Hypothetical Question from my Instructor. HELP!

[MakeItHappenCaptain]

Ah, but once the wheels (tyres) exceed their speed rating, you will be in trouble...

[FGD135]

Of course the aircraft can take off!

To take flight, the wings must generate lift. But the aircraft is stationary, relative to the air surrounding it - so there cannot be any lift generated.

Very straightforward when viewed in those terms.

[Volumex]

Mythbusters - Plane on a Conveyor Belt - YouTube

[Ex FSO GRIFFO]

Here's one back to the Instructor,

A boat has rope ladder hanging over the side.
Each rung of the ladder is 6 inches apart.
The incoming tide comes in at the rate of 1 foot per hour.
How long is it before the first three rungs are covered??

See yas.....

[ollie_a]

Wow I really hope the comments that an aircraft on a conveyor belt could not take off are tongue in cheek.

In case anybody is actually taken in:

Yes, the aircraft can take off.

Not up for typing the reasoning out word for word but a good explanation is here:

The Straight Dope: An airplane taxies in one direction on a moving conveyor belt going the opposite direction. Can the plane take off?

[AerocatS2A]

To take flight, the wings must generate lift. But the aircraft is stationary, relative to the air surrounding it - so there cannot be any lift generated.

Very straightforward when viewed in those terms.

Except those are the wrong terms. The conveyer belt doesn't have any mechanism by which it can prevent the aircraft from moving forward relative to the air. It can spin the wheels as fast as it likes but unless the wheels fail, the aircraft will still move forwards.

Edit: The answer to the conveyer belt question actually depends on how the question is worded. If the conveyer belt is defined as being capable of keeping the aircraft stationary (through magic or whatever) then the aeroplane won't fly. On the other hand, if all it does is attempt to match the wheel speed, ie, it is a real world conveyer belt that obeys the laws of physics, then it can't stop the aircraft from moving forwards.

As far as the theoretical stable airmass goes, a stable airmass for practical purposes is a very common occurrence. It only really needs to be felt as smooth air.

[Wally Mk2]

..geeez 'griffo' what drugs ya on there mate?

The boat displaces the same amount of water regardless how high/fast the tide rises
Good test: Place one rubber ducky in the bath tub in one inch of water so it floats with some of it's cute yellow butt exposed. Add water to tub & watch the ducky rise with same amount of butt exposed (sick buggers we are)....'till the bath overflows that is then all bets are off as mum will kick ya ass

'Aero' yr spot on there mate The wheels are the only thing that will not be normal in the T/off, they simply spin faster

Wmk2-)

[sisemen]

The question that was asked is:

Which direction should you fly to make the clouds seem like they are going the fastest past you?

I refer the honourable members to the answer previously given in post # 5

[neru1]

Attempting to answer

You have decided to go for a nice scenic fly at Bendigo, the cloud is scattered at 2000ft with a wind of 360M/15kts. You climb up above the clouds so you can skim the tops (remember hypothetical) and get that feeling of speed. Which direction should you fly to make the clouds seem like they are going the fastest past you?
All hypothetical questions are meant for checking your fundamental understanding of the concept. Hence I would respect the intention of the instructor for posing this problem. The comments (positive and negative) posed by all are justified in their place since the thread starter has not specified his level of pilot training at which the instructor has posed this problem. So assuming that a genuine answer is required i would like to make an attempt at answering the question form the scratch.
The problem is of understanding of wind effect and relative velocity. This can be answered in two ways. First, assume same AIR MASS so that only relative aspects remain or answer algebraically using the wind triangle.
Clouds drift at wind speed at same level. (I am assuming that clouds are not vertical and are at 2000 feet where the wind velocity is 360M/15kts. So flying above clouds means you are also experiencing same wind velocity. If TAS is assumed to be 200 knots (hypothetical) then ground speed of ac in direction 360 is 200-15 = 185 knots. Speed of clouds is 15 knots but in same direction. So as per vector speed calculation, the relative speed is 185 + 15 = 200 (opposite direction –ADD).
In reverse direction of 180 (ac heading) the GS is 200+15 = 215. Speed of cloud remains 15 but in same direction. So relative speed is 215-15 = 200 knots (same direction –SUBTRACT).
So in both cases along the direction of winds the relative speed is 200 knots.
Now let us take other directions. Extreme cases are perpendicular directions. i. E. 090 or 270.
Ignoring the effect of drift on ac (as TAS is not specified) the speed of ac will be 200 (=TAS no HW or TW) and speed of clouds remain 15 in perpendicular direction. So the resultant vector will be more than 185 and less than 200 (as hypotenuse is less than sum of two sides). This will be true for all other directions as well where the function of sin cos will reduce the relative speed.
Hence the answer is that the ac has to fly in line with the wind, along or opposite to get the maximum relative speed. Other directions the relative speed would be less.
Hope that I have answered the query correctly!!

[Ultralights]

The scariest part of that mythbusters episode was the UL pilot believed he wouldnt get off the ground.....


I find it amazing something so simple, can be made so difficult.

ignoring windsheer/microbursts/turbulence etc, evey aircraft flies in Nil wind, calm conditions, Ground speed is irrelevent.
the only time an aircraft is NOT in a nil wind calm condition is when its attached to something that stops it moving with the air mass. such as the ground...

even if the wind is 090 at 100 kts, the clouds wills till pass under the wing at the aircraft IAS..

not so hard.

and on the "oh geez, not this ****e again" downwind turn, again, only thing that matters is the path over the ground determined by windspeed relative to the ground.. once the aircraft is airborne, see above.. nil wind condition.. but the airmass relative to the ground is what will determine if you end up in the trees or not..

[Ex FSO GRIFFO]

Hi Wall,....

Have ya ever seen me sign orf wif 'See Yas' before......???

I was expecting 'someone' to bite.......but....? 'QUE??'

See Ya

p.s. A VERY noice 'red'.... in answer to yr question...

[Wally Mk2]

.............yeah Griffo me old buddy same red you where drinking, called 'Que'

'Ultra' have to agree there, the UL driver was surprised..............that's more amazing than anything else been said here!
At the end of the day as long as the numbers on the dial keep the machine airborne what does it matter?

Joke: Local Irish swimming pool has decided to be more community minded during a water shortage one long summer, they closed lanes 7 & 8..................still looking for my rubber ducky though


Wmk2

[Oktas8]

A very thorough post from Neru. From an instructor though I would hope for simplicity as well as thoroughness.

• The aircraft flies at a particular TAS. Direction & wind are of course irrelevant for TAS.
• The clouds fly at zero TAS. Ditto direction & wind.
• Therefore direction & wind are irrelevant to the difference in speed between the two.

Now altitude however is quite relevant to TAS - so you should fly just above the layer at 10,000' rather than just above the layer at 3000' for that real feel of speed...

Hmm. Now about that downwind turn:

• The aircraft accelerates (turns) as a function of angle of bank and TAS.
• The wind vector has exactly the same effect on an aircraft at all times, in terms of direction and speed.
• Therefore the wind cannot cause the aircraft to accelerate (turn), unless the wind itself changes.
• Therefore all acceleration (turning) done by the aircraft, in a steady wind, is caused solely by a function of angle of bank & TAS.
• This function of AoB & TAS doesn't have compass bearing in it, therefore one would not expect TAS to change in a turn, unless the wind itself changes.

Hope that works...

[Capn Bloggs]

A very thorough post from Neru.

But, as others have pointed out, wrong answer. Any relation to the ground of either clouds or aeroplane is irrelevant; the aeroplane flies only in the air immediately surrounding it. That airmass may be doing 5kts or 500kts (relative to the earth) in any direction. But as far as the aeroplane is concerned, it is merely pushing (or being pulled by the prop!) through that airmass at the IAS resulting from the thrust/drag balance.

Consider standing on the ground as a 500kt airmass passed overhead with Elfatness sitting in his C150 flying in said airmass. He may look like (depending on which way the cesspit in pointing) he's flying at 600kts! But, on his ASI, it reads 100KIAS, because it is reading the relative windspeed past the pitot head. Doesn't matter which way he's actually pointing travelling (as long at he doesn't turn too tight ). It's the relative windspeed that determines the speed of the clouds whizzing by in that airmass. The speed, or direction, for that matter, over the ground, of the whole airmass, with C150 in it, is irrelevant.

Another example. Fill up one of those half seashell tubs with water. Now, build an island in the middle (not too big, you have to pick this thing up soon). Get a motorised boat and set it to motor around the island. Boat is observed passing island at a couple of knots. Boat speed relative to the ground the tub is on is also a couple of knots.

Now, pick up tub and walk around backyard. Tub is now doing 5 knots in various directions! But, relative to the island in the tub, the boat is still only doing a couple of knots. Even though the water mass is moving relative to the earth, the boat still only does a couple of knots relative to the island. Sama Sama a moving airmass with aeroplane in it.

[Oktas8]

Should I have said "from an instructor, I would expect simplicity and a correct answer, as well as thoroughness?" I confess that neru's post was so long I didn't read all the way through to his incorrect answer. Oops.

I think, just maybe, you didn't read my post either... which is actually correct. But nice examples all the same. I learned quite a bit about these principles from considering aircraft ops from an aircraft carrier. Bit like your boat-in-a-portable-bathtub example.

[Ex FSO GRIFFO]

Re
"Now, pick up tub and walk around backyard. Tub is now doing 5 knots in various directions! "

You betcha...mostly vertically down...at 32ft/sec....

Unless said tub is also full of ice into which a few tinnies have been carefully placed....

[Checkboard]

Originally Posted by neru1

Ignoring the effect of drift on ac...

Originally Posted by neru1

So the resultant vector will be more than 185 and less than 200 (as hypotenuse is less than sum of two sides).

I hope you see what you did, there. ie. Ignored the drift in one case, and then compared it to a case with drift ...