Sunday, January 1, 2006
Name That Bird (Cont'd)
Two suggestions for naming the VH-71 U.S. presidential transport:
1. Harpagornis (New Zealand Eagle)--This now-extinct bird weighed 20 lb. or more and possessed a wingspan of about 9 ft. It was the largest eagle ever. It had talons as large as tiger's claws.
2. Hesperornis (Toothed Bird)--It was an aggressive, prehistoric sea bird that could dive into the sea and fish. It possessed a jaw full of teeth (modern birds have no teeth).
David John Barrett
Head, Airframe Technology Branch
U.S. Naval Air Systems Command
Patuxent River, Md.
I think any of the following might be considered as a name for the new VH-71: Reliant, Independence, Statesman, Chieftain or Guardian.
Kildeer Police Dept.
Tailwind & Settling?
I wish to comment on Johan Nurmi's article on settling with power (Settling With Power," May 2005, p. T10). He stipulates that a condition necessary for this phenomenon is a tailwind and repeats this in the few examples he gives.
To state that a tailwind is necessary is both false and dangerously misleading, in that some pilots might think they are safe if they have only a few knots of headwind or none at all. The only speed element involved in order to develop this condition is that of having a low airspeed. It will vary with the helicopter type, but it will certainly be below translational lift, which is more likely to be less than about 15 kt. than 30, as he suggests.
It is also disturbing to see the flight maneuvers of quick stops and steep approaches mentioned in the same breath as a tailwind without soundly condemning the practice. During a flight test, if a student ever conducted an approach or a quick stop downwind without realizing it and taking the proper precautions, I would fail him automatically.
ATPL Helicopter (10,000+ hr.)
Grade 1 Instructor
Johannesburg, South Africa
I also would like to comment on Johan Nurmi's article.
First, there are three conditions, not four, that must be met simultaneously for settling with power to occur. If one is not present, it is not aerodynamically possible for settling with power to occur. They are 1) little or no airspeed, 2) some power applied and 3) a rate of descent greater than 300 fpm. The only time that a tailwind becomes a factor is while flying downwind at or near the same speed as the wind, at which point one of the conditions (little or no airspeed) would be met. However, flying downwind can be hazardous if the pilot is not paying attention to airspeed. With zero airspeed in a downwind, the aircraft will still have ground speed. This movement over the ground is frequently mistaken for airspeed.
Second, asserting that the helicopter will become uncontrollable "if you don't act immediately" upon entering settling with power is incorrect. When I demonstrate settling with power, I get the helo into the vortex ring state and keep it there so the student learns how to recognize it, how quickly it escalates, and how to get out of it. I've demonstrated settling with power in MD-500s, MD-600s, AS350s, UH-1s, and R22s and have never had one react violently, much less uncontrollably.
Third, Mr. Nurmi's assertion that "pilots performing 360-deg. pedal turns at altitude will encounter tailwind at some point, with the risk of getting into settling with power if they allow a descent" is also a bit mistaken. Wind is not always present. That notwithstanding, a helicopter at hover doesn't care whether its nose or its tail is into the wind. Whether a headwind or a tailwind, the airspeed is the same, that being the speed of the wind itself. The additional power of which Mr. Nurmi spoke would be required while the aircraft is turning and the tail rotor is either pushing or pulling the tail of the aircraft into the relative wind.
Fourth, while it is true that settling-with-power accidents do happen during confined-area and pinnacle approaches, I would hope that it is not because the pilot is "unaware" of his slow forward airspeed. Slow airspeed is the nature of these types of approaches. However, it is not always a forward airspeed. While not preferable, it is sometimes necessary to make a downwind approach into a confined area or to a pinnacle. During such approaches (and depending on the wind speed), the aircraft may actually be going backward, relative to the wind.
Fifth, while settling with power may be induced by lowering the collective and establishing a rate of descent of greater than 300 fpm. (as long as the two other criteria are also present), further lowering of the collective will not exacerbate the condition. Mr Nurmi stated that "if you further lower the collective, the blades' root sections go deeper into stall and the tip sections further lose lift." This doesn't make sense since, in this context, going "deeper into stall" and "further lose lift" are the same thing. These statements are nonetheless inconsequential, because they are not true. As far as I know, you cannot aggravate the stall of any airfoil by reducing its angle of attack. Moreover, lowering the collective is one of the steps in recovering from settling with power.
I have been reading Rotor & Wing for many years, but have never felt this compelled to rebut an article because of its inaccuracies and inconsistencies. I am disappointed that you would publish this article on such an important issue as settling with power without a great deal of proofreading for accuracy. This is such an important training issue that the article should be rewritten and republished as a service to your readers.
One final note: contrary to what Mr. Nurmi stated, one cannot generally tell the wind direction in the desert by doing pedal turns at 8 ft. agl. I know. I've been flying helicopters in the desert for a long time. A trick that does work is getting down low enough to make a dust cloud, then flying away and watching which way it moves.