The issue is one Galileo considered and answered in his Dialogs!

It is not a dumb question, at all.

The aircraft cabin represents a bit of space that is a "reference frame" to study. If the frame is accelerating, then something must impart that acceleration on everything in the frame, or else the object will leave the frame.

In the cabin, during takeoff, the frame is accelerating, so you are pushed by the seat back, relatively firmly. The seat shoves the typical 170 pound passenger by about 40 pounds of force. Kept up for 40 to 60 seconds, this leads to a lift off for the plane, passenger and seat.

That shove from the seat is not provided to the hovering helo, so it will rapidly accelerate backwards in the cabin reference frame, and slam into the back wall as if it "fell" aftwards.

If the flight performance of the little guy allows a .2G acceleration via its rotor, the operator will tilt it forward and it will maintain position as it works like a dog to stay in position. No stability system in the helo will position keep, since the frame does not impart its acceleration on the helo. Like a pencil that you drop during takeoff, the helo must fight for itself.

Yes, Nick! Galileo placed butterflies in his ship for a reason. In an unaccelerated airplane, at any speed, the helicopter hovers as it would at rest. However, when the aircraft (Galileo's ship) accelerates forwards, the helicopter (butterfly) is pushed towards the back.

More to the point, consider a helicopter hovering over a stationary destroyer deck on a calm wind day, when the ship skipper elects to "throttle forward" without (as usual) communicating with the helo crew, the hapless Naval Aviators shall see the deck slipping away from beneath them while everyone else on the ship (including all shoes) leaves with the ship herself; to recover aboard, the 'forementioned crew shall have to apply forward cyclic and reconnect with their lunch box.:hmm:
The above explanation while a very exacting recount of real events is rendered redoundant by Mr. Lappos far simpler one.
Non-PC, if you are on the Autostrada you better be going the speed of light or be mercilessly run down, hence the uselessness of your rearview mirror since objects would not be visible being of reflected light and not able to reach you.:E

Hmmmm
you could tether it to the front of the cabin with an invisible line, any crop duster would tell you the recipe, then be prepared to increase RPM to counter the vertical acceleration as the A/C lifts, and again as the atmosphere becomes more rarified at the usual 10,000ft pressure altitude.

Then scan for very impressed nubiles to get a good line going.

Unless of course you depart from Darwin in the summertime where there usually is no vertical acceleration, the A/C simply flies straight ahead and the ground disappears downwards due to the curvature of the earth. If you fly a fair way it is usual to note that the A/C has to circle for a fair while to get back down to the ground!:ok:

But then again I suppose if you guys had just come off a long line operation you would have figguuured it all out before you started scratching your head.

Won't the increasing pressure inside the cabin affect it's ability to hover? Whats the stats on an R/C helicopter hovering at 6000-8000 p.a. .... and surely there must be an airflow inside the cabin due to the air con, so won't the wind direction affect it..... dunno, just a few random factors that baffle me in this conundrum. :confused:

MD :ok:

I am a bit confused here....and as I see it all ya'll fail to address the concept of reference for the wee heli thing. Being a real helicopter pilot I am able to transition control of the helicopter by switching from one reference to the another thus why are we all that concerned about which way the airplane is moving? I see this as basic hovering exercise one....keep the same spot under yer hind end (granted Nick's frame of reference in that regard provides him much latitude compared to some of us). My concept therefore allows the helicopter to not only be free to move about but do so in multiple directions at one time (assuming time is not a constant when analyzed for point of reference and speed).

As to objects moving at the speed of light....the opposite effect can be demonstrated by watching Nick trying to find the coins in his trouser pockets when the bill for a round of drinks arrives. Theorists of Black Hole dynamics must have attended some of Nick's informal lectures re engineering prinicples and micro economics.

Sorry Nick, you didn't have a few of the facts required to get to Deemar's response. What you said is mostly accurate for a full control RC helicopter where it is the operator that controls the hover. However, I believe the original question was concerning the yaw and collective mini RC helicopter's that you can get for $20.

The other points to consider are:

- The air inside the fuselage moves with the fuselage. That's why the passengers don't feel wind on take off. This has an effect on both types of helicopters. The obvious one is flapback on the full control RC helicopter that provides a bit of speed stability.

- The second point is the mini RC helicopter's dynamics are set up to stabilize in a zero air speed hover. Sounds incredible but it works. On Saturday a bunch of flight testers got together to play some poker, but when the mini RC helicopter was brought out, we had to qual eval. We would toss it across the room with hover power applied and eventually it would come to a zero airspeed hover. Many different launch conditions. Slight injuries to the FTE.

When you consider the above two points, the story changes. As the jet is accelerating, the helicopter isn't, so the helicopter starts moving backwards with respect to the fuselage. Since the air isn't moving wrt the fuselage, the helicopter is now moving wrt the air. Dynamics of the helicopter reduces that airspeed (flapback for one, but its a bit more involved for the mini RC). Of course the jet continues to accelerate, so I would expect the helicopter would continue moving back, perhaps accelerating until the jet reaches a constant speed. At that point the mini RC helicopter would stabilise in a zero airspeed hover.

The helicopter would sink, as overtauk pointed out, but once the jet starts to climb, then the helicopter would need to climb as well. Of course, the air in the fuselage is climbing with the jet, so what happens to the helicopter?

That question is probably a good direction for this thread to go.

Matthew Parsons,
have a look back at Overt Auk's post a little earlier on to understand why the little helicopter will not move all the way to the back of the plane.

If the plane is accelerating with a constant acceleration, then effectively the little helicopter is seeing* a "gravity" force comprised of the actual gravity force, and a pseudo force** towards the back of the plane on account of the plane's acceleration. The resultant is a force:

• Slightly stronger than normal gravity
• That acts down and a bit towards the rear of the plane

Neglecting transient "air sloshing" effects, the air in the cabin of the plane will be stationary with respect to the plane.

As you have mentioned before, these little helicopters are stable with respect to motion through the air***. We can see this by hovering them around in our living room. So the helicopter will end up still relative to the plane, and because it's stable orientation is with the rotor force opposing the "gravity force" it sees, its stable orientation will be with it's disk tilted slightly forward.

If the collective is kept in the same position as when the plane started moving, then obviously the lift won't be enough to oppose the slightly stronger than gravity force that the helicopter sees, and so it will descend.

Seen by an observer watching the plane accelerate down the runway, the helicopter will be seen to be tilting forward at the angle required to allow it's acceleration to match that of the jet it happens to be flying in. One way of looking at it is that the still air relative to the cabin of the plane provides a feedback mechanism to synchronise the helicopter's acceleration to that of the plane.

All of this falls apart when we start to consider model helicopters with full cyclic control. These are generally designed not to have significant flapback effects****, and so require active piloting to keep them stationary relative to the plane.

Finally, for those who'd like an extra challenge with regards to the aerodynamics of the little helicopters:

1) Why do they accelerate forward when turning one way, but stop when turned the other way?

2) Why is there a weird amount of phase offset between the stabiliser bar and the main rotor? It's not 0 degrees, and it is no where near 90 degrees. Lu Zuckermann would be most displeased.

Daniel

* My apologies for blatant anthropomorphism.
** Pseudo-force - a force that appears in an accelerated reference frame, proportional to an object's mass and the acceleration of the reference frame, but isn't actually a force (see http://www.xkcd.com/123/)
*** Actually this depends upon exactly where the CG is. They are usually trimmed with the CG a little forward so they move slowly forward and can be navigated around the room.

Matthew and Deemar,

Next time you take a Physics course, pay attention!

I suggest that you try this little experiment:

Strap on roller skates

Stand in the front of a standing bus, near the driver

Ask driver to accelerate bus at maximum rate

Hold on to nothing

Hit back of bus as bus rolls out from under you


Deemar, you forget that an observer outside the airplane will see the helicopter STANDING STILL in the larger reference frame as the airplane accelerates out of the frame. You think the helo "knows" the plane has begun accelerating, as if the space around the helo somehow is shifted with the airplane. This thinking is what Galileo had to refute 350 years ago. It is sad that in the 350 years, we collectively have not moved forward one inch.

I have no idea how the non-physics that you folks "understand" gets in your heads. Does EITHER of you ever feel the seat push you when you take off in a plane, or do you think that seat push comes from the air in the cabin??

Nick, spend $20 on a mini RC helicopter before you humiliate yourself any further.

Please Gentlemen, let's go back to the three main priciples of dynamics; they are all relative to a reference system.
Generally, this system is considered Earth's gravitational field (the apple......Sir Isaac Newton.......), however we are free to choose whatever reference system we wish, and make our considerations relative to that.
Insofar, we have considerd the RC model hovering in reference to the tarmac even when the 737 starts accelerating down the runway; if the hovering model keeps its reference to the ground outside, it shall keep on hovering and eventually slam itself against the aft bulkhead.
If the hovering reference is something inside the 737 cabin, then the model shall need to accelerate at the same rate as the aircraft going down the runway to avoid the outcome of the first case.
An observer standing bravely on the side of the runway, shall see the RC helicopter in a fixed position sliding to the back cattle class seat rows in the first case or departing with the "free hot towels" class in the second.

In any event, the explanation in my previous post is nothing but a real world example of the very same thing, the reference system being the ship deck or the water around it.
We must also consider that the hovering helicopter is considered a system of its own, and not connected to the accelerating airliner.

It's junior high stuff.

Is it not the friction of the skate rollers that causes the rearward motion?

Since there is a certain amount of friction and rolling resistance created by the skate rollers as a nature of their design it would seem to me that would account for the movement "rearwards" being described by some.

Would not the "friction effect" exceed the acceleration of the skater caused by the acceleration of the air noted by some.

Since drag increased exponentially with speed increases....until the aircraft reaches a very high airspeed would the drag overcome the friction effect?

Hooloovoo

Glad someone agrees with me!:}

PR

In turn (and Lord protect me from these poor souls who find physics so confusing!)

Matthew, I wonder why you think a $20 helicopter will deconstruct the laws of physics. Will you please just fly that little guy in your car while you do this same experiment? Watch out, those little rotors will beat themselves against the windows as the car drives out from under the helo. Of course, if you keep controlling the helo while using the car reference frame, and if the helo has the flight acceleration to duplicate the car's acceleration, the helo can be flown in "formation" with the car and prevent rotor/glass contact.

svenestron, you do not feel a gust of air in your face on an airliner (unless you open the cute little vent) because the air is trapped in the bottle/cabin, and is accelerated with the entire cabin.

Hooloovoo, you don't think it is a similar experiment because you have no idea what you are talking about! The force that holds the helo up comes from its rotor, the force that holds the skates up comes from the floor, but are necessary to keep the object (helo/skater) from tumbling down to the center of the earth. The helo is not in total isolation, because it is merely supporting itself by making the momentum of the air lift it up, that air hits the floor of the cabin and adds the weight of the helo to the total weight of the aircraft. Nobody asked yet, but if the hovering helo weighed 50,000 lbs, the airliner would be over weight and would crash instead of lift off, because the weight of the helo is part of the weight of the aircraft, regardless if it is hovering or laying on the carpet.

Obviously, hooloovoo you have never seen an object that was stored under a seat shoot aftward as a takeoff occurred - because if you had (it happens very often) you wouldn't go on this way! You magically think a hovering helo has cut its strings and no longer obeys physics. Such a shame. Says much about your schooling.

Sasless, shame on you, the friction slows the rearward motion of the skater, it is the only force that causes the skater to accelerate slightly forward with the plane. Typical rolling friction is about .01 or about 1% of the weight of the skater, so the friction would be perhaps 2 pounds , not nearly enough to prevent the poor skater from smashing against the rear bulkhead.

For you all, it is a shame that you have not learned, or have forgotten the way to draw a free body diagram about an object and trace all the forces it is experiencing, otherwise this would not be so painful for you.

Now hang on Mr. Lappos,

Does not a body at rest remain at rest until acted upon by a force?

Our skater is stood there in the aisle in First Class....and at rest.

The aircraft accelerates taking the cabin air and our intrepid skater along with it.

Since there is drag/friction generated by the skates, our skater although still at rest (only the aircraft acceleration is operating upon him) our skater is actually accelerating at a lesser rate than is the airplane.

Aa +/- Sf = Sa add a variable of Time and then we can calculate Velocity....right?

Thus the skater is accelerating "forward" but in "Time" will be over run by the toilets in the rear of the airplane. Thus Time is the critical measurement.

As we all know time varies as a function of speed thus we would need to calculate space time for both the airplane and skater to achieve the real time it takes for the collision to occur.

The one thing that confuses me is trying to conceive where the collision will take place as the airplane and skater will be in two different places after the intial acceleration occurs.

I some books by Steven Hawking that discusses similar concepts....thus perhaps I should cite his Tomes as a reference.
A being Airplane, S being Skater, a being acceleration and f being Friction

The Lord has not protected me!

Sasless, the body (skater) does stay at rest, and the airplane drives out from under him! You foolishly see the airplane as a new "system" and expect everything inside the airplane to "know" it is time to move forward. That is the foolish thinking that even the ancients knew was wrong.

Everything inside the airplane that is not bolted down slides aft (in the airplane reference frame) and remains still (in the earth reference frame).

Of course, those carts the attendants push have to be bolted into their frames (look at the massive dogs that hold them) so that the airplane acceleration can be forced onto the cart, via those hard metal bolts. In your Dogpatch Airliner where the carts sit idly by as the airliner takes off, the carts would all hit the aft bulkhead doing 100 miles per hour (actually, the bulkhead would hit them doing 100 mph!)

You confuse yourself with "friction": because it allows your sloppy analysis to seem more valid. The friction on a rolling item is about 1%, and is insignificant for this discussion.

Nick said, I'm quite sure that it won't, Nick. However since you don't read all the words of our posts, I thought perhaps you could discover the stability of these aircraft yourself.

If you did shell out the $20 you would find you only need throttle to control height, trim the yaw and then discover that the helicopter hovers without user input (provided c of g is good, otherwise you get a low speed flight). If you push it laterally while its hovering, then it will start moving in a straight line (remember physics class, and F=ma?).

Now for the outstanding part. Without user input, the helicopter will slow down, until it returns to its stable hover. You're probably thinking of something Isaac Newton said, "...objects in motion stay in motion..." and thinking that this helicopter does break laws. However, the rest of that line is "...unless compelled to [change] by a force impressed upon them."

That force comes from the rotor system. When the rotor senses some forwards velocity, its natural stability causes it to pitch up, and slow down.
Don't trust me. Spend the $20.



Deemar, you are correct. The net acceleration of the helicopter will be the resultant of the gravity and the movement of the jet. This well help the helicopter reach its zero airspeed stability point much more quickly than I stated.

I think Svenestron's thought experiment with the helium balloon is pertinent.

Nick,
I have no disagreements with you in the case of roller skates on the bus. An observer outside will see the rear of the bus slam into you as the buss moves off, and you'll be in pretty much the same position relative to the outside as when the bus was stationary. The small amount of friction in your roller skates will accelerate you a very small amount, but it certainly won't be noticeable.

The forces that the bus can exert on me through the rollerskates is minimal relative to my mass.

But what happens to a neutrally bouyant helium balloon if I let go of it at the moment the bus starts to accelerate. I'm sure you'll agree that the balloon won't make it all the way to the end of the bus. Why not? Because the drag forces from the air on the balloon are large relative to the mass of the balloon.

The forces that the bus can exert on the balloon through aerodynamic drag forces (as the air inside the bus accelerates with the bus) is significant relative to the mass of the balloon.

As Matthew says, grab one of these little helicopters and have a play around with it. The reason they can be flown by people with no experience (i.e. most of the people in my office) is that they are stable with respect to motion through the air. They are almost indestructible, so you can try things like chucking them across the room with the rotors spinning, they don't go very far.

I'm tempted to take one of my little helicopters on the train with me tomorrow. I can use the ultimate method for settling disputes about physics - I can try an experiment.

Daniel

Nick wrote:

You think the helo "knows" the plane has begun accelerating, as if the space around the helo somehow is shifted with the airplane.


My response:

The space around the helo is not shifted with airplane.

However, as you pointed out earlier, the air is.

The helicopter "knows" about it's motion through the air. For exactly the reasons you have pointed out before, the helicopter will initially remain stationary relative to an outside observer. As the air in the cabin starts to rush past the helicopter you will get the usual responses of a helicopter to a strengthening of the relative wind.

Normal model helicopter (weighs from 0.5 to 5 kg or so depending on the model, designed to be flown outdoors and at high speed)
Flapback effect is minimal - > helicopter is not accelerated much by the relative airflow -> helicopter impacts the rear of plane

PicoZ or similar micro heli (weighs about 20gm, is impossible to fly in any wind - it is affected by the motion of a person walking past)
Flapback effect is huge - > helicopter is accelerated significantly by relative airflow and tries to achieve zero velocity relative to the air -> helicopter initially moves backwards slightly as the plane accelerates achieves zero velocity relative to cabin air.

Daniel

Deemar, Matthew and svenestron (my fingers get tired typing usernames!) I do understand your point more fully, you do not argue (anymore?) the physics, you argue that the extremely strong speed stability of the helo would stop its rearward slide as soon as any rearward air velocity is detected by its rotor.

OK, I could buy that, if the machine has the excess thrust to keep up with the airplane (which uses about .4 G during takeoff).

But the helo will not keep station, because it must sense the velocity to respond, and the velocity can only be gained when it slides rearward in the first place. In order to sense the airplane's acceleration, the helo must CREATE the rearward wind by sliding out of position. Unlike your living room experiments where the room is stable and you vary the wind, in this case the wind is stable (at zero) and the room is accelerating. An observer will watch the helo slide rearward. The helo will then tilt itself to correct that rearward slide, and (if the thrust available is sufficient to get to 1.4 G's) it will maintain a new more rearward position in the airplane.

Nice concept question, guys!

Regarding the balloon in svenestron's experiment, I will wait, but it is a very nice question, too!!