Helicopter Urban Myths
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Matthew,
Don't forget that at these speeds the air is to all practical intents and purposes incompressible. I like to visualise ground effect as a mirror image virtual upwash which is going back up through the rotor system. The rotor system causes this virtual upwash to expand to cover the whole disk, in the same way the downwash suffers wake contraction. The net effect is that for the same lift thrust the blade pitch, hence rotor torque, hence power is reduced.
What causes the air to travel out horizontally at the ground? Well the real downwash and the virtual upwash meet at the ground to produce zero vertical velocity. The downwash is effectively suffering complete wake expansion at the ground, which results in the horizontal outwards flow. In truth streamlines require extensive computing power to work out, since you need at least the Biot-Savart law to consider the resulting flow from the blade tip vortices. This is why before CFD, wind tunnels were required.
Don't forget that Navier-Stokes also tells us that the air has viscosity, so the downwash will also reduce in stregth as you get further away from the rotor. As a physicist you will appreciate that this is just one of those problems where the complexity soon becomes such that the average person needs a much simpler model to visualise. The pressure bubble is just that reduced model, but no aerodynamicist will seriously consider it when doing calculations.
However, the mirror analogy works well to explain the long grass phenomena, where you need more collective (thus manifold pressure (~0.5") or torque) over long grass. Effectively the ground is no longer a perfect mirror, but has some mechanical impedance. This reduces the strength of the virtual upwash, so hover power goes up slightly in comparison to concrete.
On balance, the reflected virtual helicopter is more intuitive to me.
Don't forget that at these speeds the air is to all practical intents and purposes incompressible. I like to visualise ground effect as a mirror image virtual upwash which is going back up through the rotor system. The rotor system causes this virtual upwash to expand to cover the whole disk, in the same way the downwash suffers wake contraction. The net effect is that for the same lift thrust the blade pitch, hence rotor torque, hence power is reduced.
What causes the air to travel out horizontally at the ground? Well the real downwash and the virtual upwash meet at the ground to produce zero vertical velocity. The downwash is effectively suffering complete wake expansion at the ground, which results in the horizontal outwards flow. In truth streamlines require extensive computing power to work out, since you need at least the Biot-Savart law to consider the resulting flow from the blade tip vortices. This is why before CFD, wind tunnels were required.
Don't forget that Navier-Stokes also tells us that the air has viscosity, so the downwash will also reduce in stregth as you get further away from the rotor. As a physicist you will appreciate that this is just one of those problems where the complexity soon becomes such that the average person needs a much simpler model to visualise. The pressure bubble is just that reduced model, but no aerodynamicist will seriously consider it when doing calculations.
However, the mirror analogy works well to explain the long grass phenomena, where you need more collective (thus manifold pressure (~0.5") or torque) over long grass. Effectively the ground is no longer a perfect mirror, but has some mechanical impedance. This reduces the strength of the virtual upwash, so hover power goes up slightly in comparison to concrete.
On balance, the reflected virtual helicopter is more intuitive to me.
Last edited by Graviman; 24th Nov 2007 at 14:55. Reason: Tidy up only
Graviman,
I read it before the edit, and after - and I still dont understand. Perhaps I need to understand what the Biot-Savart law is - or perhaps I need more beer.
I do like the idea of virtual upwash as an explanation - it is something I havnt come across before.
I read it before the edit, and after - and I still dont understand. Perhaps I need to understand what the Biot-Savart law is - or perhaps I need more beer.
I do like the idea of virtual upwash as an explanation - it is something I havnt come across before.
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FB,
Like Mathew i have studied physics, and so refer to techniques that will make sense to him. I have also studied engineering, including aerodynamics, so often refer to principles which are probably not widely understood. Interestingly i find that physicists often do not understand engineering, and engineers often do not understand physics! There is at least a common ground in Newton/Bernouilli physics.
I think this forum would be a good place to try to develope a new model which, while compatible with the aerodynamics, would also help all pilots gain intuitive understanding of what is happening. Visualisation is useful, because it stays with you in an emergency.
The other two visual models i would like to develop are VRS (which i see as a bound vortex or aerodynamic doughnut), and LTE (which i see as the tail rotor caught up in it's own bound vortex). Considering tail rotor downwash vorticity also, i believe, explains why a tail rotor with marginal power works better with front-up rotation (Magnus effect reduces TR VRS).
This post has probably confused half it's readers, but may lead to a clear diagram or two...
Like Mathew i have studied physics, and so refer to techniques that will make sense to him. I have also studied engineering, including aerodynamics, so often refer to principles which are probably not widely understood. Interestingly i find that physicists often do not understand engineering, and engineers often do not understand physics! There is at least a common ground in Newton/Bernouilli physics.
I think this forum would be a good place to try to develope a new model which, while compatible with the aerodynamics, would also help all pilots gain intuitive understanding of what is happening. Visualisation is useful, because it stays with you in an emergency.
The other two visual models i would like to develop are VRS (which i see as a bound vortex or aerodynamic doughnut), and LTE (which i see as the tail rotor caught up in it's own bound vortex). Considering tail rotor downwash vorticity also, i believe, explains why a tail rotor with marginal power works better with front-up rotation (Magnus effect reduces TR VRS).
This post has probably confused half it's readers, but may lead to a clear diagram or two...
Last edited by Graviman; 24th Nov 2007 at 10:53.
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Originally Posted by NickLappos
1) Vortex Ring State (VRS) can happen at as little as 300 foot per minute descent, it does not have to be a higher descent rate
2) VRS is more likely at high altitude and high gross weight
3) Hovering with the nose off wind consumes much more power
4) Blade stall is always preceded by vibration
5) Winds affect the power we require when we are in forward flight
6) Downwind takeoffs are absolutely forbidden
7) The Height Velocity curve is a precise guide to the engine failure danger zone
8) Engine failure is the most common accident cause, so full CAT A is the most cost effective safety enhancement we can incorporate into new helicopters.
9) The legal definition of VFR is sufficient to assure flight control and safety using outside references
10) "They" sometimes hide things from us. We should not trust them, the only reliable information we can trust is our own wits.
11) The helicopter is perched on a ball of high pressure air when close to the ground, and "falls off" this ground cushion when it moves forward.
12) Phase lag is cause by gyroscopic precession, and is always exactly 90 degrees
13) LTE is when you run out of power pedal and can be experienced by any single rotor helicopter.
14) NVG are dangerous and should only be used by gifted military pilots.
15) You have to first learn to fly fixed wing before you take helicopter training
16) Torque limits, overspeed limits, temperature limits, hours and airframe limits have huge safety factors built into them by the engineers, so it is OK to bust them every now and then.
2) VRS is more likely at high altitude and high gross weight
3) Hovering with the nose off wind consumes much more power
4) Blade stall is always preceded by vibration
5) Winds affect the power we require when we are in forward flight
6) Downwind takeoffs are absolutely forbidden
7) The Height Velocity curve is a precise guide to the engine failure danger zone
8) Engine failure is the most common accident cause, so full CAT A is the most cost effective safety enhancement we can incorporate into new helicopters.
9) The legal definition of VFR is sufficient to assure flight control and safety using outside references
10) "They" sometimes hide things from us. We should not trust them, the only reliable information we can trust is our own wits.
11) The helicopter is perched on a ball of high pressure air when close to the ground, and "falls off" this ground cushion when it moves forward.
12) Phase lag is cause by gyroscopic precession, and is always exactly 90 degrees
13) LTE is when you run out of power pedal and can be experienced by any single rotor helicopter.
14) NVG are dangerous and should only be used by gifted military pilots.
15) You have to first learn to fly fixed wing before you take helicopter training
16) Torque limits, overspeed limits, temperature limits, hours and airframe limits have huge safety factors built into them by the engineers, so it is OK to bust them every now and then.
Number 10) i'll put down to aliens - they seem to get into everythang.
Number 11) has frightened away the other players - again.
Number 12) looks like fun. I would say that each blade on the rotor behaves as a system in resonance, so its maximum movement occurs about 90 degrees after the maximum input. The exact phase depends on the blade natural frequency compared to revolution frequency, and the damping of the resulting frequency. This damping is dependant on the aerodynamic forces to inertia (Lock number).
Number 16) Miner's law of fatigue damage says that if you're an occasional bean nibbler, the jar will soon be empty...
Actually i'd like to put in a couple of my own:
17) In hover the cyclic controls the direction the lift vector is pointing.
18) There are certain regimes of flight where smaller diameter rotors are advantageous.
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Graviman, not sure where you're going with #17.
For #18, the implication is that greater rotor diameter is always better. Not so. An increased diameter also means heavier blades. At some point you'd get diminishing returns.
As far as the pressure "bubble". I didn't want to go down this road. But since we are...incompressible doesn't mean you can't have a pressure gradient. It is an early assumption made to make calculations easy, but it doesn't change the physics. Also, viscosity is not derived from the N-S equations, it is an input.
Your explanation for ground effect sounds great, but it doesn't address whether there is a pressure gradient. You can't say that you'll ignore the pressure gradient, so therefore there is none. Remember, I'm not claiming this is necessarily something causes a flight phenomenon, just that it seems like it must exist.
Instead of talking about what happens to the helicopter, why don't you tell me how a chunk of air changes its direction by 90 degrees when it hits an open surface squarely (ie downwash hitting ground).
For #18, the implication is that greater rotor diameter is always better. Not so. An increased diameter also means heavier blades. At some point you'd get diminishing returns.
As far as the pressure "bubble". I didn't want to go down this road. But since we are...incompressible doesn't mean you can't have a pressure gradient. It is an early assumption made to make calculations easy, but it doesn't change the physics. Also, viscosity is not derived from the N-S equations, it is an input.
Your explanation for ground effect sounds great, but it doesn't address whether there is a pressure gradient. You can't say that you'll ignore the pressure gradient, so therefore there is none. Remember, I'm not claiming this is necessarily something causes a flight phenomenon, just that it seems like it must exist.
Instead of talking about what happens to the helicopter, why don't you tell me how a chunk of air changes its direction by 90 degrees when it hits an open surface squarely (ie downwash hitting ground).
Last edited by Matthew Parsons; 24th Nov 2007 at 20:10.
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For the record, there is no pressure gradient below the rotor, in fact, that lame conventional explanation doesn't even hold water for a millisecond when you realize that ground effect only works on the induced power. How does a "pressure bubble" single out induced power as the only recipient of its wonder?
In fact, pressure bubblers have some difficulty explaining how ground effect works for an airplane at 250 knots, when the "pressure bubble" is about 1/4 mile behind the wing.
In fact, pressure bubblers have some difficulty explaining how ground effect works for an airplane at 250 knots, when the "pressure bubble" is about 1/4 mile behind the wing.
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That's a good point
What would be a better explenation than the conventional one?
I can see the point that ground effect slows induced flow and therefore increase the AoA, but it does seem as though the pressure is higher in a hover than on the ground when not producing lift.
So how does ground effect diminish the size of the vortex rings? I was taught that it was the downwash pulling the vortex rings out away from the helicopter. I was told this is the reason that tall grass diminishes ground effect, the grass slows the outward flow and lets the vortex rings get bigger. Of course after reading some of the explanations so far it seems this is not true.
Of course, all of this is in an effort to explain, at least in part, why the helicopter requires more power at the point just before ETL than any other time.
I can see the point that ground effect slows induced flow and therefore increase the AoA, but it does seem as though the pressure is higher in a hover than on the ground when not producing lift.
So how does ground effect diminish the size of the vortex rings? I was taught that it was the downwash pulling the vortex rings out away from the helicopter. I was told this is the reason that tall grass diminishes ground effect, the grass slows the outward flow and lets the vortex rings get bigger. Of course after reading some of the explanations so far it seems this is not true.
Of course, all of this is in an effort to explain, at least in part, why the helicopter requires more power at the point just before ETL than any other time.
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Fine, there's no pressure gradient. Just tell me what changes the direction of the air.
By the way, this myth was about a sensation the pilot felt while hovering in zero wind conditions, not about power required or ground effect.
By the way, this myth was about a sensation the pilot felt while hovering in zero wind conditions, not about power required or ground effect.
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Interesting, my 15 year, and 7000 hour chief pilot uses the pressure bubble explanation to explain how to do a normal take off.
"Start accelerating slowly so as not to fall of the bubble and pull it along with you. As you continue to accelerate you will start to out run the pressure bubble and experience a dip just before ETL. At that point you may use a slight amount of up collective to avoid dipping too much, then lower collective to previous position."
I am very interested in learning a more accurate explanation for all that we have been discussing.
"Start accelerating slowly so as not to fall of the bubble and pull it along with you. As you continue to accelerate you will start to out run the pressure bubble and experience a dip just before ETL. At that point you may use a slight amount of up collective to avoid dipping too much, then lower collective to previous position."
I am very interested in learning a more accurate explanation for all that we have been discussing.
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southernweyr:
Maybe you should get a hold of Shawn Coyle's "Cyclic and Collective":
Dear Shawn, Mr. Publisher, dear mods: if this should violate any copyrights and isn't appreciated to be posted, please remove it immediately or have me remove it - I just can't explain it better
)
Maybe you should get a hold of Shawn Coyle's "Cyclic and Collective":
Dear Shawn, Mr. Publisher, dear mods: if this should violate any copyrights and isn't appreciated to be posted, please remove it immediately or have me remove it - I just can't explain it better
)
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Thank you for the post. It makes sense now. I suppose that information is not in question due to testing in a wind tunnel or something?
Any ideas on why the vortex rings are diminished when in ground effect? Is it just due to slower induced flow, or does it have anything to do with the down wash moving horizontally outward and "pulling" the vortex rings away?
Any ideas on why the vortex rings are diminished when in ground effect? Is it just due to slower induced flow, or does it have anything to do with the down wash moving horizontally outward and "pulling" the vortex rings away?
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I'm enjoying Shawn Coyle's C&C now! It's worth getting hold of...
Mathew, difference between static pressure and stagnation pressure:
Static pressure is pressure moving with air free stream (static port).
Stagnation pressure is pressure when stream is stopped (pitot tube).
This bothered me about meteorological systems too. What you have to remember is that at subsonic speeds there is no real resistance to flow. The viscosity affects how the air flows over a surface, but the bulk air just finds a different path. Air hitting the ground tries to set up a static pressure gradient, but this never develops before the air has been diverted. Otherwise there would be a change in density.
What you are thinking about is the stagnation pressure. I've included a link for everyones benefit (particularly mine
). A "perfect" altimeter will read the same static pressure altitude under a hovering helicopter as in free space (otherwise airspeed would affect it). The static port is positioned where local flow is close to free stream velocity (a wing reduces static pressure because the velocity is much higher than free stream).
http://en.wikipedia.org/wiki/Stagnation_pressure
For more information about pitot-static systems:
http://en.wikipedia.org/wiki/Static_port
Actually, this post has helped my understanding!
Mathew, difference between static pressure and stagnation pressure:
Static pressure is pressure moving with air free stream (static port).
Stagnation pressure is pressure when stream is stopped (pitot tube).
This bothered me about meteorological systems too. What you have to remember is that at subsonic speeds there is no real resistance to flow. The viscosity affects how the air flows over a surface, but the bulk air just finds a different path. Air hitting the ground tries to set up a static pressure gradient, but this never develops before the air has been diverted. Otherwise there would be a change in density.
What you are thinking about is the stagnation pressure. I've included a link for everyones benefit (particularly mine
). A "perfect" altimeter will read the same static pressure altitude under a hovering helicopter as in free space (otherwise airspeed would affect it). The static port is positioned where local flow is close to free stream velocity (a wing reduces static pressure because the velocity is much higher than free stream).http://en.wikipedia.org/wiki/Stagnation_pressure
For more information about pitot-static systems:
http://en.wikipedia.org/wiki/Static_port
Actually, this post has helped my understanding!
Last edited by Graviman; 25th Nov 2007 at 11:05. Reason: I've just done a bit of homework, since i was getting rusty...
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Hope i'm not going to have to start paying thread tax...
Just, easier to break this into two posts.
Agreed about your point on #18, there is eventually a point where parasitic drag takes over. For a practical design (droop stops etc) i can see that mass would increase as length^2 for strength or length^3 for stiffness. What is not always appreciated is that rotor diameter is often limited to less than ideal by the mission requirement. This may include transport, or stiffness requirements. This is where the Mollers of this world get a look in...
What i was trying to get at with #17 is that the simple model is that the cyclic controls the tilt of the rotor, which explains why hover is so hard to master. The problem is that it just is not true. The cyclic actually does control the pitch and roll rates of the aircraft, but the compliance in the rotor system leads to a delay in response. It is this delay which makes hovering such an art. I'm often met with suprised reactions when i discuss this with other pilots - it took me a while to get it.
Originally Posted by Matthew Parsons
Graviman, not sure where you're going with #17.
For #18, the implication is that greater rotor diameter is always better. Not so. An increased diameter also means heavier blades. At some point you'd get diminishing returns.
For #18, the implication is that greater rotor diameter is always better. Not so. An increased diameter also means heavier blades. At some point you'd get diminishing returns.
What i was trying to get at with #17 is that the simple model is that the cyclic controls the tilt of the rotor, which explains why hover is so hard to master. The problem is that it just is not true. The cyclic actually does control the pitch and roll rates of the aircraft, but the compliance in the rotor system leads to a delay in response. It is this delay which makes hovering such an art. I'm often met with suprised reactions when i discuss this with other pilots - it took me a while to get it.
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Pressure ball myth
Matthew
Took some time, but I did some detailed pressure calculations concerning the pressure ball myth
Basis for calculations: My Scientific simulator for R44 (detailed dynamic model, precision better than 1%)
Assumptions R44-I, ISA , sea level, TOW 1000 kg
OGE IGE (3 ft) IGE (0ft)
Induction speed vi m/s 7.1 6.0 5.2
Pressure below rotor mB 1014.14 1014.23 1014.29
The ground effect blocks the air flow of rotor wake, this reduces induced speed and as a result increases pressure below rotor by 0.09 mB.
What are the dynamics now.
1. Vertical changes
The pressure ball is stable with respect to vertical variations :
(a) If hight increases pressure drops from 1014.23 to a minimum of 1014.14 and the heli sinks again.
(b) Conversily if heli sinks until skids touch ground, pressure increases to 1014.29 and the heli will climb
2. Changes in cyclic
Move cyclic 2 degrees forward produces two effects
(a) Decrease of lift by 0.1% because rotor trust is not fully aligned with vertical and starts producing a forward acceleration of 0.035g
(b) Slight decrease of pressure (increase of induction), also of the order of magnitude of 0.1%, because rotor wake is less blocked (wake deflects and starts escaping more to the rear, see publish drawning of Shawn by Phill77)
3. Changes in forward speed
Forward speed has two opposing effects:
(a) gradual disappearence of ground effect with loss of pressure and increase of induction speed (again because wake starts escaping to the rear)
(b) gradual increase of translational lift, with increase of pressure and decrease of induction speed
Calculation IGE (3 ft), maintaining IGE collective
Forward speed Knts 0 5 10 15 20 30 50
Induction speed vi m/s 6.0 6.0 5.5 4.9 4.3 3.1 2.0
Pressure below rotor mB 1014.23 1014.23 1014.29 1014.31 1014.34 1014.38 1014.43
Climb potential (ft/min) 0 10 91 200 306 416 576
4. Conclusion
The negative effects of pressure loss and effective lift loss because of cyclic movements are rapidely (as of 5 kts) counteracted by the positive effects of translational lift (the power curve). If cyclic were used more aggresively (say 5 -10°) the first effect will of course be more pronounced
Edited : don't seam to get the tables aligned
Took some time, but I did some detailed pressure calculations concerning the pressure ball myth
Basis for calculations: My Scientific simulator for R44 (detailed dynamic model, precision better than 1%)
Assumptions R44-I, ISA , sea level, TOW 1000 kg
OGE IGE (3 ft) IGE (0ft)
Induction speed vi m/s 7.1 6.0 5.2
Pressure below rotor mB 1014.14 1014.23 1014.29
The ground effect blocks the air flow of rotor wake, this reduces induced speed and as a result increases pressure below rotor by 0.09 mB.
What are the dynamics now.
1. Vertical changes
The pressure ball is stable with respect to vertical variations :
(a) If hight increases pressure drops from 1014.23 to a minimum of 1014.14 and the heli sinks again.
(b) Conversily if heli sinks until skids touch ground, pressure increases to 1014.29 and the heli will climb
2. Changes in cyclic
Move cyclic 2 degrees forward produces two effects
(a) Decrease of lift by 0.1% because rotor trust is not fully aligned with vertical and starts producing a forward acceleration of 0.035g
(b) Slight decrease of pressure (increase of induction), also of the order of magnitude of 0.1%, because rotor wake is less blocked (wake deflects and starts escaping more to the rear, see publish drawning of Shawn by Phill77)
3. Changes in forward speed
Forward speed has two opposing effects:
(a) gradual disappearence of ground effect with loss of pressure and increase of induction speed (again because wake starts escaping to the rear)
(b) gradual increase of translational lift, with increase of pressure and decrease of induction speed
Calculation IGE (3 ft), maintaining IGE collective
Forward speed Knts 0 5 10 15 20 30 50
Induction speed vi m/s 6.0 6.0 5.5 4.9 4.3 3.1 2.0
Pressure below rotor mB 1014.23 1014.23 1014.29 1014.31 1014.34 1014.38 1014.43
Climb potential (ft/min) 0 10 91 200 306 416 576
4. Conclusion
The negative effects of pressure loss and effective lift loss because of cyclic movements are rapidely (as of 5 kts) counteracted by the positive effects of translational lift (the power curve). If cyclic were used more aggresively (say 5 -10°) the first effect will of course be more pronounced
Edited : don't seam to get the tables aligned
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Wow, this is good work D3! I'd like to know more about your Scientific simulator for R44 - it sounds interesting.
Lets use your figure of min to max pressure change in hover, so from 1014.14mB to 1014.29mB. Now an R44 has a rotor radius of 198" or 5.03m. Lets assume wake contraction and hub cutout half the disk area, or that pressure change is halved at rotor. This pressure change of 0.15mB or 15N/m^2
produces a lift increase of 1192N/2 or 61kg.
http://robinsonheli.com/r44iispecs.htm
This all looks pretty convincing for the pressure ball theory, but what you haven't stated is are you quoting static or total (stagnation) pressure.
Yes the total pressure under the heli does increase, since an ASI would register a reading. No the static pressure under the heli does not increase, since an altimeter at ground height in hover center (stagnation point) would not register a change. To get into the aerodynamic nitty gritty the terms have to be used precisely.
Matthew, this has got under my skin and i'd like to provide a good answer. I'm going to discuss this in detail with some aeroengineers. I'll get back to you when i'm convinced that i know what i'm talking about!
Meanwhile...
Lets use your figure of min to max pressure change in hover, so from 1014.14mB to 1014.29mB. Now an R44 has a rotor radius of 198" or 5.03m. Lets assume wake contraction and hub cutout half the disk area, or that pressure change is halved at rotor. This pressure change of 0.15mB or 15N/m^2
produces a lift increase of 1192N/2 or 61kg.
http://robinsonheli.com/r44iispecs.htm
This all looks pretty convincing for the pressure ball theory, but what you haven't stated is are you quoting static or total (stagnation) pressure.
Yes the total pressure under the heli does increase, since an ASI would register a reading. No the static pressure under the heli does not increase, since an altimeter at ground height in hover center (stagnation point) would not register a change. To get into the aerodynamic nitty gritty the terms have to be used precisely.
Matthew, this has got under my skin and i'd like to provide a good answer. I'm going to discuss this in detail with some aeroengineers. I'll get back to you when i'm convinced that i know what i'm talking about!
Meanwhile...
Last edited by Graviman; 25th Nov 2007 at 17:47.
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Graviman
Thx.
1. The pressure quoted is static pressure, as inferred from Bernoulli. If uniform induction applies (ideal hover rotor, no tip losses), then an underpression of apprx 1/4 of the weight (converted to pressure using rotor disk area T/A) is found above the rotor. The rotor gives an impuls to the pressure equal to the trust (or weight in hover), resulting in a 3/4 T/A over pressure (see also Leishman p 42)
2. In that sense I think there is a fundamental difference between the plank analogy Nick made an the heli. For the plank indeed the pressure change only comes from a very local change in speed (slowing airflow below the foil). In a heli the same blade pulses over and over again on the same air mass.
3. The pressure increase under the rotor cannot be used directly to compute gains in trust, because the pressure above the rotor may (and will) also increase, it is the differential that counts.
4. For me the induction speed is the important parameter.
5. Restictions of my model
5.1 It does not used aero-elasty. I am sure I could build this in, because right now I already use a full 4th order aerodynamic blade section model with full stall modelling that is input to full dynamic equations (to "all moving parts"). Computing power however is already max for real-time simulation (which was the prime motivation of my work, see the delta3 discussion with good old LU, the low-g roll simulations, different autorotation simulations, retreating blade stall simulations etc).
3.2. The ground effects are modelled as average effects, also consistent with empirical data by Leishman. As can be seen from the drawings by Shawn, two effects can be distinguished
(a) a more overall effect of more or less blocking of wake, observed quite a bit downstream of the wake (5.03m radius versus 4.60 m rotor to the ground in the given example)
(b) a temporary indigestion of the forward vortex by a small section (forward sector) of the rotor.
Both produce in the given scenario an increase of induction, so a decrease of trust with fixed collective.
Since my data seams to be consistent with my personal (restricted) measurement capabilities, I think the R44 may be less susceptable to forward vortex ingestion, because of low induction speeds and a relative high position of the rotor. This may be different for other geometries.
d3
1. The pressure quoted is static pressure, as inferred from Bernoulli. If uniform induction applies (ideal hover rotor, no tip losses), then an underpression of apprx 1/4 of the weight (converted to pressure using rotor disk area T/A) is found above the rotor. The rotor gives an impuls to the pressure equal to the trust (or weight in hover), resulting in a 3/4 T/A over pressure (see also Leishman p 42)
2. In that sense I think there is a fundamental difference between the plank analogy Nick made an the heli. For the plank indeed the pressure change only comes from a very local change in speed (slowing airflow below the foil). In a heli the same blade pulses over and over again on the same air mass.
3. The pressure increase under the rotor cannot be used directly to compute gains in trust, because the pressure above the rotor may (and will) also increase, it is the differential that counts.
4. For me the induction speed is the important parameter.
5. Restictions of my model
5.1 It does not used aero-elasty. I am sure I could build this in, because right now I already use a full 4th order aerodynamic blade section model with full stall modelling that is input to full dynamic equations (to "all moving parts"). Computing power however is already max for real-time simulation (which was the prime motivation of my work, see the delta3 discussion with good old LU, the low-g roll simulations, different autorotation simulations, retreating blade stall simulations etc).
3.2. The ground effects are modelled as average effects, also consistent with empirical data by Leishman. As can be seen from the drawings by Shawn, two effects can be distinguished
(a) a more overall effect of more or less blocking of wake, observed quite a bit downstream of the wake (5.03m radius versus 4.60 m rotor to the ground in the given example)
(b) a temporary indigestion of the forward vortex by a small section (forward sector) of the rotor.
Both produce in the given scenario an increase of induction, so a decrease of trust with fixed collective.
Since my data seams to be consistent with my personal (restricted) measurement capabilities, I think the R44 may be less susceptable to forward vortex ingestion, because of low induction speeds and a relative high position of the rotor. This may be different for other geometries.
d3
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So basically what delta 3 says is:
1) The idea that ground effect reduces the induced power is wrong, the rotor is just a pressure pump.
2) That the SAME effect on an airplane's wing at 250 knots simply doesn't fit, so we must simply ignore it!
3) That he can ignore the effect of the reduction in angle of attack and suppression of the induced drag losses (which are the REAL cause of ground effect) because he calculates a pressure rise on his desktop universe.
Nice. Delta 3's popcorn aerodynamics PROVES the hardiness of the MYTH!
May the pressure balls be with you.
1) The idea that ground effect reduces the induced power is wrong, the rotor is just a pressure pump.
2) That the SAME effect on an airplane's wing at 250 knots simply doesn't fit, so we must simply ignore it!
3) That he can ignore the effect of the reduction in angle of attack and suppression of the induced drag losses (which are the REAL cause of ground effect) because he calculates a pressure rise on his desktop universe.
Nice. Delta 3's popcorn aerodynamics PROVES the hardiness of the MYTH!
May the pressure balls be with you.
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Nick, that's a bit of a leap. All that delta3 has shown is that there are pressure changes beneath the helicopter associated with height, forward speed, and cyclic activity. This didn't surprise me. What hasn't been shown is effect of the pressure gradient. Just because we can predict something with formulae or even measure something with instruments, doesn't mean that measurement has any effect on anything.
As far as urban myth status, I think we'll have to refer to Mythbusters terminology and call this one plausible (choices are busted, plausible, or confirmed). However, even though it feels like falling off a pressure ball to a pilot, and it appears there might be a ball of pressure, I still think that the pressure gradient is not big enough to cause this effect on its own.
As far as ground effect machines at 250 kts, I'd be interested to know how far in front of the machine they cause changes to the air. Anyone have a computer simulation that could help?
Hey, speaking of mythbusters, we could try bust this. In a few months I should have an instrumentation pallet available to me with a trailing static bomb. It might be possible to give the bomb a short leash and measure static pressure closely under a helicopter during transition to forward flight. I'll let you know if I get a chance to do this.
As far as urban myth status, I think we'll have to refer to Mythbusters terminology and call this one plausible (choices are busted, plausible, or confirmed). However, even though it feels like falling off a pressure ball to a pilot, and it appears there might be a ball of pressure, I still think that the pressure gradient is not big enough to cause this effect on its own.
As far as ground effect machines at 250 kts, I'd be interested to know how far in front of the machine they cause changes to the air. Anyone have a computer simulation that could help?
Hey, speaking of mythbusters, we could try bust this. In a few months I should have an instrumentation pallet available to me with a trailing static bomb. It might be possible to give the bomb a short leash and measure static pressure closely under a helicopter during transition to forward flight. I'll let you know if I get a chance to do this.