hofer

Hi together,
As mentioned a few posts above ; there is a bibel about basic aerodynamics of an airplane which should have been read by each and every pilot :
Title :STICK AND RUDDER
Author :Wolfgang Langewische
Editor: Mcgraw Hill
The book is over 60 years old, still distributed and has never been topped concerning the straight forward explanations to all the bacics of piloting an airplane !!(be it model or FS)
99% of the Q`s and A`s in this whole thread are brought to the point !!
see you
hans

vasek

My Feedback: (4)

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haikt

My Feedback: (11)

I have a question
In DS soaring the plane adds speed with each loop and can achive speeds up to 200mph. If steady wind does not have an effect on the airspeed (lets say speed of air over the wing) and it is only ground speed (our perspective) then why does the plane fail (wing or fuse damage)
what am i missing

flythesky

You increase power and fly up.
I think you're dumb too.

Flyfalcons

Tisk, tisk, Larry. Think of this as a wind triangle. To offset the 20 knot headwind, the aircraft must be making 20 knots of forward progress through the air (the X axis). By flying a vertical line with reference to the ground, you must angle the plane up (now think of your new flight path as the hypotenuse of the triangle). Would you mind explaining how exactly the hypotenuse of the triangle is exactly the same measurement as the x-axis when there is any angle involved? Bonus points for using cosine calculation in your explanation. Good luck, we're waiting for you.

Flyfalcons

Actually those crazy SOB's are hitting 400 now. Bring your kevlar jacket! Anyway the reason that DS planes are hitting these speeds (and those speeds are measured on the back side, in calm air, so they're really going that fast) is because they are flying through a shear layer over and over. They fly on the back side of the mountain (on the lee), where the lower air is calm and the air above the top of the ridge is moving quickly. Traditional slope soaring is done on the front side of the hill, with (in comparison) a fairly steady wind.

Steve Graham

So now that this thread has deteriorated into a shambles only vaguely resembling the original intent wrap your minds around this one.

Turbulence inflight is normally managed by setting your IAS to something called maneuvering speed. This speed, in theory at least, is low enough that any gust will stall the wing before it fails due to overloading and at the same time high enough to give you protection from an inadvertent stall. Inflight turbulence is always handled without reference to the wind direction relative to the flight path.

There is one notable exception that I know of re. this concept. Orographic turbulence, that created by wind moving over an object on the ground be it buildings or the ground itself in the form of a hill or mountain, is more dangerous when you are flying downwind. Take a mountain wave. This will create a bump in the airflow that is stationary over the object creating it. The air itself is flowing but the WAVE is stationary and in this wave are the shears that create the turbulence to an aircraft passing by and attempting to maintain a straight flight path. It is the groundspeed in this case that determines the severity of the turbulence experience by the aircraft. Therefore, downwind turbulence in this admittedly rather rare case is worse than upwind. Cool huh?

Well stated on the DS Flyfalcons. In a sorta related way I was able to recently keep my EZ glider aloft for an extended period of time over a flat field by exploiting wind gusts. Climb in the gust and glide in the lull. Somewhere I was reading about Albatross who do the same thing over waves in the open ocean in order to conserve energy.

Lnewqban

rcjets_63

My Feedback: (4)

Lnewqban,

Excellent graphic of a pelican flying inverted in Australia.

Jim

Rob2160

Because you are firmly planted on the ground..

your perspective to the helicopter / aicraft is being changed by the wind constantly..

If you were floating along with the wind at a steady state with the helicopter then the control inputs would be the same.

Rob2160

This is all good in theory and I have instructed the same as your argument many times in full size aircraft..

But here is one to try to explain..

Kinetic Energy is calculated by KE = 1/2 x Mass x velocity squared..

So even though you maintain a constant flying speed around a 360 degree turn in a steady wind.. your Kinetic energy is constantly varying..

Here is an example..

Assume a 20 kt steady state wind..

You are in a plane gliding at 60 kts airspeed.

You face into wind and glide straight into a a tree.. you impact at 40 Kts..

Now imagine you were flying with the wind.. you would hit the tree at 80 Kts.

Double the speed. but FOURtimes the kinetic energy and hence a much more DANGEROUSimpact.

So something is different when you turn downwind.. you have LOWER kinetic energy when flying into wind and as you turn to face downwind you are gaining kinetic energy.

Doug Cronkhite

My Feedback: (34)

Which has about as much relevance to the downwind turn myth as the cost of a cup of coffee.

cfircav8r

My Feedback: (1)

Your kinetic energy relative to the ground changes but not in reference to the air. Think of it this way the tree is moving at a varying rate compared to you depending on the wind and direction. The air you are impacting while flying through an airmass is always moving the same rate and speed, assuming steady winds, therefore your kinetic energy relative to the wind does not change.

flythesky

So this thread is still roaring along. Good. It gives me the chance to admit Iwas I was totally wrong in my thinking. My square loop example is pretty much junk science.

Thanks to all who tried to make me see the light.

cfircav8r

My Feedback: (1)

It took me 2 years to figure out P-Factor. It finally just hit me at 2 in the morning after many sessions of instructors trying to get it past all of the preconceived notions and improperly relating unrelated knowledge. No one thinks anyone is stupid for not getting it. We all understand that there is years of knowing what we know and seeing what we know we are seeing getting in the way. The other problem is you don't know who I am and what my experience is. I can say I am a CFII with 15 years of experience but I could be a 15 year old kid trying to mess with you. I can't speak for the others, but I imagine they feel the same. The only reason I am involved in these discussions is to try and help make others more successful, by providing them with as much correct information as possible. I fortunately had some mentors with a wealth of knowledge and patience to help guide me along. I am just trying to pass it on.

Lnewqban

Rob,

The kinetic energy of the plane, just like the wind associated to it, is always dependent of a frame or point of reference.
There is no absolute value for kinetic energy, since it is based on variation of speed (acceleration or deceleration).

Energy can only be accumulated or transferred.
Since the airplane moves with and within the air, there is no transfer of energy from the air into the plane (upwind or downwind).

At the moment of impact with the tree, the transferred energy (or work) to the tree from the plane is added to the energy of the wind within which it flies.
If the plane would crash against a tree while flying upwind, the opposite would happen.

Copied from http://en.wikipedia.org/wiki/Kinetic_energy:

"The kinetic energy of an object is the energy which it possesses due to its motion. It is defined as the work needed to accelerate a body of a given mass from rest to its stated velocity. Having gained this energy during its acceleration, the body maintains this kinetic energy unless its speed changes. The same amount of work is done by the body in decelerating from its current speed to a state of rest.

The speed, and thus the kinetic energy of a single object is frame-dependent (relative): it can take any non-negative value, by choosing a suitable inertial frame of reference. For example, a bullet passing an observer has kinetic energy in the reference frame of this observer, but the same bullet is stationary, and so has zero kinetic energy, from the point of view of an observer moving with the same velocity as the bullet. By contrast, the total kinetic energy of a system of objects cannot be reduced to zero by a suitable choice of the inertial reference frame, unless all the objects have the same velocity. In any other case the total kinetic energy has a non-zero minimum, as no inertial reference frame can be chosen in which all the objects are stationary. This minimum kinetic energy contributes to the system's invariant mass, which is independent of the reference frame."

Tall Paul

.
There is a tangible speed difference to the pilot upwind and downwind, and this tangible speed difference does alter the effect of an impact, whatever the airspeed might be.
He can see this speed difference, but not the airspeed, so it's easy to believe the myth.

Tall Paul

.
This one got lost in the forum reshuffle yesterday...

rjbob

My Feedback: (8)

Explanations, right or wrong, are sometimes very difficult to understand without being able to draw them out on paper.

Let's fly model airplanes now, Larry.

Sport_Pilot

Doug Cronkhite

My Feedback: (34)

P-factor, Torque, Gyroscopic Precession, and Spiral Slipstream are probably the most confused and incorrectly attributed subjects in aviation.

RZielin

My Feedback: (1)

Do you all see what what's happened here, what you've all done now? This thread was tearing up bandwidth, creating real interest UNTIL:
Reason got the upper hand. Great, now we all agree on the damn truth. I hope you're all happy with yourselves, debunking that myth. Now what have you got? Nothing to say, that's what! Not even an upside down pelican in Australia today. PATHETIC!

Now I've got nothing to do, but write.......THIS.

I'll tell you what this thread, this hobby, this nation needs: someone to pipe up and say something STUPID. Without a few dumb misconceptions to jump on, all you smarty pants correct and knowledgeable people are so QUIET and BORING.

HH couldn't kill this thread even with appeals for mercy to the moderator. I guess it's finally died of natural causes. [sm=cry_smile.gif]

PS: Has anyone noticed that GUSTS are more dangerous when flying downwind than upwind?

eddieC

My Feedback: (2)

fulps

Its all about inertia. In steady conditions the wind doesnt matter and has no influence on the true airspeed. Only the groundspeed depends on the wind. BUT if there is a sudden change in windspeed (including a change of the direction like in the 180 degrees turn) the airplane wants to maintain its speed due to inertia.
Yesterday on my way home from the US I noticed this effect again. During cruise flight we encountered a change in windspeed. The windspeed drop 20 to 30 knots. Due to inertia our true airspeed increased close to vmax. Leading to a powersetting close to flightidle at crusing level and mach .82. Its like standing on a skateboard on a conveyor belt. If the speed of the conveyor belt changes (quickly) - you move! Its the same effect on the downwind turn.

Does anyone understand what I mean? Dont ask me about the loop-problem....



best regards

Jochen

RZielin

My Feedback: (1)

I think I follow how inertia is a factor during gusts and lulls. The plane cannot accelerate or decelerate as fast as the gust does, so there is a transient change in air speed, until the the plane comes back to "steady state" air speed. The result is that a gust would cause a transient rise in airspeed when headed upwind, and a transient drop in airspeed when headed downwind. Lulls, just the opposite.

I'd bet that gusts and lulls have a faster acceleration at their front than they do at their tail. (Perhaps this "asymmetry" is more pronounced for gusts than lulls?) The gust hits (accelerates) fast, like a puff, then slowly dies back to average wind speed. The lull hits (decelerates) abruptly, then wind speed gradually climbs back up to average wind speed. This combined with the planes inertia means that the plane will be abruptly affected by the front of the gust or lull, with a rapid change in airspeed. The plane then recovers to previous airspeed more gradually as the gust or lull "dies out" and the inertial effect "dies out".

SO...gusts are more dangerous when the plane is headed downwind because you get an abrupt drop in airspeed, then a slow airspeed recovery, possibly too slow to recover if the plane hits stall airspeed. Going upwind, the gust is no problem because the plane "jumps" up quickly and gradually settles back to previous altitude as the airspeed decays back to baseline.

Note however that a lull when headed upwind is just about as unsettling and dangerous as a gust when headed downwind. In general, I'd say that upwind turns in wind are just about as dangerous as downwind turns in GUSTY conditions.

Whenever turning in windy AND gusty conditions, you never know when a gust or lull will hit your plane. It will not hit the plane at the same time it hits you. (Sailors can see gusts and lulls coming by the surface of the water. They are very "local" phenomena, often only about 30 to 200 feet in length and width).