Coffee Break Conundrum (Principles of Flight)
 

Nothing too heavy here but maybe something to ponder over coffee (or beer)

I was recently given a tiny RC heli approx 5" in length with a 5" rotor diameter. The toy is designed to be flown in zero wind conditions i.e. indoors. After days of frustration it is now with limited success able to whizz round the living room. It has zero collective controllability so all climbs/ descents are achieved with RPM adjustment.

One charachteristic has become apparent and I wonder why it occurs. If the thing is flown up and hovered very close to the ceiling (lets call this ICE - In Ceiling Effect) it can remain there despite a very large decrease in RPM.

This is probably a venturi effect but I'd be interested to hear thoughts on the blade element theory behind it.

R1Tamer

Is it a Picco Z? One of these?

http://gizmosforgeeks.com/images/2006/12/piccoz1.jpg


Cheers

Whirls

It looks about the same size but with a 300 CB fuselage. Rotor mechanism looks identical so I guess you can fly yours up to the ceiling and see the effect for yourself!

Great fun to play with when I should be prepping for an interview !!!

R1

I usually crash mine into the ceiling :} But then, the ceiling height is only 5' 9"!!! I prefer to hover around shoulder height; less dangerous!

Cheers

Whirls

Off to charge up the batteries!

Interesting question...
Recalling the "venturi effect" you probably mean the "ground effect" - i.e:

A helicopter hovering close to the ground will require less power than when hovering out of 'wing in ground effect'.

Now... when hovering close to the ceiling your heli should require more power, because ground effect (ceiling effect!) reduces the lift generated by the rotor. If your heli remains close to the ceiling despite a very large decrease in RPM i guess that's probably because of temperature raise of the air close to the ceiling (and density decrease). Am I wrong?

I'm guessing the occupier of the apartment above yours has a whopping great big subwoofer on the floor, and the magnetism from that is keeping your heli on the ceiling!

Having just had a little play with my Picco, I think it's because of the way the controls work. If you push forward on the power control (and it only works in one dimension), the heli keeps on accelerating (in my case, crashing into my low ceilings), therefore, to stop it accelerating heavenward, you ease off the power. Heli will start to sink, increase power etc until you find an equilibrium point which you are constantly adjusting with micro-movements of thumb!

After that, you're concentrating on it not knocking over the flowers, keeping it out of the cat's reach and not smashing the rotor blades on the light fittings!

Cheers

Whirls

Could it be possibly because of a reduction of induced flow?

The same thing should happen if you hold your hand above the rotor disk.

If on the other hand you where to blow gently on top of the rotor disk it should decend due to the increased induced flow.

R1Tamer,

I have a PicoZ too and I know exactly what you mean. I used to teach basic helicopter aerodynamics to RAF students and have pondered about this effect; it's almost as if the Pico is stuck to the ceiling! When it does come down, only after a major power reduction, there follows a sudden and rapid descent because of the deficiency of power for a free air hover!

I think it's because rotor inflow is markedly reduced when close to the ceiling, increasing the angle of attack for the (set) rotor pitch and making the rotor system much more efficient, allowing a big reduction in rpm / power.

Right, I'm off to charge the PicoZ's batteries for another play :O

suction


what you think:}

Hay whirls , try those copper top batts , they work really well in most things

have fun with ya toys

Could both explantions be correct? i.e. a reduced power requirement occurs due to the angle of attack increase caused by the presence of the ceiling (Shy Torque) and also, for related reasons, it is difficult to accelerate the helicopter vertically because the proximity of the ceiling makes it difficult to increase the induced flow (Platinum; R1 Tamer).

You have the same problem trying to accelerate the aircraft from the hover to the ground unless a faily large amount of lever is lowered. But that isn't recommended practice :ouch:

TT

I offer the venturi effect (bernoullis theory?) as follows;

When the rotor operates so close to the ceiling it has a reducing mass of air available directly overhead to literally pull down i.e. the available air mass from which it can induce a flow is not freely available in a column directly above.

It presumably is literally sucking (inducing) the available air mass from slightly outside the disc and above it across the top of the disc and down through the disc.

Clearly to cover this kind of distance the air is accelerated and hence pressure reduced since temp remains the same. The subsequent pressure reduction consequently pulls the aircraft toward the ceiling or vice versa the greater pressure below pushes the rotor toward the ceiling above.

Or something or nothing!!!

R1Tamer

Possibly totally off track here but my theory is that the movement of air from above the rotor disc to below produces a bubble of low preassure above the disc.
When the aircraft is placed in a position where the ceiling is within that bubble of low preassure the aircraft tends to suck itself to the ceiling.
The added upward thrust means that the engine does not need to work as hard and therefore the RPMs can be reduced.

Standing by to be shot down in flames.

LM

My hypothesis: The establishment of a ground plane near the disk is the same above or below. The ground plane reduces the induced drag, even if it is at the inflow side of the disk.

How do I know this? Not because it is published somewhere, but rather because R1Tamer your elegant little experiment proves it.

Forget the low pressure stuff, which is the same as the "high pressure bubble" below an IGE helo in the mythological world of pop-aerodynamics.

Like IGE, the proximity of a smooth surface to the disk controls the flow, and reduces the induced power losses. I wish I had one of those little helos, I would try to quantify the power "gain" based on the proximity of the surface. If it is the same as ground effect, the power should get to a reduction of up to 25% at 10% of the rotor diameter from the ceiling.

Go_on_Nick,_treat_yourself,_they're_great_fun

Cheers

Whirls

I think I've got it...
 

I have been wondering about this ever since it was demonstrated to me at work 6 months ago. But, I had other things to think about, and I didn't think too hard about it.
When the rotor is winging around, by the Bernouli theory of lift there is low pressure on the top of the blade, and high pressure underneath. Now, a helicopter hovering in free air is going to experience a couple of things. First, the low pressure area above the blades will tend to pull the ambient pressure air from above the disk down through the blades. Second, the air ejected out the bottom will tend to jam into the ambient pressure air, causing a local area of high pressure. Differential pressure = lift.
Now consider ground effect. We all know that ground effect reduces induced drag. My own little pea brain also likes the 'ground cushion' description, which I interpret as the outflow gets jammed into the ground, creating an area of local higher pressure.
So what happens at the ceiling? Inflow into the disk is crap. There is a ceiling in the way, so you don't have an infinite reservior of air to get pulled into the low pressure. So, the local pressure just above the rotor disk gets really low, because it's hard to get air back above the rotor disk. It has to come from the sides, and the spanwise flow tends to be outward rather than inward.
I would reject Nick's hypothesis that it is the same as ground effect because what I've seen is a 50 to 75% reduction in power (estimated). It's way stronger than ground effect, especially with a coax with no flybar where it can get really close.
Meanwhile, I'll get my team of crack test pilots to work on the problem some more.
http://i198.photobucket.com/albums/a.../mkcflying.jpg
http://i198.photobucket.com/albums/a...amRChelo04.jpg
-- IFMU

IFMU, Hey, what does that young upstart Nick Lappos know about helicopters, anyway? :rolleyes:

This time he must be right though, because he agrees with my theory..

I think you might find normal ground effect much more effective too, if you could get the blades a couple of millimetres above the surface (preferably emulsioned, matt white).

P.S. Like your carpets, very Art Deco ;)

Hey, cardboad landing pad! Why didn't i think of that? :confused:
I have to stuff "collective" down to stop my machine rolling over when skids catch floorboards...

IFMU, i am impressed with your start 'em young training program.

Prouty would speak of vector addition of induced flows from rotor tip vortices and images of rotor tip vortices in ground or ceiling. In essence imagine a mirror image rotor operating in the ground or ceiling plane. Heli has to operate in upside down wash of ground plane or upside down inflow of cieling plane. Pressures don't change (still 1g lift, right?) but the rotor induced flow is altered by the mirror image. This means flow through rotor will decrease, and collective goes down. The mirror image rotor could be thought of as generating downforce in either cieling or ground, but the actual mechanism is good old bernoulli venturi effect

I don't trust my skills to try it, so i'll let the test pilots tell me if i'm wrong! :}

ShyTorque,

Good point. We need to modify one of these for inverted flight, so we can get closer to the ground. We can hand-launch and hand recover. Or just use my 4-year old's technique of chopping the collective wherever and letting gravity take over.

I still think CE will be more effective than GE, because I think it will be easier to eject air out in ground effect vs. pulling air in by the ceiling. Spanwise flow is going to want to go out.

-- IFMU

My 2cents worth..
similar to recirculation caused when hovering close to building/hangar. what happens if object causing recirculation is located above rather than adjacent?? pretty expensive to prove/disprove with real eggbeater :\

other bernoulli thing equally plausable. narrower gap to ceiling, greater speed of relative air, greater drop in pressure above aerofoil, less angle of attack required?

or could just suck back on a beer and say Coooooooooooooolllllll :}

NB