will it take off?

[turnkey4099]

... AND, you have post #1000 in this thread! Congrats!!!

Yeah. My apologies. It was unintentional and I didn't notice it until after I posted. Almost deleted but decided not to. 1,000 should have gone to someone with more posts than I.

Harry K

 

[RobS]

Yeah. My apologies. It was unintentional and I didn't notice it until after I posted. Almost deleted but decided not to. 1,000 should have gone to someone with more posts than I.

Harry K

Hey, don't apologize. As a platinum member you are more than worthy! Besides, your contribution to this thread cements your status as a TBN trivialist.

 

[tallyho8]

There's always post # 2000 to look forward to.

 

[beaverhouse]

So just what is the downside to that?

Harry K

No downside at all...Just another excuse I can use !

 

[daTeacha]

Pat: Regarding the various birds "running" over the surface of the water while taking off, and please excuse the anthropomorphic explanation -- Is it not within the realm of possibility that they are simply constantly trying to see if they have enough airspeed to escape the ground effect (water effect?) and using the additional upwards push generated by slamming their webbed feet into the surface to increase vertical acceleration (not actually aerodynamic lift, but effectively the same result) and not forward speed? I have seen very few webfooted birds actually run like a pheasant might, even when chasing gulls on a beach. On land they generally just take a few steps and are airborne.

Or perhaps they need a longer takeoff when coming off the water so they can shed the water droplets clinging to their feathers and become light enough to avoid stalling, followed by the legendary crash and burn. That latter would be interesting to see, wouldn't it? Particularly with something like an albatross with it's huge wingspread.

Have any of you read "Jonathon Livingston Seagull" by Richard Bach?

 

[schmism]

wow... still going .....

btw vbullitin boards max at 500 pages and 15K posts.

so keep it up we have a ways to go.

ps Ih8mud is on there 26th such thread
(hear is the 25th one)
The ultimate thread! (possible NSFW) (#25) - Page 500 - IH8MUD.com Forum

 

[daTeacha]

For argument sake, think of the running takeoff by certain seabirds as being quite similar to the rolling takeoff of the plane that started all this. The forward propulsion for the bird comes from the action of the wings, not the feet. The feet, despite their flaying of the water surface, do not contribute to the speed of the bird, they just support it until there is sufficient airspeed to take off.

Now, if said bird was attempting to take off on our magically moving lake or a river, and the speed of the water was equal but opposite to that of the bird, the bird would definitely be able to take off. He might get blisters on his feet, but he would get airborne.

 

[tallyho8]

For argument sake, think of the running takeoff by certain seabirds as being quite similar to the rolling takeoff of the plane that started all this. The forward propulsion for the bird comes from the action of the wings, not the feet. The feet, despite their flaying of the water surface, do not contribute to the speed of the bird, they just support it until there is sufficient airspeed to take off.

Now, if said bird was attempting to take off on our magically moving lake or a river, and the speed of the water was equal but opposite to that of the bird, the bird would definitely be able to take off. He might get blisters on his feet, but he would get airborne.

But what if the bird was wearing rollerskates?

 

[patrick_g]

But what if the bird was wearing rollerskates?

Same as A/C on wheels unless you mean the bird trying to take off from the water and then the skates might weight him down and sink his feet into the water, or worse.

I will go on record saying that water birds in some instances definitely do get forward thrust from their feet on the water and are not just holding themselves up to get their wings out of the water.

If the bird were trying to take off with a large rearward water speed the contact of his feet on the water would transfer a rearward force which could be adjusted by impeller speed to hold the bird in place WRT the land (and in a no wind condition, the air) I assert it is not particularly difficult to move water water faster than a bird can run.

Pat

 

[SPYDERLK]

I thot that was a very insightful response regarding 0 G environment effects on natural convection. Ever since it popped into my head one day that 'heat rises' is only true in a fluid within a gravity gradient, I have been getting funny looks when I replied, "Not on the space station" to that 'truism'. Your post provides many interesting ramifications. You have done more thinking on it than I, I think.
On the issue of rolling vs skiing I have done some thinking and rethinking trying to fit it within my experiential and intuitive base. Below are my thots worked into your reply:

Larry, I am pretty much "with you" on the above down to the ++++ including having the rapidly spinning wheels help accelerate the plane if the conveyor slowed down.

Pat, I must admit I am a bit baffled in some instances on how to deal with thrust. It seems that an engine that can produce constant thrust, such as a jet or rocket would effectively make more HP the faster it moved..[Force X Distance/T] ya know. I would really like to get a handle on it - and we need to in this case because it enters into the ski vs wheel comparison. ---------- To me these are not analogous. Wheels store energy both translationaly and rotationally. In the case of wheels, the conveyor manipulates the plane by thrusting the contact patch on the wheels. Acceleration of this contact patch is resisted by the mass of the wheels factored against their moment of inertia. In our case the acceleration is rearward in order to cancel the planes thrust via the connection at the axle. [[In a perfect sense, relying only on accelerating rotational mass for the counterthrust, the wheels are storing energy at the same rate as the engine is putting it out.]] The energy doesnt come from the engine - it comes from the conveyor resisting the thrust of the engine. The energy stored [HP x T] is available to be fed back to the plane if the conveyor is slowed at a rate that prevents a peel out.

+++++++++

Where we part company (in our analysis) is at your statement, "With skis, as with wheels, the thrust of the plane can only be countered up to the point at which they slip."

With skis, as with wheels, the thrust of the plane can only be countered up to the point at which they slip. With skis on hardpack Im guessing that dynanic slip occurs at a force of about 0.01 to 0.02 of the weight the ski supports. With tires on pavement this value is static since the tire doesnt have to slip to move, and is usually high within the 0.5 to 1.0 range - enuf to counter the thrust of most planes. We are talking a multiple of 50 over the skis. It doesnt seem like ski drag would ever increase to the point of balancing a thrust-to-supported-weight ratio of anywhere near what even a normal plane has. While acceleration of the conveyor has no effect, since with skis there is nothing for the acceleration to work against because of slippage, the speed of the conveyor may. Thats where I have a problem. The quandary of low drag at high enuf speed being power, but having no apparent ability to cancel thrust in excess of said drag. I would really love to have this "click".

I don't agree with yor statement as applied to the case of wheels or skis.

There also seems a disconnect about movement in response to thrust since I perceive you using it in self contradictory ways in your argument. I know you know this, but to recap pertinent physics; a) Thrust is Force, b) Work is Energy ..= F X Distance = MVV/2 etc, c) Power is how fast Energy is delivered... FD/Time or F multiplied by Speed or FV. In other form, Torque X revolutions is equivalent to FD or E... and thus Torque X Rev/min is power This is our most common and easily understood form. It says that continuous torque at increasing RPM requires increasing power. Extrapolating comfortably from this, one realizes that a continuous rate of acceleration requires linearly increasing power. [[Note that my [[ above quote from my previous post]] was incorrect. The conveyor must apply constant counter thrust, therefore required conveyor HP increases linearly with time and quickly exceeds the planes HP and keeps rising. The rate that energy is stored in the wheels increases with time.]] .... Making the move over to thrust, I think of it as torque applied in a line rather than a spin. In tractor or auto terms, torque applied at the axle yields thrust at the ground contact. The thrust available is limited to engine max torque as transmitted thru the gearing in each gear. Max thrust varies with gear and decreases as the ratio provides for speed increases. - - A jet or rocket engine operates differently - they produce a thrust by throwing mass out the back end. During the acceleration of this Mass the engine experiences a thrust forward - - F=MA. Regardless of speed, these engines throw mass[fuel] at about the same rate and with the same acceleration. This causes a constant thrust force and gives constant Acceleration [actually, as you make reference to later, A increases some since thrown mass depletes the mass of the vehicle]. Since its speeding up, the product of thrust X distance, FD, [delivered Energy] is increasing from one second to the next. Energy per second is increasing. Thus the power of the engine increases with speed.

The conveyor would have to speed up dramatically after the tires began to slip in order for the slipping tires to create the lateral force required to equal the prop's thrust where in reality a blowout would soon follow from overheating, wearing right through the tire to the metal wheel, and such minor difficulties but for our theoretical idealized case we will assume the tires to be pretty magic too.

The tires would never slip if their coefficient of friction at the belt contact was greater than the thrust to weight ratio of the plane. If the plane has enuf thrust to overcome tire traction there is no way the conveyor can prevent the plane from moving. Once breaking quasi static friction conditions the traction lessens, the force at slip presenting, as I said, the upper limit that you can apply a CounterThrust to the plane via the wheels. With high hysteresis rubber compounds the traction can be increased somewhat at the point that quasi-slip begins and the tractive benefit can be extended into actual slip. This is the realm of racing tires, where a graceful balancing act exists between the traction benefit and unavoidable decreased rolling efficiency- since the retarded rebound of the rubber moving up away from ground contact [leaving the contact patch] gives an effect of running uphill. -- In tires made for normal load bearing situations such as ours there is no tractive benefit at slip. Slipping just makes heat without increasing the reduced CT. This is quite noticeable on ice and wet surfaces. Youre right about slip causing a blowout shortly but the plane still wouldnt take off due extreme rolling resistance of flats. No magic from the tires is necessary to hold the plane still for a limited time. The acceleration needed from the conveyor is the hard part.

It takes more energy (per unit time or looked at another way a greater distance) to slide skis rapidly than to slide them slowly.

Agreed.

Even if the coefficient of friction DID NOT CHANGE with speed (which it surely will in our high speed sceanario approaching relativistic speeds) the lateral force applied to the skis by the weight on the skis, the coefficient of friction, and the distance traveled per unit time, increases with speed. In the case where we have a magic conveyor, the conveyor runs up to the speed required to produce the lateral force equal to thrust.

Lots of power from the conveyor- just like the rocket going faster at constant thrust develops increasing power. However the only power the plane must use against this slip 'CounterThrust' is the small CT force times the planes speed in its own inertial frame. Plenty of power left for acceleration and takeoff regardless how much heat energy is generated by the conveyor slip against the skis.

In your rocket example which I DID NOT EXAMINE REALLY CLOSELY, I suspect that on close examination you will find that the equivalent HP of the rocket thrust does not change over time. The rocket will accelerate at a non linear rate with static thrust due to the rocket's mass being reduced at the rate of mass ejection out the nozzle. Simply firing a rocket engine for an extended time is NOT a way to make it more powerful. (Now for a probably unneeded caveat: the above rocket talk assumes non-relativistic, i.e. Newtonian physics.)

A continuous thrust rocket makes more power the faster it goes, just as a continuous slip force uses more power as slip speed increases. [Same thing, yet you seem to assert one and deny the other.] A rocket in space [or a jet neglecting air resistance] would accelerate forever against a fixed counterforce [such as low coefficient slip] less than its thrust. A rotating engine applying force by mechanically pushing against something cannot achieve this because as the rotating pusher[wheel] is geared to be driven faster the torque to drive it decreases. Thus with conventional engines driving wheels, any steady state drag will limit max speed.

Too bad it doesn't work like that. It would be sort of a reverse reciprocity effect where the slower you expend a rockets fuel the more work you get done.

You expend the fuel as slowly as possible by reducing the rate to the point where every particle that leaves can be let go at a velocity near c. Truth is tho, I didnt say that you got more work done when expending fuel at a slower rate. I said that an engine producing constant [or non diminishing] thrust produces more power as speed rises. I didnt say there was any difference in HP-hrs derived from the fuel at a slower use rate.

Applying the argument of reductio ad absurdum, we see then that as the rate of burn approaches zero the ultimate work done approaches infinity so if you would only be willing to start a long time in advance, accelerating very slowly, you would ultimately approach the speed of light with the rocket.

Not sure how you got there from ...... where?

If you disagree with the above rocket science conclusion then it is incumbent on you to find a flaw in either the logic or the premises or both. I think the logic is pretty air tight and the premises are yours. I guess maybe NASA hasn't overlooked a great boon to rocket science and you and I will not be going to Sweden to collect lots of $ for this idea.

Oh darn. Sure would be nice to be able to defy the laws of Physics.
There are flaws in the way you are dealing with friction. I believe it is a reference frame problem.
Also, there are disconnects that I cannot follow in your extrapolation of what I have said.
larry