounding air.
EVEN in a thermal or ridge lift, the glider is decending in the air around it. If the decent rate of the glider is lower than the upward rising thermal air you are in, the net result is you find your altimeter climbing. But you are still decending in the air that your plane is in.

I was told by my good mate Eddie the instructor that if you push the stick forward you go down, if you pull back on the stick....you go down ;) You may for a short time go up in the surrounding air, but your airspeed will fall away and you will indeed go down. This also sits true for powered flight. I guess this is why Eddie called it pitch control, not an "elevator" That is what an engine is for :D


Well, I guess the most controversy here was caused because I based some of my assumptions on dynamic soaring where the glider actually gains altitude flying upwind, then sinks downwind and may keep flying in that fashion as long as the wind lasts… but I realised that's another story...

What happens if you don't turn back down the hill? Keep it going straight up and see what happens, it comes back down when the airspeed falls away. So it is only gaining altitude "upwind" due to the speed you already entered that mass of air. It is not magically "gaining altitude because it is going upwind".

P.S You like that one I made up? There is a second paragraph. "the one who dosen't listen to the Aio's of this world, will find learning a lot harder than the one who does :D

[HELModels]

I'll shut up soon, but did Eddie also teach you to use spoilers on landing like throttle to control approach? ...getting too low, close spoilers...too high open spoilers, somewhere in between just right...start to flare, full spoilers :D

forgot to add - touchdown, close spoilers and rollout off the grass

[gouch]

Did you miss the memo that said I have trouble keeping the damn thing flying straight and level :D

Seriously though, he did, and I guess like every good instructor probably does, he explains every move he makes and why, although for some reason it was "his plane" at around 5K ft AGL :D Self preservation I guess ;)

Besides, I am still trying to keep that bloomin' string parallel to the fuse in turns :rolleyes:

[biber]

Estorch, that is the way it is taught and it works out fine.

biber

[adam_one]

I was told by my good mate Eddie the instructor that if you push the stick forward you go down, if you pull back on the stick....you go down You may for a short time go up in the surrounding air, but your airspeed will fall away and you will indeed go down. This also sits true for powered flight. I guess this is why Eddie called it pitch control, not an "elevator" That is what an engine is for Hey gouch, that one reminds me something that still puzzles me, how can you get your gliders up there without any prop to move them forward? could you tell me your take-off secret pls. :D


What happens if you don't turn back down the hill? Keep it going straight up and see what happens, it comes back down when the airspeed falls away.Ha, ha, but if it comes back down then it will increase the airspeed, no? ;)


So it is only gaining altitude "upwind" due to the speed you already entered that mass of air. It is not magically "gaining altitude because it is going upwind".I know I know, your glider has no motor so it uses the gravity to move forward putting you at the wind's mercy. :D

[Aio_1]

...Besides, I am still trying to keep that bloomin' string parallel to the fuse in turns :rolleyes:
I know the feeling. For some reason I always got left and right mixed up with the rudder pedals which doesn't help! It always seemed to me that the rudder pedals should work in the opposite sense, i.e. you should push on the side that's on the outside of the required yaw. Apparently no else agrees.

I did about 35 winch launches last year in K13s and a K21. I got to the stage where I was in control for the whole flight including launch and landing not that they were all smooth! I was living in England at the time near lots of old RAF bases several of which are now used by gliding clubs. Unfortunately there's only a couple of clubs in Ireland and it's more expensive too but I plan to get back to it at some stage. I think the secret when you're learning is to fly regularly. If I left more than a week or two between flights I was all over the place. You may remember the data but you tend to lose the feel of the controls when you leave it too long between flights.

Aidan

P.S.
Thanks for the compliment in #68 - I'm honoured!

[Aio_1]

Aio_1
I do really appreciate your effort but as I told previously I understand the difference between airspeed and ground speed.
You may check out what I wrote on my post #21 (http://www.rcgroups.com/forums/showpost.php?p=5530199&postcount=21) just disregard the first paragraph (it's the source of controversy which has been clarified for me now).
I was basing my suspicion that you hadn't quite got to grips with things on your more recent posts. The way you described a plane being "blown backwards" and thus perhaps gaining altitude among other comments leads me to think you have something confused (as much because of the way you phrased it as the relationships described). I might be wrong about that but do consider the possibility that I'm not just in case ;).

Aidan

[biber]

I know the feeling. For some reason I always got left and right mixed up with the rudder pedals which doesn't help! It always seemed to me that the rudder pedals should work in the opposite sense, i.e. you should push on the side that's on the outside of the required yaw. Apparently no else agrees.The funny thing is, I know at least one modeling buddy, who said just the same on the weird mixed up rudder pedals. He expected them to work the other way around, too. Maybe it is, because he was modeller long before his first flight in a fullsize motorglider, as you where, too, I guess.

And I think I can recall similar thoughts of my own, when gliding was new for me, but seems it's not that hard to get over it.

biber

[Aio_1]

Estorch, that is the way it is taught and it works out fine.
That's often the case.
One often missed fact is that while a good pilot may know exactly how to make a plane fly well it does not mean they understand exactly what's happening.
I had lectures with and from guys who worked in the Empire Test Pilots' School (http://www.qinetiq.com/home_etps.html) and it was clear that one of the big skills in aircraft testing for both the test pilots and the test engineers is to avoid letting the pilot's interpretation pollute the data. A good test pilot it seems has to un-learn a lot of what's taught from day one of pilot training in order to accurately relate what happened duing a flight. Not because what they were taught was bad but because the analysis used by the pilot is sometimes a fast and efficient simplification which is more practical for operation (often because it seems "right" and intuitive) but disguises the actual mechanism involved.

I got a good demonstration of this when I was at the gliding field and an instructor overheard me explaining something to two other novices. He became very angry because he thought I was potentially putting their lives at risk by explaining something I didn't understand myself. I can't remember what the particular topic was but he understood it as a pilot and and I understood it from the flight dynamics perspective. We were both right but a pilot would apparently never be told what I was saying.
He really was quite angry but we managed to clear things up after explaining that the the 3 of us were studying flight dynamics and this was stuff that we were already pretty familar with and needed to understand in detail.

Aidan

[Aio_1]

The funny thing is, I know at least one modeling buddy, who said just the same on the weird mixed up rudder pedals. He expected them to work the other way around, too. Maybe it is, because he was modeller long before his first flight in a fullsize motorglider, as you where, too, I guess.

And I think I can recall similar thoughts of my own, when gliding was new for me, but seems it's not that hard to get over it.
Yeah I'm pretty sure RC was responsible fo my confusion too. I got over it pretty quickly but anytime I took a break for a few weeks I tended to get it wrong again next time up.

[biber]

I mean, it's not to simplified to think of the elevator (kind of wrong term for that thing) as the tool to dictate a speed, while the airbreaks are not the tool to eat up speed, but to dictate the glidepath/sinkrate.

However, I can confirm your statement on the testpiloting.
In the Idaflieg the Zacherprotokoll is a known as a standard method to examine the flight characteristics of gliders and motorglider systematicly.
Aero studends of the German Akafliegs do these testings every year on the Idaflieg Sommertreffen, aswell as there are done flight performance measurements.
The Zacherprotokoll has not only the goal of collecting data on gliders of all manufacturers, but is also considered as an educational measure.
And I assure you the average glider pilot has again to learn to fly in a different way, for that stuff.

biber