Tilt rotors and the difficulty to get RPM and pitch right

[meatbomber]

Hey folks,

i have followed the VTOL discussions since i saw Larrys thread on his Osprey.

Seems so far one of the biggest problems is that you either use heli blades which are good for hover but have too little pitch for forward flight, or use props which are very in efficient in hover... as well as being hard to get.

Anyone had a closer look at using VPP (variable pitch props) currently coming up in the Shockflyer scene ?

You could use pretty big blades to reduce your disk loading in hover running at medium high RPM. When you tilt to forward flight you increase blade pitch while reducing RPM that puts you in a quite inefficient range for the electric motor but you should need only partial power anyways in forward flight...

Thisw also gives you the advantage (disatvantage?) that you have 2 ways of controlling torque... RPM and pitch.

http://213.23.113.132/xtCommerce_3.0.3/images/product_images/popup_images/105_0.jpg
http://www.mamo-modelltechnik.com/shop/images/artikel/MAMOVP9.jpg

Maybe a thing to consider..

[MustangAce17]

that is something i strongly considered for my project when i first saw the Rimfire vpp and motor for $80 US you have some points there that in the future seem like they could be put to good use

[v22chap]

Yep ,,I have looked hard at those ...even did one yrs ago with tail rotor units.But for anything more than a parkflyer type VTOL it just isn't going to work (just my 2 cents ). But now with the newer brushless motors and lite lypo's ,,,it might just make its way into the lite VTOL world. :cool:

[Tuner]

Basicly my issue is I need a larger Prop size 18-20" and none of the VPP I know of can handle that size.
It is a simple build. My motors have 6mm shaft and I can find hollow shafts for it and I have been considereing creating my own VPP to utilize 20" Blades it is a rather simple design whe a hollow shaft is used.

My other thought is to use 3 servo CPPM type mixing with a hacke up TREX head but that adds weight.

Who makes the second VPP in your post??

[meatbomber]

http://www.mamo-modelltechnik.com/

it's only a 9" prop tho.

IMO parkyflyer VTOLs are the most achieveable (sp?)...

[v22chap]

Scott
Why not go with some of the cheap fixed pitch heli heads ? or even the cheaper lighter cp heads like the blade ,,etc.
Larry

[Tuner]

On the books I want to do this badly.
Actualy as I said earlier I am curious if you remove all the mixing and connect the head directly to the swash plate without any flybar would this be to unstable or sensitive in a multi rotor type VTOL. This would be lighter weight yeild better thrust and a shorter

Got any suggestions as I have the $$ and will be the guinea pig. I have seen other setups like this only they use the whole complicated heli mechanical mixing.

I need variable pitch at the very least so fixed pitch heads are not an option. Using Cyclic controls are an option.

I will gladly order a couple heads shafts and what not tomarrow and overnight them. I need to tackle this issue soon as I either need JUST Variable Pitch or I need Full Cyclic control. If I go just variable pitch I would like to use the Hollow shaft concept as its simplicity and low number of components is irresistable.

I Need a 2 and 3 blade Hub along with a Swash plate for them.
It would be preferable that they have a 6mm main shaft.
If I can get this I will build a direct drive system that swing a 20" blade like Tumbler is using (its the blades for the Dragon Fly XPro)

[v22chap]

Tuner.
This is a hard question .
The airplane prop only has been done and works. Brain has done the non collective with cyclic and it hovered good.
So I am thinking that multi blade head and cyclic are the way to go... I am also thinking that MS composits makes a 3 bladed head that would handle the 20 " dia. rotor disc .... the only problem is the main shaft is only 3 mm.
flybarless is always more sensitive to conrol ,,but not unflyable and now they have mixers that allow you to use gyro's on the cyclic controls to smooth them out.

If you go 3 bladed heads you lose the flybars and still can have collective and cyclic controls ,,,but you will still need the one or two extra servos ,,,depending on what type of collective you do.

The MS heads are only 19.99 apiece for the hub and then 4.99 for a set of two blade holders and 4.99 for the bearings for two.

I will measure my MS heads tomrrow ( they are at work right now :-) ) and see if they can be drilled out to 6 mm
Larry

[BigS]

Why not use a zoom mainshaft? its hollow. Or a Trex mainshaft (5mm).

Shannon.

[v22chap]

Checked the MS 3 bladed head and it is only 7 mm total dia. so drilling it out to 6mm is out.

[avianaut]

My guess is that a collective pitch head with cyclic is the way to go, in terms of controllability. Collective offers far better response than fixed pitch and both rotors can stay at the same speed, and cyclic will avoid gyroscopic effects. With fixed pitch and two rotors, when you increase thrust on one and decrease the other, say for roll control, they run at differing speeds and cause unwanted yaw, and the gyroscopics are no longer balanced, so you'll have undesireable coupling from that too.

Cheers.

[Tuner]

Ok I was banking on that no fly bar would still allow it to fly.
I was also banking on that flybarless 3 blades are less sensitive than flybarless 2 blades....

Hmm all I need is a 3 blade head that holds 20" blades and accepts a 6mm dia shaft... Some how I think I will find a 2 blade head like this before I find a 3 blader.

Maybe I will buy and MS Holders and Head take my calipers and copy the MS head only make it accept the proper shaft?

Google and I will have some fun tonite maybe I will turn something up??

In the end I should just machine my own head because I want the RPM to be higher than normal for forward flight. 4,000 RPM max.
last thing I want is a vasectomy by heli blade.

Thanks everyone I think this needs to be taken head on as you are right Collective + Cyclic = one hell of a VTOL. I think once somone is successfull hear other will follow.

Scott

[Tuner]

Check out this photo I wounder if these are larger than 10" props???
http://www.nongnu.org/paparazzi/images/cam4_bench.jpg

I am slowly getting into this project and I am talking with them I will post any info I get on these props.

Sorry a tad off topic.

[rdresch]

I just got to ask. If we use a normal cyclic CP heli head and limit the pitch in helimode to 9 degrees, but then add pitch as the rotor move to airplane mode and speed increases I think you could use heli blades. On my particular setup I have 3 blade head using ccpm so I can get over 40 degrees of pitch. I limited negative pitch to - 3. Probably can't get that with a flybar and Bell Hiller though. If I am wrong about this I can't see why. The only difference between a prop and a heli blade is twist at the root. This is because the inside of blade turns at slower ft per second so more pitch is required to be efficient. This compensated for to a degree by adding chord to the inside of the blade nearest the hub, though some twist here would help. This is fundamental here so I'd like to get your thoughts. I am leaving for a few days of fishing with the grand kids so I'll see your replies when I get back.
Meatball, When you add pitch for forward flite your airspeed will have increased so your rotor rpm will stay the same and not bog your motor, as long as you only add pitch to match your airspeed. I could all wrong about this , but I don't think so.
Rudy

[v22chap]

Rudy ,,do a quick search of propeller problems and such and you'll find some articles that talk about tip speeds verses root speeds and such and how twisted blades too , reach a limit that they can no longer go any faster ... so flat blades are going to reach this limit sooner as they are not designed to "screw "through the air like the twisted airplane props are and the tip speed is going to be higher at a lower pitch setting and RPM.

Then on top of that ,, the full scale has 47 degrees of twist in the proprotors and I have been told that on top of that they can go up to 47 degrees of collective also .That is a total of 94 degrees ... the flat blades can't go that much even if they are without the flybar and hiller and they would no longer be pulling air but pushing it side ways.

Although the XV-3 (bells first tilt rotor ,, back in 47 ) did use bell 47 heads with the short flybars and the semi symmetrical blades of that day and it did do the full conversion ,,,but could only carry the pilot and copilot ,,not the full crew of 6 that it was designed to carry. The pilot also had to somewhere in the conversion slow the engine down ,,,, clutch and shift a two speed gear box in order to make the conversion..... can you imagine being at 3,000 feet altitude and idling back the engine ,disengaging the drive shaft and shifting gears to the only thing keeping you in the air :eek: and I assume this was back before we had synchronized gear boxes so I assume a lot of double clutching ,grinding and cussing probably went on in that cockpit . ;) :rolleyes: :o

Then lastly I have tried 60 degree conversions with the semi symmetrical heli blades and not been able to become wingborne ,,,not until I started using twisted blades. Now maybe I just then hit the magic pitch number and am able to do it now,,,but every engineer I have talked to says it is next to impossible to do with normal heli blades and probably more so for R/C as you are not reading gauges for RPM and able to feel what is happening to the craft .
I remember reading an article about the XV-15 not having to have the gearbox as they have the excessive turbine power that can be idled back in hover and opened up to turn the excessive blade pitch needed to move fast ....now here again we don't care to move fast either ,,,,just move is our main plan.

So with all that said I am not even close to an engineer and can't tell you one way or the other ... maybe some one who is knowledgeable will read this rambling and be able to tell us all what is up with propellers. :D ;)
Larry

[Tuner]

I just ordered one of the MS composit heads and 3 blade holders.
I have a plan.
The blades I have are fixed pitch heli blades they are very thin and have a large chord and taper from center to tip but dont twist.
I will aslo take some folding prop blades and make an adapter to mount these to the blade grips. I like the folding props because they flap slightly.

I aggree flat bottom or semi symetrical heli blades are not good at all. It makes common sense to me if you just think about the air flow at high pitch/speed. this is where chord helps.

I have always found that thin blades with chord work very well at low to medium speeds. Slow flyer props are a great example. the key is stiffness thats what I like about these fixed pitch blades for the dragonflyer XPRO.

[rdresch]

I guess only some experimentation will tell. Take a look at the blades on a C130. They are without twist but do have a very wide chord, and are short aspect ratio. They derive their thrust from the pitch which I assume is pilot controlled. thet can even go to reverse pitch. I guess I am being stubborn about this, But till I get an explanation as to why one works and the other doesn't I'll keep on trying this with heli Blades. My next experiment is take a T Rex modify the head to get 35 degrees pitch and turn it 90 degrees and hold on to it while my wife drves my truck and see how fast I can drive while the blades are still pulling. Oh Oh, My fish are done. Time to eat.
Rudy

[v22chap]

Didn't know that about the C-130 ,,other than the pilot controlled reversing ,,but stubborn in VTOL's is the only way to be as all the rules don't apply here and sometimes in R/C you can break those rules too. ;) :D
Your T-rex experiment ...should shade some light on this .. let us know the results.
Hope you don't get picked up by homeland security for testing suicide booming technigues or something .....if you do ,,give them my number I will vouch for you Rudy. ;) :D

"Yep he is one of us ,,certifyably off ,,as he is an R/C VTOL'er ;) :) :rolleyes: :D :cool:

[avianaut]

Actually, C130 prop blades DO have twist, AND cambered airfoils, as do virtually all full scale propellers, and most model props too. Heli rotors are really quite different, and prop-rotors (as in V22) are different again. Most model helis use symmetrical airfoils with no twist because they want same performance inverted (negative pitch) as upright. But I have used cambered airfoil blades on my heli and they are noticeably more efficient (when upright), if they had some twist they'd be even better. Many full scale helis have it. Heli rotors work well when the speed of the airflow through them is low, propellers work best at much higher airspeeds, relatively speaking. A prop-rotor must fall somewhere in the middle to work at it's best, it's a compromise. Furthermore, props are normally run at much higher RPM than rotors. Again, a prop-rotor is in the middle of the RPM range. I believe the full scale V22 runs different RPM in hover and fwd. flight, high RPM for hover and lower RPM in fwd., though it doesn't need a gear change 'cos it has a free turbine powerplant.

Food for thought huh?

Cheers.

[v22chap]

avianut.... thanks for the ,,,(sounds like inside scoop maybe ) It will take some more reasearch the way it sounds. We'll have to wait and see what Rudy comes up with ... but I am pretty sure that he is going to find the T-rex is not going to pull any where past 35 MPH or so.
Rudy
Also make sure to lower (if it isn't already ) the RPM of your T-rex to the 1600 or so that we run normal .46 size birds to ,,,as some T-rex guys are running in the 1800 to 2000 RPM area ,,,and that might make a difference.

[avianaut]

v22chap, I've been reading about the V22 since it's inception, bit by bit you come to know a thing or two. I have convinced myself that a fully functional model will need a carefully designed prop-rotor. Not a propeller, not a heli rotor, however a clever combination of the two may prove satisfactory. As you said, in R/C anything goes!

Keep at guys.

Cheers.

[v22chap]

avianaut
I too am convinced it will take a special prop .Both from what I have read and been told by V-22 engineers.But with Rudy having the excessive power of a turbine ,,,he might over come the flat blade with just raw power. :)

I have been taking a standard wood (525mm) concept blades and cutting the balsa trailing edge almost off . I leave 1/16 " of the bottom side uncut and the last 2 " of the tip. I then bend the trailing edge over a 45 degree form and clamp it in place .I CA the bottom of the gap it makes and then fill the gap in with slow cure epoxy and sand it to a round airfoil on top.I then turn the blade over and epoxy 1/8" balsa filler into the bottom "V" of the hard wood leading edge and balsa trailing edge and this then reinforces the bottom and gives it a flat shape like the bottom of an airplane prop. I then weigh and balance them .These seem to work well so far ,,but have yet to try and go full airplane mode with them. It would be nice to find someone that could make them out of fiberglass or carbon so they would be precise and easier to come by.
Larry

[rdresch]

I am not arguing that the heli blades would be more efficient with some twist to them. I realize that the speed of the blade at the base is much lower than that at the tip. So one would assume that it would need a different airfoil shape as you move toward the tip. That said I still think that adding enough pitch as airspeed increases will allow higher airspeeds. But I will test and see. I am pretty busy right now so the test will have to wait till later this week. If I could find a blade that would work believe me I'd buy it. I am just afraid that trying to design one myself may end being less efficient than a straight blade.
Rudy

[avianaut]

Larry, I once came across a website with the airfoil shape of the full scale blades, it is Bell-Boeing's own design, cambered high lift shape. I understand they use the same airfoil for the whole blade but the thickness ratio changes, it's thinner at the tip and thicker at the root, and as we know it's heavily twisted.

Rudy, I have a Zoom 400 and I've had a look at how blades are made (I've busted a few of 'em!). I wouldn't worry too much about making my own blades, I'd just copy the same construction as the stock ones, nothing tricky in there. I really do think the twist is gonna be important, though. I'm interested to hear the results of your tests. Good luck.

Cheers.

[v22chap]

avianaut
Yep ,,I once had an engineer send me the coordinates for the proprotors ,,but have since lost where I put them .... It's tough to get old and forgetful. ;) :rolleyes:
I have had some pretty badly made wood ones and they still flew so on the R/C I don't think it precision is a problem.
I also had one of those engineers give me an airfoil program to work with to choose the wing airfoil ,,,the one I finally choose was the same foil that the Bell XV-3 used ,,, heavy and reciprocal engine with no large amount of torque. And it was the web page you talk about where I found this out ,,,later on after I had started using this foil I ran across the web site .It was stored on a hard drive that crashed and I lost it ...if you can find it I would like to have it again.
I also have a NASA manual on the XV-15 that I think gives its coordinates too.Probably a little different than the larger V-22's ,,,might be more what we need.

Larry

[flypaper 2]

Maybe what might work is a constant speed prop. I made one for a 50cc Quadra gas motor. Worked on the test stand but never got around to trying it in a plane. What would happen is, when the wing tilted forward and the plane started to accelerate, the blades would automatically go into coarser pitch and the motors would stay at the same RPM, thus the constant speed. I was thinking about trying it on the electrics but never got around to it. Works on centrifugal force making the blades go to coarser pitch and springs making it go to finer pitch. The right RPM is a balance between the two. I'll have to give it another go. :p

[BigS]

If anyone finds those proprotor airfoil coordinates, I would like to have them too. I have a copy carver at work that does opposites from wood, so I can make 1 blade then copy as many as I like from it in opposite pairs.

I think since most of the working Vtols will fly on a model aircraft prop, the model V22 proprotors would be more likely to be closer to a prop than a rotor (confused yet:rolleyes: ).

Idea: the proprotor blades look more like wind turbine blades to me more than anything else. :rolleyes:

Shannon.

[rdresch]

The adjustable pitch is a great Idea. On my V22 the rotorheads are adjustable pitch. It is a ccpm 120 degree head. I was thinking a mixer tied to the pod rotation channel to add pitch as the pod goes to airplane mode might help. The spring weight idea might work well on a electric that is fixed pitch.
XN 28 XN 18 XN 12 are the bell airfoils for the V22 I think. I'll try to find the co-ordinates for them. If I could get a wood master of a Clockwise and Counterclockwise I'll build molds to match. Lets keep on top of this, it could help alot of us. Keep in mind this change from a heli style blade to a combo prop WILL require a change in rotor rpm. Larry what have you found? I did a preliminary test with the T-Rex at about 35 degrees pitch. Results are interesting but still inconclusive. Unless I was above 50 MPH the motor would bog. All that pitch takes a ton of power to turn unless at speed. When I got back I found it had broke one of the servos. I have repaired the damage and will try again. If anyone else tries this I suggest you make sure that you don't allow the main blades to contact the side of the auto. Not good for T-rex or Auto,
Rudy

[avianaut]

Larry, Shannon, I might do a search of my favourites tomorrow to try to find that site with the airfoil, but don't hold ya breath, my system is a mess! Also, it only gives the airfoil, no other details on the blade design so it probably isn't much use to us at this stage.

Flypaper2, most model helis are, in effect, a constant speed prop. We control blade pitch and use Tx. mixing or a governer to control throttle to maintain constant rotor RPM. Airplane type constant speed props use a governer to adjust prop pitch to maintain RPM when the throttle (power) or airspeed changes, it's basically the same thing. More than one way to skin a cat! Constant speed has several advantages over fixed pitch, I hope it works for ya.

Rudy, at 35* pitch the inboard section of the blades is way past stalled, no lift and high drag, most of the middle of the blade is at a very high AoA producing some lift but high drag. Only the tip is at a reasonable AoA. So it's understandable that it 'bogged' untill you reached high speed. It's that twist thing again. I'm glad you done the test, and thanks for posting the result. It helps us all to know rather than guess. I hope your car is okay!

Just a thought, it wouldn't be too hard to shorten the blades and rev it up some (a lot!), would it? I still think heli rotors are too big and too slow to handle airplane mode.

Cheers.

[v22chap]

this change from a heli style blade to a combo prop WILL require a change in rotor rpm
Yes and no ,,I first mounted my twisted concept blades to my VR 46 pod and boom and right away I noticed to high of RPM and lift off to soon.
I moved the throttle curve back a lot ,,,,still same RPM that I hovered at before ,,but different lower curve.
The pitch curve of course was a lot less too. Instead of -3 at low stick ,+6 at 1/2 ,and +10 at full... it now was -6 at low ,+3 at 1/2 and +7 at full.

Rudy ,,hope the car and T-rex survived ,, It sounds to me like what maybe is happening with that 35 degree set up is you have to drive fast enough to start autorotaing the props and it helps the motor along so it don't bog so much ,,,maybe ??????Something is not quite right there ????

[Tuner]

I got in the MS composite head but frigen $200 for head and grips YIKES those grips are expensive heck I can buy a whole motor for the price of 2 of them.

But that is the way it goes. Looks Like I wil be machining an identicle Hub as MS composites only I will add holes for 2 3 & 4 blade grips.
I will give www.emachineshop.com my firts try as I figure this part is as simple as it gets.

I will make 2 hubs on with 4mm hole for my current motor shaft and one with 6mm hole for my future 400-600watt 18tooth 40mmdia 15mm H 9ounce Crocodile motor whenever it gets produced and shipped. I cant wait for this motor it will ushure in a new wave of direct drive helis in my opinion.

I just cant see a way around buiding a custom blade.
Problem is that seems to be one of the most difficult things I have looked into doing.
Now yes I can make 3d model of a blade but to make one that is properly engineered is well an artform.

After reading everyones perspective hear and doing some other research I have come to the conclusion that the most practicle methode I can find is to use these fixed pitch blades and use a combination of increased RPM and increased pitch no more than 10deg especialy since fixed pitch blades have a pitch built into them with considerable chamber. I plan to run the rotor head at twice the average RPM of around 4,000 rpm.

What would be very helpfull is picture of the rotor blades of the V-22. I have done some 3D Modeling and It is amazing how accurate you can recreate something with 2 or 3 pictures taken from different angles those in combination with an actual airfoil spects could yield a lot of helpfull information.

I can generate the 3D model and even help pay for molds if somone can build the composite blades or has somone that can do it.

I wounder if you could get any of the Heli Blade Manufactures to get into this???? I got $1,000 for custom blades if anyone wants to pool in and just pay a heli blade company to custom design one.

Scott

[v22chap]

You could have got the MS plastic blade grips a whole lot cheaper and they still would have worked o.k.

Problem with pooling together on blades is the fact everyone is doing a little different scale ,,,, you need 20" rotor ,, I need around 42 " and maybe more for Rudy ,,,etc. The wood modified blades that I do for mine are really not all that hard to do and work o.k. and I have built whole new blades for the micro birds (hornet , flybarless collective when we started play with it ) and even put lead weight in them . I think I would try that approach until you find an airfoil that works.... doing the R/C the same as the full scale ,,,usually doesn't work.The Reynolds numbers are all off and besides the full scale not only wants to be able to go into airplane mode ,,but they want the most speed that they could get out of the bird. All we care about is getting our over weigh ,unpowered , R/C model to go into airplane mode.You have to learn how to keep it really ,really simple with our models.
I can give you that build link ,,,I think if you are interested.I think you will find that in the model world and especially the micro or mini world you can be less exact and still get by ... just look at what they do with the fan fold profile ,flat winged parkflyers.Those things are 3-D nuts and just a piece of flat foam :eek: :o ;) :rolleyes: :D

I don't think you will get a blade company to do it ,,,the amount of money they have to put into a mold and then only make 20 blades or so ,,,probably wouldn't temp them to much...... but it doesn't hurt to ask I guess.
Rudy and I have a company that is designing an onboard mixer for us,,,, but then again he is interested in making a kit of the v-22 in mini form some day .
Just my 2 cents
Larry

[Tuner]

Ahh Yes are you referring to the TH-2.
This mixer I plan on getting in deep with as it has a lot of potential simple and lightweight.
I have yet to downloadthe Development kit for this mixer but plan to. Unfortunately I am trying to learn the gumstix/robostix pieces first.

I asked the guy who makes the TH-2 if he could supply his source code or some examples that I can use to build a 4-rotor heli mixer.
He was not so excited to give out his source code (I never understand this being a programmer) But he sounded more than willing to help me in any other way, nice guy great mixer!

Yah I would love to see what you have on making blades. It is simple enough to build a heli blade but a blade with a twist in it does not sound so easy but PLEASE let me know what you can about bulding my own Props/Blades.
Only think I know is make a 3d Model Machine a mold and make the blade. I just though the Old fasion Carvinga blade would be to time consuming and would yeild difficult to balance blades that product asymetric lift.

You made a great point that I to live by In the end really really simple works best especialy with the reynolds numbers we are working with you can make a box fly if you wanted to, well a Pizza box at least.


In a perfect world I would take the APC Slowflyer Prop and produce the size I need and make it variable pitch for Cyclic and minor Collective.

Those APC slowflyer Blow all other props out of the water for static thrust. They have thin airfoil and a minor twist.

[rdresch]

Looked into the airfoils for the rotors on the V22. They are inboard to out xn28, xn 18, xn 12, xn09. However I could find no data on the net or any data points for them. The XV15 has a rotor with data available, sort of. I found it at one point used naca 64-935, 64-528, 64-118, 64-12-208 from root to tip. These can be generated in compufoil 3d with data points. I'll buy the program and the naca generator option with DX file option if that is what is needed.
Shannon check out Compufoil and let me know if it works for you. I did one more test with the T-rex. A complete failure. No way is that motor going to pull anywhere near 30 degrees pitch. That is turned into the wind at 50MPH. I will try again with less pitch. My neighbors are starting to look at me funny.
Rudy

[v22chap]

Scott .. The way I make my twisted blades is to use the wood helicopter blades that go to your size bird. Almost cut the balsa trailing edge off but leave 1/16 on the bottom , then poke you exacto knife into the last 1/16 " every so often to give it a serated effect .Make sure to leave the last 2 " in of the tip uncut completely.I then make a clamp out of 3/4 " pine wood and clamp the blade at this 2" mark. Then I make a 3/4" pine wood pattern off the front of the blade and the 45 degrees that I want to twist the root end to.This thin form gets clamped to the blade at the leading edge and then I start to carefully flex the balsa trailing edge down to this pattern .... working all along the trailing edge a little at a time and soon you have it twist to the pattern at the front and you fasten it to the pattern with another small clamp.You then sight along the trailing edge to see that it flows in a nice arch up to the tip.CA the bottom of the "V" this forms between the balsa and hard wood .You then mix up some 30 min epoxy and fill this "V" area to the top.
Once this is dry unclamp the blade and flip it over and measure from the root to the area that just starts to flatten out ( a little in front of the 2" mark) and cut a piece of 1/8" balsa as wide as the blade is now and to the just measured length. Mix up a bunch (and I mean a bunch of 15 min epoxy) and put it into the "v" that this bottom part of the blade now makes( puttin more into the root end.) Place the 1/8 " balsa strip into this V and push it down until it looks like it is even with the leading edge and the trailing edge and hold it until the epoxy dries ( I now use 5 min as it is easier to do and I don't have to hold it so long)Once this dries you can rasp it down (by hand with a rough wood rasp )to where the rasp just touches the leading edge and trailing edge making the bottom of the blade flat in a twist ,,,just like the bottom of an airplane prop.Stop a little before hand so you can use med / down to fine sand paper to finish it up smooth.

Once this is done you can file the top of this blade to a nice round shape.(by hand with a rough wood rasp,, until you get close and then use med / to fine sand paper to smooth it up ) I always put marks on to the hard wood leading edge and balsa trailing edge to use as a guide on how far to rasp .I put the mark at 1/4 " from the hardwood epoxy line up on to the hard wood and down on to the balsa by 1/4" at the root end .
Then go out to the point where the blade starts to get flat again and measure a 1/8" on each side of the balsa/hardwood joint and draw lines from root to tip on these marks as a referrence to rough file to ,,so I remove about the same amount on each blade.

Then comes the fun part ... you need to weigh and balance the blades again.
On the mini helis I think you can just get them to weigh the same and balance them again . On the larger ones you must do the Kavan type balance where you have to find the CG of each blade and then take weight off the heavy blade to make the CG's come out within a few thousands.
I can now make a set of blades (2 ) balance them and cover them in about 5 hrs.

Now the blades we made for the hornet were made from spruce 3/16" X3/4" stock with 5/8"balsa trailing edges and CAed together on a flat surface.Cut to length and 6 oz of lead put in the leading edge about 1 " from the tip. I will try to find the article on that.Hopefully it is still in the hornet bulletin board data base as I don't think I have the hard drive that was stored on any more ( darn computer crashes) and I doubt that I can find it in the hard copy I made .
I can maybe make this a little more like instruction and draw out some hand plans that might make it more understandable. :rolleyes: :D
Larry

[v22chap]

Rudy ... on your test with the Rex ... do you start out slow ...in the car ,,,say around 20 MPH and then set the Rex at low pitch and try to go up to 30 degrees collective slowly ,then go back down in pitch and try 25 MPH on the car and go up to 30 degrees on the Rex again ...or do you just hit 50 MPH and put the Rex out the window and try to turn it at full pitch right away?????

I would think that you would have to work up from slow car MPH and low collective (one that lets the blades turn with the motor freely ) and work at it a little at a time to get a good sense of what the blades are doing and not going to doing.
Larry

[Tuner]

I understand it took a couple reads because I have honestly never used wood blades.

Good Point starting out from existing blades is a great Idea.
I will look for some good flat bottoms that are thin and wide.

Thanks

Scott
Time consuming though. I have never balanced in the methode you mention or you made it sound different.
Were you refering to the balance of the blade itself from leading to trailing edge or the balance of two blades?

[rdresch]

No Larry I tried slow starting at 10 mph adding pitch till I felt a pull on the model. Then as the car accelerated I increased pitch noting which speed it quit pulling. On the last test it maxed out at about 45 or 50 mph. As soon as I decreased pitch it would show a strong resistance to the airflow. Motor was extremely hot and this is after only about 15 secounds so it is maxed out. I tried with different amounts of pitch from 5 to 30 this last test. I suspect that holding a complete T-Rex turned with the topside facing the airflow is just to much drag for the motor to pull. I had The governor engaged so it had 90% throttle. I tried switching it to idle up to be certain it wasn't a pitch throttle curve issue.
Rudy

[rdresch]

Tuner, Remember if you are using counter rotating blades you may have problems finding them in flat bottom. Does any one make them?
Rudy

[BigS]

WOW, this thread has really taken off..

I am thinking of making the blades out of fibreglass then carbon fibre when the foil/twist is right. I could make the larger ones out of wood on the copy carver at work but the small ones might be a bit thin to cut without splitting/snapping.

I think we need to design them to fly as airplane props then use them as rotors and not the other way around. Most of the micro ospreys here have used airplane props to hover but I have yet to see airplanes using heli blades to fly forward. The variable pitch plane props that use flat blades are generally shorter than the vtol blades and they seem to have a narrower tip than root.

If it is possible to calculate the forward speed needed to fly (well above stall speed hopefully), then I think the best way about designing the proprotor would be to start with designing in the twist needed for the required pitch speed at the operating rpm in foward flight. Kinda like the prop twist to give a pitch speed :confused: ?. Everyone let me know what you think.

Rudy,

thanks for the polars on the XV-15, I can use profili to print out foils to make proprotor mould from. Will post some pics as soon as I have it started. :cool:

Shannon.

[BigS]

Posted a new thread instead as it got a bit big and might change the topic too much in this thread.
http://www.rcgroups.com/forums/showthread.php?t=507956


Shannon.

[avianaut]

Scott, those 3 blade rotors on the MS hubs look great. What head speed are you gonna go for?

Larry and Rudy, I did some calculations today based on the Zoom 400 and it doesn't look good in airplane mode. At 1800 RPM and 30* pitch at 50 MPH the figures indicate that the inboard half of the blade would be at negative pitch with the root probably stalled, the outboard half would be at a positive AoA but the outer section near the tip would also be stalled. I don't think it would pull the skin off a rice pudding!

If there was a practical way to change the gearing on these micro helis to get 4-5,000 RPM we might get into the ballpark, using blades off a slowfly prop. I wonder if I could put a heli rotor on my 3D Foamy plane? Got brushless and 6.6 : 1 gears on it. Hmmm... That gives me an idea!!

I can't find my links for the V22 foils anymore, sorry. The XV15 shapes should be fine if we can calculate the correct twist, and the overall size for our respective projects.

Does anyone reckon emachineshop could make molds for blades?

This thread is showing great potential, keep it up guys.

Cheers.

[v22chap]

Woow guys ,,been busy this morn and could n't get on line till now and it looks like some of you have really been doing thier home work :D

avianaut
Were your figures for just heli blades ???? I gathered that was what you were figuring.The difference between the outboard and inboard half was what the article I read was talking about ....and it even said that twisted props hit the wall at a certain MPH.
butttttttttttttttttttttttttttttttttttttttt.., you have to remember that all the figures are for full scale and we being smaller can get by with things that the full scale can't and you also have to consider the fact ,,that this bird don't rotate off the ground ,,,it is already in the air and almost airborne speed from the heli part .
I don't think we need the RPM for the speed we need to transition at ,,,they go for the max speed so they can cruise at a high speed.

You can do variable propellers on the parkflyers ...but it doesn't have cyclic control just collective ,,,so you can actually fly (hover inverted ,,,nose down ) with a parkflyer.
Larry

[rdresch]

Check out Big S's thread link. Great way to make your molds. Simply cover the wax with fiberglass tooling Gel then glass and you have your first mold half.
Avianaut. Seems like you know what your doing with these prop profiles. Maybe you could figure a twist for a 27 inch diameter rotor at about 2400 rpm. I would guess around 50mph forward speed should be plenty. Use the XV-15 airfoils for a start.
If this works out I'll try and build some 48 inch Diam blades for myself. I can admit when I am wrong. My tests on the T-Rex show it is just mainly beating the air to death. You guys proved me wrong. At least I know what has to be done. I'll need about 100mph top speed. Any Idea's on twist?
Rudy

[Tuner]

Ok so we Got Rotor speed witch is perpedicular to airspeed.

If you rotor speed is 100mph and your airspeed is 10mph. If the blade has a pitch of 0 then the angle of attach is -10 degrees.

So just take Prop Speed witch is Circ. * RPM = X Ft/M
Take (Prop Ft/m) / ( Desired Airspeed Ft/m) = Angle of attach of incomming air.

Apply this in steps from center to tip say every 1/4inch.

Repeat this for multiple airpseeds & RPM to get an idea of the mean pitch that yeilds a good angle of attach for desired airspeeds/RPM range.

Im no aerodynamicist I just love my math & physics this should be accurate.

[flypaper 2]

On the constant speed prop, it doesn't need a governor to control it. It's its own governor. Once it's calibrated for a certain RPM the motor will maintain that RPM whether the plane is in a hover or doing 50 MPH. It's calibrated by using the RPM where the motor is running at it's optimum amp draw. Guess I'll have to make one for a show and tell :D

[v22chap]

flypaper ,,don't have to do that Kress has them already made for electric motors. Forgot all about these until you mentioned it.
I did a quick search and didn't find them on his website ,,so I assume they don't make them anymore ,,,darn it .You might contact him as he might have a few laying around .... So I guess it would be neat for you to "show and tell " after all ;) :D
Kress Jets (http://kress-jets.com/index.htm)

While searching Kress's page found a neat thrust convertor for his fan units.
thrust convertor (http://kress-jets.com/electricfans.htm)

[rdresch]

Tuner help me out here. For example if tip speed wass 100mph and the wind flowing into the prop (airspeed) was 100 mph then wouldn't the resulting angle of attack be 45 degrees? I was drawing up the resulting triangles as a ratio of base to rise to figure angle. I think your concept is right though. I did run into a problem though. If I am correct at 100 mph the airspeed is 8,800fpm. On a rotor w/ a 48 inch diameter at 1600 rpm the tip speed is 20,096 fpm. Which would be about 20 degrees. Can this possibly be correct?
I need help here. Is my math right?
Rudy

[flypaper 2]

I still have two of Bob Kress's RK 20 ducted fan units still in the boxes, I bought back in the 70s. Maybe I'll have to hang them out on the nacelles :D

[v22chap]

Rudy ,,if your math is right ,,,then the pitch is about right as that is about what I had on the last two birds. 42 " rotors turning 1800 RPM and 15 degrees pitch.
Larry

[BigS]

I am working on a spreadsheet for Excel that will be able to convert blade length/rpm and pitch speed into angles at various locations along the blade.

I will post the file here once it is done to a good standard. I have it so it calculates at the moment but it needs some tidying up.

Shannon.

[rdresch]

You just got to love this forum. Seek and you shall find. Thanks all.
Rudy

[BigS]

You just got to love this forum. Seek and you shall find. Thanks all.
Rudy

So is there a prop pitch calulator out there already :confused: :rolleyes: :D

Shannon.

[v22chap]

Shannon
I have found several prop calcs...but they are only interested in thrust for an airplane and usually will not handle anything above a 16 " prop size , and non that do the pitch angle at various places along the lenght of the blade.
yours will be the first to do heli blades that I know of.

[rdresch]

No, not that I know of. I have been trying to find info on this for a long time. It was your spreadsheet that I was so happily referring to.
Rudy

[BigS]

Well here it is, I hope a few of you can find this usefull.

Let me know of any bugs, suggestions etc. :cool:

Shannon.

[v22chap]

O.K. Big S
I have played just a little with your program ,,,need to get going soon ,,but it looked to me like for my 42 " props turning at 1800 RPM that I need about 45 degree twist with 30 degrees of pitch setting ,,if I am doing it right ... which is about what I have and it seems to work ,,well I only have 15 degrees pitch .
I also didn't understand what the pitch speed is suppose to be ,,especially in metric ,,had to use a converter to do even the rest of it....like I said math is not my bag ,,,baby ( in Austin Meyers high raspy voice ).
I used 200 for the pitch speed as it gave me the MPH tip speed that we talked about last night.
Thanks for the spread,,,I will learn ;) :D
Larry

[BigS]

I could do a version with it all in Mph/inches etc..just to make it easier then :D

The pitch speed is in Kilometers per hour so 200 Kph works out to around 120miles per hour. This is the speed that the proprotor is flying forwards through the air.

Shannon.

[v22chap]

O.K. that is what I thought ,,but wasn't sure ,,, and the conversion site works o.k. to make the metric understandable ,,, so don't bother with making another u.s. only version ,,this one works fine and us older folks that are not bilingual will just have to cope. ;) :D
Thanks
Larry

[rdresch]

Believe it or not I did find another calculator www.nclra.org/Programs/PropCalculator.html
Yours is way better since it automatically plots all positions. Results seem to be the same and also correspond to the math I did earlier. So now we know what we need. Question now comes to mind is how much power will this take to turn? I want the 48 inch diameter to keep it scale. I am sure that all that twist and section will take a lot more power to turn. Looking for a program now that will tell me. I saw one once.
Rudy

[rdresch]

I found a site that does power calcs for props.
www.jimsrc.com
But what I see is scary. A 3 blade prop with the riht pitch (47.5) gives a speed of 90 mph, good so far. But produces 48 lbs of thrust and needs like 16 hp to turn. This numbers are based on APC profile props, but you see the problem. Our props will need a much narrower chord but still will suck gobs of power if built to scale diameters. The more you learn the more you need to learn.
Rudy

[v22chap]

Rudy .. finally got back .. I don't think those figures apply to our prop and situtation .they are longer than a model and wider. You also will not be using the full 47.5 degrees of pitch,, the full scale uses it because they want to go as fast as possible ,,,all we want to do is get airborne.

I in fact noticed that I had to remove throttle when I put the twisted blades on my pod and boomer and a whole lot of pitch had to be removed to hover at 1/2 stick and to fly around like it use to.

Calculations are a good start ,,but until you actually go out and try it ,,you will never know for sure and especially in the model world .... we have done stuff that a full scale just can't do ,,, like hover on the prop ... if you calculate that out ,,,it is impossible to do but guys do it every day .Get your pod and boomer out and make a set of wood twisted blades like I do and you can see what the effects are ... I didn 't take the time to document all the things I saw with that setup ,,,just that it worked and the blades didn't fly apart when I looped and rolled them and did hard banking manuvers ,,trying to tear them apart.Maybe make up a holding device to mount it on the top of your car and drive down the road like you did with the t-Rex ,,,,,,,, then your neighbors will for sure call the men in white coats ;) :D

[rdresch]

Larry, The sites I am looking at are for models but the power calculator uses normal model props to figure the loading. These are different from what we will need since we are half prop and half rotor. The props are pretty wide. the blades on the osprey are pretty thin and measure a aspect ratio of 14 to 1. In other words they are 14 times longer than wide. the props are around 4.6 to 1. If I figure the loss of blade area the numbers start look really close. Remember to I am using 3 blades per head. The blades I have now are 2 inches wide if I reduce them to 1.5 inches in width I'll be scale to. I am going to try to build the blades with the right airfoil sections all the way to the tip so it will be a lot of work. Two blades then four molds. I want to be as sure as I can that they are what I need.
Rudy

[v22chap]

O.K. that makes a little more sense ... good luck on the blade build... you also got the HP there to swing some serious props too.

[BigS]

Rudy,
Thanks for the link to the prop pitch calculator, good that mine works out the same too. :cool: :D

I also checked the drawings I have of the V22 and for an individual blade the span/chord ratio works out at about 7 to 1, I take it that you mean the overall proprotor length with a ratio of 14 to 1 ?:confused:

One thing I did notice is that when I put the real V22 blade and airspeeds measurements into the calculator, the tipspeed increases dramatically when in forward flight. I always did wonder why they decreased the rpm for forward flight, it seems it is to keep the proprotor tip speed from going supersonic.

I also think there is a fair amount of compromise that will go on between the rotor and prop parts of the flight envelope. This would be evident in a reduction in the amount of overall twist in the blade.

I am not sure what pitch speed to design for as there is always an amount of 'slippage' when a blade is flying through the air. Say a 100mph pitch speed might only fly at 85mph. Some examples of how much to expect would be handy. Anyone? :D

I will test a proprotor out on my trex as soon as I am happy with the calcs, then it should end up it the ballpark for flying. I might just make the blade mold using the wax method (shape wax to the reverse of the blade then mold straight off the wax) then I can prototype blades even faster. Might try the Trex in the airflow trick with it too, my Mum has a convertable sports car that would do the job :D

Shannon.

[rdresch]

Shannon, If you are using the drawings the V22 site they are not anywhere near scale. I don't know which you have. I found a picture of the Osprey with a good head on shot with the rotors frozen (high speed shutter) I measured the width and length of a individual blade. I got 14 to1. I wish we could find real info but it's the only way I could think of. At any rate I don't think we could turn 7 to 1 twisted. I'll be interested in your results with the T-Rex blades. If you are using T-rex heads I'd design for the speed you need, you can always add more with collective adjustment. It's getting the twist right that was important. Your spreadsheet makes that a lot easier. At least the quess is a educated one (maybe)
Rudy

[rdresch]

Shannon, Found another picture with the blades stopped also a head on shot.this shows 10 to 1 ratio. Maybe the blades were at pitch on the other picture so they measured narrower. That is close to the blades on my Raptor so I'll go with that. Still pretty much a guessing game.
Rudy

[BigS]

I guess we need the actual blade dimensions then, I have a 1/48 scale model here I can check on, but I'm not sure how accurate that is though. I have some osprey drawings off the russian site airwar.ru, they seem to be a bit more accurate and some of them are over a 1 meg in size.

I think if it is a head on shot the blades will have quite a lot of pitch on them due to the osprey being in forward flight, so that will throw the measurements off a bit. I also think that the twist obscures the amount of taper that the blades have as the root will appear narrower than it really is in a head on shot.

I have a video clip here titled "V22 music video" and on it the blades of the osprey definately seem to be between 10/1 and 7/1 ratio.

I might end up putting around 8/1 to 7/1 ratio on the proprotor blades.

I still need to know what the flying speed will need to be for a set wing area and weight. I am not really sure what works in this respect.

Shannon.

[rdresch]

Remember if the picture was taken without a really fast shutter speed they will look wider than they are. They will also look wider at the tip than the base. The one good shot I have shows them not turning but with the rotorpods at the airplane position on a carrier deck. Maintenance I guess. the shot is almost straight on. I can't say what the pitch setting is but the aspect ratio is 10 to 1. Try this site
http://adamone.rchomepage.com/cg_calc.htm
Hope the link works, there is a spreadsheet at the bottom left that figures wingloading and stall speed
What did use to open that russian drawing?
Rudy

[BigS]

I think photoshop will open the drawings, if you dont have a copy I can send you jpeg version of the pictures.

Shannon.

[BigS]

While searching the web I found these,

http://halfdome.arc.nasa.gov/publications/files/Johnson_ERF01.pdf
This one on page 4.3 has the blade twist and chord sections for the very accurate 1/4 scale TRAM model of the V22 proprotor. :cool:

http://halfdome.arc.nasa.gov/publications/files/Johnson_AHS01.pdf
This one is mainly illustrations, wake models, etc.

http://halfdome.arc.nasa.gov/publications/files/Young_AHS99.pdf
Here is some more info on the TRAM model

So it looks like the chord is about 8.9% at the tip and 15% at the root.
So it looks like we both could be right but were looking at opposite ends of the elephant ;) :D
The TRAM model had a 32degree twist too.

Shannon.

[v22chap]

BIGS
Good find. I remember having the first one bookmarked ,but lost from a HD crash ,before I hard copied it . Good info ------Newer than the XV-15 stuff I have and definately good enough for what we are doing .I don't think it is what they are using now ,,but it should work for us as it obiviously was working back in 2001 for them. ;) :D

[rdresch]

No I was wrong. The numbers I was using were a ratio of chord to span. Measurements from pictures put the chord at the tip at 15 to1 or 6.66%
In reality the ratio is 10.36 to 1 or 9.65%. That plus it gets fatter as you move to the root. At the 30% station it increases to 7.4 to one or 13.5%. Wider still at the very root. The twist is nearly 40 degrees. The table shows 32 at the root, but nearly a -8 at the tip. I suppose there is a reason they used -8 instead of 0 for the bottom of the chart besides trying to trick us.
By the way I blew up the carts 400% and used calipers for my measurements,so there may be error from innacurate blow up percentage. But not much. This should be close enough to work with. I do worry about the power required to turn one of these scale props though. Each blade would have twice the area of a normal heliblade plus the drag of the twist in hovermode. And 3 blades verses 2. a OS 50 has maybe like 2 hp. How is the same Hp going to turn 1 extra blade plus twice the blade area and the drag of the twist? Food for thought. Larry, do you have any pictures of your blades from the top, It would help.
Rudy

[rdresch]

I forgot, could you please email me that Jpeg drawing
Rudy

[v22chap]

Rudy
I don't have any good pix of my blades and right now we are headed for a day of college Graduation (my wifes BSN nursing degree ) :D So it will be later tonight and the camera will probably be dead ,,so may be Mon.
That is interesting that they use -8 ...I noticed that ,,but kind of assumed I was reading the chart wrong...... I think I will change my angle to the 40 degree or so . Your turbine should be putting out 5 to 8 hp depending on what version of the wren you got ?? ( 2 or 3 ??) ,,,so I would think you should be o.k.
Larry

[avianaut]

Guys, this is moving too fast for me now, a lot of very usefull information that will take a little while to absorb. Thanks particularly to Shannon for the spreadsheet and the NASA info.

I have a friend who has offered to help me do a spreadsheet, he's an aeromodeller and is competent with computing and maths. It gets a little tedious crunching the numbers with a pocket calculator, pen and paper! There are more parameters I want to include, such as adding AoA to the pitch angle, and showing results for two different RPM's side by side, and I'd like to skip the bolt hole and take all station measurements from the hub centre. There's more, but later.

Rudy, I did the sums for your T-Rex and find it very similar to the results for my Zoom 400. In short, way too much pitch and too much RPM. Your blade root would be near 40* negative AoA, 0* AoA at 5" out from the hub, approaching stall at 8" out, and around 17* AoA at the tip, so the outer third of the blades would be heavily stalled, making not much lift and a lot of drag. No wonder it bogged! It looks better at about 20* blade pitch and 1800 RPM. It would be even better to shorten the blades and start over. This is not much use, I know, but it seems to confirm the benefit of twist.

Larry, you may be surprised to learn that there are some full scale aerobatic aircraft that are prop hanging nowadays, and full scale helis that can loop and roll, and pull negative pitch. For model helis there are some rotor blades for .60 to .90 size helis that have cambered airfoils AND twist, intended for F3C flying. They are much more efficient than 3D blades, that's why you have to take out pitch or lower your throttle curve.

Rudy, if you were to design a proprotor for a Raptor 50 you would find the diameter will be quite a bit smaller, the total blade area will shrink as a consequence. It will need to spin a lot faster but the tip speed will still be similar. With camber (better lift/drag) and twist (the twist reduces drag and improves efficiency) the blades will be much more efficient than 3D blades, and with 3 blades the aerodynamic loading may be not much more than 2 large blades, and will probably make a similar amount of lift using the power of a stock OS 50 SX-H. It won't like being inverted, but the important thing is it will go a LOT faster in forward flight. You just need the right figures!!

Cheers.

[rdresch]

Larry,My congratulaions to your wife!!!

Rudy

[rdresch]

Avianaut, Agreed I will have to build twisted blades. I actually made one quite some time ago but was not happy with it. This is after building molds etc. I really want to feel more confident in the design and this thread is helping me to zero in. I agree that the diameter will need to be smaller. Though I have a turbine for power (8hp) some of that will be lost since I want to limit it to 120000 rpm. Also transmission losses to the rotorheads. I will probably end up with about 3 hp to each rotor. If I made the rotorhead scale in chord to length then it would almost double the blade surface area. I am sure it would be much more efficient in fast forward airplane mode ( required actually)but I am not sure about in heli mode. I think I'll plan on 550 mm blades built to correct twist. Larry it does look like 40 degrees twist from root to tip is about right. I am going to wait a bit to see if any more info comes to us first. Looking forward to that spreadsheet. It will help with all the what ifs. I am really enjoying this thread.
Rudy

[Tuner]

In response to that question on my post way back thier a yes 45 degrees is the angle the wind would be coming from and when I said 10 deg. oops hehe sorry that was the slope I never converted it to degrees.

Thats all null and void now thanks everyone.

I will contribute my peice of info.

http://www.araa.asn.au/acra/acra2004/papers/pounds.pdf

probably has all of what has been discussed and more.

Scott

[BigS]

Avianaut,

The spreadsheet you want to make sounds good, you can use mine to start with if you like. Its protected but there is no password to disable the protection if you want to change it.

I like your idea of using two Rpm's side by side, I also think having the pitch angles for the same Rpm but different pitch speeds, side by side would be good too.
I left the bolt hole spacing in as I find it easier when it comes to make the blades if you use the bolt hole for finding former locations. Otherwise they would all have an offset to calculate. I could make up a seperate table that tell the chord locations from the shaft center easy enough though.

Do you have any more information on the angle of attack of the blade? It would be handy to have that to add but I suspect it is a variable that changes with the thrust of the proprotor. I think that as the blade approaches pitch speed through the air the resultant thrust will head towards zero and if the pitch speed is less than the airspeed you would have negative AoA and negative thrust.

Now you've done it... I'm getting ideas. A chart that plots multiple pitch angles and airspeeds for a given blade twist and gives the percentage difference at each airspeed from the optimum.etc ....just you lot wait and see what I come up with ;) :D :D

As someone said though, (Larry? :confused: ) it is still all theory and untill we make something and test it, we can't be sure. :rolleyes:

Tuner, your link offers some good food for thought. :cool:


Shannon.

[v22chap]

sorry double post ,some how

[v22chap]

Ya... I'm the one who always puts water on the fire ... but I have been down the road for the last 16 yrs and have bounced my head (not pencil /paper or computer )off a lot of these problems already to long. :o ;) :rolleyes: :D
It still don't hurt to figure as much as you can in advance ,,, just don't give up because the figures look bad. :p :) :D

[rdresch]

Larry, Thanks to you we at least have a idea of where the major problems are. I am a firm believer in designing as much as you can on paper. That said, being in construction and working with blueprints every day, we have a saying in contruction. The drawings are only the starting point to modify from.
Shannon, I got the drawings from the russian site and they are great. Wish I had them a long time ago. I am not sure how accurate the scale is on the rotor though, it doesn't seem to match the tram data. End of story, it's about time to shoot the engineers and build the darn thing.
Rudy

[v22chap]

Hey , thanks Rudy ,, the wife and I had fun Sun at the graduation and dinner ,,got to see a lot of old friends and eat a lot of food.
Didn't break the camera with anyone ... so tonight I hope to get you some pix of the blades I have been using

I think the trouble with data we get off the internet and some that I get from people that work in different places with the V-22 ... it is what someone thinks it is or interupts it a different way and it is not the real data.And some of it is outdated material ,,that is no longer used .This is usually the case...because most of what they use is classified and not released to the public for a few yrs.
At any rate it is nice to know that we are in the ball park and it give us a little more security and that helps a lot when you got that much money in the air. :D
Rudy ,,could you email me the Russian site stuff too.

[avianaut]

Shannon, I got to spend some more time with your spreadsheet today, it's a great starting point. I need my friend's help 'cos I've never used xcell before. The reason I want two RPM's side by side is that I believe it's a primary factor for proprotors. My reasoniong on the bolt hole was that it may not be the same on various machines, if we just took measurements from hub centre it would be easier to compare different blade shapes, apples to apples.

The angles that are given are actually zero AoA, or blade advance angles. We will need to add blade AoA to them. Relative airflow speeds should be calculated from the blade advance angles though. Most common airfoils have a best lift to drag ratio around 3* to 4*. If you have a specific airfoil in mind the L/D graphs should clearly show it, or just have a stab in the dark. Sure, this is variable in flight, but a prop or rotor (any prop in fact) needs to be close to it or it'll just soak up horsepower beating the atmosphere to death! Another thing to think about is that this is a variable pitch rotor so it might be nice to see how the numbers look as we change blade pitch while keeping blade shape and RPM the same. Further down the line I want to calculate how much power is required. If we don't design for it, it'll be trial and error in the end, right? Another issue here is that we don't know what the target airspeed is, we dunno how fast a model V22 will fly. However, it is just an airplane so whatever empirical data and experience works for conventional models should work well here too. Everyone is on there own because we're all doing different sizes here.

Tuner, that's a very usefull link, I already had it and was gonna post, you beat me to the punch.

Rudy, I also have faith in the engineering process, so I'd also like to see good results on paper before cutting metal. So don't shoot the engineers yet, I'm still busy!!

Cheers guys.

[v22chap]

avianaut
This new expanded spreadsheet sounds really good.

I know that I have to have atleast 39-40 MPH to lift my 20 lb bird into the air off a grass strip.This I tested with a single Super tiger .75 engine on the front of my red and white bird ,,with an eagle tree data recorder onboard .I also know that I have went onto the wing at this same MPH in doing the conversion to 60 degree position many times.So it is going to have to be in the 40 to 50 mph range ,,I would guess to do what we want ,,but here again it is going to be somewhat different for everyone because of the different scales being used and weights.!!!??????

[v22chap]

Rudy ,,here are the pix of a blade that I removed the covering so you could see the split and all .This was one of the last ones I built and you can tell it was done in a hurry.

[v22chap]

And a few more ...it is hard to capture the twist on picture ...if you want a different angle ,,,just say so.
Larry

[BigS]

Here's another chart that uses multiple pitch speeds so you can see the variation in the blade twist for a range of speeds.

I also included a conversion table for those of you with imperial tendencies, you will still need to move the figures over to their appropriate boxes though ;) :rolleyes: :D

Edit: modified table to include AoA differences compared to a set pitch speed.

Let me know what you think.

Shannon.

[avianaut]

Larry, very good that you have flown the airframe as an airplane, and with data logging!! :cool: Can you share a little info on the model, particularly scale, weight, wing airfoil and wing area? It's possible to figure out a few things based on the performance of that model and use that info on models of different sizes or wing areas.

Shannon, you've been busy, haven't you? :rolleyes: That's a lot to absorb. I can see that changing speed has a significant effect. Again, I need more time with that to really 'get the picture'. Well done, and thanks. I have, just tonight, started learning Xcell! I want 'In' on this! ;)

Any thoughts on calculating total lift, and drag? That would be a necessary step to enable calculation of power required. Is it possible to calculate the total area under a curve in the graphs? That would represent the total force, wouldn't it? I dunno if Xcell works like that.

Cheers.

[rdresch]

Thank you so much for the new spreadsheet. Very helpful!!! I too am wondering about power requirements. It might be lot more difficult to figure though. Especially if the rotor blade has a taper with varying chord thickness. I am learning so much from all of you and enjoying it immensely.
Rudy

[rdresch]

Larry, Thanks for the pictures. I have it now. So you are splitting the blade about 3/4 inch behind the leading edge and across the grip. The part I didn't see before was that you were filling in the low spot to smooth out the airfoil shape. Also you are running the same blade section width all the way across the length of the blade, so the area stays the same. Now I can understand why you have enough power to turn the blades. I was thinking your blade were wider at the root than the tip like scale. The picture on you website made it look like you just added a triangle section at the back of the blade wider at the root and tapering to 0 at the tip. Kind of like a wing with a flap at 45 at the back, but tapering in to 0 at the tip. That's why I wasn't real excited about the blade profile. I now understand. Sometimes when things don't make sense there is a reason.
Rudy

[v22chap]

Yep you got it Rudy.. I split it right where the hardwood leading edge meets the balsa .. and I don't split it all the way thru ,,that way it doesn't completely seperate from the leading edge.I usually leave about 1/16 " uncut and then just poke the exacto knife thru it to searate the bottom 1/16 " so it bends easier.
Yep .. I just try and keep it simple ,,,after all it is just a model and once I prove it can do the conversion ,,then I will work on making it all scale and do it .These blade really make some serious lift .

Scott or some one ,,I think I didn't finish answering your question on the Kavan type balancing . The old 1980 era birds had you build the blades too. You actually glued the balsa to the hard wood leading edge and then had to sand it to shape and balance it.

The quick version of this is mount an old style single edge shaver blade or a flat exacto knife blade in your vise and lay the heli blade at a 45 degree position on this knife blade and get it to balance. When it does push the blade into the knife enough to make a mark ( and be sure to be careful when around this setup that it doesn't leave a mark on your fore arm or fingers....don't ask me how I know this :(----)
Then you rotate the same blade 90 degree to the first position ...so that when you balance again and push down on the blade ,,it makes another mark and this one is across the other one leaving an "X"
Do the other blade and then measure the weight of each blade and the distance from the root to this X ....

Now comes the hard part ,,,you have to decide which blade to add weight to ,,if one needs it,,, and also which end to add it to so that the "X" (CG ) will be close to the other one and also weigh the same .
When you do this you come out with a pretty good set of balanced blades.
If I get my scanner fixed I can scan the Kavan instructions and it has some pictures that help in deciding which blade to do and which end to put the weight in .
Larry

[rdresch]

Lets not let this thread die. This is really important stuff. Any one tried to do any tests or working on any blades yet?
Rudy

[avianaut]

Rudy, I'm still tagging along. My friend here is helping me learn Xcell, and I have some notes on how I want to build on Shannons spreadsheet. I can't give it top priority, but I'm still on it.

Cheers.

[BigS]

I have made half of my blade mould for the Trex size proprotor. I'm not really happy with it, so I will make another one.
I have been building my new 5S Trex so haven't done anything on the osprey for a while.

I hope to get back to building the osprey in the next couple of weeks.

Shannon.

[rdresch]

It will be interesting to see the result. I have concerns about the power requirement required to turn a blade with a lot of twist in it at normal heli rpm speed. I am afraid we will need to come up with a compromise. Half prop and half heliblade. Time will tell
Rudy

[BigS]

The other thing to remember is that the small fixed pitch helis have a lot of camber on the blades, and they can hover on a small amount of power. My guess is that a lower headspeed will be needed to hover.

Shannon.