"static thrust" raises it's ugly head again - but in relation to helis..

[Andy W]

Over here..
http://www.rcgroups.com/forums/showthread.php?t=423749

I'm very confused why a larger, high performance heli needs more power PER POUND to hover than any other.

As stated in my most recent post there, the somewhat useless* static thrust calcs and measurements many modelers seem to want to gather for their aircraft (*3d models excluded!) are particularly appropriate to a heli in a hover. Slight efficiency variations aside, how can it take 200-300% more power to hover a high performance heli?

Very confused..
..a

[Tom Harper]

I think you have two different situations.

Often when static thrust is measured on a conventional craft the prop is stalled. Thrust is a brute force function of RPM. Changing the pitch of the prop will have little or no impact on static thrust.

The rotor of a hovering helicopter is in it's normal operating range. It is not stalled. As you point out, observed performance is a factor. A lot more power gives you better performance. It is not required for equal ability.

For equal configurations wieght increases as the cube. The required power increases as the 3.5 power. The .5 is a little kicker that I don't understand. Other than that, the relationship between power and weight is essentially linear. It is as you would expect.

[markdrela]

Over here..
http://www.rcgroups.com/forums/showthread.php?t=423749

I'm very confused why a larger, high performance heli needs more power PER POUND to hover than any other.


For static thrust, the Power/Thrust ratio (i.e. Power/Weight for a heli) depends only on the disk loading. Assuming consistent units, the equation is

P/T = sqrt[ 0.5 (T/A)/rho ] / eta

where

T/A = thrust/disk_area = disk loading
rho = air density
eta = hover efficiency < 1

"eta" accounts for nonideal blade loading and blade profile drag. Typically it will be in the range 0.6 - 0.75. Having junk in the propwash (e.g. large heli fuselage) will further decrease eta.

[jrb]

Probably sounds funny; but Andy I’m always personally offended when I read one of your postings like above with respect to static thrust measurement.

Took a few years, but worked very hard to earn both a BS and MA in AE with my primary concentration on Propulsion.

I spent another 20 years working in Propulsion Test being around when 1000s of full scale engines were tested statically; and you know what thrust was the 1st parameter we always looked at!

I know way back when you learned that not all of what you see statically applies well in flight (Cox 5x3x3 on S400/Zagi) but for the folks I worked with and me, as referenced above, static test are very important, even measuring thrust.

Also, I’m glad Astro Bob spent many hours doing static tests and even measured thrust years ago; without his efforts E-Flight would probably not be where it is today.

Static thrust and other measurements are as important today as they were in the past for both full scale and E- flight.


Jim

[vintage1]

Its almost impossible to measure anything BUT thrust on a jet engine...although efflux velocity is a necessary thing to find out power, mostlty weith efflux velocity well in excess of aircraft speed, the only thing that will affect the performance is the thrust, and due to high efflux velocity and probably ram charging, its not going to decrease with speed.

Its not the same on a geared prop engine. Those will get serious loss of thrust as they appraoch pitch speed, and thats why the figures are less meaningful.

Its arguable that all that IS relevant on a helicopter IS thrust. If the heavier models need more watts per pound its because their rotors are not big enough. Bit like a high wing loading plane...

[Bill Mixon]

Another thing to look at it seems to me is that helicopters must use straight blades (no twist). So the tips are really working good while the blade further in isn't doing a whole lot. Is that what you mean by nonideal blade loading, Mark?
If you made heli blades with a section and twist optimised just for lifting at a fixed setting you would be able to use much less power.

[Andy W]

That doesn't make much sense either - the inned portion of a flat blade is simply not going to be working as hard as the tip section, which requires less power to begin with. I can't believe blade efficiency on a 60-size heli is 1/3 of that on my humble, fixed-pitch corona. That's what I'm being told, however..
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[markdrela]

Heli blades are almost flat because of the forward-flight requirement. As a result, the inner parts of the blade carry much less load than is ideal for the most efficient hovering.

The attached PDF shows a 4-blade rotor which is designed to do nothing but hover. Note the very large blade chords towards the root. The second PDF shows only one blade viewed end-on, which shows the hefty pitch angle increase towards the root.

This rotor requires roughly 10-15% less power to hover than a conventional heli rotor of the same size. But in fast forward flight the wide blade roots would cause mainly drag.

[Bill Mixon]

That doesn't make much sense either - the inned portion of a flat blade is simply not going to be working as hard as the tip section, which requires less power to begin with. I can't believe blade efficiency on a 60-size heli is 1/3 of that on my humble, fixed-pitch corona. That's what I'm being told, however..
..a
The inner portion also isn't doing very much because the speed is so much slower, yet the pitch in degrees is the same as well as the chord length and airfoil section.
Comparing the corona to the typical 60 size heli isn't a good way to look at it IMHO. The corona has blades with twist and a longer chord towards the inside much like as shown in Mark's attachement.
The typical 60 size collective pitch heli has blades with (1) a constant chord, (2) fully symmetrical blades, and (3) no twist. All of which are bad for creating lift, but good for forward flight, and aerobatics (good performance in +and -).

Having a high lift airfoil and that twist in the blades makes a huge difference. A perfect example is a variable pitch prop on a 3D fixed wing model. On a model that I was flying with a 10X3.8 apc SF prop, the thrust and vertical performance was pretty good on only 90 watts of power.
When I switched over to the variable pitch prop that has symmetrical blades with no twist it would barely climb out good at 90 watts. I ended up pushing around 170 watts through it to get 1:1 thrust to weight out of it.