helicopter aerocynamics [Archive]

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Fiesty

11-24-2003, 06:07 AM

Discussion  An overal increase in angle of attack on a rotor disc will result in an increase in rotor RPM :)

Fiesty

11-24-2003, 06:09 AM

lateral vibrations indicate out of balance rotor  and vertical vibrations indicate out of track rotor.

Fiesty

11-24-2003, 06:12 AM

:)Vertical autorotations result in higher rotor RPM than autrotation with forward flight

Fiesty

11-24-2003, 10:52 AM

???Incorrect tower height or coning of blades  will result in a 2 per rev  

Fiesty

11-24-2003, 11:00 AM

:oHigh frequency vibrations are always associated with the tail rotor

gyromike

11-24-2003, 12:04 PM

Randy, Are these questions? Or statements? Just checking.

Fiesty

11-24-2003, 10:51 PM

???Mike they are statements for discussion. Disagreeaments and why.

Fiesty

11-24-2003, 10:57 PM

:-/Rotors are statically stable an dynamically un stable. *They resist any change in airspeed by flapping up in front as airspeed *increases resisting the change. Damping is by airframe.

Fiesty

11-24-2003, 11:01 PM

???What is the cause of retreating blade stall? *Where does the stall start ?

Hognose

11-26-2003, 11:53 AM

OK, I'll have a go... An overal increase in angle of attack on a rotor disc will result in an increase in rotor RPM If powered, false. Increasing angle of attack will increase lift AND drag on the airfoil, slowing it down. If in autoration, true -- to the point of rotor stall.                                           lateral vibrations indicate out of balance rotor *and vertical vibrations indicate out of track rotor. True                             Vertical autorotations result in higher rotor RPM than autrotation with forward flight Hmmm. Thinking... the RPM would be the same, the rate of descent would be higher for a vertical autorotation?                             Incorrect tower height or coning of blades *will result in a 2 per rev Almost anything in the rotor being wrong shows up as a 2/rev?       High frequency vibrations are always associated with the tail rotor Hmmm. Could be, but on a piston helicopter I'd start in the drive train. TR can be mid or high freq vibration, but so can oodles of things in the motor. Now, on a turbine, any vibration will be too fast to feel, usually... you'll hear if a turbine is out of whack.                             What is the cause of retreating blade stall? It usually begins with excessive airspeed (excessive airspeed/density altitude too; most helis VNE trails off as altitude goes up).  Basically, the rotor needs to have sufficient relative speed through the air to generate lift. WIth the helicopter moving forward, the rear-moving side of the rotor disc sees its relative speed (and therefore lift) decrease.   * Where does the stall start ? It spreads from the stall region at the root of the rotor -- there the effects of relative wind are most pronounced. OK... There are my answers... tell me how much more ground school I need before I tackle a helicopter written! cheers -=K=-

Fiesty

11-27-2003, 11:19 PM

Kevin"Hognose"O`Brien, Very well done except for a couple of discussions. Vertical autorations result in higher rotor RPM than autrotation with forward flight Answer:  An overal increase in rotor disc angle of attack results in an increase in rotor RPM.  In vertical autoration the angle of attack is greater than in forward flight. Incorrect tower height will result in a 2 per rev. Answer:  Yes.  as blades are lowered or raised in relation to the feathering axis it will raised or lower the CG location in the roror disc resulting in a 2 per rev.

Fiesty

11-28-2003, 05:57 AM

Al., Item #1. Comparing the angle of attack on rotor blade it will be the highest in vertical autorotation. *The lift vector is at 90degrees to the relative wind so it will be inclined forward increasing the thrust on the rotor blade resulting in a higher RPM. The driving region of the blade moves towards the tip reducing the propeller region. Item #2. *The same answer applies as angle of attack increases the rotor RPM increases until stablized.  Most small helicopters the pitch is lowered completely when you go into autorotation so it is imposible to lower pitch any more. Item #3. *On most helicopters it is the feathering axis as blades are hinged in line with the blade. *On the early Bell types with semi-rigid hubs they are under slung *and the feathering axis is above center line of blades like a teetering pivot on the autogyro. I am not * expert. *I only use my experience of 53 years as a helicopter flight instructor and test pilot *and I could make mistakes *in terminlogy and other terms used for gyros as I am not gyro instructor. I started this subject as discussion and a learning section *where we can come to a standard understanding .

Fiesty

11-28-2003, 06:42 AM

What type of hub   has a rotor system in which the blades are fixed to the hub, but are free to flap and feather? What is Transverse-Flow effect? A rotor hub that rotates below the top of the mast, as in a semi-rotor sytems is __________________________?

Fiesty

11-28-2003, 06:57 AM

Al.  This is not a discussion of autogyro aerdynamics.  Helicopter and gyro have a lot of similiar things which happen and result, but not all are the same so please stay with helicopter aerodynamics  in this subject.

Al_Hammer

11-28-2003, 07:03 AM

everything I said includes both gyros and helicopters, Randy, and I was careful to explain that. The only difference is the last remark where I said that a gyro has the feathering axis below the rotorhead. On a helicopter the feathering axis has nothing to do with the placement of the cg or undersling, so my criticcism of your answer still stands. It is not my intent to annoy you , but  if I believe you are wrong, I will say so, so that others may read this disccussion and decide for themselves what is correct.

Fiesty

11-28-2003, 07:15 AM

Al. Translation increases airflow over the rotor sytems, so more lift.  like in any aifoil.  more air set in motion in a unit of time. equals more lift, so reduce rate of descent. I believe Chuck was speaking about settling into disturbed air like in Vortex-Ring effect. In increase airspeed in aurorotation will reduce rate of descent until you reach some where around sixty MPH then it starts to in crease.

Fiesty

11-28-2003, 09:50 AM

Al. Why do you say we shed vortices?  I agree that the advancing blade reduces vortices, but the retreating blade vortices increase.  The rotor is more efficient  and treated as a disc it has more air flow in a unit of time, so more lift.

Fiesty

11-28-2003, 10:16 AM

Al. Lets hover a helicopter at 25" MP and lock the throttle at that amount. Now move cyclic forward slightly and machine settles, increase forward cyclic and helicopter will start accelerate and climb at *12 MPh and as speed increases it climbs faster * untill you reach about 60MPH. *As speed increases above the magic *number(60?) the helicopter starts to descend. *It is possible . to fly from a hover around the traffic pattern , slow helicopter and approach to a hover without changing the 25"MP.

Fiesty

11-28-2003, 10:29 AM

Al. *It seems like is saying the same thing about forward flight. I think we have beat this to death, so we should go to another  subject.

Fiesty

11-28-2003, 11:11 PM

Al. *I agree he did good. *By beating to death , we have spent enough on one subject as we are saying same thing over and over. Lets try the other open questions and statements.

Fiesty

11-28-2003, 11:20 PM

Question:  Can you have a 2 per rev vibration in a rotor with more than two blades?

Fiesty

11-29-2003, 05:43 AM

Welcome aboard Steven.  How about entering the discussions. Jumpin the water is warm.

Fiesty

11-29-2003, 05:45 AM

Question: Can anyone explain the Rotormatic rotor system? Al__________________Steven____________________

Al_Hammer

11-29-2003, 06:58 AM

Welcome Steven. :D The  vibrations associated with a rotor(in addition to the simple 1/rev) are generally in the form of N per rev where N is the number of blades. In adddition to what Steven said , there is a nice explanation here about the Hiller paddles and the Bell-Hiller system used on models. http://www.w3mh.co.uk/articles/html/csm9-11.htm The Hiller paddles are controlled by the pilot and they then control the rotor. There is a certain amount of lag involved. The Bell hiller system combines the paddles with swashplate control via a mixer so that the control is speeded up yet still retains the stability of  the paddles.

Fiesty

11-29-2003, 12:11 PM

;)Good information. *Thanks Steven and Al. *In 1950 thru 1954 I flew the H-23A and H-23B teaching U.S. Army Test Flight procedures at Tulsa, Okla. Randy Question:  Who invented the Stablizer Bar used on the Bell 47 Helicopter?  Give purpose and technical aspects.

Al_Hammer

11-29-2003, 05:22 PM

Since my plan is to primarily fly gyrocopters (2 place, with my wife as passenger), what kind of training would you recommend? Gyro training somehow comes to mind, Steven :P You can always learn helicopters later. Its not necessary to fly a helicopter  first because the two skills are substantially different. Its cheaper, to get a fixed wing PPL and then get a gyro add-on later. The only snag with starting in gyros is the transition to solo in your own machine after what usually is minimal hours dual in a heavier machine like the RAF. The light machine with open cockpit will feel very diferent and it is good to have some tail dragger time or any kind of fixed wing solo time to give you some basic flying skills. A gyro tow glider is a great way to go, if you know anyone who has one(and there aren't many.) I personally got a lot out of using a simulator (with homemade helicopter controls) when learning to fly a helicopter, and that's a topic for another post sometime.

Fiesty

11-29-2003, 10:55 PM

Steven:  As Al said gyro glider is cheapest.  I would recemmend that you start with fixed wing tail dragger.  This will develop your air sense and control of the aircraft.  I do not believe that gyro training will help you to fly helicopters. A note: Colin Mill is giving theory from a RC model concept.  His theories are good, but some do not apply to man carrying helicopters.  For example the the Bell-Hiller control sytem has never been used  except on model helicopters.

Fiesty

11-29-2003, 11:00 PM

Al. how about giving us some information on your Helicycle.  Construction etc.

Al_Hammer

11-30-2003, 05:32 AM

Randy, thanks for asking. The  turbine is being used in 99% of the machines, but I opted for the Rotax. My engine is not mounted yet, so technically I could still go with the turbine,  The construction is otherwise mostly complete except for wiring, painting the cabin, installing fuel tanks and engine. There is almost every kind of construction required in the kit, except welding. You quickly learn to use metal snips, rivet tool, cleco pliers, clamps, drill press, reamer, grinder, sander, paint gun, etc. Lots of fun if you like to build. For example, the builder fabricates the main rotor root end "doublers" from sheet stock and bonds them to the blades. http://was.kewlhair.com/hammer/jpegs/doublers.jpg Then the end plugs, tip weights, lag bolt attachment, prepping , and painting must be done. http://was.kewlhair.com/hammer/chopper/ForSale.htm

Fiesty

11-30-2003, 06:39 AM

Al the helicycle looks wonderful.  It should be a buy for some one.  Thanks for information.

Fiesty

12-02-2003, 09:24 AM

Steven- The Bell Stablizer Bar did not stay level with horizon.  It followed the mast in certain time period.  This was accomplished by two hydraulic shock absorbers bolted onto the mast with connections to the bar.  In rigging they had a special timing tool for adjusting the following time.

Fiesty

12-13-2003, 09:11 AM

Al: You are speaking about gyros.