Quick way to prototype F/glass vtol blade plug/moulds

[BigS]

Well I was going to post this in another thread, but it might be a bit too long and change the topic a bit too much so here goes.

There is quite a bit of discussion about making or purchasing counter rotating blades for a V22 and similar Vtols.

Here is how I will be making some composite blade moulds.

To start with I have used the foils that Rudy has posted here
http://www.rcgroups.com/forums/showpost.php?p=5361834&postcount=34
and plotted them using profili airfoil program.

I then printed them out on sheets of paper. I printed them on the opposite sides of the sheet thinking that I had to make the bottom half too :rolleyes: but forgot that I was only doing this to show how it's done. :o (thats why the airfoil names are jumbled up in the pic)

I then marked out the pitch angles of the foils on the sheets of paper and cut them out. Then I glued the paper onto some card and when dry cut them out.

Then using some scrap balsa I glued the patterns to a flat piece of wood so they sat vertical in their corresponding locations.

Then I sheeted the area with some balsa wood, just enough so i could fill the area with wax.

Shannon.

[BigS]

I then started to pour wax onto the plug and ended up starting to make a mess. I figured that I had to put a dam around the plug to stop the wax running away till it cooled :rolleyes: .

I then let it cool off for a bit, then cut away the dam, and used the back of a hacksaw blade to scrape the wax down to the level of the card templates.

Came out pretty good for a rough demo attempt.

I will need to make four of these for a set of composite proprotors (two tops and two bottoms, opposite handed) but they don't really take long to make.
they would also need the blade bolting areas and glass layup runoff areas.
I would allow for the trailing edge thickness when drawing the foils so that the chord doesn't end up narrower when molded.

Other options would include a slight dihedral on the blade grip mounting to allow for coning of the proprotor disk.

The dimensions on this one were just thrown in to make it look like a proprotor but shorter. If anyone is curious the chords/pitch angles are as follows from root to tip, 50/20degrees,50/10degrees,45/5degrees,35,0degrees.
Spanwise spacing is 50mm for a total span of 150mm.

I think these could be done in any size and molds made out of epoxy or polyester resin (more info on mold making in the composits forum) ;)

Shannon.

[BigS]

Another thing to do would be to soak the template edges with CA to stop them from pushing down so much when shaping the wax.

The best thing about using wax is that you can glass right on top of it and it will always come out (if it does decide to stick for some silly reason, you can just melt the plug out of the mold)

I have changed my mind about the micro size V-22 that I am building and I am going to make it using Trex parts.
http://www.rcgroups.com/forums/showthread.php?t=477578
I may decide to build this as a BA609 :cool:

The blade grips for the Trex cost about the same as ones for the hornet etc but the precision is much higher due to the larger ball links etc. Now I will need to work out the approx AUW and the wing loading so I can ballpark the proprotor pitch speed to fly in airplane mode. then I can start making the plugs/molds for the proprotors.

I think for a RC proprotor the chord will need to be reduced to around 50%(more :confused: ) of the real one and maybe even the spanwise twist reduced to suit(pitch speed calcs anyone?) :D

Shannon.

[rdresch]

Very impressed. You don't mess around do you. First the angle of the inboard section of the real V22 is about 45 degrees. I am sure Larry will jump in with the actual number.From all the pictures I can find it looks like the chord width stays pretty much the same for the entire span. since the inside is at a 45 degree angle perhaps a slightly wider chord so it is even from a top view. Some quick info. The tip speed on the real V22 is 662 fps. Most scale heli models use about half that number. For example a 4 ft diameter rotor at 1600 rpm is about 332 fps. a Trex with 215 mm blades at 2400 rpm = 277.33 fpm. Remember this is a combination of a prop with the diameter of a heli blade. The rpm it turns at will be much lower than a prop. Still the larger diameter will give you the tip speed. A 6 inch diameter prop at 14000 rpm will give you a slightly higher tip speed as your T-Rex 325 mm blades at around 2500 rpm. I would guess you could copy the pitch angles from a 6 inch prop that gives the speed range you need for a starting point. Note I am making many assumptions here that seem logical to me but may well be false. Still, I have asked many times on many forums for info on this subject and drawn only blanks.
Rudy

[v22chap]

Been to busy today to get on the puter ,,but boy it looks like some of you have.... :D
Yes I have been told ,,don't have figures on paper ,,,,that the root end is 45 degrees of twist and that the collective then goes another 47 degrees in fast airplane with the throttle pulled back to keep the tip speed down.We can't do that with our low power IC engines...now Rudy may have the ablity to do that with his overpowered turbine as it will be like the real one.
I don't have the education to do the math and have just been poking in the dark ...but what I have done with my modified heli blades seems to show that twisted heli blades will work. I have been wingborne with these new blades I am sure ,,,as I one time hit the wrong switch and turned all my heli controls off and still flew level clear across our long flight field ,,and a heli would n't do that on its own and won't even stay in the air at the same pitch settings with the flat blades.
I think Rudy is right on the 6 " prop copy ,,we in the model world don't have to be so precise as they do in full scale.

And the reason you have drawn blanks with your forum question before Rudy ,,is the fact no one else knows ,,,except for the real V-22 engineers and they are probably not on our forums ,,or can't give out that info any way. Lots of stuff that I get comes from a worker down the line that says they didn't know where this info came from and if I say it came from them ,,they would deny knowing me .... ;) :rolleyes: :D
My son has his degree in aeronautics and he can't seem to come up with much to help either .,,, so it is kind of vodoo ,dark magic stuff that we are messing around with here. :rolleyes: :D
Maybe that is why it is so addictive ;) :cool: :D
Good stuff guys keep it coming ,,,maybe together we can crack this nut.
Larry

[rdresch]

Larry, I am sure that the V22 probably has an additional 45 degrees of pitch available. But that would put the blade at 90 degrees. Just what you would expect for a fully feathered prop. I can see no other reason why there would be that much pitch in the blades otherwise. Of course I've been stricking out a lot recently.
Shannon. 50mm is just about right for my 48 inch rotors I think the T-Rex blades should be around 30 or less.
Rudy

[Tuner]

Nice I am alway worried about hand made mold as I have never gotton the best join of the halfs. Good enough for fuselages but not wing or props.

Much repsect this wasa great overview! Makes me want to send a block of wax or plastic to the machine shop???

[rdresch]

Tuner, One of the guys at my field builds carbon fibre blades from aluminum molds he machined. He uses slow setting epoxy then the carbon fiber on both halves then clamps them together. He does use a foam core. I've never seen one of his blades split at the seam.
I did build a mold for a twisted blade from glass, and made a couple blades with a similar technique without the foam. They broke widthwise before the joint failed. They are a ton of work, though once you have the molds made you are looking good. It is too much work to do on a blade design you have no assurance will work. I do think this thread is zeroing in on a reasonable starting point. With all the great responses and help coming from this forum, we'll git er done.
Rudy

[Tuner]

I would love it if you could post those profiles or when you finalize the profiles you will use please upload them so others can have fun playing with wax!! :)

[v22chap]

This is great stuff guys ,,and I am setting here probably one of a few non government civilians that has been allowed inside the 109 hanger at Pax River .MD and have been standing in front of a disasembled V-22 and the rotor head and even reached out and touched the rotor head ,,but didn't remember to look at the airfoil or for that matter snap a pix...of course they were pretty guarded of anyone having camera's in the shop area.
I guess I was just awe stricken to the point of being drunkenly blind and mute.
I had 4 days and talked to a lot of people and that is probably the problem as I talked about the model to so many people cause they wanted to know about it ,,,and that didn't leave me time to talk about what I wanted to .
Wish I could be more help on the blade making process ,,but I have went about as far as I can go with it ,,,unless my wife gets done with college soon and lets me play around more. ;) :rolleyes:


Rudy ..if the blade angle is measured at the tip where it is 0 degrees when the blade root is 45 degrees ... it would put the root at 90 ,,but not the tip... and your right I am sure they don't use it all

Here is some interesting info and maybe usefull to someone that I found in my NASA technical memorandum on the XV-15
Heli mode tip speed (ft/s) is 740 ,,,RPM 565
Airplane tip speed (ft/s) is 600 ,,,,RPM 458
heli overspeed test tip speed 818 RPM 625
airplane overspeed test TP 865 ,,,RPM 661
Couldn't find anything about the collective amount

[rdresch]

Larry I have been usung 662 fps as tip speed. I guess thats close enough.

[v22chap]

Hey Rudy ,,, right on this time ;) :D There is so much information out there and different guys giving it out that one has trouble knowing what to believe or what to do.
I am hoping ,,like you that this thread may bring us in to the right direction on the prop rotor deal and it sounds like we are headed in the right direction and thanks in part to your attempts with the T-rex and its sacrific :o :D
Thanks
Larry
P.S. I did find in writing that the XV-15's twist degree is actually 43 degrees of non - linear twist ,,but here again this material is dated 01- 1975 :D
I think that I will back my twist back a little ,,maybe to 40 degrees and see what happens ,,, the last attempt I did ,,,seemed to really bog the engines and actually was the reason for the complete demise of the gray bird that survived the spiral to the ground in the video.

[rdresch]

Larry I think you are right about the twist being non linear. I have been looking hard at all the pictures and am ready to print a few that have really good angles for measuring and do just that
Rudy

[BigS]

I also think it is non linear as the pitch angle difference needed is dramatically more as you get close to the root of the blade. I think its more like an exponential pitch increase. It should be very similar to a propeller.


Shannon.

[BigS]

Just had an idea of an even quicker way of moulding the blades, instead of making the plug, just make the mold from the wax as I showed above and use that to prototype a blade or three. You would have to be careful when removing the first couple not to damage the wax surface of the mold and ruin the chances of making more blades from it.

The other option would be to remove the finished blade from the two mold halves and sand it smooth, paint etc. Then put it back in one of the wax mold halves and take a mold off the (now better quality) finished blade. Do the same for the other side and you have a finished high quality mold.

I think I will be making my first few blades by making the mold from wax then taking the blades directly off that as it will save a bit of time when prototyping them.


Shannon.