400mph flight - tricky business it turns out

[Algy]

Even DARPA is struggling (http://www.flightglobal.com/blogs/graham-warwick/2007/10/darpas-highspeed-heliplane-hit.html)with this one.

[traumajunkie]

Could just be me, but they seem to have neglected to attach a TR or other anti-torque system to those nice flash models.. should make for some interesting hover performance :eek:

[NickLappos]

trau,

They are gyrodynes, with tip thrusters providing the power. No main rotor torque, so no anti-torque!

That being said, the fastest rotorcraft was the Sikorsky XH-59 ABC, which went 300 mph back in 1973.

[traumajunkie]

Nick,

Cheers, missed the bit about tip jets the first time through! :O

[rotortorque]

What about transmission drag? When tip driven wouldn't there still be a bit of drag in the same direction of the main rotor?

[scooter boy]

Isn't retreating blade stall the major factor limiting maximum forward speed with any rotary wing aircraft?

SB

[ScrumpyLuvver]

This may be a daft question but I am still trying to get my head around rotor aerodynamics.

As long as the retreating blade is travelling faster than your fwd velocity + wind speed/direction etc, retreating blade stall is negated? :confused:

Be kind if I am completely off the mark.. the physics learning curve is proving to be the toughest :}

SL

[Graviman]

Scrumpy, in this design the wings begin to unload the rotor in cruise. Most likely the hingeless rotor still provides pitch and roll control, but the wings will be supporting maybe 60% of the weight. Otherwise you are right that the retreating blade sets the speed limit.

For pusher prop gyros the rotor is optimised (twist and taper) for autorotation, which normally means that the retreating lift is more evenly distributed. These often fly above mu (forward flight to rotor tip speed) of 0.5.

The latest approach is to have counterrotating rotors, so the retreating sides both unload. In S69 this was done by either each rotor having a differential roll to load up the advancing side, or by reducing the pitch lead angle (revectoring the forward cyclic position to cause differential roll). X2 is the one to watch now...

[Dave_Jackson]

The problem for rotorcraft:

How to derive an improved lift-for-power from the region of reverse airflow that is greater than the resulting deterioration in the lift-for-power from the region of conventional airflow; for the projected cruise - hover ratio.

Specifically, how to achieve; Optimal airfoil profile for both directions of airflow. Optimized angle of attack at all azimuths and radii.

[waspy77]

Scrumpyluvver


As long as the retreating blade is travelling faster than your fwd velocity + wind speed/direction etc, retreating blade stall is negated?


A simplistic view is that the advancing blade generates more and more lift the faster you go. In order for the disc to remain stable in roll, the retreating blade also has to produce the same lift, but in condiditions of relatively lower airspeed. This means a higher angle of incidence, and ultimately classic stall.

As Graviman says, a design like the X2 has advancing blades on both sides and so this is no longer the limit. However the individual discs do see this, and so the hubs need to be particularly stiff. It is certainly the project to watch.

[Graviman]

Dave, the solution is simple: you unload the retreating side to present 0 deg AOA to relative airflow. So retreating side pitch caters for accent or descent.

Waspy, to some extent you can use centrifugal acceleration. The problem with this is that there are efficiency reasons to slow down Nr as cruise speed goes up (compressibility and optimum AOA). S69 had an incredible 12% effective hinge offset, so X2 probably exceeds Comanche 15%. This means blade root can be stressed to handle more fixed wing like bending moments.

All in all i can see why a mechanical system would quickly become just to complex for this aircraft...

[Dave_Jackson]

Mart,Dave, the solution is simple: you unload the retreating side to present 0 deg AOA to relative airflow. So retreating side pitch caters for accent or descent. Unfortunately, it ain't that easy.

There will only be one (or two, depending on the azimuth) points on the span of today's retreating blade that will experience 0-deg AOA. In addition, a blade with a sharp leading edge (reverse airflow on a conventional airfoil) will stall at a small AOA.

Dave

[Graviman]

Dave, this is where your Independant Root and Tip Control comes in. ;)

Blade twist will be a compromise to cover every flight condition. I imagine it is low to allow for high speed with reduced Nr (whether 2 speed or variable), with the compromise being hover figure of merit. Most likely the retreating tip will be angled to present 0 deg AOA relative airflow, since the root can be more easilly strengthened against reverse flow divergence. At speed the root will likely be stalled and produce a downwards lift, but this is the compromise that comes with such a high speed capability. Still if X2 successfully commercialises this approach (and i hope it does) then the development will be ongoing to overcome the compromises.

Actually the biggest problem is likely to be the eigenmodes associated with the high effective hinge offset required for reduced Nr. I did a simplistic calculation somewhere on this forum to try to estimate how blade natural frequency combines with rotational stiffening. Reading Prouty and Newman i realised that calc was not that accurate, and you really need FE to get the frequencies correct (i thought about doing a simple model in Ideas to capture Comanche blade dynamics). The practical upshot is that with high hinge offset first bending may be getting close to 4P (which may be why Comanche opted for a 5 bladed rotor system). X2 may have gone back to something like a 12% hinge offset to avoid this problem. An alternative would be the active tip servo technology, which (if failsafe/reliable enough) ensures the blades never resonate in the first place.

Eitherway it's one project i want to read up on...

[waspy77]

Graviman,
Simon Newman published a number of papers concerning blade sailing. If you haven't already they're worth a read. I attended a lecture he gave on this subject and it was interesting to see how the natural modes of the blades can be come dominant during rotor startup and stopping.

[Ptkay]

It looks like the Groen Bros try to steal the show from Jay Carter.... :(

http://www.cartercopters.com/

They claimed to have broken the mu barrier some time ago.

[Dave_Jackson]

Mart,

One concern that I have with the aerodynamics of the X2 craft is its use of the coaxial configuration.

Consider the side of the craft where the upper blades are advancing and the lower blades are retreating, during cruise. The use of ABC means that the upper (advancing) blades are providing most of the thrust. The lower (retreating) blades are meeting the airflow with a sharp leading edge.

We know that a sharp leading edge can only operate within a small range of AOA. We also know that there is an amount of time required for the sectors of an airfoil to recover from a stall. In addition, we know that the lower blades on this side of the craft are passing in and out of the thrust of the upper blades 8 times per rotor revolution.

IMHO, it is reasonable to assume that segments of the lower retreating blades will passing in and out of stall, at rates of up to 8 times per RRPM.

This is one reason for considering the Side-by-side, the Interleaving and the Intermeshing configurations.


Dave

[Graviman]

Waspy, Simon Newman demonstrates his considerable prowess as a mathematician in his book. Even he admits that, despite not having the elegance of a Bessel function, Finite Element is the most versatile method of understanding how practical design blade eigenmodes undergo rotational stiffening. Still if you happen to have a copy. ;)

Ptkay, autogyros go above mu of 0.5 all the time. Mu is defined as forwards speed divided by rotor tip speed, so it defines the diameter of the retreating blade reverse flow circle (technical stuff, but google and Dave's web site will help). All mu >0.5 means is that the reverse flow region covers >50% of the retreating blade. X2 has at least the potential to approach mu =1, depending on the control techniques used to avoid blade divergence.

Dave, keeping the retreating blade in clean air is the main reason i like your intermeshing ABC concept - you should pursue it. My objection to the interleaver has always been the increase in width for a similar performance machine, and the additional complexity in the powertrain - this means this design will struggle to be cost competitive with a comparible coaxial.

[Dave_Jackson]

Mart,

Some like the Interleaving and Intermeshing for high speed flight .....
..... but they've got to dump those wings. ;)

Kamov Ka-34 (http://avia.russian.ee/helicopters_eng/ka-34.php)
Kamov Ka-35 (http://avia.russian.ee/helicopters_eng/ka-35.php)
Kamov V-100 (http://avia.russian.ee/helicopters_eng/kamov_v-100.php)

Bölkow V/STOL Rotorcraft (http://www.aiaa.org/tc/vstol/unbuilt/bo/index_r.html)