Coffee Break Conundrum (Principles of Flight)
Avoid imitations
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For Nick L's question:
The PicooZ main rotor has a smaller, secondary co-axial rotor blade, which is tip-weighted and acts as a stabiliser / flybar, above the two bladed main rotor. This puts the main rotor approximately 15mm from the surface above it when hovering ICE (In Ceiling Effect). Main rotor's diameter is 130mm, flybar's is 70mm.
Today I splashed out on a set of new AA batteries to try out my PicooZ. It still acted a bit tired due to not being charged / used for some time.
After a short re-trimming flight the heli's rechargeable battery was rapidly going flat again. It couldn't generate enough Nr to hover OGE and could only just get off the ground in very close ground effect (which is difficult anyway at low power as it skips sideways on a very smooth surface; if on the carpet a skid digs in and it tips over. There is no way of counteracting the tail rotor force while the skids stay in contact with the surface).
Even though it couldn't do much more than obtain bare skid clearance IGE, I next hand lifted it to the ceiling, where it hovered / stuck quite well.
From this, I conclude that ceiling effect is definitely more pronounced than ground effect. As I said before, I suspect this is only due to the closer proximity to the surface than when IGE.
P.S. Nick, I'm sorry to inform you that the rotors go the "wrong way" so it's "rucky right" not "lucky left" (made in the far east).
The PicooZ main rotor has a smaller, secondary co-axial rotor blade, which is tip-weighted and acts as a stabiliser / flybar, above the two bladed main rotor. This puts the main rotor approximately 15mm from the surface above it when hovering ICE (In Ceiling Effect). Main rotor's diameter is 130mm, flybar's is 70mm.
Today I splashed out on a set of new AA batteries to try out my PicooZ. It still acted a bit tired due to not being charged / used for some time.
After a short re-trimming flight the heli's rechargeable battery was rapidly going flat again. It couldn't generate enough Nr to hover OGE and could only just get off the ground in very close ground effect (which is difficult anyway at low power as it skips sideways on a very smooth surface; if on the carpet a skid digs in and it tips over. There is no way of counteracting the tail rotor force while the skids stay in contact with the surface).
Even though it couldn't do much more than obtain bare skid clearance IGE, I next hand lifted it to the ceiling, where it hovered / stuck quite well.
From this, I conclude that ceiling effect is definitely more pronounced than ground effect. As I said before, I suspect this is only due to the closer proximity to the surface than when IGE.
P.S. Nick, I'm sorry to inform you that the rotors go the "wrong way" so it's "rucky right" not "lucky left" (made in the far east).
I have a coffee time question. Was asked this by a student once. Where is the maximum speed of airflow under the disc when in an IGE hover. let's say for arguments sake the a/c is a R22, about 10' skid height above your head, where would the maximum speed of the airflow be measured and what speed would it be?
a/ Just below the disc/rotors?
b/ Ground level where direction is more likely to change?
c/ outside the disc at ground level?
d/ none of the above or other?
a/ Just below the disc/rotors?
b/ Ground level where direction is more likely to change?
c/ outside the disc at ground level?
d/ none of the above or other?
Avoid imitations
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If the majority of the induced airflow through the disc acts like that in a "double ended divergent duct", surely it would be where the "duct" is at it's narrowest.
As for airflow speed, how long is a piece of string? Power dependent on a particular helicopter at that particular moment in time?
As for airflow speed, how long is a piece of string? Power dependent on a particular helicopter at that particular moment in time?
D'ya think I've got what it takes chaps?
http://youtube.com/watch?v=4VcsQ08OH-I
apologies for the poor quality- video and flying
http://youtube.com/watch?v=4VcsQ08OH-I
apologies for the poor quality- video and flying
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If the ICE works then we need to start building roofs over all our helos. This would have the added advantage of stopping the things leaking rain water all over the drivers. I'm off to the Dragon's Den with this one, just got to get my numbers straight...
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How about a new urban myth - the negative pressure bubble. The rotor tries to chuck air out of the bottom faster than it can get in between the ceiling and the rotor. Hence negative pressure bubble.
Nooooooooo, Arggghh
edit Oops sorry, missed Nicks post on page 1. I'll get me coat...
Nooooooooo, Arggghh
edit Oops sorry, missed Nicks post on page 1. I'll get me coat...
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I bought 3 of these little things for presents last year. Like the Vertibird replica I bought my nephew the year before I felt obligated to put the whole system through a rigorous flight test and evaluation regime before I deemed it acceptable for use and ready for customer delivery.
I too noticed the strange ceiling effect. It was often somewhat like watching a cranefly repeatedly bash itself into the ceiling. I found that to pull myself out of this effect I had to drop power quite drmatically, and the descent once out of ICE was too great to recover from.
Strangely, and despite the pilots best efforts (honest), I found they are also magically attracted to cats, for which they are no match. My 4' Maine Coon took great delight in whacking it out the air and pouncing on it before picking it up in his mouth and running off through the cat flap and into the garden with it. I also learnt that they have a decidely negative impact on female temperament during Christmas dinner.
Si
I too noticed the strange ceiling effect. It was often somewhat like watching a cranefly repeatedly bash itself into the ceiling. I found that to pull myself out of this effect I had to drop power quite drmatically, and the descent once out of ICE was too great to recover from.
Strangely, and despite the pilots best efforts (honest), I found they are also magically attracted to cats, for which they are no match. My 4' Maine Coon took great delight in whacking it out the air and pouncing on it before picking it up in his mouth and running off through the cat flap and into the garden with it. I also learnt that they have a decidely negative impact on female temperament during Christmas dinner.
Si
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ICE
Gaseous
So as you are in stating we have a negative pressure bubble myth.
Bernoulli like reasoning goes as follows (in fact vacuum cleaner effect) :
As the rotor gets closer to the ceiling the intake streampipe gets blocked, instead of being of multiple of the rotor area it becomes just part of a cylinder. Around the border of that cylinder the air accelarates more creating high speeds and a static pressure drop. In the cylinder volume above the rotor the air will diffuse over the MR which will slow it down again as it bends and flows "vertically" through the MR-disk, providing for the higher rotor efficiency, but I think again with an increase of static pressure.
If reasonable uniformity of inflow persists, which can be questionable if the rotor gets really close, vacuum above the rotor could increase from -1/4 T/A to -T/A.
Due the low pressure differentials I think compressibility of the air still will play only a minor effect (in that sense it is not a real vacuum cleaner).
As far as strength of the effect, I guess you get the rotor closer to the cieling in ICE than in to the ground in IGE...
d3
changed : corrected for major errors and not fully comfortable...
So as you are in stating we have a negative pressure bubble myth.
Bernoulli like reasoning goes as follows (in fact vacuum cleaner effect) :
As the rotor gets closer to the ceiling the intake streampipe gets blocked, instead of being of multiple of the rotor area it becomes just part of a cylinder. Around the border of that cylinder the air accelarates more creating high speeds and a static pressure drop. In the cylinder volume above the rotor the air will diffuse over the MR which will slow it down again as it bends and flows "vertically" through the MR-disk, providing for the higher rotor efficiency, but I think again with an increase of static pressure.
If reasonable uniformity of inflow persists, which can be questionable if the rotor gets really close, vacuum above the rotor could increase from -1/4 T/A to -T/A.
Due the low pressure differentials I think compressibility of the air still will play only a minor effect (in that sense it is not a real vacuum cleaner).
As far as strength of the effect, I guess you get the rotor closer to the cieling in ICE than in to the ground in IGE...
d3
changed : corrected for major errors and not fully comfortable...
Last edited by delta3; 11th Dec 2007 at 08:15.
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D3,
I would have thought the streamtube shape would reduce the effectiveness of ICE over IGE. The inflow comes from all directions, while the outflow is directed downwards.
After the last discussion, i would agree that there should be a slight reduction in static pressure (ie density) above the rotor, as well as the expected reduction in flow velocity. But the pressure reduction is only due to the rotor forcing a direction change in the inflow. The incompressibility model is just a model intended to make bernoulli calcs simpler.
The blocking is not a friction effect through air viscosity, however. It blocks the inflow from above the rotor, so that the inflow now has a mild upwards component. Basically, the air is being forced to change direction more than if it were free stream. Or another way of putting it is the rotor is creating a flow field similar to descending. Hence the power reduction.
The reason why some machines are more sensitive than others is probably due to the twist and taper of the blades. Some machines will work better in this flow field, while others lose efficiency.
BTW Leishman on order. Probably won't get a chance to read it properly for a while though. I'll have a skim through when i get it. I'm still interested in your work, but have to fit it in with my own - oh then there's the day job...
I would have thought the streamtube shape would reduce the effectiveness of ICE over IGE. The inflow comes from all directions, while the outflow is directed downwards.
After the last discussion, i would agree that there should be a slight reduction in static pressure (ie density) above the rotor, as well as the expected reduction in flow velocity. But the pressure reduction is only due to the rotor forcing a direction change in the inflow. The incompressibility model is just a model intended to make bernoulli calcs simpler.
The blocking is not a friction effect through air viscosity, however. It blocks the inflow from above the rotor, so that the inflow now has a mild upwards component. Basically, the air is being forced to change direction more than if it were free stream. Or another way of putting it is the rotor is creating a flow field similar to descending. Hence the power reduction.
The reason why some machines are more sensitive than others is probably due to the twist and taper of the blades. Some machines will work better in this flow field, while others lose efficiency.
BTW Leishman on order. Probably won't get a chance to read it properly for a while though. I'll have a skim through when i get it. I'm still interested in your work, but have to fit it in with my own - oh then there's the day job...
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Hi Y'all
I don't think the Pico suffers from Vortex Ring for some reason I cannot explain. R1 and Simon, I was flying mine in the hangar the other day (expensive full size model on the pad outside). It will fly to maximum altitude, 25 ish feet limited only by the ceiling, I chop the power and it falls vertically. The blades slow to virtually stationary but on application of full power, it will recover and fly away.
Try BluTac on the nose for serious increase in speed!!
J9
I don't think the Pico suffers from Vortex Ring for some reason I cannot explain. R1 and Simon, I was flying mine in the hangar the other day (expensive full size model on the pad outside). It will fly to maximum altitude, 25 ish feet limited only by the ceiling, I chop the power and it falls vertically. The blades slow to virtually stationary but on application of full power, it will recover and fly away.
Try BluTac on the nose for serious increase in speed!!
J9
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I think it does suffer from vortex ring - If I do the same as above, when re-applying the power the blades dont provide lift... seem to have stalled? And it doesn't fly away?
MADY
MADY
