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

The inflow is small, because the space between the ceiling and the disc is small, air will want to fill this low presure area (above the disc)from the side, giving translational lift, in turn, reduced power.

Ding Dong

Given nil wind, and the helicopter stationary at the ceiling, where is this "translational lift" coming from?

noblades

"Less angle of attack, therefore less power required": Surely it is a greater angle of attack and reduced induced flow vector and drag due to the presence of the ceiling surface?

TT

Reduced induced flow=reduced induced drag=reduced power requirement. Same as an IGE hover. Only instead of below the rotor, you have a surface above the rotor limiting the tip path vortices as well as limiting induced flow. I would also have to say that there is some translational flow going on helping to reduce the power requirements. My two cents or euros or pfennigs or won or yen or francs; all depending on what part of the world you hail from. MAC

My son bought one of these helos in California and tools around his apartment with it. I asked him to try the ceiling-effect maneuver and report his experience. He said that there was no apparent ceiling-effect at all, even under close scrutiny. His was a small, coaxial helo with two side props for thrust/yaw control. I will get a model number from him.

In the interest of truly scientific prattling around, can we get a report from each of the pprune model flyers as to who has tried the close to ceiling power check, what type of helo, what ceiling proximity and what were the power observations?

theory B
 

My little Bell 47 lookalike is big enough to chip spikes off the artexed ceiling, increasing the mass thrown downwards.

Additionally, small black pieces of rotor disk are part of a dynamic weight reduction exercise, increasing power to weight ratio ...

... off to re-decorate before 'er indoors finds out ...

Okay,

Let me perhaps throw in the first set of figures.

The model is a small schweizer 300, 5" rotor blade with small sperate electric motors controlling main and tail rotors. I have no idea what speeds the rotors are spooling around at but the observations are as follows.

The little craft flys round my living room describing approximately an 8 foot circular track with a fixed rpm setting. Depending how accurately I can make the thing fly it climbs maybe 6 to 8" per orbit. It can do this from nearly ground level. However at approximately 8 to 12" from the ceiling the climb becomes far more pronounced with no unaltered power setting. It literally starts to accelerate toward the ceiling.

A reduction in the power (rpm) setting (unquantifiable... sorry) seems to have no effect in retarding this increasing rate of climb in the orbit. Unless I apply a very large reduction in rpm the craft eventually starts to brush the rotor hub against the ceiling. After maybe a full circle or two like this with the rotors eventually crashing against the ceiling I reduce sufficient rpm that the craft falls away from the ceiling but is unrecoverable and seems in my limited knowledge to be in vortex ring state i.e. even if full rpm is then applied the craft falls to the ground unable to regain lift.

Unfortunately Mrs R1 is away for the evening otherwise I'd have her video it and see if I couldn't post it some where for you to view. I can't fly and video closely enough at the same time. I'll try and get some footage in the next day or two.

R1Tamer

My 'research'
 

As a former unmanned VTOL professional pilot, I decided to brush off the old 'skills' and get some video.
Not much in the way of instrumentation. But here is what I found. The single rotor air-hog, I think the same thing as a picco-z, does not seem to show a lot of ICE:
http://i198.photobucket.com/albums/a...h_DSCN0721.jpg
The dual rotor Saber Strike, which has no flybar and puts the rotor verrry close to the ceiling, exhibits extreme ICE:
http://i198.photobucket.com/albums/a...h_DSCN0723.jpg
I certify that no 6-year olds were injured in the filming of this scientific experiment.
-- IFMU

IFMU, any likelyhood then that tail rotor upsets ICE?

Other thought: Inflow could be affected by ceiling to set up conditions for VRS, same way as being close to a wall does. The Saber Strike has one rotor operating away from ceiling, so VRS does not affect ICE. Air-hog rotor gets into VRS so ceiling effect is reduced.

Help!
 

After one too many 'carrier' landings mine has developed a severe lack of tail rotor power. I've installed the spare but it's still not producing enough thrust and the slightest touch on the throttle sends the thing spinning away under the sofa. My four year old is getting mighty pissed off at being sent under to retrieve it (muttered comments about Victorian chimney sweeps and Childline).
Anyone any ideas to solve this? May be something to do with the trim control but I have no idea how it works.
Long winter shifts on standby way too far north beckon and I need a solution!
Yours in desperation,
Trolleys
ps Solutions other than 'Buy a new one!'. I'm saving for a proper toy!

Mart,

I don't think so. There is a little flybar rotor, maybe 40% of the diameter is the main rotor, which stands the main rotor away from the ceiling. This flybar rotor isn't doing squat for producing lift, the tip speeds are way too low I think. The saber strike has no flybar, I think it uses magic for stabilization. Or maybe there is a cat with buttered toast strapped on its back, suitably miniaturized, inside. With no flybar the blades can get nice n close to the ceiling.

Ours do that sometimes. Hover it and use the trim until it stops spinning. Be patient, it takes a while. Also, I'm sure you have pulled the t/r off and removed the cat hair/ect.

-- IFMU

IFMU, just reread your scientific experiment post. I get it, so ICE only works when hovering very close to ceiling.

This makes sense, since inflow is ingested from all directions while downwash goes... well... down. This means that IGE easilly affects hover power, while ICE has a very weak affect. Same reason tail rotors are usually pushers instead of pullers (since vert stab counters tail rotor). Trouble is ingesting turbulent air makes for a noisier tail rotor - i suspect ICE makes a lot more rotor noise than IGE too.

I couldn't hear any more tail rotor noise. The main rotor was a lot noiser as it was trying to plane down the ceiling.

-- IFMU

Electric trim
 

Hey Trolley there are electric trim buttons located under the TR control. You press the buttons down to establish 'balance' then fly the actual control lever. A lot like a real H300/MD500.;)

just wondering if there is any effect if, when in "ice", you get it near a wall aswell..... :rolleyes::ugh:

simfly, although a rotor downwash goes straight down the inflow comes from all directions. Ever seen a bell mouth inlet? Same thing. If you put a wall nearby the inflow starts to pull the downwash back upwards, until eventually recirculation happens. The odd result is that lift can be lost near a wall, so i imagine ICE would also be reduced.

Ok, now I'm lost. What joke?

-- IFMU

IFMU,

By "plane" i thought you meant as in knocking off all the little ceiling features - i wrote a serious response, then realised you might be commenting about how close you needed to get for ICE. Doh! :\

Main rotor will be noisier, since it is ingesting air that has flowed across the ceiling. The ceiling features will cause small scale turbulence, so that the rotor is not running in clean air. Normally the boundary layer seperation begins to occur at the rear of the airfoil (adverse pressure gradient for flow direction), but this ingested turbulents trips it sooner. The noise is the result of the small random changes in the downwash velocity becoming very small pressure variations (flow is only approximately incompressible).

Pusher tail rotors will make more noise as they pass into the vert stab shadow, because they are seeing sudden changes in flow. Coaxials and pusher props have the same problems. Coaxial because the lower blades are seeing sudden AOA changes and pushers because they are ingesting the turbulent layer from the fuselage.

Just wondering....does the texture of the ceiling have any effect on ICE?!!!:}

for the attention of mr lappos,

my own bad science observations with the Picoo Z, as pictured at the start of this thread, ICE was very noticeable and quite strong if I look at this in terms of throttle travel as a percentage from full off to full on.

1.the rotors do not turn at all on mine until about 20% travel at which they turn rapidly
2. it takes about 70% travel to get it to the ceiling
3. ICE kicks in and it bumps against ceiling, throttle needs to be reduced to about 50% to get it to break the ICE, at which point it descends rapidly

what does this prove ? with this helo ICE seems stronger than ground effect to me.

I would presume a textured ceiling would reduce ICE, but I dunno mine isnt :(

however you look at it, it is a design miracle, you can even see it sits right skid low in the hover (clockwise rotor)

regards

CF

That is exactly what I meant. There is no more aero noise from the rotor, which are nearly silent anyway. Primary noise from them little buggers is motors/gears. When planing down the ceiling features, you can't hear the motors/gears anymore as they are masked by whacking the ceiling.

-- IFMU