Basic Aerodynamics

[apereg]

This question launched an ongoing debate in my respectable Airline company...:confused:
An aircraft, any aircraft if flying in steady but very strong wind.
Starting point is with head wind.
The aircraft starts to turn, fixed thrust (only compensating for the turn), fixed altitude, same bank angle, same wind direction and speed - all parameters are fixed.
Will the aircraft's AIRSPEED change when moving from head to tailwind?
Remind you: Very strong wind (may be stronger than the A/C airspeed...)
Thanks

[False Capture]

Oh no! Don't wake the beast.

The mythical "downwind turn" rises from the dead yet again.:eek:

[Clandestino]

It's not even about the aerodynamics, it's about basic phisics and relative motion. There's special phase of flight where airspeed changes with turning up/downwind and it's called taxiing. As soon as your wings take over from wheels, you move through the air and together with it - airspeed will stay the same but groundspeed will vary. It's counterintuitive for ground dwelling mammals but your inertia is now relative to airmass, not to ground.

[Mad (Flt) Scientist]

It's counterintuitive for ground dwelling mammals but your inertia is now relative to airmass, not to ground.

If that were true then windshear, gusts, etc., would have no impact on airspeed.

[one dot right]

If that were true then windshear, gusts, etc., would have no impact on airspeed.
not quite true i'm afraid 'cos gusts,windshear etc are all relative to airmass as a whole, which,once you are airborne,you are too!

[barit1]

It's a dynamic phenomenon - depends on the rate of change. Gusts, updrafts/downdrafts are short-term and DO affect IAS since inertial speed (i.e. groundspeed) cannot change instantly.

On the other hand the downwind turn is a longer-duration event. The same physical laws apply, but the rate of change is much slower - at least in a relatively fast plane at a standard rate turn. Energy must be added to increase the groundspeed, but it's all within the everyday minor turbulence.

The problem arises in a slower aircraft in a faster-than-standard rate downwind turn. In order to increase its inertial speed, energy must be added at a fairly high rate; either drop the nose or add power.

[Superpilot]

Can I show my goofyness and ask you fellas to try it on a PC Flight Sim. If modelled correctly you will see the true outcome before your very own eyes.

[Clandestino]

inertial speed (i.e. groundspeed)

This is true for aircraft physically connected to earth surface or using antigravity instead of aerodynamic lift. Earth is just another non-privileged inertial sistem. Once you slip its surly bonds you wouldn't care a bit about its position if it wasn't for the gravity.

Now everyone who thinks that airplane "knows" its speed and position relative to ground and its airspeed is threfore affected by turning downwind - think harder. If you think hard enough, you'll soon find the way of simply distinguishing gravitational and inertial force acting on airborne aircraft. Patent it as soon as you find it, there are gigabucks to be made by making entire gyroscope industry obsolete!

I apologise if I used some incorrect terms, but my science classes were not in english and neither was my ATPL theory.

[Mad (Flt) Scientist]

The original post invited us to consider an aircraft moving relatively slowly compared to the windspeed. So let's consider a case where the airspeed and groundspeed are identical:

Aircraft heading N at 100m/s airspeed, 100 m/s headwind : net groundspeed zero.

After the turn the aircraft has a ias still of 100m/s and a 100m/s tailwind : groundspeed 200 m/s

Now, in real physical terms, groundspeed is what matters (in as much as we have an inertial reference frame to work with, it's earth-based (we'll ignore Coriolis forces and the like for now)).

Therefore our aircraft has actually been accelerated, in real terms, by 200m/s in the time taken to complete the turn. Where does this energy come from?

Well, one source is the tailwind itself, which will tend to 'push' the aircraft South (as it has been doing throughout the turn) - at the 1/2 way point, there's a 100m/s sidewind trying to push us South, plus there's our engines adding energy. So, if we turn in a sedate enough fashion, we can actually extract energy efficiently enough from the atmosphere to enable our speed to remain essentially constant.

But, suppose a very fast turn - say 9 deg/sec, completeing the turn in 20 secs. That means we have been accelerated in groundspeed terms by 10m/s^2 in order to get from 0 to 200 - that's approximately 1 'g' acceleration. There isn't that much spare energy floating around in a typical aircraft (unless you're a fighter, your T/W is nothing like that, and unless you're a balloon or parachute your drag is also nowhere near that). So, if the drag isn't high enough to extract enough force from the wind, and your engine not powerful enough to add energy (assuming you went to full power, of course) then the only way to resolve the energy balance is to slow down. So you'd exit the turn more slowly than you entered it, but then find you accelerated back to your original airspeed as the steady state equilibrium was restored.

Reduce the turn to a ridiculously short timescale - say 1 second! - and you'll see that there's simply no way to impart the kind of forces you'd need to get close to a constant airspeed case.

[Volume]

Just use a much more simple example to think about this phenomenon :
If you walk throug an airport terminal and step onto one of those typical conveyours, you note your sudden loss in walking speed due to a change in reference system speed.
Now change direction, and walk back against the conveyours movement. You don´t need different forces compared to doing this on solid ground, all that makes it feel strange, are optical illusions. Try it with your eyes closed, you will notice no difference between doing it on a conveyor or on solid ground.
It is the same type of optical illusions that trick you if flying the circuit in strong winds with a light aircraft.

Even simpler : let´s assume you live close to the equator, then you are traveling eastbound with 1666 km/h (40,000 km in 24 hours). If you walk eastbound and change your direction 180°, your speed relative to the center of the earth has changed from 1670 to 1662 km/h eastbound, but if feels like just changing from 4 km/h eastbound to 4 km/h westbound.

[TyroPicard]

Mad FS
So let's consider a case where the airspeed and groundspeed are identical:
Aircraft heading N at 100m/s airspeed, 100 m/s headwind : net groundspeed zero.

Would you care to start again?
Renaming myself TyroPedant..!

[chornedsnorkack]

Suppose that an airplane makes a turn in a strong but steady wind. The airspeed is kept constant. The groundspeed increases from a small value (flying into headwind) to a large value (tailwind).

The aircraft gains a lot of real kinetic energy with respect to ground. Where does it come from?

It comes from the wind. As the plane turns in the crosswind, the wings are banked and the wind is carrying the plane downwind.

The airspeed, and kinetic energy in the air reference frame, stays constant (the engines simply compensate for drag).

However, if the wind is not steady, then ground is an inertial frame, but air is not. It is possible to lose airspeed to windshear or turbulence - or gain airspeed from there.

[Brian Abraham]

Where I used to fly (Offshore helo) it was not unusual to have 60K of wind and with climb speed of 74K a turn to downwind after take off you wouldnt know the difference if it had been light and variable. False Capture has it right. Oh no! Don't wake the beast.

[overstress]

accelerated, in real terms etc etc

MadS: I've been flying for longer than I care to remember and I've never heard such gobbledygook. No-one cares about the real terms as when you are flying, everything is relative to the ac.

[Mad (Flt) Scientist]

You also probably haven't done many 2g+ 180 deg turns in conditions where the windspeed is of the same magnitude as your airspeed, so it might not be possible to extrapolate your real experience - which I noted is of slow enough changes that any effects are much smaller.

But wind does matter - otherwise we wouldn't specify that our climb performance tests be conducted headwind/tailwind (some other companies do them crosswind instead) - sometimes it makes a difference.

[Brian Abraham]

Mad, I'm afraid your barking up a tree. Do a search for downwind turns to get an insight to your misconception. A study of the principles behind inertial platforms would aid your understanding as well.

[Mad (Flt) Scientist]

Mea Culpa.

I guess I need to spend less time in meetings, it looks like they do indeed atrophy the brain.

I guess the explanation in my simplified brain is that the aircraft which cannot safely conduct the 'downwind turn' case also can't manage the still wind case, so it's not the wind causing the problem, its the turn.

[Brian Abraham]

so it's not the wind causing the problem, its the turn

NO, NO and NO. A pilot deprived of his NAV instrumention and operating in IMC conditions has no idea if there is wind blowing (assuming steady state wind) or not, and if there is wind he has no idea of its strength, whether it be one knot or 250. If VMC he can determine by reference to the ground (not when flying above an 8/8 undercast) visually drift and depending on wind strength the variation of groundspeed when orbiting. A steady state wind has no influence on the dynamics of a manoeuvering aircraft. I hope you get the drift - pun not intended.

[chornedsnorkack]

But wind does matter - otherwise we wouldn't specify that our climb performance tests be conducted headwind/tailwind (some other companies do them crosswind instead) - sometimes it makes a difference.
I think that with respect to definition of climb performance, the ground often matters, so naturally the strength and direction of wind would then be relevant... At the same airspeed, and climb rate defined as metres per minute, an airplane might climb vertically up in a strong headwind, or at a very shallow angle in a strong tailwind...

[sikeano]

mad fs has a very good theory the original question was about i quote
"An aircraft, any aircraft if flying in steady but very strong wind.
Starting point is with head wind.
The aircraft starts to turn, fixed thrust (only compensating for the turn), fixed altitude, same bank angle, same wind direction and speed - all parameters are fixed.
Will the aircraft's AIRSPEED change when moving from head to tailwind?"
there is only one speed and that is speed of the aircraft through the air TAS all other so called speeds are pressures
and the question says same bank angle same speed is maintained i think the ans is in the question :ok: