Not to rain on your parade there, but IAS is typically what is used.Quote:
Originally Posted by focus
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Not to rain on your parade there, but IAS is typically what is used.Quote:
Originally Posted by focus
...and the beatings from the engineers continue...
I said the exact same thing 13 months ago.Quote:
Originally Posted by AsheanMT
Which begs the question, why am I still arguing about it now?
....:confused:
It isn't my parade, it's pechelmans.Quote:
Originally Posted by P_McPoser
And tell me, does it matter? IAS -- Indicated Air Speed, is by its name an estimation of velocity. Same principle, just insert IAS where pitot tube was.
Whoa, how the fuck did I miss that? Thanks for crowning yourself dumbass of the year with that remark. I guess I can no longer perform my function as a ventilation engineer since you decided to rewrite the laws of thermodynamics right there.Quote:
Originally Posted by focus
Guess I can no longer measure air veloticies by measuring Velocity Pressures or anything.
Nevermind, thought you were claiming IAS was useless. I was just saying that's what's used. And for aircraft/boats etc, yes, it's an estimate dependent on either windspeed or current. So no argument there. :)Quote:
Originally Posted by focus
My above post, however, still stands. I can measure AIR VELOCITY (as in wind speed) very accurately with a pitot tube.
Absofuckingloootely was overstating it. The point remains. Pressures, airspeeds, etc. are not what the treadmill is measuring. Measuring them is a tool for estimating velocity. Just a tool. The plane's actual velocity is something that may or may not be reflected by these measurements. The treadmill is measuring the PLANE'S (or cars) actual velocity, not airspeed, not pressures, not wheel speed. Dumbass of the year? I'll wear it. Doesn't change the fact that you still don't seem to fucking get it.Quote:
Originally Posted by P_McPoser
;)
edit: do you get it? I don't know, and I'm not going to reread all your posts right now. Maybe I'm arguing with the wrong guy. It still blows my mind that people who work with this stuff, have degrees in this stuff, can't read the question correctly and can't fit their minds around the concept. All you engineers (the ones handing out "beatings") are overanalyzing the crap out of this thing.
Quote:
Originally Posted by pechelman
A car propels itself by pushing against the ground. An airplane propels itself by pushing against the air. An airplane overcomes gravity by creating a pressure differential between the top and the bottom of the wings. Airflow is required for this.
The question, as it is asked, is unanswerable. There is simply not enough information provided. Everybody is (mostly unknowingly) inserting their own information into this question to make it answerable. Insert different information, get different answers.
Since the engines provide thrust, the plane will move forward unless another force counteracts the engine's thrust. The other force, in this example, is generated by the treadmill and transmitted to the plane through friction. It can be assumed that the wheels are spinning at the same speed as the belt on the treadmill, so there is no kinetic friction between the wheels and the treadmill; only static friction. So the only other place kinetic friction could result in a force being exerted on the plane is in the bearings. Frictionless bearings=plane takes off normally, assuming the treadmill is long enough and the tires don't fly apart due to inertial forces.
If the bearings are of this earth and are affected by friction: then, if the thrust provided by the engines is sufficient to accelerate the plane to a speed that creates enough airflow across the wings to create enough lift for the plane to take off...the plane takes off. But, if the counteracting force generated by the friction in the bearings is not able to be overcome by the engines...it doesn't.
As I said, the question, as it is presented, is unanswerable. More information has to be introduced and people are unconsciously doing this based on perceived characteristics of devices(i.e. a treadmill makes objects remain stationary/jet engine makes airplane take off) without really considering the individual forces involved.
I'm sure there are dozens of engineers and pilots just sitting back and laughing at this discussion.
Isn't the force vector from most planes engines not stictly horizontal? I thought they actually pointed down a bit. If this is true, a non vectored thrurst plane with enough thrust could leave the ground even if it was stationary given MEGA-powerful jets.
:eek:
Also I'm no expert on bearings but I'm pretty sure their friction would increase with velocity in the real world, due to heat expansion, chattering of the inner components, ect(likely not continuously). I know physics texts say that static and kinetic frictions are constant but in real world applications the friction due to systems of stuff probably isn't. So it is possible to have a treadmill spinning super-duper-mega fast that could provide enough drag to counter the thrust forward. The bearings would probably fail long before this though!
:tongue:
Hang out for a second pechel, I'll help explain it.Quote:
Originally Posted by pechelman
Your statement "if the plane isn't allowed to move forward..." Why do you believe that this is true?
Actually we agree damnit. Just not on the car thing. :)Quote:
Originally Posted by focus
BUT my head hurts so fucking much from this thread that I haven't given it much thought.
so im not going to get in any further of an argument, but I still dont see the plane moving.
the acceleration thats counteracting your air mass from the turbines is from inertia, ie gravity.
heres my thought process (so you can either correct me or figure out what im saying)
normal plane normal runway
engines spin up, thrust applied to plane, plane rolls, rolls fast enough to take off
normal plane on conveyor
engines spin up, thrust applied to plane, plane rolls but instantly so does the conveyor, and the plane never gains any airspeed.
if the plane isnt allowed to move forward, and gain airspeed and lift, by a moving ground, then it doesnt takeoff.
seriously, not trying to get in an argument, so theres no need to get so pissy and defensive. If you guys are right, which Im not 100% certian yet, Id like to understand why. Im not there yet.
Go reread my post on page 7.Quote:
Originally Posted by MeatPuppet
woah
howd you quote me before i posted? ;)
I deleted that and clarified my post
hah
anyway, "not allowed to roll forward" because the conveyor matches the plane's speed at the same instant. Im thinking 100% in velocity here, because thats whats important for a plane to takeoff.
Pechelman, think of it this way, say the plane is landing on the treadmill, which is rotating at the same speed as the plane, it would be no different then landing on a stationary strip, his wheels will just spin faster. The conveyer is not applying a force that counteracts that create by the engines.
and a pitot tube will give you the absolute air speed measured from pressure, not the relative ground speed.
this is why racecars have pitot tubes in addition to wheel speed sensors.
There's the problem. Say the airplane begins moving at 1 mph. The treadmill moves backward at 1 mph. The wheels are free spinning and do not transfer any force to the airplane. The airplane continues to roll. The treadmill cannot go faster than the airplane to increase rolling friction and stop it.Quote:
Originally Posted by pechelman
Once the plane begins rolling, the conveyor cannot stop the airplane. The frictional force between the conveyor and the wheels is almost insignificant.
Quote:
Originally Posted by DJSapp
Dude, why didn't you tell me to go look at your post before I went through the trouble of writing all that down?!? You even used numbers and pictures...for all the good it did...:biggrin:
LMAO!
Page 20 here we come.
gonna have to disagree againQuote:
Originally Posted by P_McPoser
the plane when landing already has velocity
in this scenario i just dont see it moving.
i just dont see your guys' logic yet.
gah maybe i need to let this settle for a bit
it was always these problems i sucked at anyway
Dude, if it doesn't move forwards, the conveyor doesn't roll backwards. In your world, nothing will ever move. Ever. Not the plane, not the wheels, not the treadmill. There's so much tension in your world, man.Quote:
Originally Posted by pechelman
The treadmill matches the plane's forward speed. No more, no less. It isn't overcoming anything. The conveyor is not charged with the task of keeping the plane stationary. It just rolls backwards at the same speed that the plane is rolling forwards. The wheels do double-time.
The conveyer matches the velocity of the plane, but the only connection between the plane and conveyer is a roller mang, thing back to dynamics. Even if you want to talk friction beteween the tires and the conveyer, and the wheel bearings, that horizontal force element is never going to be significant compared to that of a jet engine.Quote:
Originally Posted by pechelman
my understanding of "instantaneously" means at the same timeQuote:
Originally Posted by focus
if the plane would normally move forward at a certain velocity and acceleration on a normal runway, then the conveyor would match this velocity and acceleration profile exactly and at the same time indicies.
instantly. at the same time.
thats where i think we're assuming different things or reading it differently
maybe?
And this is your problem.Quote:
Originally Posted by DJSapp
The treadmill cannot go faster than the forward velocity of the airplane. Let me say that again - it cannot accelerate faster and faster and faster to stop the airplane. It matches speed exactly with the forward velocity of the airplane.
The conveyor is not there to stop the airplane. That's not what it does. Less than insignificant, frictional force really has nothing to do with the problem.
again, not arguing
just explaining my thought process and reasoning so i can get to the bottom of this and sleep tonight :p
Net forces cause accelerations. The engines cause a force forward due to them throwing air molecules backward(like throwing a basketball in a wheeled office chair) The spinning treadmill creates a force backward due do friction in the whole wheel system. Most likely at all treadmill speeds the force of friction is never bigger or equal to thrust(though it may be at crazy speeds assuming the wheel system doesn't break apart, see my above post) thus there is a net force forward and thus an accelerationQuote:
Originally Posted by pechelman