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Old 09-05-2016, 09:51 PM   #12 (permalink)
VTX Designs
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Originally Posted by usaf_aggie View Post
Yes! I would love to see this as a fellow engineer. Something that always plagues model aircraft when scaling down designs is low Reynolds numbers. However, for a 700 sized blade at 2000 rpm, I calculate a tip speed above incompressible flow speeds, and a Re above the critical Re of 500,000, so I would guess the flow is predominantly turbulent, and pretty atypical for model aircraft wings!

EDIT: I had the Reply window open for along time and didn't see that you had already posted in reply to Dennis. You covered a lot of the same ground I was asking about. Sorry!

How does that affect model aircraft rotor design? I am sure you don't want to give away any trade secrets, but it must be a challenge to get any sort of efficiency back at these aerodynamic regimes!
Hey there - you clearly know a thing or two about aerodynamics! Exactly right, we're running totally turbulent flow on all the VTX blades. Good word for our low Re: "plague". I agree! In fact, early in the design study I was looking at active boundary layer control using micro-vortex laid into the blade surface. But it wasn't necessary...

Efficiency comes in two flavors here: the sectional efficiency that is the L/D of the airfoils themselves isn't great compared to what can be had with cambered sections, but I simply cannot target L/D ratios if we also want to fly hard smack maneuvers. So instead I design for superb planform efficiency with the sculptured blade and exotic tip shape.

How does that affect model aircraft design? Hmmm... I guess I'd say it forces an astute engineer to target the more important stuff and not let the second and third-order factors run the show. It also requires lots of communication with pilots; the better ones can elaborate on technical issues with good specific/descriptive language. Jesse Kavros and Ryan Witchey are among the best I know. They both did extensive test flying for us. Without them, this entire effort would have floundered.

Prototype 3a: Howdy! No we didn't consider 3 blade heads due to production issues (this was really first and foremost a business decision from VTX Designs) and I can tell you as an engineer a 3 blade head isn't superior to 2 blades in some important areas. From my view, the 3 blade head can achieve better cyclic physical ratios; better cyclic speed, and the blades are shorter for a given weight, so that reduces tip speed which is good, but does not appear to me to be superior in terms of straight collective. Its also not as efficient overall aerodynamically and must expend more energy to achieve similar results. I AM impressed with the cyclic speed, though.

edit: I forgot to answer your other question. Head speed: all of the VTX blades are designed around atmospheric baseline parameters of 1000' MSL and 75dF. At those conditions, the 697 can run with decent efficiency up to 2050 rpm. That's really pushing it - and that's the speed Mirko turns (he runs the 697 on the new Logo700, turns 2050 and says it can maneuver too fast for the eye to follow in some cases). The 717 needs to run a little slower, due to its length. 2000 rpm will put the 717 exactly in the same aero envelope as 2050 will do for the 697. You CAN turn them faster, but as I explained earlier this will result in mostly making more noise, draining batteries faster and will only yield inconsequential gains in "smack-ablity".

edit #2: I gave a 2200rpm number earlier in the thread as a practical top-end limit. I was sort of just thinking in general across the entire blade spectrum: 477 to 717. So that was really an approximation. As our blades become available, I will post more specific info as to the design speed envelope. Each blade is designed on paper with a target head speed (center of envelope) but its best if we wait until after real-world testing is done to publish speed targets. Theory is great, but nothing trumps reality.

Last edited by VTX Designs; 09-06-2016 at 08:20 AM..
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