birch blades

Hoskins type rotors

Hoskins type rotors

Here's some photos of the cross section of my blades. Hope this helps.
 

Attachments

  • photo.jpg
    photo.jpg
    38.8 KB · Views: 10
  • photo 2.jpg
    photo 2.jpg
    34.7 KB · Views: 10
  • photo 3.jpg
    photo 3.jpg
    37.1 KB · Views: 10
SpyroGyro - Hey that's cool, thanks for posting the pics of the cross section. Hmm, that design differs from what the Hoskins plan in my book describes. The design shown in my book has a solid laminated plywood spar with a steel strap embedded in it. See my earlier posts on this thread for pictures. I suppose it's possible Hoskins "upgraded" his design later and went with the aluminum leading edge like that. But I wouldn't be so sure about that. The previous owner may have gone out on his own with that design. Not that it is necessarily bad, I can see where it would require a smaller nose weight (or even none at all). But to be honest the airfoil profile on the leading edge looks somewhat crudely shaped and smacks of amateur construction. And I'm not aware of anyone else flying blades quite like that (someone correct me if I'm wrong), so IMO I'd be leery about being the first to test fly someone elses unproven design. The quality of the gluing/bonding is a matter of life and death; I'd be concerned about that based on the quality of the shaping work. Not trying to crack on your blades, just food for thought.

PS>> You show cross sectional views of an uncompleted blade segment and also a pic of what appears to be completed blades. Do you have two sets?
 
Last edited:
SnoBird - Thanks. I know your concern comes from a good place. I have several hours of video showing these blades flying from the previous owner, so I know at least they've been broken in successfully. I think you may be right about the liberty he took with the GyRotor plans. Although in a bird strike scenario I would prefer a solid aluminum bar vs. a strap. The bonding agent is some sort of epoxy and the finished blades are wrapped a few times with fiberglass. Hopefully this sandwich construction would give some advanced notice before any delamination. I think I'll go ahead and fly them since I have proof that they're in working condition, but you're right about untested rotors, don't trust em'. The previous owner tells me that he had some SkyWheels on there (which he sold) and they didn't give a very smooth ride in turbulence. He said these wood rotors ride like a Cadillac and soak up windy days like some kind of "wind sponge". There are some pretty heavy lead weights on the end of the blades so I'm guessing they compensate for the lack of weight elsewhere and account for the smooth ride. I only have one set. The builder made the cross section because he wanted to show people later how they were constructed. He didn't really spend as much time or detail on the cross section compared to the finished blades.
 
Thanks Spyro, I'm glad you weren't offended by my post, again wasn't trying to bust on your blades. I like wood blades and like to learn more about them, so it's interesting to see this variation. The fact that they have been flown repeatedly is definitely a reassuring factor. The fiberglass wrap sounds like an improvement too. And I agree the aluminum leading edge would do a better job at chopping up birds than a wooden one :). Did the previous owner specify what type of aluminum he used for the spar? Or how he completed the bonding process? The main thing I worry about when it comes to buying/flying something someone else put together (and possibly designed) is in materials and workmanship. Once the blades are built, you can't see if the builder properly prepped and cleaned the bonded surfaces prior to gluing (which is absolutely critical). You can't be sure they used the proper adhesive or clamped them the required amount for the required time. And unless you can see something printed on the aluminum spar that tells you they used the required grade of aluminum, that's another thing you have to wonder about. Unless you personally know the seller and trust them, you gotta go out on a limb that's hundreds of feet up in the air when trusting that they knew what they were doing.

Sorry to sound like a doubting Thomas or a wet blanket. I only have 11 hours in a gyro (dual instruction) and although it was thrilling flying them, I also experienced a healthy dose of fear. My worst nightmare would be a rotorhead coming off or a blade coming apart in flight, so I think very conservatively along those lines. Good luck with your Bee and keep those pics coming.
 
My engineering tummy tells me not to trust an adhesive bond alone (especially if it connects two very different materials with thermal expansion coefficients which are probably orders of magnitude apart) unless it was crafted under the supervision of one real expert. I'd always go for a rivet connection (for which there are pretty straight forward design rules) and use the adhesive as a belt and braces approach.
There are huge cyclic shear loads in the aluminum/plywood bond. Did anyone do some sort of analysis for the fatigue life of the bond? Are fatigue data available for the bond?

DON'T USE THE BLADES UNTIL THESE QUESTIONS ARE ANSWERED!!!


Just my two cents,


Juergen

PS: still I love the "rough and ready" approache of that design..;-)
 
Last edited:
Snobird - I've read somewhere that rotorcraft (in general) are 8X more dangerous than general aviation aircraft. Gyros don't nearly have as bad a safety record as EMS or Logging Helos. I you're going to fly rotorcraft, you're going to take more risk. As for rotor blades, today's modern epoxys simply must be better than the glues available on Bensen blades in the 1960's and I've never heard of Bensen blades delaminating in any significant quantities. With wood-specific epoxies it's usually the wood which breaks 1st instead of the bond. Wood is also not life-limited like aluminum. This is why many helicopter flight schools in the 70's & 80's used wood blades instead of aluminum. They would get 3X the life out of them. However, wood takes more care and inspection so lifetime is based solely on condition. Assuming condition is good, you can place every bit as much faith in them as other materials. The reason you don't see any wood rotors around anymore is because the process for making wood rotors does not lend itself to modern mass production techniques (like extruded aluminum). Wood rotors require far too much eyeballing, sand finishing, and sealing to be mass produced by todays standards. But for someone wanting to spend $200 instead of $2,000 it might be just the ticket.
 
My engineering tummy tells me not to trust an adhesive bond alone (especially if it connects two very different materials with thermal expansion coefficients which are probably orders of magnitude apart) unless it was crafted under the supervision of one real expert. I'd always go for a rivet connection (for which there are pretty straight forward design rules) and use the adhesive as a belt and braces approach.

There are huge cyclic shear loads in the aluminum/plywood bond. Did anyone do some sort of analysis for the fatigue life of the bond? Are fatigue data available for the bond?

DON'T USE THE BLADES UNTIL THESE QUESTIONS ARE ANSWERED!!!

Just my two cents,

Juergen

PS: still I love the "rough and ready" approache of that design..;-)

For what it's worth, I agree but I offer that the results of an "analysis" should take a backseat to the actual testing - to destruction - of one of the sets of blades. What I'm suggesting here is to build at a minimum, two identical sets of blades, build a whirl tower and and actually test one of those sets to destruction. Once you know how many hours it takes for fatigue to start to show, divide the time by two and then you'll have a reasonably safe figure for flight time on the remaining set of blades. To me, to simply go up and hour by hour or better, minute by minute take your chances on a questionable construction method, that is insane. There is plenty of advice on the Internet with regard to how to build a whirl tower, electric motors are cheap and so is electricity.

tyc
 
There are lots of arguments for tyc's approach under the circumstances. My last question
Are fatigue data available for the bond?
is the weak point because all the input data necessary for a proper analysis will be next to impossible to obtain. Yet an analysis could be helpfull in one special way: If separation of the bond starts inside the blade you will not be able to watch the progress of the fatigue damage. The first thing you will see is one of the blades desintegrating (and the other one shortly thereafter). Having said that I would like to offer two ideas for discussion:
a) Us an analysis to find out where the maximum stresses in the design are. My guess is they are insde the blade so you might want to make some inspection holes in the lower side of the blade (which are taped over while the blade spins) to find out where and when separation begins.
b) Use each blade of the "expendable" set in the whirl tower test separately with a counterweight. That way you have 100% more test data from one set of blades..;-)

Cheers,

Juergen

PS: it just occurs to me that there is one real snag: The largest contribution to fatigue damage is probably from the cyclic bending of a blade that is advancing and retreating on a rotorcraft in *forward* flight. How do we get that cyclic load in a whirl tower test?
 
Last edited:
... The largest contribution to fatigue damage is probably from the cyclic bending of a blade that is advancing and retreating on a rotorcraft in *forward* flight. How do we get that cyclic load in a whirl tower test?

As I understand it, the cyclic bending of a given blade is the greatest cause of fatigue for a rotor blade. At flight speed, the up and down cycling of the collective puts considerable stress on the blades and all associated components. Simply spinning blades around in a whirl tower, hour after hour will provide very little in the way of knowledge with regard to what it will take to fatigue the blade; it's the blade grips which will "be under load." In aproperly set up whirl tower testing program, the deliberate up and down cycling of the rotor blade/s as they are spinning, rotating at flight speed will be included. This will test those blades and all associated components of a given rotary design. The whole idea of such testing is NOT to see how long the the blades will last, rather it is to see just what it will take to begin to begin to destroy them and reasonably close observation by even untrained eyes will show evidence of the beginnings of delamination and/or the beginning of fatigue well before delamination begins to seriously get underway and cause catastrophic failure, when fatigue in its myriad of forms causes parts to begin to fly apart.

tyc

.
 
Last edited:
tyc wrote:
There is plenty of advice on the Internet with regard to how to build a whirl tower, electric motors are cheap and so is electricity.
To incorporate cyclic loads the whirl tower obviously has to be equipped with some sort of mechanism to achieve this. Now I have been looking around a bit but didn't find any good design. I'd appreciate if anyone who has a link to a good whirl tower design could post it in the forum.
Thanks,

Juergen
 
To incorporate cyclic loads the whirl tower obviously has to be equipped with some sort of mechanism to achieve this. Now I have been looking around a bit but didn't find any good design. I'd appreciate if anyone who has a link to a good whirl tower design could post it in the forum.
Thanks,
Juergen

Don't rely on the Internet for your information - do it the old "tried and true" way; i.e., To cause the collective to rise and lower every sixty seconds or so, hour after hour, day after day, a simple cam mechanism will do it. A first year basic engineering text will give you a fiew good ideas in this regard, plus a few other "tips" as well..

As for a whirl tower design, find a place a mile or so away from inhabited structures; good safey zone or belt should parts start to come off.

Four sturdy six by six or laminated eight by eight PTLs well mounted on sono tubes which go down at least six feet or 180cm. Rows of old tennis court nets attached from the inside of the PTLs, from the top to a foot or so from the bottom will do the trick here.

Mount an electric motor of suitable size/HP in the middle of the PTL poles, then attach you mast and rotors, then go from there.

Hope this helps.

tyc
 
Last edited:
Beeing a mechanical engineer I think your suggestions are very sound. Yet, ....there's a lot of detail to take care of from that point on and I hoped to find some nice drawings that would spare me quite some of that work. Apart from that Germany is much smaller than Canada and it's not easy to find a spot to set up a whirl tower from which a blade could detach without hitting someone...;-)

Cheers,

Juergen
 
Last edited:
... I hoped to find some nice drawings that would spare me quite some of that work. Apart from that Germany is much smaller than Canada and it's not easy to find a spot to set up a whirl tower from which a balde could detach without hitting someone...;-)

Save for the actual installation and assembly of the stripped down sub-assembly for the powered rotor and associated parts, you shouldn't need drawings for a whirl tower. Just keep it as simple as you know how, get as many old tennis court nets or the like and startring at the top of the polls, hang them up. If you over-engineer such work you'll never get the job done..

As for vacant land there in "downtown" Germany, the last time I was there, outside of Munich (one of the prettiest cities I think I've ever seen; good beer and fantastic frauliens!) it seemed to me there was there were plenty of acres/hecters of farmland which can be leased for a year or two or are you over by France or up there near the Baltic coast?

tyc
 
Last edited:
Kolibri you're not really going to build a whirl tower and spin a set of wood blades to destruction like those shown earlier are you? That could take years and years of effort and cost many thousands of euros. Wouldn't it be a whole lot easier and cheaper to just buy a set of fully tested and proven Dragon Wings? :)
 
will a wheat stone bridge work on wood? it's used on rotors durind tests for strain measuring and the info is passed to a recorder via slip ring, frank had them all over the R-22 prototype ,inside otuside top bottom -all over.....
 
As for beer and fraeuleins Duesseldorf can compete with Munich easily, as a city probably less so....
Now to be serious (and answer SnoBird's question) I don't want to spin full scale blades but a model rotor of some 6' diameter. I want to design a rotor with a trailing edge flap for which torsional stiffness is a crucial parameter. My last postings to this thread were motivated by genuine concern about using those bonded blades and I just wanted to share some thoughts on how to test them properly (if someone wants to).
As for strain gauges (for which the wheatstone bridge is an auxiliary circuit to read them) I think there should be a way to use them on wooden parts. You might have to go to some length though.

Cheers,

Juergen
 
Last edited:
Slip rings

Slip rings

would probably be required to pass the signal from the strain gauge to the bridge circuit, unless you use wireless instrumentation which would include the bridge circuit and transmitter being built into the blade. I would think that in both cases your measurements would alter the characteristics of the blade you are measuring with less impact, of course, if you used slip rings.
 
Top