Re: Speed of light cycling

dkahn400 said:

Phil Barker said:

dkahn400 wrote:

Quoted message said:

Quoted message said:

Cycling at the speed of light the top of the wheel would be
travelling at twice the speed of light.

No it wouldn't. You're assuming that you can calculate relative
velocities through simple addition (e.g. person on road side sees
rider is going at speed 0.5 relative to ground; top of wheel is going
at speed 0.5 relative to rider, therefore top of wheel is going at
speed 1 relative to person on road side). This assumes that distance
and time measured by the rider are the same as those measured by the
person on the ground. This isn't the case. If you move quickly, space
and time are distorted in a way described by Einstein's theory of
relativity, which has its centenary this year, see
http://www.einsteinyear.org/facts/special_relativity

Yes, I believe you are correct. The out-of-roundness of the wheels is
also associated with this, I believe, so that the closer you get to the
speed of light the more the bicycle is shortened and the more closely
all parts of it appear to an observer to be moving at the same speed.

Yes, but be careful. The wheel is accellerating: special relativity descibes
only what happens at constant velocities, to work out what happens to the
wheel you would need general relativity ... which is where I get off :-}

Quoted message said:

It would not appear distorted to the rider though. I'm happy to be
further corrected on any of this by someone who really knows what
(s)he's talking about. I sympathise with the quote from Chaim Weizmann
after he'd made an Atlantic crossing of several days with Einstein.
"Einstein explained his theory to me every day and on my arrival I was
fully convinced that he understood it."

dkahn400 said:

Phil Barker said:

dkahn400 wrote:

Quoted message said:

Quoted message said:

Cycling at the speed of light the top of the wheel would be
travelling at twice the speed of light.

No it wouldn't. You're assuming that you can calculate relative
velocities through simple addition (e.g. person on road side sees
rider is going at speed 0.5 relative to ground; top of wheel is going
at speed 0.5 relative to rider, therefore top of wheel is going at
speed 1 relative to person on road side). This assumes that distance
and time measured by the rider are the same as those measured by the
person on the ground. This isn't the case. If you move quickly, space
and time are distorted in a way described by Einstein's theory of
relativity, which has its centenary this year, see
http://www.einsteinyear.org/facts/special_relativity

Yes, I believe you are correct. The out-of-roundness of the wheels is
also associated with this, I believe, so that the closer you get to the
speed of light the more the bicycle is shortened and the more closely
all parts of it appear to an observer to be moving at the same speed.

Yes, but be careful. The wheel is accellerating: special relativity descibes
only what happens at constant velocities, to work out what happens to the
wheel you would need general relativity ... which is where I get off :-}

Quoted message said:

It would not appear distorted to the rider though. I'm happy to be
further corrected on any of this by someone who really knows what
(s)he's talking about. I sympathise with the quote from Chaim Weizmann
after he'd made an Atlantic crossing of several days with Einstein.
"Einstein explained his theory to me every day and on my arrival I was
fully convinced that he understood it."

Trevor Barton said:

LSMike said:

"Simon Brooke" <[email hidden]> wrote in message
news:[email hidden]...

Quoted message said:

in message <[email hidden]>,

LSMike

Quoted message said:

Quoted message said:

Quoted message said:

([email hidden]'😉 wrote:

> "Simon Brooke" <[email hidden]> wrote in message
> news:[email hidden]...
>> in message

<[email hidden]>,

Quoted message said:

Quoted message said:

Quoted message said:

>
>> Helicopters can fly at at most not quite half the speed of sound

for

Quoted message said:

Quoted message said:

Quoted message said:

>> exactly this reason.
>
> Not quite, I think the record is some 400km/hour (that's gone up

since

Quoted message said:

Quoted message said:

Quoted message said:

> I
> last read). I thought the main limiting factor was the reducing

lift

Quoted message said:

Quoted message said:

Quoted message said:

> from the retreating blades.

The speed of sound is 1,237Km/h.

Depends on the conditions, surely?

Quoted message said:

The limiting factor is putting the tips
of your rotor blades through mach 1, because the stresses involved
would break them.

I rather think it's much more complex than this. I'm sure it's

possible to

Quoted message said:

Quoted message said:

make rotor blades that can go faster than the speed of sound since

under

Quoted message said:

Quoted message said:

certain conditions the tips of some aircraft propellers will go

supersonic.

Quoted message said:

Quoted message said:

IANAE but I seem to remember that there are all kinds of

aerodynamic

Quoted message said:

Quoted message said:

disadvantages to having part of a wing (i.e. rotor blade) super

sonic and

Quoted message said:

Quoted message said:

part not.

Yeah, the stresses involved tend to break them.

Quoted message said:

Quoted message said:

You do need to adjust the pitch of the retreating
blades, because going relatively slower they need to have courser
pitch.

IIRC here is the problem. Low blade airspeed and coarse pitch on

the

Quoted message said:

Quoted message said:

retreating blade = stall over some of the inner (and thus slower

moving)

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part of the blade. The faster you go the further out the stalled

section

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moves. Hence I still believe this is the limiting factor.

No, stall happens at an angle of attack, not a speed. For the same

angle

Quoted message said:

of attack (or pitch) the blade is either stalled or not, but the

speed

Quoted message said:

matters not, at least to first order.

This is not the case with aircraft wings, which are similar aerofoils
to propeller blades / heli rotors. The whole point of having flaps on
wings is to reduce the speed at which the wing stalls at a given angle
of attack, so you can take off and land at a speed that doesn't require
a 10 mile runway.

The Luggage said:

Trevor Barton said:

LSMike said:

"Simon Brooke" <[email hidden]> wrote in message
news:[email hidden]...
> in message <[email hidden]>,

LSMike

Quoted message said:

Quoted message said:

> ([email hidden]'😉 wrote:
>
>> "Simon Brooke" <[email hidden]> wrote in message
>> news:[email hidden]...
>>> in message

<[email hidden]>,

Quoted message said:

Quoted message said:

>>
>>> Helicopters can fly at at most not quite half the speed of sound

for

Quoted message said:

Quoted message said:

>>> exactly this reason.
>>
>> Not quite, I think the record is some 400km/hour (that's gone up

since

Quoted message said:

Quoted message said:

>> I
>> last read). I thought the main limiting factor was the reducing

lift

Quoted message said:

Quoted message said:

>> from the retreating blades.
>
> The speed of sound is 1,237Km/h.

Depends on the conditions, surely?

> The limiting factor is putting the tips
> of your rotor blades through mach 1, because the stresses involved
> would break them.

I rather think it's much more complex than this. I'm sure it's

possible to

Quoted message said:

Quoted message said:

make rotor blades that can go faster than the speed of sound since

under

Quoted message said:

Quoted message said:

certain conditions the tips of some aircraft propellers will go

supersonic.

Quoted message said:

Quoted message said:

IANAE but I seem to remember that there are all kinds of

aerodynamic

Quoted message said:

Quoted message said:

disadvantages to having part of a wing (i.e. rotor blade) super

sonic and

Quoted message said:

Quoted message said:

part not.

Yeah, the stresses involved tend to break them.

Quoted message said:

> You do need to adjust the pitch of the retreating
> blades, because going relatively slower they need to have courser
> pitch.

IIRC here is the problem. Low blade airspeed and coarse pitch on

the

Quoted message said:

Quoted message said:

retreating blade = stall over some of the inner (and thus slower

moving)

Quoted message said:

Quoted message said:

part of the blade. The faster you go the further out the stalled

section

Quoted message said:

Quoted message said:

moves. Hence I still believe this is the limiting factor.

No, stall happens at an angle of attack, not a speed. For the same

angle

Quoted message said:

of attack (or pitch) the blade is either stalled or not, but the

speed

Quoted message said:

matters not, at least to first order.

This is not the case with aircraft wings, which are similar aerofoils
to propeller blades / heli rotors. The whole point of having flaps on
wings is to reduce the speed at which the wing stalls at a given angle
of attack, so you can take off and land at a speed that doesn't require
a 10 mile runway.

No, the sole purpose of flaps is to increase the lift of a wing at
a given speed. While there is a direct relationship between speed
and angle of attack for an aerofoil when it's used as a plane wing,
that is simply because of the balance of forces involved - if the
plane slows the angle of attack of the wing *must* increase so that
the lift remains the same, and in level flight the lift generated
by the wing has to be the same as the weight of the plane. Therefore
as the plane slows the angle of attack increases, and at some particular
angle of attack the wing will stall. This relationship leads to the
common mistake of equating stall with speed - and it's not helped in
that an aircraft's stall point is usually quoted as an airspeed for
a given wing configuration.

However, that's not the case for a helicopter blade, each point of
which is at a pitch (or angle of attack) that is determined by the
controls and not by the dynamics of the aircraft. The angle of
attack is unrelated to the speed of the blade through the air, unlike
the wing of an aircraft, where the angle of attack is determined by
the speed through the air.

In fact, the angle of attack at which an aerofoil will stall depends
on the speed. Typically, an aerofoil will stall at a lower angle
of attack at higher speeds. It's possible to stall a wing at very
high speeds on an agile plane compared with the same wing's stall
characteristics in straight and level flight.

--
Trevor Barton