Helicopter design theory vs reality

[StevieTerrier]

How accurately does "theoretical performance" relate to actual performance when designing and building a completely new helicopter model?

Can the designers / engineers / boffins say for example that "given an engine producing x horsepower in an airframe weighing y kilos and a 5-blade rotor system creating z lbs of lift etc etc etc our new SportiCopter will travel at 136.2 kts, carry a load of 600 pounds for 6.1 hours and fly up to 20,000' '"?

And if they do, how near do they get to reality?

[Shawn Coyle]

As far as hover performance goes, they get pretty close. Forward flight stuff can get a little more complicated, due to a whole host of things, but again, they get pretty close.
One of the issues, from my personal viewpoint, is that very little wind tunnel work gets done on helicopters in North America, and this doesn't help.
There are a lot of reasons for this, but in comparison to fixed wing development and aerodynamics, we are way behind.
Ask Nick how close Sikorsky came to their stated design aims with the S-92.

[StevieTerrier]

Alright, Shawn, I will!

Nick - how close did Sikorsky come to their stated design aims for the S-92?

[Dave_Jackson]

Shawn said;One of the issues, from my personal viewpoint, is that very little wind tunnel work gets done on helicopters in North America, and this doesn't help.

WIND TUNNELS OF NASA (http://www.hq.nasa.gov/office/pao/History/SP-440/contents.htm)

Isn't their large one, shown in the picture, about to be, or has just been, shut down ?

[Lu Zuckerman]

They installed a Cheyenne helicopter in the wind tunnel at Ames and the rotor suffered a divergence and wiped out the inside of the tunnel.

The wind tunnel at the Arnold research center in Tullahoma, Tennessee is so large that you can install an F-15 in it and fly it at altitude (the aircraft is mounted on a plinth with three degrees of freedom). The tunnel can only be run at night as it consumes as much electricity as the City of Chicago. The air stream is diverted while the pilot gets into the aircraft and starts the engines and then the airflow is allowed to pass through the tunnel. The drive shaft for the fans must be constantly rotated in order to prevent the shaft from going out of alignment. The hot exhaust from the engines is diverted and weighed and then a corresponding amount of air is allowed into the airstream.

:E :E

[RDRickster]

The Alfred Gessow Rotorcraft Center (http://www.enae.umd.edu/department/research/agrc.html) at the University of Maryland has a wind tunnel and they do quite a bit of rotorcraft research. The Glenn L. Martin Wind Tunnel (http://www.enae.umd.edu/department/industry/glmwt.html) specializes in tandem & single rotors, jet VTOL, tilt wing, etc.

[Rich Lee]

Lu,

One of the blades when through the 40 x 80 wind tunnel wall, a few offices, hit and slid down a file cabinet and then into a trash can. The vehicle was pretty well trashed. What ever happened to the good ole days.....

[NickLappos]

Stevie,
Our luck with general performance (power, speed and rate of climb) is very good. We usually beat the predictions by a bit (we added over 650 pounds to the S-92 because we were a bit better than we predicted). Our speeds are always within about 1 or 2 knots, as well.

The place where the prediction tools are imprecise is in the low speed handling, especially where the horizontal tail is involved. I now of no helicopter that has entered production with the same tail it had on first flight (Rich Lee, help me out here!) The behavior of the main wake and the spillage from the main rotor pylon are a mystery, generally.

We have found that wind tunnels are of limited use, since the normal flight conditions need a powered rotor, which does not scale very well. In a tunnel, the rotor is fully restrained so the model hums along at a fixed thrust and moment condition, while the fuselage is fully supported by the stand. The designer must calculate the needed moments, inducing errors. Airplanes are nicely behaved, by comparison, and provide good data when in a wind tunnel. Were tunnels of more use, they would be used more, Shawn!

[CRAN]

....and it doesn't stop there. The most sophisticated aerodynamics tools available today are known as Computation Fluid Mechanics methods (CFD). These methods seek to solve the fauve-averaged Navier-Stokes equations (or a reduce form of them) in a numerical grid based manner. Such methods are in routine use for fixed wing design and provide huge in site to fixed wing designers.

However, as with the wind tunnel, these methods are of limited use to the rotorcraft engineer, since the calculations become enormously computationally expensive (weeks on a supercomputer), and will still give you the wrong answer as the method cannot conserve vorticity correctly and hence the wake is incorrectly represented. As i'm sure most of us are aware, there are few flight conditions in which the wake is not of huge significance to helicopter aerodynamics.

So, the best experimental techniques are difficult to apply and so are the best computational techniques. What to do?

Well what is actually done is that most of the design work is done using lower order methods [comprehensive codes] and then CFD is used to focus in on particular design features for which more physical insight is required, for example tip flows, engine bay flows etc.

In addition, hybrid methods and very high performance methods are being developed within the community. i.e. Combining the virtues of high order CFD methods and explicit wake modelling ~ lot's of work using this approach being published at the moment, in the US and Europe. Another approach being pioneered in the UK by two universities is the development of CFD algorithms that can conserve vorticity. it's still early days at the moment and some of the most highly respected researchers in the world (the fathers of the subject) are consumed in working on the problem! There is a link here to some work being done using Imperials Vorticity Transport Model, ~ a CFD algorithm that can conserve vorticity and capture wakes...

www.ae.ic.ac.uk/research/rotorcraft/HFD.html#wake

I suggest you navigate around the page a little they have lots of interesting stuff, start with:

www.ae.ic.ac.uk/research/rotorcraft/

Impressive stuff but it has a long way to go!

Cheers
CRAN
:ok:

[Dave_Jackson]

Alternative to wind tunnels and computer code. :O

http://www.UniCopter.com/Temporary/Test_Trailer.jpg

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Edited re: The web page mentioned by CRAN

Interestingly, the wake does not appear to contract. :confused: