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extrapilot
05-21-2012, 11:23 PM
Well, the subforum has been quiet, so I figured Id post some data that might be of interest.

This particular data export shows the measured relationship between tail rotor angle of incidence (purple), motor current (red), and resulting tail rotor thrust (green). The units are degrees, amps, and grams, respectively. This is for a stock 450 turning 2800RRPM.

The vertical grey line in each window defines the rotor stall onset (21deg AOI, or approx 18deg AOA due to established inflow). What is shown in the chart is what is well established in aero- that at stall, you can still generate thrust, but the power/thrust ratio increases dramatically. By about 30deg AOI, increasing AOI yields no additional thrust though power demand continues to increase. The thrust trend begins to decrease, and would do so rapidly above 40deg AOI, though power would continue to ramp up.

Cheers guys-

okcalla
05-22-2012, 09:39 PM
so I'm no engineer or anything but i do like physics an figuring out how things work, so forgive me if i come off a lil slow here. I may answer some of my own questions but here goes.

when you say tail rotor angle of incidence does this simply mean pitch or is this somehow different or relative to something else?

I'm assuming the heli was in "3D mode" so all blades were rotating at a constant speed correct? 2800 RRPM.

"rotor stall on set" I'm not sure what to make of this term as it doesn't seem to correlate to 0 pitch as I'm reading the graph unless i have misread it or is it to say the pitch is too steep as may be the whole point of this graph.

AOI and AOA are not acronyms I'm familiar with...

If the blades are spinning at the same speed throughout the graphs time period whats causing the delta in amperage? Increased blade pitch?

as you probably figured out i'm a little bit of a newb... but thanks in advance for humoring me.

SARBoy
05-23-2012, 04:52 PM
AOI - Angle of Incidence. I'm assuming this is pitch angle on the blades relative to the heli.

AOA - Angle of Attack. This is the angle of the relative air flow to the blade. The blades creat an induced flow which modifies the relative air flow the blades see.

Induced flow is explained here. http://www.helifreak.com/showthread.php?t=410100&page=2 post # 16

Hope this helps.

desertstalker
05-24-2012, 01:00 AM
when you say tail rotor angle of incidence does this simply mean pitch or is this somehow different or relative to something else?

Pretty much yes, its the angle of the blade relative to the direction of motion.

"rotor stall on set" I'm not sure what to make of this term as it doesn't seem to correlate to 0 pitch as I'm reading the graph unless i have misread it or is it to say the pitch is too steep as may be the whole point of this graph.

Stall, is when an airfoil stops behaving "nicely" drag goes up dramatically and minimal lift/thrust is produced. This starts (the onset) for this example when the power consumption goes up much faster than the thrust (which is fairly flat here. The full stall occurs when the thrust drops of drastically (note there is still high power consumption here) but that is not really shown here.

If the blades are spinning at the same speed throughout the graphs time period whats causing the delta in amperage? Increased blade pitch?

Increased pitch causes increased drag, so to maintain RPM you need more power, hence the rise in current (please dont call it amperage, the unit may be Amps but the quantity is current :thumbup:)

(think thats mostly correct, if basic, my fluid mechanics is a bit rusty)

kimmik
05-31-2012, 12:12 AM
Very interesting and useful experiment thank you. One point i want to raise is the effect of using cf vs plastic blades.

Which type did you use?

I'm thinking a stiffer blade will not feed forward when placed at high AOI. A soft blade when placed at say 20deg, might undergo torsion deformation and the tip is actually say 25deg AOI. This makes the blade less efficient since the blade tip is increasingly stalling while middle segment is not.

My prediction is a CF blade will take a higher AOI before stall and have better thrust to power ratio.

Also, because the AOA is dependent on both AOI and air flow, setting a tail at 35-40deg isnt a bad idea.

If say youre doing a super fast funnel, you're going to need the extra tail pitch to maintain authority. Would be good if someone could work out the AOI vs AOA at various speed given say 250mm rotor diameter and 8000rpm.

Kim

extrapilot
05-31-2012, 09:19 PM
Kim

Im glad you find this interesting- there is not a lot of published measured data for things like tail rotor efficiency, thrust vs AOI, etc.

The construction material implies nothing about blade profile, chordwise CG, or other important factors (i.e. damping) that may matter. These happened to be Align 450Pro standard blades…

Sure, it is possible to have a blade with bizarre behavior along the span- but realistically, the foils have some inherent properties which prevent the divergence you suggest. For example, CRF acts on the blade simply as a result of its mass distribution- and attempts to twist the blade into the plane of rotation. For blades with a chordwise CG ahead of the CP, you have aerodynamic stability. In a case where you have some divergence, the flap itself causes the blade’s local (sectional) AOA to drop.

You could use a high speed strobe to validate for a given blade/AOI/RPM.

In your example, a 250mm rotor at 8000RPM at 45deg AOI has nearly all the outer half of the blade stalled at an axial inflow of 60mph. It would need to be operating in an axial inflow of approx 90mph to have its tips unstalled, and that assumes established inflow from the thrust produced at this AOA/RPM.

Defining a graph for this doesn’t make a lot of sense. For one, the AOI is expected to be nearly constant along the span, so the AOA changes just as a function of radius. So where do you measure AOA? And, the inflow velocity changes in a way that is not linear with radius (tip loss). But the bigger challenge is that the inflow vector is basically impossible to define in a meaningful/representative way. Even in hover, you have recirculation from the main rotor, tip vortex interaction, etc. In something like a high-speed funnel, you have asymmetry depending on the direction; the anti-torque thrust is always the same direction, but the inflow can reverse. So yea, just really difficult to extend data like this to a general rule.

One can hope the system managing the TR is not stupid enough to throw 45deg AOI at every problem, such that you have this pitch available for the very unusual conditions, but are not seeing it in stuff like piro hover entry/stop. Unfortunately, I have logged systems with reputable FBLs and tail servos that do see tail stall in some basic maneuvers. So, this is something to consider as a tuner hoping to maximize efficiency of the system-

Regards

okcalla
05-31-2012, 09:53 PM
thank you for the reply's. desertstalker this may require a PM so as not to hijack this thread but now i'm curious. I always thought amperage was a unit of current draw. Although re-reading what I wrote your point may have been more to the way I phrased the question. I believe I should have said delta in current draw not delta in amperage. correct? :banana

kimmik
06-01-2012, 06:09 AM
Thats a good explanation. Sounds like from your quick calculations, and very roughly extrapolating, that setting around 35-40deg tail pitch is enough for nearly all situations inscluding very fast funnels. This is the default anti-torque tail pitch on both the 600pro and tdr and looks like it was chosen very deliberately (as you'd expect lol).

When i suggested the aoi vs aoa curve, i think a reasonable simplification is to use the maximum non stall pressure point on the blade for calculation.

OverTemp
06-01-2012, 06:25 AM
In your example, a 250mm rotor at 8000RPM at 45deg AOI has nearly all the outer half of the blade stalled at an axial inflow of 60mph. It would need to be operating in an axial inflow of approx 90mph to have its tips unstalled, and that assumes established inflow from the thrust produced at this AOA/RPM.

I'd say this isn't uncommon, especially with today's large powerful electrics!!! I know I punish mine!

Keep in mind, power required is a function of drag, which as you know has 2 components: induced and form. In situations where the tail system is demanding very large pitches, the induced drag stays high but the axial inflow actually decreases form drag. Sweet for us.

I always tell people who try to limit their tail throws (as per the compass manuals)... If you are actually stalling your tails in flight then they are too short or you aren't spinning them fast enough. There should be enough mechanical authority available so that your gyro can never ask the tail for more than its capable of... within some reason.

curmudgeon
09-12-2012, 07:24 AM
...rotor stall onset (21deg AOI, or approx 18deg AOA due to established inflow). What is shown in the chart is what is well established in aero- that at stall, you can still generate thrust, but the power/thrust ratio increases dramatically. By about 30deg AOI, increasing AOI yields no additional thrust though power demand continues to increase.
How does one apply this information in practice? Should I limit the tail blades throw on the tail slider, and not used the maximum mechanical throw? If so, what is the maximum recommended angle of incidence?

OverTemp
09-12-2012, 09:14 AM
Don't limit. If you are stalling your tail, you need bigger blades or higher RPM.

DannyvG
07-29-2013, 06:34 AM
The vertical grey line in each window defines the rotor stall onset (21deg AOI, or approx 18deg AOA due to established inflow). What is shown in the chart is what is well established in aero- that at stall, you can still generate thrust, but the power/thrust ratio increases dramatically. By about 30deg AOI, increasing AOI yields no additional thrust though power demand continues to increase. The thrust trend begins to decrease, and would do so rapidly above 40deg AOI, though power would continue to ramp up.

Did you take the rotational speed of the helicopter itself into account or was this measured with a helicopter at standstill?
Or isnt it relevant?
Because I can imagine (with my limited knowledge on aerodynamics) that the additional inflow of the yawing helicopter will reduce the AOA and thus one shouldnt limit the tail pitch to the "static" stall angle since in practice you also have the extra inflow.

extrapilot
07-29-2013, 03:56 PM
Hi Danny

It’s a good question. The initial post was not about optimal blade AOI limits, it was to show how power is related to stall, and that even post stall, you can generate substantial lift (with a much higher power consumption).

Certainly, if you have a high axial flow velocity component at the TR, this will alter the AOA. Even just in hover, the induced flow changes the AOA, and because that velocity is not symmetrical along the span of the blade, and the blade has no real twist, the AOA differs along the span…

There are at least a couple of scenarios where you may need 45deg TR AOI- very high speed lateral flight (i.e. nose up/down funnels), and piros at high lateral speed (lateral being perpendicular to the main rotor axis).

If you care about tuning your machine, it makes sense to either enable full travel on the TR, and adjust the gyro/FBL as best as possible so as to not get TR stall on piro stops etc, or to limit the travel initially to say 30deg and see if that poses any problem for your style of flight. Often, people will see a tail kick on a punch, and assume they need more travel. Sometimes, it is the opposite; the gyro throws too much travel, the blades stall, you get less thrust, the TR pulls too much power from the system, and the gyro adds yet more pitch to try to fix the situation, etc. So maybe they will be better off with larger TR blades at lower pitch, etc.

Best regards

Steve Graham
02-13-2014, 10:11 PM
So is it the general consensus to trust the heli designers and set as much mechanical pitch as you can get short of binding or risking over center of the pitch change mechanism?

My ALEES for example allows me 45 degrees of CW pitch and probably high 30's CCW. A friend noticed this yesterday and indicated he felt I was at risk for stall and losing tail authority. I'm not sure I buy this. It appears that while the blades become less efficient at extreme angles but don't really go off a cliff and quit making thrust in the way we might understand the term stall as it applies to a fixed airfoil. Still there does seem to be a strong arguement here for limiting tail pitch to maybe 30 degrees max?

I love the theory but it rapidly escapes me when it becomes more complex. What I'd like is something that gives me a functional rule of thumb, flawed from an academic standpoint that they always are.

OverTemp
02-15-2014, 05:52 AM
Remember that your Heli isn't always sitting still. What looks like 45 degrees sitting in the bench can easily become 15 degrees of actual angle of attack (angle between the chord and relative wind) in a falling or FFF piro.