Article: 742 of rec.aviation.student Newsgroups: rec.aviation.student Path: newshost.ncd.com!ncd.com!olivea!spool.mu.edu!agate!ames!riacs!faf3.arc.nasa.gov!user From: Brent_Wellman@qmgate.arc.nasa.gov (Brent Wellman) Subject: Re: Helicopter question Message-ID: Followup-To: rec.aviation.student Sender: news@riacs.edu Organization: NASA Ames Research Center References: <3295@esun179.gdc.com> Date: Tue, 29 Jun 93 17:31:23 GMT Lines: 87 In article <3295@esun179.gdc.com>, farmer@gdc.COM (David Farmer) wrote: > > I have a question for the Helicopter experts out there: > > I'm flying along in a Robinson, and the Low Rotor Speed Alarm goes > off. The book says to lower the collective and or use back cyclic. I > am trying to understand why back cyclic will help increase the rotor > rpm. I can understand why it is true while in an auto-rotation since > more air is being forced through the rotor disk. It would seem that > under powered flight, this more air would translate to more drag. > > In particular the books warns that an airplane pilots normal reaction > to the horn would be to push forward on the cyclic compounding the > problem. > > Can anyone provide any illumination? > > Thanks, > David Farmer > COMM RW, training for CFI. OK Dave, here goes. You are correct when you assume that this has something to do with autorotation, but it also has to do with the issues of induced drag, profile drag, required power and available power. As your R-22 travels through the air, it has energy related to its altitude and movement. Anyone who has done a pitch-up in a Cessna is familiar with the concept of trading energy between the two. The situation is not so simple as this seems, because there are non-conservative forces at work, namely DRAG. Drag is non-conservative because while you can recover the energy due to altitude, the energy loss due to drag is gone (quite literally forever). This robbing of energy due to the profile drag of your helicopter, which, as is typical of rotorcraft, has the aerodynamics of a streamlined anvil, is countered by the powerplant. The energy of the powerplant and drive train are turned into energy of movement, thrust, by a transfer of momentum from the rotor to the surrounding air. Now the rotor blades transfer momentum to the air, generating lift (just like a wing) but also generating induced drag (also just like a wing). The blades also have some profile drag and skin drag that is thrown into the mix. This drag impedes the blades rotation. It acts like a force on a crank and is seen as a torque at the rotor shaft. Torque times RPM (times a constant) equals horsepower. If you fly at some airspeed, this translates to a drag force; this drag force must be countered by thrust, if the helicopter is to fly at constant speed; this translates to a thrust required for the condition, which implies a particular power setting. Certain variables such as air density and rotor efficiency (called figure-of-merit by us rotorcraft types), also come into play in a minor way, but for simplicity sake I'll ignore them. If you attempt to command more thrust from the rotor than the engine at its fuel setting will allow, induced drag will cause the rotor to slow down and if you are not alert trip the RPM alarm. The immediate action should be to reduce the induced drag on the blades, reducing the torque on the engine, allowing it to come back up to speed. Both reducing collective and backing off forward cyclic in forward flight do just this. The first action simply reduces the angle of incidence and therefore the angle of attack of the blades, causing them to generate less lift and consequently less induced drag and therefore less induced torque. The second action unloads the rotor blades by tipping the rotor system slightly aft and reducing forward thrust. And yes, it _is_ counterintuitive to a fixed wing pilot. Also, be careful with these procedures at low altitudes. Autorotation entry is nothing more than an extreme case of this. When the engine quits, it puts out _no_ power to counter induced torque. Since it is imperitive to you that you maintain RPM (and thereby live to defy gravity another day), your immediate action is to drop collective and haul back on the cyclic both to unload the rotor and to facilitate autorotation entry. There is some optimum airspeed you hit (I don't know what it is for the R-22), usually right in the power bucket (a whole other explanation of that), and you establish some descent rate, regulating RPM with the collective (you don't want to spin it TOO fast!). It is easily shown that the autorotative rotor has approximately the same drag as a parachute of the same diameter. Near the ground, you trade rotational energy of the rotor for enough thrust to halt your descent and your forward speed by bringing in enough collective to do the job. Induced torque slows the rotor down drastically, but hopefully you're safe on the ground before you lose lift entirely. Brent Wellman