Originally Posted by
Ascend Charlie
Sorry it's a long post, but for the benefit of
Robbiee and his mates, here is a gathering of
Nick Lappos's Urban Myths, plus some notes from the late Shawn Coyle, both these gents being highly qualified test pilots:
Helicopter Urban Myths
These Urban Myths pervade our understanding of helicopters and how they operate. Each is fundamentally incorrect, but most are generally held as gospel, because training, lore and reference documents have repeated them long enough that they are simply accepted.
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.
Davy07,
The books say so, but they are quite wrong. VRS can only be induced by descending at least as fast as 75% of the downwash velocity of the helicopter and at a forward speed of not more than about 8 knots. For a Robbie this is at least -750fpm rod. For an S76 it is about -1500 fpm.
Why is 300 taught? Because one can start a VRS event by entering a hover with too little power, slip into an overpitching event and in short order, enter VRS. In a helo with scads of OGE hover performance, the 300 fpm is truly a myth, in one without, it is misleading as a VRS cue, but good word as an overpitching warning.
Manfrom Uncle, what you are correctly saying is that going faster uses more power. The wind has nothing to do with it. 80Kts airspeed uses the same power both up and down wind.
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.
Maybe you should get a hold of Shawn Coyle's "Cyclic and Collective":
http://i159.photobucket.com/albums/t...g?t=1195957331
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 )
delta 3,
OK, your work is truly great, and I can see the reduction on angle of attack and power. It is truly a desktop universe, and I mean that in a good way.
The problem is that without careful explanation, one could actually think you show how the "pressure bubble" under the rotor is what ground effect is, and that would be untrue. Ground effect is when the ground plane flattens out the flow, causing the rotor to behave as if the blades were much longer. As a result, the tip losses for the blades are sharply reduced, causing a sharp reduction in the induced power requirements. The reason why ground effect does not show very well at 100 knots is that induced power is very low there, but ground effect is still present at 100 knots.
For an airplane, the case is easier to picture. As the wing comes down into ground effect, the tip wash reduces sharply, and the wing behaves as if it were infinitely long. This requires less angle of attack (which had caused a large drag rise prior to entering the ground effect). The induced drag wastes power, so ground effect shows itself as a reduction in angle of attack and a reduction in the power required.
Ground effect is not caused by a pressure rise, nor is the advantage to the aircraft due to any pressure that pushes the aircraft up (or any other such pressure bubble nonsense.)
Ground effect explained
Misinformed Instructors who propel the pressure bubble myth should be asked to do a quick Yahoo search using "induced drag ground effect" so that they can learn how to tell their students the truth, and not convenient myths. We would be all the better for it. Pressure bubblers, please note:
Induced drag explained:
http://galileo.phys.virginia.edu/cla...ero/node5.html
http://www.grc.nasa.gov/WWW/K-12/airplane/induced.html
Ground effect explained:
404 - File or directory not found.
http://www.pilotfriend.com/training/...g/aft_perf.htm
"Many pilots think that ground effect is caused by air being compressed between the wing and the ground. This is not so. Ground effect is caused by the reduction of induced drag when an airplane is flown at slow speed very near the surface."
The quiz for today, for the bright student:
Define induced drag
define aspect ratio
explain how aspect ratio affects induced drag
explain how proximity to the ground affects aspect ratio
I love this block, we can go round and round it forever!
Some points:
1) I never said that ground effect was a myth, I said that the "pressure bubble" was a myth! Ground effect is very real, and amounts to a typical 15% power savings for most helicopters (where the rotor cannot get closer to the ground that about .3 radius.
2) Those who love the pressure bubble theory are welcome to it, believing myths is not against the law! However, ask yourself why ground effect disappears when you hover over long grass - and don't tell me that grass absorbs pressure! The grass slows the outflow, which is the mechanism that changes the blade angle of attack and reduces the hover power.
3) The pressure bubble theory cannot explain why ground effect only reduces the induced drag of the blades, if that theory is correct, more velocity makes more pressure, and thus saves more power. If the air velocity gets banged against the ground, and pressure builds, then a higher velocity should make even more pressure, and more ground effect, right? No, wrong! Maximum ground effect is about +15% power, regardless of how fast the downwash velocity is. In fact, ground effect is no different for highly loaded rotors, with faster downwash, than it is for low disk loading rotors with gentle downwash.
4) If pressure bubbles push the aircraft up, then high speed airplanes, with the wake hitting the ground hundreds of meters behind the aircraft, should see no ground effect. But they do! That is because the effect is on the wing, where the angle of the flow around the wing is changed by the presence of the ground.
5) Ground effect makes the wings or blades act as if they are longer, and this cuts the tip losses that make the induced drag. They have nothing to do with the pressure under the blade or wing, they have everything to do with the reduction in outflow, and the reduction in the tip vortex pattern due to that outflow.
With my respect for Shawn unabated, I must respectfully disagree with him. There is a meaningful difference between the two cases, light vs heavy. But first we must clarify what we mean by Vortex Ring State (VRS) - and what is meant by a different problem called "Settling with Power (SWP)" or "over pitching" or "insufficient power to Hover OGE"
Most accidents where a hovering helicopter falls and crashes and VRS is blamed are actually cases of SWP or over pitching, where the hover performance is marginal, and insufficient reserve power (power margin) is available to allow moderate climbs and descents while OGE. The aircraft "falls through" the hover, hits hard (usually with just a bent helicopter and bruised ego) and then someone says "It was VRS." Sometimes the mistaken person is an official accident investigator!
In a helicopter at high MGW, with only slight or no margin between the power needed to HOGE and the power available from the engines, "over pitching" is more likely than in a lightly loaded helicopter where lots of power above hover power is available. When lightly loaded, there is much extra power available above the hover power, so the lightly loaded helo is much less likely to experience "over pitching" and thus the lightly loaded helo is much less likely to be mistakenly labeled as a VRS accident.
Now the truth: Since true VRS involves the descent of the helicopter into its own downwash, and since in a light helicopter the downwash velocity is quite a bit less than in that same helicopter when heavy, a lightly loaded helicopter needs much less rate of descent to experience true VRS.
Thus, heavy helicopters require more descent rate to get true VRS, and so are less likely to enter that state, but heavily loaded helicopters have more over pitching power control accidents that are too often labeled "VRS", so the mistaken pilot lore says heavy helicopters are more likely to experience VRS.
For the record, no helicopter can experience true VRS unless it is descending nearly vertically at about 800 to 1000 feet per minute.
Also for the record, most helicopters can experience SWP or overpitching at rates of descent near zero if they have little hover power margin.
Also, heavily loaded helicopters have less propensity to enter VRS because they need more vertical descent rate than lightly loaded helicopters, which need less descent rate to get into VRS.
Hmm,...are you sure these aren't "Rural Myths". I never heard them training in the city.
By the way, since you guys keep pointing this list out to me, is there any one particular "myth" you all think I'm perpetuating?