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Nov 11, 2013, 04:51 PM
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Quote:
 Originally Posted by mnowell129 There are too many unknowns to reach this conclusion. Depends if you are at the same airspeed. Adding a lb means that to fly at the same trim you will have to increase power to fly faster so the rotor sees a higher AOA due to airflow. To fly at the same speed as before you have to make some more lift somehow, your solution was to trim up and increase the AOA of the rotor, causing it to make more lift (and drag) from AOA. Either solution takes more power. There is no free lunch, more weight takes more lift, thus more power to fly level. You can use that power to fly faster or at a higher angle, your choice.
But does this not suggest a very lightly loaded rotor will operate more efficiently at a lower shaft angle ?
Nov 12, 2013, 10:23 AM
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Quote:
 Originally Posted by Tom Wright But does this not suggest a very lightly loaded rotor will operate more efficiently at a lower shaft angle ?
Not to me. The efficiency of the rotor is a function of its geometry and construction. Changing the weight of the model does change the efficiency of the rotor. It does change the rotor operating point. The rotor's efficiency will vary with operating point but it is not assured that lower loading is necessarily more efficient. Its possible that at a lower loading and lower rpm the reynolds number drops sufficiently that the rotor efficiency is poor, but as long as it is enough to fly the model, who cares? It is also possible at a higher loading the reynolds number changes or the airfoil is operated in a better L/D position and the rotor efficiency goes up, but the aircraft uses more power because its heavier. But it uses less additional power than it would if the rotor were operating at the other less efficient operating point. It's very difficult to talk in absolutes because things have to be ratioed against other variables to have a apples:apples comparison.
Re: shaft angle, this doesn't tell you a whole lot because it seems that it is mostly related to aircraft weight. The design corner that gets reached is that the small rotor must have some minimum angle to continue autorotation and this may be greater than the angle that would be needed just to provide enough lift to fly the aircraft. At that point from a total aircraft point of view you might be better off to go with smaller/higher aspect ratio/fewer blades so that the rotor is more efficient so that the drag produced by the rotor tilted the minimum amount to continue autorotation is the minimum drag for the aircraft.

mickey
 Nov 12, 2013, 01:20 PM Registered User Joined Feb 2012 680 Posts Mickey Thanks for that summary it all make sense. I think what you have identified here is my original intention to wast potential efficiency in the interests of airframe simplicity , hence the over size rotors operating at less than optimum angle. Today I did go some way to testing a more efficient set up eg more mast angle and higher AR blades , the results were encouraging and in line with your predictions, so taking things one step further will also improve the ROG performance. This further step though will require a new design which will be accessed against the original. I think this diversion from the the thread topic has been useful for me and I hope the same can be said for others looking in? Tom.
 Nov 13, 2013, 05:23 AM Brisbane, Australia Joined Apr 2013 145 Posts All good, this is how we learn. Still pondering the effects of changing the "fixed" rotor blade negative angle of incidence compared to the rotor axle, an experiment is underway, I will have to pop a question soon. Last edited by Ian444; Nov 13, 2013 at 05:37 AM.
 Nov 19, 2013, 12:22 AM Brisbane, Australia Joined Apr 2013 145 Posts Still pondering the effects of varying the rotor blade negative angle of incidence. Mick, I'm not making any progress on this, can you explain the effects of varying the blade incidence angle? All I can come up with is this: With a more negative angle of incidence the rotor axle will require more backwards tilt to get the same blade AOA as before the angle of incidence was changed. As a result, the TPP will be tilted back more causing less lift and more drag. With a less negative angle of incidence the rotor may not spin up. So we are stuck between two places, rotor won't spin up at one extreme, and increasing drag/less lift at the other extreme. Is that all there is to it? Are my assumptions even correct? What about rotor rpm, is it affected by blade incidence angle at all?
Nov 19, 2013, 07:41 AM
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Quote:
 Originally Posted by Ian444 Still pondering the effects of varying the rotor blade negative angle of incidence. Mick, I'm not making any progress on this, can you explain the effects of varying the blade incidence angle? All I can come up with is this: With a more negative angle of incidence the rotor axle will require more backwards tilt to get the same blade AOA as before the angle of incidence was changed. As a result, the TPP will be tilted back more causing less lift and more drag. With a less negative angle of incidence the rotor may not spin up. So we are stuck between two places, rotor won't spin up at one extreme, and increasing drag/less lift at the other extreme. Is that all there is to it? Are my assumptions even correct? What about rotor rpm, is it affected by blade incidence angle at all?
As usual nothing is really simple.
Some key points to consider. The lift on a blade is described by this equation:
What this means is the lift is proportional to the blade area; is proportional to the lift coefficient (related to the AOA), but is proportional to the blade tip speed squared.
This squared relationship give some people some heartburn. For example lets suppose that you reduce the blade area by 10%. You'd think the lift would go down by 10%. But just suppose that by reducing the blade area by 10% that the rpm went up by 5% as a result. Because the lift is proportional to the rpm squared the lift goes up as a result of rpm by 1.05 * 1.05 = 1.10 = 110%. So a smaller blade makes the same lift. If the RPM goes up by 1% more, to 6%, then the lift due to rpm goes up 1.06*1.06 = 112%, so you end up with more lift with the smaller blade.
There is another fact that figures in. The lift to drag ratio of an airfoil is not always the same number. It varies with angle of attack and it's not a straight line. So sometimes a small increase in angle of attack causes a large increase in drag. Sometimes a small decrease in angle of attack causes a large decrease in drag.
This has an effect in that the RPM is very much a function of drag.
So suppose you reduce the lift coefficient by 10%, you would expect a loss of lift of 10% based on the equation, but suppose that for that 10% loss in lift you get a 20% reduction in drag. For simplicity's sake assume that a 20% loss
in drag gives you a 5% increase in RPM. Back through the same math as before gives you the same net lift.
So all of this would be time consuming to model mathematically, but it gives you a general trend to consider.
The predominant thing to remember is that there are more lift gains in RPM than in blade area or angle of attack.
Also, operating the blade at an optimum angle of attack to achieve the best L/D is important and this angle is at a much shallower angle of attack than the maximum lift angle.
More blade area is not necessarily better. If the increased drag of the larger blade creates a slower RPM then you lose more than you gain.
So back to your original question.
There is some optimum combination of blade size, blade angle, and RPM that provides the most efficient flight and this might take experimentation. Startup may cause you to compromise the most efficient flight condition, but that is a price to be paid without pre-rotation.
What appears to be a more negative angle might be the more efficient way to produce lift.
The aft tilt of the rotor is going to ultimately depend on the L/D of the blade, thus the aircraft efficiency is also going to depend on the L/D of the blades.
Also keep in mind that the parasite drag of the blades due to surface roughness plays into the overall drag, as well as the form drag due to the thickness of the airfoil. You can't discount these factors as they directly affect the RPM, so airfoil selection is important and blade finish is very important.
Sorry to have stirred up the mud in the pond again, but hopefully this will point you in the right direction.
I've been more successful with smaller higher aspect ratio blades at higher RPM than larger slower turning blades. My G2 is now fitted with blades that are ~7mm narrower chord than the stock blades and it takes off easier and flies slower.
 Nov 19, 2013, 10:26 AM Registered User Joined Feb 2012 680 Posts Mickey Your explanation fits the results I get from flying many blade formats. I recently fitted a model designed for 2" chord blades with a set of 1.5" . The model flew better and was able to lift more load , perhaps a good example of the benefits of RPM . Tom..
Nov 19, 2013, 11:21 AM
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Quote:
 Originally Posted by Tom Wright Mickey Your explanation fits the results I get from flying many blade formats. I recently fitted a model designed for 2" chord blades with a set of 1.5" . The model flew better and was able to lift more load , perhaps a good example of the benefits of RPM . Tom..
I think many of the designers are still intuitively trying to lower the wing loading with big blades. This is not the right approach for the optimum performance. In effect leads you to more and more area and slower and slower blades. The problem with this is that the retreating blades gets slower and slower and is less able to make enough lift thus the rotor flaps back further and further to compensate. Keep in mind that the lift asymmetry due to advancing and retreating blades gets smaller as the rpm goes higher, therefore it takes less flap back to compensate.
It might seem cute and comfy to see a big slow turning rotor, but this isn't the great advantage that it seems.
To add insult to injury the wide blades suffer more from centripetal flattening and thus are more difficult to get to the correct pitch. The tips, where you get the most bang for your RPM buck effectively try to de-pitch themselves.

I'm still of the opinion that torsionally stiff, high aspect ratio blades, cg corrected with a good finish, that run at a higher RPM should be the design goal. If the goal is a beginner model then tip weight should be used to make the model less responsive. Just my 2 cents/pence worth.
Nov 19, 2013, 02:47 PM
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Quote:
 Originally Posted by mnowell129 Keep in mind that the lift asymmetry due to advancing and retreating blades gets smaller as the rpm goes higher, therefore it takes less flap back to compensate. .
And the resultant excessive roll trim that results from slow blades , looks...
 Nov 20, 2013, 05:51 AM Brisbane, Australia Joined Apr 2013 145 Posts Thanks Mick, that is a lot of info, much appreciated, very helpful.
 Jul 17, 2014, 08:59 PM Registered User Joined Mar 2011 775 Posts Does anyone have an idea of how much harder the propellor works as compared to the rotor?