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Old 09-10-2016, 05:59 PM   #61 (permalink)
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Originally Posted by RGorham View Post
Bill,

Thank you for putting so much time and effort into designing this revolutionary airfoil for the hobby industry.

Three questions;

•What is the maximum allowable head speed that these blades are designed to withstand?

•Are you using lead at the outer tip of the rotor blade and if so are you using a safety wire that connects to the root?

Thanks,

Rg
Hey there Robert! The "max head speed" question is for sure the #1 issue that comes up about the blades. I've done a pretty thorough job of addressing that up-thread. Its important to reiterate again: VTX Designs does not build these blades and we have not been involved in their structural design. For those reasons, we cannot make claims or publish data defining rpm limits. Please review comments up-thread for more details...
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Old 09-10-2016, 07:43 PM   #62 (permalink)
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10/4

Thank you.
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Old 09-15-2016, 02:06 PM   #63 (permalink)
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Default Max AOA degredation in compressible flow

A member recently PM'd me with a question about airfoil Max_AOA capability as a function of Mach number. I thought it best to compose an answer to this question in-thread rather than PM so y'all could benefit:

The history of all this really began back in the 30's and by the 40's Prandtl-Glauert and Karman-Tsien developed non-linear compressibility corrections that are still used today. Even back in the pre-war years, aerodynamics as a science was advancing rapidly and these were the days when the really solid ground work for modern fluid flow mathematics was being developed. Aerospace science in the United States, in the 1920's was clearly falling behind other world players - namely Europe - and so the US Congress commissioned the National Advisory Committee for Aeronautics in the early 30's if I remember correctly. NACA then went to work and developed a series of airfoils that represented rational thickness and camber geometries - but none of these were ever developed for a specific application (did you know that?) So the NACA00XX symmetric sections that were used prolifically on heli rotor blades was basically only a lab rat for its day... but since reliable data was obtained and made public, it got used and re-used across the industry. To my amazement: those old NACA sections are now nearly 90 years old) our RC heli industry has and does make significant use of them. But I digress..

"Compressible flow" is a term that aerodynamicists use when flow speeds become high enough to begin to physically compress the flow ahead of a body in a non linear way. An airfoil when operating at fast enough speed compresses the air ahead of it enough to begin to non-linearly alter the flow field, thus changing the other variables in the flow. Dynamic pressure - or the pressure created by flow speed - varies as the square of velocity (Q=1/2*rho*V^2) so as I said earlier, these compressible flow effects are non-linear: they're related to the square of the velocity. Remembering that ideal gas law drives linear pressure-volume relationships, but compressibility adds a non-linear term into the physics of high speed aerodynamics.

"Mach number" is the factor we use to measure flow speed when analyzing compressibility because mach is defined as the flow speed expressed as a fraction of the speed of sound. You should understand that the speed of sound in dry air is only a function of temperature.

And this is about the time you say "but I thought the air was always being compressed ahead of the airfoil - so what makes compressible flow any different from incompressible flow?" Well yes you're right but there are additional effects we must consider when the flow is compressed at elevated mach number.

The problem lies in the physics of ideal gasses and how sound (id: pressure) propagates through them. That's right: pressure propagates through a gas medium at a certain speed and in air we know that as the speed of sound. At standard sea level conditions that speed is about 1100 feet/second or 770 mph. When a body moves rapidly through a compressible medium, pressure waves in front of the body begin to get "scrunched up" ahead of it. Think of traffic on a freeway where you have drivers coming up from behind tailgating the cars ahead, trying to get them to go faster (but of course they don't).

This phenomenon becomes more exacerbated as the body moves faster and faster - and we must measure the body's speed as a fraction of the speed of sound. So mach number is what we call a "compressible number" because its useful in measuring flow speeds where the physical speed is a relatively high fraction of the speed of pressure propagation.

When we get moving really fast - enough speed for the compressed flow field ahead of the body to affect aerodynamics (most aero engineers will want to account for mach effects over about .3M or so) the compressed flow ahead of the body will affect how the air flows over the body. When moving over an airfoil, the air is accelerated due to the curvature of the airfoil's surfaces (Bernoulli's equation) and so its important that you understand the flow speed over the surfaces of your rotor blades is actually moving quite a bit faster than the flow is ahead of the blade. This effect is increased as angle of attack is increased, because with increasing AOA the stagnation point moves lower, underneath the leading edge and flow moving across the upper surface must therefore travel a longer distance which in turn requires faster flow speeds to make the journey and reach the trailing edge. Faster flow speeds require lower flow pressures again this goes back to ideal gas law. With flow speed ahead of the blade going fast enough, we can actually get flow over the airfoil to accelerate to near mach 1.0 and if this happens we're getting into what aerodynamicists call "super critical flow" or just "critical flow" and the related mach where that happens is called the "critical mach" number. We can make this happen at VERY fast rotor speeds on our RC helicopters when doing a rotor overspeed out of a huge dive with low collective on the way down. The blades will never be able to accelerate to tip speeds over M1.0 though, due to the very steep transonic drag rise associated with speeds over about M.80 or so on our relatively fat cross section blades.

And now to the original question (finally!) - why does max AOA decrease with increasing mach? Answer for that comes in two major parts:

1) One of the flow phenomenon associated with compressibility causes the air flow just ahead of the leading edge to increase its angle of attack. The air streams will begin to get bent upward more steeply just as they approach the leading edge as compressibility goes up. Said another way, the upwash field is made steeper in compressible flow conditions. This in turn causes an increase in angle of attack - so you should understand that with compressible flow there is a related change in angle of attack on the airfoil.
2) Due to the airfoil's upper surface curvature, flow across the upper surface is accelerated to speeds well above the free stream velocity (velocity ahead of and unaffected by the airfoil). This flow acceleration across the upper surface is a function of AOA: more angle of attack forces more acceleration. But we can only do this to a point: as the stagnation point rolls underneath the leading edge, and as we force lower and lower pressures over the upper surface, the flow will experience a more and more dramatic change from high - to - low pressure as it hits and transitions across the low pressure peak. So you should understand that, for a given AOA, compressibility causes a larger pressure differential to develop across the airfoil which presents its own difficulties for flow in the boundary layer. The back side of this pressure peak becomes VERY steep, causing a very high adverse pressure gradient on the airfoil. When the adverse pressure gradient becomes too high and steep, this will eventually cause flow separation as the boundary layer shears in a reverse direction (the flow momentum is overwhelmed by the adverse pressure physics).

On thin airfoils - or rotor blades with thinned down tips - this will initially result in an area of recirculation developing over the forward part of the airfoil. This is mach induced flow separation. The airfoil is still developing lift, but drag numbers go up disproportionately with the lift increase. Increasing AOA beyond the point of mach induced flow separation, the "drag bubble" as we call it becomes larger and then itself will detach and get sucked into the downstream flow field which becomes a huge drag pocket. This causes huge lift losses and huge drag increases. For that reason, VTX does not use thin airfoil sections where they will experience predominately 2D flow. They're too susceptible to flow instability and aerodynamic stall.

These are the two primary reasons why compressibility affects maximum AOA. The effects cannot be avoided - no way. As an aero engineer I chose to design blades that operate best OUT of the compressible envelope, which for practical purposes is above about M.45 at 93% blade span - or about 2000 rpm on the VTX717 and 2050 on the VTX697. Thats where we can maintain a high fraction of our theoretical incompressible flow efficiency: lift to drag ratios.

A couple references I'd recommend:

"Aerodynamics, Aeronautics and Flight Mechanics" by Barnes McCormick - chapter 5. The second edition has additional chapters on rotary wing aerodymamics.

"Rotary Wing Aerodynamics" by WZ Stepniewsky and CN Keys - section 6.1

Both of these volumes are mathematically intensive, but also contain graphic content anybody can look at and understand what's being shown. Bring your crayons...

Cheers,
Bill.

Last edited by VTX Designs; 09-15-2016 at 02:58 PM.. Reason: multiple edits for improved readability
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Old 09-15-2016, 11:26 PM   #64 (permalink)
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Wow!

Thanks Bill,

That was quite a read. The deeper complexities of aerodynamics escape the vast majority of us, myself included. It really illustrates that there is far more going on behind the curtain than we ever thought. My perspective having a simple mind has always been to try and find simple answers, that if not completely accurate at least give a functional understanding of the problem of flight. This allows us to quickly come up to speed as well as teach others with the goal of creating safe pilots. Still, there are realms where common sense and simplified answers fail us and can even lead the unaware pilot down dark alleys. At these times we have to give thanks that those with the prerequisite knowledge are at work giving us the tools we need to get the job done.

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Old 09-16-2016, 08:21 PM   #65 (permalink)
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Thanks Bill , i always wondered about blade design, funny you should talk about naca foils , my dad worked at naca back in the 40's and 50's , he helped build their wind tunnel , had a wood bladed prop that weighed something like 40 tons , he was given a wind tunnel section of the x-15 and other stuff they worked on , my brother is also in the aero field , has a phd in composites ...
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Old 09-16-2016, 09:49 PM   #66 (permalink)
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Can you give us more about tail blades? Will they look similar in design? Projected date when they will be ready?
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Old 09-16-2016, 10:32 PM   #67 (permalink)
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Great read, thanks Bill
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Old 09-18-2016, 03:38 AM   #68 (permalink)
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Great read thanks Bill.

Just got my first set of VTX 717 blades and as a qualified full scale test pilot, the aerodynamics makes absolute sense to me. Will try these on my TDR2 and Banshee to get real world results and report back on my experience.
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Old 09-18-2016, 03:45 AM   #69 (permalink)
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Quote:
Originally Posted by Vinger View Post
Great read thanks Bill.

Just got my first set of VTX 717 blades and as a qualified full scale test pilot, the aerodynamics makes absolute sense to me. Will try these on my TDR2 and Banshee to get real world results and report back on my experience.
Pete,

I would like to see a vid of your TDR2 with the 717's
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Old 09-18-2016, 09:56 PM   #70 (permalink)
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Originally Posted by dominator View Post
Can you give us more about tail blades? Will they look similar in design? Projected date when they will be ready?
Just got back from the Abilene Chopper Madness event, and - WOW was it a major success!! Parking over flowed, 4x flight line stations manned 100% and LOTS of awesome flying! It was a very motivational experience for me... thank y'all for putting on such a nice show and especially to Louis Malave for being behind the effort with everything a man could possibly put into it.

Tail Blades: This is Priority #1 as we speak. I have worked up a first line candidate airfoil for our tail blades but don't have planform nailed down as of yet. VTX is 100% in support of "what ever it takes" (to quote Bryan) and I know y'all are anxious but also hope you appreciate the enormous complexity of properly designing, testing and evaluating what will no doubt be several dozen test articles before a production-quality product is in hand.

As for looks: you might find it interesting, if not even a bit strange, that when designing main blades I don't know what they'll look like until Ryan models them up for me. Its all aero data-driven. The tail will be similar - and to be quite honest I cannot exactly predict how the planform will shape up considering the complexities of the inflow field on the tail system with all the spinning hardware, the boom/fin interference and horrendously low reynolds numbers coupled with high mach numbers we're running. I do have what you might call "engineers design intuition" to try and guide design/test loop decisions but until we're all thoroughly satisfied with what we have it will continue to get developed by the engineering / pilot team.

Bill.
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Old 09-18-2016, 10:09 PM   #71 (permalink)
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These blades are by far the best I have ever used
I bought two sets of the 697 at IRCHA finally had a chance to fly them this weekend at the Abilene Fun fly with Bill by my side
All I can say is WOW
Tried the 717s as well
Every maneuver stayed locked in solid
Tic Tocs are completely effortless and the heli tracks like its on rails
Amazing design and an amazing designer
Thank you Bill. See ya soon
Thomas
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Old 09-18-2016, 10:18 PM   #72 (permalink)
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These blades are by far the best I have ever used
I bought two sets of the 697 at IRCHA finally had a chance to fly them this weekend at the Abilene Fun fly with Bill by my side
All I can say is WOW
Tried the 717s as well
Every maneuver stayed locked in solid
Tic Tocs are completely effortless and the heli tracks like its on rails
Amazing design and an amazing designer
Thank you Bill. See ya soon
Thomas
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Old 09-19-2016, 04:04 AM   #73 (permalink)
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Test flown the 717 on my TDR2 and as everyone else WOW!

But getting back from the wow factor let me put it in a more comparative experience. Used to fly Radix V2 710 and for speed the XBlades 713S. Compared to the Radix I experience quicker collective, truer cyclic and the best of all less amp draw at similar rpm's. The FFF was on rails although never pushed it past the 200kmh mark, those I leave for the dedicated speed blades. Overall I think these are the best all-round blades that I have flown, from Rail to Zeals and would honestly recommend them without hesitation.

Will do some more flights time permitting and upadate as and when possible. Will also compare logged data when I'm home to get more hard facts.


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Old 09-19-2016, 11:45 AM   #74 (permalink)
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Thanks for the report Vinger. Do you think they may be as good as the XBlades for speed flying? Or did you get any hints of any limitations that you would not want to push the VTX that far since they were not specifically designed for speed. On the face of it, the VTX look like they would be good for speed.
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Old 09-19-2016, 12:00 PM   #75 (permalink)
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Originally Posted by RJ View Post
Thanks for the report Vinger. Do you think they may be as good as the XBlades for speed flying? Or did you get any hints of any limitations that you would not want to push the VTX that far since they were not specifically designed for speed. On the face of it, the VTX look like they would be good for speed.
I've talked with the engineer about this very topic. The X713S has a linearly narrowing NACA profile. Overall the X713S blades are noticeably thinner and should be better for speed especially in that 150mph + range.
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Old 09-19-2016, 01:32 PM   #76 (permalink)
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I had no bad tendencies at 2100rpm and doing about 200kmh via GPS timing, but then Radix V2 was the same, I use the Xblades for when I really want to go fast. As I have said a really GREAT allround blade. The VTX actually excell in low rpm, 1200rpm that I tried, as the collective and cyclic just do not get laggy.
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Old 09-19-2016, 03:38 PM   #77 (permalink)
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Thanks for your reply Vinger. Looks like a good, if not better, replacement for the Radix V2 that is a little harder to get now.

Quote:
Originally Posted by mkovalcson View Post
I've talked with the engineer about this very topic. The X713S has a linearly narrowing NACA profile. Overall the X713S blades are noticeably thinner and should be better for speed especially in that 150mph + range.
Makes me wonder what Bill could come up with if he wanted to. (Slightly off topic, but Mark, do you think the VTX 717 is too much blade for the TDR with a Pyro 700? I noticed you got the VTX 697 for your TDR).
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Old 09-19-2016, 03:55 PM   #78 (permalink)
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Originally Posted by VTX Designs View Post
Tail Blades: This is Priority #1 as we speak. I have worked up a first line candidate airfoil for our tail blades but don't have planform nailed down as of yet. VTX is 100% in support of "what ever it takes" (to quote Bryan) and I know y'all are anxious but also hope you appreciate the enormous complexity of properly designing, testing and evaluating what will no doubt be several dozen test articles before a production-quality product is in hand.
Bill, I would like to ask you what is the maximum usable pitch for tail blades, given 3D flying with lots of FFF, rolls, pirouettes and sideways movement. There are a lot of opinions on this subject in the forums but I would like to hear it from a professional aerodynamicist such as yourself.
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Old 09-19-2016, 07:21 PM   #79 (permalink)
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I've talked with the engineer about this very topic. The X713S has a linearly narrowing NACA profile. Overall the X713S blades are noticeably thinner and should be better for speed especially in that 150mph + range.
Mark has this right. Our thick blade sections will drag-up much more at high tip speeds than a thinner section will. However, there's no harm in trying. Although I've not done a tight analysis on speed-specific flight parameters, I'd bet my lunch that the inboard 50% of the blade is running at negative AOA. Somebody should bolt some VTX blades on and see how they do anyway... it would be an interesting test at least to me anyway. Just sneak up on the fast speeds in increments.
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Old 09-19-2016, 07:33 PM   #80 (permalink)
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I haven't really tried this but I think I will. My goblin 700C with 717s is locked in in almost all maneuvers. However during high speed 2150 rpm speed passes they seem to be slightly less locked in at full collective than my e7 with 716 rails at the same headspeed and same FBL unit. I cannot tell which is faster with my eyes but my g700c keeps me on my toes and the e7 is almost hands off.

This weekend I will flop blades on my e7 and G700c and see the results. I'll be sure to post my impressions here.

I will say this my g700c with 717s now autos as good if not better than my e7. E7 is 11lbs g700c is 12.5 lbs. I know the 717s are more efficient all around.

With that said I cannot wait for my damn 700 logo to ship with 697s so I can play with those.
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