VTX Rotorblades - Aero Engineering

[MikadoUSA]

After speaking to the VTX guys the next blade will be 617, 517 & 377. They have to order a minimum of 1000 sets per size and pay 50% upfront so unlikely theyll offer numerous sizes in close ranges with the way the heli market is currently. Last we heard they were still designing and creating tail blades but theyre not quite finished with the data/prototypes required to start production.

[prototype3a]

What happened to the rumored 557s?

[VTX Designs]

Originally Posted by Mentalic

(snip) ...
Made 8 flights back to back with them, very happy with these blades!

Wayne, thank you so very much for your superb pilot report on our new 477 blades. You've made my day! Great job there - maybe you might be able to do some very low speed testing also.. and let us know how that goes. Very well composed report there - Hoorah!

Smittylube - the VTX477's do have a brass insert in the blade grip - and I suppose it could be pressed out to accommodate a larger bolt diameter. Unfortunately, I'm not any sort of RC helicopter expert, but I do see a brass insert there. Mikado pilots should answer this one...

Gladius - All VTX blades are designed around a baseline temp of 75dF, so that's where I quote "standard" operating parameters. The design envelope of all our blades is limited by mach number on the top end and by blade loading / dynamic pressure on the lower end. Therefore, the upper end of the rpm envelope will increase with increasing temperatures (as mentioned in post #63 of this thread). In rough terms, you can turn about 100 rpm faster on very hot days, and by that I'm talking upper 90's or more. On very cold days, you will need to decrease you upper rpm limit by about the same amount. Very cold means 40dF or so. Speaking about the lower end of the operating envelope - that's purely a function of how heavy the machine is and how much dynamic pressure you're offering the blade by the rpm you run. You will have to experiment a bit to see what the lowest speed you can turn is to meet your flying demands. I can say that VTX blades will substantially outperform anything else on the market in the low speed regime. So, in summary: sea level ops will help you on the low speed end of things (thicker air = more dynamic pressure) but will not affect upper end limits since those are temp dependent only.

Prototype3a - the 557s are still in flight test. We are awaiting more prototype samples for test by our pilots here in the US. Its a slow process. More work needs to be done - and for now all we can say is the 557 is still a work in progress.

Tail Blades - guys we know y'all are chomping at the bit for tail blades. The aero engineering work is finished for our first prototype. Tail design is extremely tricky. We're working on getting a first sample made for bench and flight test. The project is under pretty strict ND so we cannot divulge any details at this time.

Bill.

[corywrx]

Originally Posted by MikadoUSA

After speaking to the VTX guys the next blade will be 617, 517 & 377. They have to order a minimum of 1000 sets per size and pay 50% upfront so unlikely theyll offer numerous sizes in close ranges with the way the heli market is currently. Last we heard they were still designing and creating tail blades but theyre not quite finished with the data/prototypes required to start production.

Heck yeah 377's! I will for sure pick up a pair or two in that size

[Laughingstill]

Originally Posted by VTX Designs

Wayne, thank you so very much for your superb pilot report on our new 477 blades. You've made my day! Great job there - maybe you might be able to do some very low speed testing also.. and let us know how that goes. Very well composed report there - Hoorah!

Smittylube - the VTX477's do have a brass insert in the blade grip - and I suppose it could be pressed out to accommodate a larger bolt diameter. Unfortunately, I'm not any sort of RC helicopter expert, but I do see a brass insert there. Mikado pilots should answer this one...

Gladius - All VTX blades are designed around a baseline temp of 75dF, so that's where I quote "standard" operating parameters. The design envelope of all our blades is limited by mach number on the top end and by blade loading / dynamic pressure on the lower end. Therefore, the upper end of the rpm envelope will increase with increasing temperatures (as mentioned in post #63 of this thread). In rough terms, you can turn about 100 rpm faster on very hot days, and by that I'm talking upper 90's or more. On very cold days, you will need to decrease you upper rpm limit by about the same amount. Very cold means 40dF or so. Speaking about the lower end of the operating envelope - that's purely a function of how heavy the machine is and how much dynamic pressure you're offering the blade by the rpm you run. You will have to experiment a bit to see what the lowest speed you can turn is to meet your flying demands. I can say that VTX blades will substantially outperform anything else on the market in the low speed regime. So, in summary: sea level ops will help you on the low speed end of things (thicker air = more dynamic pressure) but will not affect upper end limits since those are temp dependent only.

Prototype3a - the 557s are still in flight test. We are awaiting more prototype samples for test by our pilots here in the US. Its a slow process. More work needs to be done - and for now all we can say is the 557 is still a work in progress.

Tail Blades - guys we know y'all are chomping at the bit for tail blades. The aero engineering work is finished for our first prototype. Tail design is extremely tricky. We're working on getting a first sample made for bench and flight test. The project is under pretty strict ND so we cannot divulge any details at this time.

Bill.

Yes, the 477s do have the brass insert. All of the VTX blades have them..

[KevinB]

Many good quality blades have a permanent brass insert that may be bonded in place. Some have a brass spacer/sleeve as well that slides into the brass insert to take the hole down a bolt size......ie: 5mm to 4mm.

I might try a LITTLE pressure on the insert in a small arbor press, but if it didn't move right away, I wouldn't push any harder.
If you do press it out without damaging the blade root, depending on the insert wall thickness, you may or may not be left with the right sized hole anyway for a 4mm bolt???

Unless VTX gives a definite YES that the spacer in the 477 blade is able to be pushed out and leaves you with a 4mm bolt hole, I would just put the original blade grips w/the 3mm bolt back on.

Just my opinion......


KevinB

[smittylube]

Ahhh that's perfect - but............... 517's in the works. For us stretched 480's that could be gold!

It's such a great time to be into this hobby.

[Mentalic]

Originally Posted by smittylube

Do the 477's have a brass insert you can remove to fly with 4mm bolts?

I also stretched mine and have the 500 grips.

There is a brass bushing but its not removable or at least I'm not gonna try.. Had to switch grips back to stock to fly the 477's. Pic has the 500's I was flying at the top, VTX477's at the bottom.

[smittylube]

You could Remove the brass piece, add some spacers ( I assume the root is wider on the 500's) I did it to a set of blades after I stuck on the 500 grips.
Did you happen to weigh them to compare?

I tried the 697's and compared to rails they are only a few grams different with the vtx cg about 10mm in, but the 717's are 25 grams heavier than the 716 rails. That sort of difference will help auto for sure. Hope to test those today.

[Johnny31297]

+1, it's completely fine to remove the brass insert and put the bolt through the bare composite hole. Keep in mind, those come from Fun-Key.

[OliP]

377? Yes, sir!

[mverhorst]

517's will def go on my X5!

[MaxRumpf]

Originally Posted by MikadoUSA

After speaking to the VTX guys the next blade will be 617, 517 & 377. They have to order a minimum of 1000 sets per size and pay 50% upfront so unlikely theyll offer numerous sizes in close ranges with the way the heli market is currently. Last we heard they were still designing and creating tail blades but theyre not quite finished with the data/prototypes required to start production.

So the 617 are the priority now? If so, whats the ETA? I'd really really loooveee to have them :D

[HammerFest93]

Any chance that Logo 480 XXtreme kits will ship with VTX 477's in the near future?

If so, I will wait until these are in stock and be quite happy!

[Vom Feinsten]

Can we expect the tailblades this year? I love the 717 on my TDR II, but would love some tailblades in the same design.........


Cheers Janis

[VTX Designs]

Max - VTX607 main blades will be the next product in-line to follow the 557's. And remember, we're still in flight test with the 557's. Not really a "priority" exactly, but that's just the next blade VTX is planning on releasing after the 557. At this time, the 607 only exists as a large collection of CFD models and spreadsheets here on my MacBook Pro
Hammerfest93 - We did receive a first batch of production 477 blades which I think all sold out nearly instantly. I haven't been privy to how those were actually distributed... but probably soon you will see Logo480's shipping with the new VTX blades.
Vom - tail blades this year? Maybe... The first prototype is currently being surface modeled in CAD. The surface model is then used for CNC molding of the blade. Prototypes will be hand made and test flown. If we like it (really REALLY like it) VTX will then authorize production. We will keep you posted. Looking good so far ... there will be some new challenges associated with making the VTX tail blades due to the radical new shape (sorry its a secret for now )

- Bill.

[VTX Designs]

Here in the second installment discussing RBS, we’ll take a closer look at the actual disc dynamics a 700 class heli would encounter in FFF. In this article, we’re using Fast Forward Flight (FFF) as the example maneuver. We’ll examine in detail the problems the advancing blade faces in high speed operation. Flow dynamics for the retreating blade will also be examined and problems associated with retreating blade disc dynamics will be explained in detail.

We all understand the basic fact that going really fast in any direction with your RC heli introduces large differences in blade speed across the rotor disc: advancing side to retreating side. In FFF, the FBL must compensate for the lateral (left-to-right) lift imbalance by tipping the swash plate to increase AOA on the retreating side, while reducing AOA on the advancing side. There are aerodynamic limits associated with BOTH of these: mach induced speed limits on the advancing side and critical AOA limits on the retreating side.

We’ve previously discussed aerodynamic drag as a combination of pressure drag and skin friction drag. Pressure drag is due to the pressure buildup around a body in motion. If we integrate these pressure forces parallel to the flight path vector, we can calculate pressure drag directly. Skin friction drag happens because air is a viscous fluid and has frictional interaction with the surfaces its in contact with.

At higher speeds there’s another important type of drag that comes into the picture called “drag due to compressibility”, or just “compressibility drag” which I will abbreviate “CD”. At higher speeds, CD happens because the body is compressing the flow ahead of itself. This drag builds up rapidly at a certain point that’s dependent on free stream mach number, and the minimum coefficient of pressure around the body. CD can completely overwhelm the aerodynamics of a fast moving aircraft. It took 6000 pounds of rocket thrust to propel the Bell X-1 to supersonic speed; an aircraft roughly the size of a Cessna 150 having a fuselage designed around the aerodynamics of a .50 cal rifle bullet (but not area ruled - oops!) Here’s a plot showing how compressible flow drag builds up on a NACA0012 airfoil (image from Stanford.EDU):



Note the “drag divergence” point on this curve: its at about mach .65. Past that, drag increases tremendously. Also note this curve is plotted for a fairly low lift coefficient: CL is only 0.3. As CL increases - which happens with increasing AOA on the rotor blade - the drag divergence point will decrease substantially. At very high CL’s we will see drag divergence happen at free stream mach numbers as low as 0.5. So the thing I want to point out here is the presence of mach induced drag rise, and how our RC helicopters are easily capable of blade speeds that get well into this area of operation.

So now we have 3 types of drag to consider: Pressure + Skin Friction + Compressibility. In high speed flight and high head speeds, the CD factor becomes a major issue for the advancing rotor blade. (Note: on all VTX blade design/analysis I’ve included compressible flow corrections so our blades are accurately engineered for their design speed envelope and are properly power train matched to the helicopter)

Another very important thing to understand is that when considering the drag of a body moving rapidly in compressible flow, fatter / thicker / more blunt shapes build CD drag more easily and to greater levels than do smaller / thinner / sharper shapes. The plot shown above is for a NACA0012 which is a 12% thick airfoil. On VTX rotor blades, we employ thicker and more blunt nosed airfoils in the working area of the blade. What this means is that if you push any rotor blade fast enough to where the thick part of the blade experiences high mach, thick airfoils will respond with larger drag increases and correspondingly higher mechanical loads on the entire blade control system. To put this in perspective, our 717 and 697mm rotor blades will hit the knee in the drag rise curve at about .60 mach at 93% blade span. (measuring blade span from main shaft to rotor tip. The blade bolt is at ~13% span on the Logo700) That's why I suggest running our 697/717 blades no faster than .45M@93% - that will give you a little head room below where CD builds up rapidly. This corresponds to 2000 rpm for the VTX717 and 2050 for the VTX697 under our standard analysis conditions. There IS more headroom above these speeds but there will be diminishing returns in terms of performance -vs- efficiency.

So, in real world numbers how fast is this? On a 700 size helicopter, the advancing blade in FFF can easily be pressed into a very undesirable high mach situation. Consider: At 2000 rpm, in a hover our 717mm blade is moving 515 ft/sec (M.45) at 93% blade span, which is definitely doing lots of work compressing flow ahead of the blade. This is getting close where the CD curve begins to get steep for the VTX7147 airfoil. Up to this point, our airfoil retains about 90% of its aerodynamic lift and AOA capability.

Now add 150 mph (220 f/s) of FFF to the advancing blade’s rotational velocity - still at 2000 rpm - and we’re looking at 735 f/s and M.65 at 93% blade span. The absolute blade tip is running 774 f/s and M.68. With very good rotor head and control system mechanics, this isn’t necessarily dangerous territory but the airfoil on the advancing blade is definitely past the drag divergence mach number so any further speed increases and/or AOA increases will result in HUGE drag increases! Crank the head speed up to 2400 - coupled with 150mph FFF - and 93% blade speed increases to 838 f/s and M.74. Folks, this is big time into wave drag territory (wave drag is due to the presence of shock waves on the airfoil). This high mach operation will also tremendously reduce the critical AOA and max lift capability for the advancing blade.

Folks, you gotta understand this: higher head speeds will produce more lift, more “pop” and more authority - UNTIL you push the advancing blade well into high mach / compressible flow territory. At these high flow speeds, compressibility losses gang up and WILL erase those gains and replace them with a 1000 pound gorilla on your rotor head. Running a 15% thick airfoil at high AOA while approaching M.80 is ASKING for trouble. This combination of mach and AOA will press the upper surface flow speed into transonic territory which cannot do anything good for an RC helicopter. This is all due to compressibility on our fat blade sections - and is a problem for the advancing blade not the retreating blade.

Aerodynamic drag loads on the rotor head & control system also translate directly into mechanical stress. The problem with really high mach number on the advancing blade is this extra drag loads all the head mechanics up tremendously - thus driving system aeroelasticity higher (think: grips / linkages / feathering shaft / dampers / main shaft / airframe / swash / servos). This increased mechanical stress increases blade flap and FBL phasing issues due to excessive blade lag angles. We will dive into those effects next installment

Now for the retreating blade: do we have a RBS problem? Using the numbers from the previous example we have our helicopter doing 2000 rpm in a 150 mph speed pass. The advancing blade is running 735f/s M.65 @93% span. The retreating blade will be doing 295 f/s and M.26. No problem! But what about reverse flow? How far inboard on the blade do we have to go to see reverse flow? Under these conditions, we get reverse flow @39% span on our 717mm blade. The blade won’t be stalled anywhere outboard of 39% because we’re not exceeding the critical AOA anywhere along the blade span, as long as the FBL isn’t asking for too much AOA on the retreating side. The reverse flow we do have is happening at such slow speeds and low AOA’s that it really doesn’t give those far-inboard blade sections enough aero authority to cause trouble. So, yes, the flow inboard of where we get reverse flow WILL be stalled, but its not going to add up to enough aero force in total to make a difference to the rest of the disc that’s operating comfortably inside the envelope. So - to answer this issue directly: YES we do have RBS but its happening so far inboard and at such slow speed and low AOA that is has insufficient capability to cause control issues.

What about the outboard part of the retreating blade? Good question! Our rotor blades inherently run tip-loaded - that is to say that when we add up the vector sums defining disc rotational velocity plus the helicopters flight path velocity, we find that our blades have AOA distributed highest at the tip and decreasing as we move inboard. So if the FBL cranks up AOA up so high on the retreating side of the disc in an attempt to balance lateral lift loads (which corresponds to pitch control loop inputs) that we exceed critical AOA for the airfoil used on the outer blade span, then yes we’ll have stalled retreating blade tips. The resulting lift loss will initially cause the helicopter to pitch up (assuming CW disc rotation): and possibly get a ballooning flight path to even a violent pitch up. If the helicopter does not respond, the FBL might just add even more pitch to the disc making the situation even worse! However, going back to Hemp’s speed pass post, we see that the VTX blades produced an uncontrolled descent - not a pitch up:

Originally Posted by hemp

(snip) ...
Without any other changes, the headspeed was bumped to 2500 ... the model began a steady descent in spite of full back elevator being fed in to counter it. ... By dropping headspeed he was able to regain control ... (snip)

The reason the VTX717 equipped heli doing 2500 rpm speed pass pitched DOWN is because this combination of head speed plus FFF drove the ADVANCING blade into a compressible flow coffin corner. The advancing (left) side of the disc was overwhelmed by compressibility drag and probably shock wave induced flow separation which produced an uncontrolled descent. The only way out was to REDUCE HEAD SPEED, which the pilot did and recovered the helicopter. So in this specific case, what happened was related to problems on the advancing blade, and had nothing to do with RBS.

Finally - its important to point out that VTX blades are designed with extremely high AOA limits. VTX blade aerodynamic AOA capability typically exceeds the heli’s mechanical pitch capability - so with VTX blades you’re going to be mechanically pitch limited not aerodynamically limited as with other designs. What that means is that in FFF our blades are very unlikely to hit their AOA limit on the retreating side of the disc. But on the advancing side, our thick blunt nose airfoils are more likely to run into compressibility issues, when compared to other designs with thinner sections across the blade span. 2400+ rpm on the VTX717 coupled with FFF is not safe - please stay away from these high head speeds with our blades.

The airfoils used on VTX blades, coupled with the wide chord planform illuminates our design ideology. Aerodynamically, our design philosophy does make certain tradeoffs, but I think they’re very smart ones. I just want you to really understand this in detail, so that y’all have the knowledge to operate your VTX equipped helicopters to achieve the very best performance, while keeping things safely inside the operating envelope.

Bill.

[Raptorapture]

Thanks Bill. Great post and very informative. I'm glad I came across this. Going to have to try some VTX blades!

[Yfbb]

I'm interested in trying a pair of VTX blades. I was wondering, What is the AOA for the maximum Cl at 2450 rpm?

[VTX Designs]

Yfbb - 2450 rpm will produce .556 mach and 793000 Re @93% blade span (VTX717 blade) in a hover. The airfoil's alpha_max will be reduced to only 7.9 degrees under these conditions. Adding any significant horizontal flight speed to this will push blade mach past the compressibility divergence point resulting in sharply degraded performance, and risking a crash as noted in post 277 above. Not good.

Its very important to note here that physical blade pitch (as you would measure with a pitch gauge) is NOT equal to the blade's AOA - ever ! Blade AOA is dependent on physical pitch angle plus blade rotational velocity plus flight path vector plus inflow velocity. Except for physical blade pitch, these parameters vary considerably across the blade span and all are dynamic: they change with the head speed and maneuver type among other things.

Given the typical situation where the heli is actually moving in the direction its disc thrust is applied, and given that the disc inflow field is always developed in the same direction as the thrust vector, actual blade AOA will almost always be significantly less than physical pitch angle.

So for example: even if you're cranking on 14 degrees collective, the blade at most might only be seeing 7 or 8 degrees of aerodynamic AOA depending on the maneuver involved.