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#1
03-04-2012, 03:57 PM
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Torque vs. RRPM at spin-up
I would like to learn about the torque required to spin a rotor at various RPM's with no wind and no forward motion (as might happen at the beginning of the rotor spin-up). Obviously, this is something that varies with several parameters such as blade length, airfoil, rotational inertia, etc.
I thought that perhaps rotor blade manufacturers would produce curves or data reflecting rotational inertia and aerodynamic drag for their various products, but thus far, I have not found this to be true. So, I think maybe I'm not looking in the right place? Does anyone have such data or suggestions as to where one might obtain such data? At this point, I'm not really picky as to what manufacturer or what blade length. I'm just looking for a starting point. In Igor Bensen's book "A Dream of Flight" he discusses the max RRPM he is able to achieve with various prerotator schemes, but it is focused on input power not required rotor power. Ralph Taggart also presents a chart of "plausible" RRPM achieved with various horsepower in his research using model airplane engines to spin rotors up; again input power focused, not requirement focused. Anyway, any data, suggestions, or discussions would be welcomed. Doug 
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#2
03-04-2012, 05:50 PM
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Doug,
As a ball park figure I've used the HP3.xls program ( http://www.rotaryforum.com/forum/att...6&d=1236136429 ) to estimate rotor h.p. and r.p.m then used those numbers to enter a cube law equation into my graphing calculator to plot out a rotor rpm vs hp required graph. The figure I use for 23 ft Dragonwings is 16 hp at 340 rpm and the plotted graph is close to what my experience with gas engine prerotators has shown. .
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#3
03-04-2012, 06:45 PM
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Doug ... please allow me the opportunity to think out loud for a moment .... I feel you have asked a valid question ....
(As a helicopter guy) .. I always ponder the torque on the Gyro fuselege (during rotor run-up) .... so I watch the mast flex and twist from the torque .... the gyro components often seem to hold up just fine ..... and then I watch the landing gear (wheels) .... ...It appears that a pre-rotating gyro requires decent wheel brakes ... and the high speed pre-rotation gyro's requires REALLY GOOD BRAKES ... plus lots of engine power (and torque) to achieve high rotor rpm on the ground. Just thinking out loud Doug ..... I look forward to all opinions on this matter .... I hope to hear from everybody ..... .... in summary .... It is my opinion that the successful high rotor rpm (RRPM) gyros ..... and the successful jump take off gyros .... come to the torque issues in the fuselage (like helicopters) .... and that becomes the dividing line between continuing with high speed gyro pre-rotation without going into helicopter anti-torque territory ..... something like that .... I hope everybody can make sense of what I just said .... and I hope everyone will chime in with their own thoughts ... You have asked a valid question Doug ... I look forward to every-bodies responses. ... I hope to learn more. Quote:
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#4
03-04-2012, 07:20 PM
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Alan,
I'm not sure I follow you. I studied the HP3 download a little. (cool spreadsheet, by the way) So, do I understand that you plug in numbers (like, say, 23' rotor , 5" chord, 500lb., 50mph IAS for example) until you get results that compare with your experience in real life, full scale that produce 340 RRPM and that, according to the spreadsheet, requires 16hp. Then you plug that point into a cubic equation of RRPM vs. Hp and plot the function back to zero? Something like that? What form does the cubic take? Something like y = A x^3 + Bx^2 + C ? Please bear with me. I think I missed something. Doug
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#5
03-04-2012, 07:34 PM
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Alan - I read your article and understand some of it .... especially about drag (in flight) of a rotor blade .... makes sense to me .... but what about on the ground (pre-rotation) ... what you call rotor drag in your article .... I would call torque ... (on the ground) (pre-rotating) .... hope that makes sense ....
I wonder if there has been a study (graph) of drag (torque) between different blades when it comes to pre-rotation power requirements of various blades ... do you know .... ?? .... Anybody .... ?? Most of my knowledge on the subject comes from pilots who report the hand starting abilities of various blades. ... That is fine as far as pre-rotating by hand .... but my question is as follows .... I appears to me that the best flying blades .... are hard or impossible to hand start ... does that sound right ...???? anyone know ... ? ..... and the easiest hand-start blades are not necessarily the best flying blades .... I must confess that I have an ulterior motive to ask that question .... not necessarily related to gyro requirements .... but the gyro pilot will be intrigued by my experiments .... Quote:
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#6
03-04-2012, 07:54 PM
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Quote:
I'm really interested in the part way before that! I contend there is an inertial component and an aerodynamic component that resist spinning up the rotor (pre-rotating). A little torque applied for a long time will overcome the inertial part (depending on your patience) and accelerate the rotor, but at some point, the little torque can't overcome the (steadily increasing) aerodynamic drag and the rotor stops accelerating. To have gotten to that point, you have to have produced enough power (= torque x RPM) at all the points along the way. So, for instance, if your gearing is not optimized, you may not get there, even though your pre-rotator has more than enough power. (It just isn't in the right combination of torques and RPM's to "break through".) Torque vs. RPM curves are readily available for most motors (gas or electric) which SUPPLY the power. I'm trying to understand the torque vs. RPM curves for the spinning rotor which REQUIRES the power. Doug
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#7
03-04-2012, 11:31 PM
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You laid it out much better than my attempt Dogman (Doug) . I expect many aviators will be grateful
Below is a recent , and excellent jump-gyro demonstration that shows some torque issues such as we discussed. These videos keep disappearing from the internet for some reason .... I hope this one survives ... at least for a day or two for the sake of this discussion .... take a look at about 2:35 (power up) and the sideways strain on the landing gear (torque - drag).... then around 3:14 when torque is released .... fuselege twists opposite .. you will see it... ..... then again at 4:27 torque applied (pre-spin) ... and at 4:46 power is released and take-off begins .... watch the fuselage twist on the landing gear from torque and then release of torque .... . I am using basic terminology to describe. This is a great example of torque issues in a gyroplane or helicopter fuselage when power is sent to the rotor blades .... I hope this video survives ... it is a total of 14 minutes in duration. Not one boring moment. The camera man and film editor should get a raise .... Other things may come into play .... and not discussed here .... such as propeller air flow over the rudder and how it may help to compensate for some torque issues in the gyro in the video. Different subject altogether .... in a helicopter we install a live rudder commonly known as a tail rotor .... which amounts to a sideways-mounted adjustable propeller that deals with torque on the fuselage .... ..... foot pedals operate the pitch of the tail rotor blades on a helicopter ..... foot pedals operate the pitch of the rudder behind the air flow propeller of a gyro ... both trying to accomplish the same thing .... different methods .... helicopters can do it sitting in one spot (hover)... that's all. I love tail rotors . they do as they are told .... most of the time anyway ..... Below is a gyroplane venturing into helicopter territory ..... nothing wrong with that ....but I repeat my earlier question .... where is the dividing line between helicopters and gyroplanes .... I dont much care myself ... but for the enthusiast of gyroplanes ..... and the enthusiast of helicopters .... we must consider them both ... they are certainly closely related . Anyone who has read this far is an enthusiast in my opinion. May you have a kind and wonderful day. I thank Vance Breese for teaching me the use of the word enthusiast. I will be forever grateful. Arnie . Autogiro L.F.I.N.O. - YouTube! Quote:
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#8
03-05-2012, 01:13 AM
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Doug, this Excel spreadsheet can help you?:
http://www.sendspace.com/file/kjo37a
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#9
03-05-2012, 01:07 PM
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Doug,
Please remember that to 'spin-up' the prop to some rpm you are dealing with two effects: the static rotational inertia of the blades themselves and the inertia of the airfoil section moving through air with some density. The aerodynamic drag (force) is far greater at significant rpm. The static inertia is simply that of a bar or thin plate rotating around a center point; the second result is derived from kinetic-energy conservation rules. Key point is that to solve without approximations, you have to know drag coefficient(s) of the rotor - mfg's are not going to supply that info generally; the NACA foils are defined. The math is covered in some texts on Helicopter Aerodynamics - I can suggest a few if you want. Sorry to be longwinded; PM if you desire. Chris
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#10
03-05-2012, 01:54 PM
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Quote:
T= torque, I= mass moment of inertia, a= angular acceleration For a two-blade rotor, I is somewhere near l*l*m/12 (m=mass, l=length) http://en.wikipedia.org/wiki/Angular_acceleration http://en.wikipedia.org/wiki/Moment_of_inertia#Examples Basically, rotor acceleration is constant until drag begins to occur. At that point, it becomes a computer iteration to calculate rather than a simple one-answer formula. I would like to see references to textbooks too. Rotor drag I could not find a formula for, but seems to depend on the cube of rpm.  Youtube video: Jump take-off at 7:30, 8:15, 9:25, 10:25 (impressive climb!) and 13:05 , transition to autorotation very few seconds later as indicated by the rearward tilt of the rotor. That shift in rotor airflow from down to up requires some delicate piloting skills. The frequency change indicates rapid decay of rotor rpm at 2:20 and 3:10 - rotor brake ? "dividing line between helicopters and gyroplanes" I would define a helicopter as a rotorcraft whose rotor is always mechanically powered to provide lift (with downward airflow) and thrust (the few tip thrusters included). An autogyro uses passive rotor and upward airflow to provide lift most of the time. Rather confusingly; FAA chapter 16 writes "upward flow" but shows downward in the picture. Also Newton's 2. law seems to require air mass sent down to keep aircraft up, regardless of lift method. The recent Sikorsky X2 and Eurocopter X3 may translate into marketable products such as S-97, which would be among the first series production of helicopters with alternate thrust. Lift remains by powered rotor pushing down air, perhaps supplemented by small wings. As development progresses, the two areas helicopter and autogyro may see more overlap - the tip thrust hovering Rotodyne may get a modern successor in an autogyro with a gear-driven rotor for hover and windmilling rotor for cruise (Groen?). 
Last edited by noflynoob; 03-05-2012 at 02:14 PM. Reason: afterthought
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#11
03-05-2012, 02:15 PM
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The subject of Vertical TakeOff and Landing is an interesting one, particularly when trying to apply it to the gyrocopter, and/or trying to reducing the complexity and cost of the helicopter. Perhaps there is an answer somewhere to the creation of a Short TakeOff and Landing craft that comes close to offering the best of both.
Quote:
 Doug, Jean's and Alan's spreadsheets may offer the best answers. The following may provide some additional knowledge but it does not consider the rotational inertia etc. The following uses Alan's top 5 values [3 blue & 2 yellow], which are then inserted into Prouty's 'Combined Momentum & Blade Element Theory' for helicopter hover. The blade used for calculations is the symmetrical NACA 0012. There is no twist, or taper and the root cutout is 5% of span. At no lift;
At hover;
Dave
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#12
03-06-2012, 07:50 AM
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Quote:
Thanks for the link. So far, I've been unable to open it, although I'm behind a pretty restrictive firewall. Later today I will get a chance to try another network. Any thing else it might be? Does your spreadsheet contain what Dave call's "Prouty's 'Combined Momentum & Blade Element Theory' for helicopter hover". That appears to have some of the numbers I'm looking for. Doug
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#13
03-06-2012, 08:34 AM
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Quote:
Has anyone ever measured this drag by measuring several points of time and RRPM as the rotor spins down? Doug
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#14
03-06-2012, 08:51 AM
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See if you can open this link.
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#15
03-06-2012, 12:38 PM
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Quote:
I do not know the input data or methods that his spreadsheet is built on, other then the calculations behind the boxes. Perhaps those who have built prerotors have some data. Perhaps AID ~ Airfoil Investigation Database and NVFoil Version 1.01, plus some assumptions will do the job. Dave
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