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Old 09-05-2016, 07:52 PM   #6 (permalink)
VTX Designs
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Originally Posted by Dennis Everett View Post
How do you go about designing heli blades ? Does it require the use of a wind tunnel ? All theory? Trial and error?
I haven't tried yours yet , but plan to with all the great reviews they have received .....
Dennis - thats a really big question! Blade design is a very complex undertaking with a great many variables. I can hit on some of the highlights though...

I use a combination of tools to analyze various elements of blade performance: A combination of CFD and my own hand written aero analysis methods are first used to work up planform shape. I just rough it in to begin with - targeting flying weight, disc diameter, expected blade loading, and propulsive capability. Blade planform is really important in the overall design process. Drag on the blade tips (the outboard 10% of span) *can* account for well over 50% of the total main shaft torsional load, in hard maneuvers and high speed. Right off the bat, I knew tip drag had to be tightly controlled so as to yield efficient, predictable performance for the pilot. Tip drag is largely related to what we call "induced drag" which is very much a function of planform shape.

On VTX blades, area is distributed where its most efficient. The big fat part of the blade - I call that the "paddle area" is where we really make things happen. The tip shape strongly influences planform efficiency inboard on the blade, so thats where a skilled aerodynamicist can massage planform metrics to optimize area where it counts most. There are lots of dynamic moving parts and they're nearly all connected together in ways that are mostly counter intuitive.

The planform solution also lets me pretty tightly predict blade weight and mass distribution. The gyroscopic moment is important to know because it directly affects cyclic acceleration and max cyclic rate. Right away, you can figure out there's a critical relationship between blade area - which is used to create aero force - and blade weight. More area means more lift, but also more mass. This tradeoff must be put in balance and match the propulsive system's capabilities.

For example, on the VTX717 the blade is power-train matched to the helicopter. The 717 blade - if pushed really hard - will extract about as much power / gear train stress / ESC amps the heli can handle. Its a close match as far as I can tell - now that we have some pretty good pilots (!) pushing it as hard as it can go. So, blade area / blade mass / propulsive capability all must be sort of mapped out and I work toward a solution set there. Did you see the video where Johnny31297 and one other guy got their ESC's to shut down by extracting too much energy from the blades? The blades didn't give up - the helicopter's ESC did. That was pretty telling wasn't it?

Once I get the disc dynamics more well defined, I go about designing mission-specific airfoils for each major part of the blade. All VTX blades are designed with heli-specific airfoils that work with the planform, head speed and blade loading to yield maximum performance. I do airfoil analysis using fully compressible, viscous CFD - its really the only way to go these days. Wind tunnels are WAY out of the budget here! (side note: even the small Lab tunnel at NASA Ames is on the order of $5000 a day )

The outer 1/3 of the blade is operating at very high speed which pushes us well into the compressible flow regime. Its nothing to push tip speeds past 0.4 Mach and even up to 0.6M if running really fast. There are some very very difficult issues that come up with compressible flow at low Reynolds number (due to the short chord lengths) - as mach number goes up maximum lift coefficient goes down (quite a lot) and boundary layer instability can be a problem. Pushing into higher mach number also reduces maximum AOA at which flow will remain attached.

So, pushing head speed faster is good up to a point. As compressibility issues ramp up on the blade (and its not a linear ramp up its exponential), aerodynamic performance begins to degrade. Maximum lift coefficient, along with maximum AOA capability fall off... in a non linear fashion. The VTX blades are designed to operate well across an unusually wide range of head speeds. But there's a point they will noticeably begin to fall off in performance, will suck batteries down quicker, and load up the drive train pretty impressively. Tip drag left unchecked gets HUGE.

One important point to make here: A designer can make choices as to what airfoil and planform they use to operate in different speed ranges. I chose with all the VTX blades to stay out of the higher-end compressible flow speeds, and by that I'm talking about roughly 0.45 mach as measured at 93% blade span (from the bolt). By doing so, the airfoils I worked up are pretty fat, and they have soft noses. They work VERY well in their design speed envelope and are quite happy at speeds well below also. Due to the relatively thick sections, I do not recommend running them too fast (2200 rpm if you want a number for starters). You will be able to tell where that is, with experience. Or do the math The specific speeds that are best (for a given physical rotor rpm) will vary with air temp... hotter days you can turn the blades a bit faster. And of course your power train should also be optimized to match peak rotor efficiency.

Taking note of all the blade popping I witnessed before launching this design effort, it was clear what was happening. Blade popping is a result of flow separation and thats a sign of a blade that's being pushed beyond its max AOA capability. The result is sloppy maneuvering, possible pitch instability, lots of current draw and a loss of feel in sticks. I'm sure there are other factors that experienced pilots could point out, but from my perspective those seem to be prevalent when blades stall.

You should know the VTX airfoils are all designed to operate at the highest possible AOA, while developing lots of lift. The only way to do this is by designing with boundary layer stability issues as a first-order parameter. Each and every airfoil designed for VTX blades undergoes hundreds of CFD runs, simulating every possible combination of flow speed, angle of attack and reynolds number. Airfoil design isn't a closed-loop solution. I have to make educated moves to work from one candidate to the next, run the whole analysis and evaluate if its the best I can do. Our first production blade - the 697 - was the result of about 2 years of work... probably a net 1000+ CFD tests, about a dozen major planform revisions, prototype blade testing by company pilots ... back to the drawing board: more CFD runs, a manufacturing process change, more planform revisions (6 I think) and finally the product you have now.

Last but definitely not least - the blade tip shape designed into all VTX blades is something special. I can't divulge how it works in detail, but I can say its a very thin section out there that works at almost any mach number you can throw at it... and we're developing a vortex on the tip that's used for boundary layer control on adjacent inboard blade sections. Hence the name "VTX" short for "vortex". Proof is in the CFD and now in the flying. Its also a technique used on some well known aircraft like the F16 - the leading edge strakes used to throw a vortex across the wing roots and maintain attached flow up to around 35 degrees or so.

There will be more to come from VTX - I have the 607 analysis here on my desk along with first candidate tail blades as we speak.
Stay tuned...

Last edited by VTX Designs; 09-06-2016 at 08:03 AM..
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