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Old 09-04-2016, 09:47 PM   #1
VTX Designs
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Default VTX Rotorblades - Aero Engineering

Hi folks, my name is Bill Wightman - aero engineer for VTX Designs. This is my first post in your forum and let me say right off I'm impressed! What a tremendous group of fliers who represent an incredible cross section of experience and flying skills from all over the world.

I've been humbled by your response to the new blades, and have had the pleasure to even meet a few of you at IRCHA this year. My engineering background has been 100% in the full scale world - over 30 years in aerospace engineering to date. Its been quite a brisk journey getting spooled up on helicopter flight dynamics, and sharpening my tools to design the VTX blades you're flying now.

Model helicopter blades run in an aerodynamic coffin corner, and are tasked to operate at high angle of attack while sustaining high surface loading. Its been a tough design challenge, and a very long road with many hundreds of hours of engineering, flight test, production issues, and re-work.

I decided to poke my head in here because I wanted to let you know that VTX wants to support their product, and has given me *some* latitude to converse about the blades, from a customer support point of view.

So I'm here as time permits to talk about OUR blades, answer your questions, and give you an opportunity to share your ideas, constructive critique and feedback (esp feedback!) on the new VTX blades.

Thanks for your most warm reception on the market. Its been fun so far and there's much more to do!

- Bill
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Old 09-05-2016, 01:30 AM   #2
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Hey Bill, great to read you around here! Welcome among the Freaks
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Old 09-05-2016, 08:33 AM   #3
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Always a good thing to have comunication between design/manufacturer and end user. Good job.

I have yet to run these but looking forward to see a 600 size.
Also if I go ahead with my Mikado Hughes 500 it will be swinging VTX 697. Interested to see how they'll do for lower rpm (1600) on a 5-5.5Kg machine.
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Old 09-05-2016, 05:56 PM   #4
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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 .....
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Old 09-05-2016, 07:30 PM   #5
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Is there any chance you could tell us the airfoils used? Maybe show us some polars that we could compare to NACA foils?
It would be awesome to see why you chose to design them the way they are

I have a set on the way to give a shot. Cant wait to try them

Last edited by dominator; 09-05-2016 at 08:35 PM..
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Old 09-05-2016, 07:52 PM   #6
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Quote:
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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Old 09-05-2016, 08:07 PM   #7
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Quote:
Originally Posted by dominator View Post
Is there any chance you could tell us the airfoils used? Maybe show us some polars that we could compare to NACA foils?
It would be awesome to see why you chose to design them the way they are
Sorry, I cannot do that. The airfoils are all proprietary. I am restricted by NDA from releasing specific airfoil data. I can tell you we make a NACA 0014 look like its chained to the ground.
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Old 09-05-2016, 08:30 PM   #8
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Bill is an awesome man to talk to and an incredibly polite and unassuming individual. If you talk to him for 5 minutes, you will know why the blades are as good as they are He lives and breathes these blades!
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Old 09-05-2016, 08:40 PM   #9
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Quote:
Originally Posted by dominator View Post
Is there any chance you could tell us the airfoils used? Maybe show us some polars that we could compare to NACA foils?
It would be awesome to see why you chose to design them the way they are
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!

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!

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!

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Old 09-05-2016, 08:52 PM   #10
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Wow thank you for the detailed response! I cant wait to fly them.
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Old 09-05-2016, 08:57 PM   #11
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What is the maximum recommended collective pitch for VTX blades?

Did you look at 3 blade rotor heads and very low headspeeds like 1100rpm?

I saw you said a max of 2200rpm for the 700 size blades but what headspeed would you recommend for maximum efficiency?
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Old 09-05-2016, 09:51 PM   #12
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Quote:
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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Old 09-05-2016, 10:36 PM   #13
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I'm actually more interested in 3 blade heads and VTX for "drone" type usage scenarios. As I understand it, 3 blade heads have less vibration (or maybe just higher frequency?) and the higher solidity ratio is good for heavy lift but I have always heard that the 2 blade head is more efficient.
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Old 09-05-2016, 10:43 PM   #14
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Interesting discussion! Glad to see someone talk about why faster head speed doesn't always = better since there is always a performance window, which is what you sometimes hear around here.

I can't believe it took this long for blade designers to think the way VTX and a small select few have about planform shape! Not saying that I have, because I definitely have not lol, but it is one of those things that you go "duh! why haven't we been doing that!" The full scale blades have gone in that direction, and of course fixed wings use planform shape, twist, fences, and all sorts of fun stuff! Twist.... hmmm that would be an interesting one to study.... probably wouldn't do a 3d bird much good, but for speed blades, you could look at more asymmetric airfoil and planform geometry! (of course if it could be built cheap enough!)
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Old 09-06-2016, 04:34 AM   #15
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@VTX Designs, thank you for engaging in this manner with the community. It is so nice to see such great input from one of the manufacturers. Please keep this up as it adds so much value and is only going to enhance not just sales but loyalty. You will of course in time come across the keyboard warriors who will be less than courteous to what you say so please ignore those people, we generally have a great bunch of folk here on Helifreak and I know they will hugely appreciate your input. Keep going and many thanks (ps I am getting my Logo 700 tomorrow with the 697 VTXs- can't wait!)
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Old 09-07-2016, 07:27 AM   #16
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I just bit. Ordered 717s.

After reading the OP's explanation, I couldn't resist. Some serious thought went into these. Gotta see for myself.

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Old 09-07-2016, 08:26 AM   #17
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Bill,

It was nice to meet you at IRCHA as well sir! (I was the 2nd guy to do the blade trial with my g700c). I put 717's on it and I love them! I got my friend a set as well and he loves them! I should shoot a floating auto video with them on my G700c - its amazing how a decently heavy 12.6 lb machine can float down even at 6000ft ASL where I live!.

They also track amazing in high speed passes - very well settled machine.

I love your logo also - the hat looks awesome.

Saludos!

-Cory
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Old 09-07-2016, 10:40 AM   #18
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Quote:
Originally Posted by UCPharmD View Post
I just bit. Ordered 717s.

After reading the OP's explanation, I couldn't resist. Some serious thought went into these. Gotta see for myself.

Thats a true endorsement - thank you! However, now its time to fly those babies. Let us know how it goes.

rb30cory: Ah yes I think I do remember... you were the first one I spoke to with a Goblin. Later that afternoon, another guy came by who had them on a TDR. That surprised me; it was the first dedicated speed machine AFAIK with VTX on it. Both of you guys are saying what I hear 100% of other pilots say about FFF tracking - its tight. You machine is heavy, but about the same as the old Logo700. No problem! Thanks for your PIREP also - these pilot feedback reports are incredibly important to us. Cheers!

edit: I was just thinking about how high your flying elevation is and looked up Saltillo on a map - looks like it can also be hot. So I ran a quick evaluation on the 717 blade to see how it performs under those conditions. Due to the density altitude, you're losing about 17% of the max rotor lift at 6000ft / 95dF (real hot day). At that temp, you can run a little more head speed and get away with it due to mach being a function of air temperature. So the upper end of what I'll call the "efficiency envelope" for the 717 at that temp will be 20 rpm faster, or 2020 rpm. That will get you some of that lost performance back.
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Old 09-07-2016, 11:40 AM   #19
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I was the 1st guy to demo them at IRCHA on my Protos Max(717mm), and I put a set on my TDR(697mm) later.

Sorry Cory, couldn't resist!

I came over and shook your hand and thanked you for creating them and am waiting patiently for the 557's and tail blades to be released.

I don't have my TDR setup for speed and doubt that it has ever been over 120mph, but it really likes the 697mm VTX blades. It's 10lbs, 9oz and those blades feel perfectly matched for it.

I just setup my TDR2 specifically for speed ( 12lbs 13.4oz) and am currently running X713S blades on it.

Do you think that the VTX blades would actually work better in speed runs than the X713S speed blades at 150mph+ speeds?

Related to that question can the 717's withstand 2700 rpm head speeds?

I wouldn't want to run them past their design limits. I saw what happens when you do that on the flight line at IRCHA when an overspeed blew a set of SAB blades apart.
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Old 09-07-2016, 11:45 AM   #20
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I test flew some 717 at IRCHA on my Max. Definitely some awesome blades compared to the Switch 713s. I'll be getting some 697 for my Max (I prefer a slightly higher disc loading for hard 3D) and, most likely, some 697 for my 700N as well eventually.
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