Two-bladed vs. Multi-bladed rotors.

[Kugel]

I posted this on the General Discussion Forum. Lots of folks have looked at it, but only one reply. I thought maybe I posted it in wrong place, so I'm trying it here. NOTE TO ADMIN: If this is wrong place for it, please feel free to delete it. THANKS!! :)

I've been lurking for a while, and I have been interested in been interested in Autogyros for a long time.

I have a few questions, based on observations of the many rotorcraft that I have looked at:

1) Why are there not more (or any) 3 or 4 bladed gyros?

2) What are the advantages/disadvantages of a 3-4 bladed gyro?
a) more lift for smaller diameter, or is it all about disk loading?
b) more speed with smaller disk diameter?
c) higher/lower rotor RPM's? higher RPM = Smoother Stick?
d) does small disk diameter = higher cruise speed?
e) more blades = Smoother stick?

3) A super simple system would be a totally fixed rotor head, except for a manual "rotor head angle (pitch?) trim" (kinda like a flap handle on a FW airplane), with a 2-Axis Control system (Rudder and Elevator only).
a) what kind of control characteristics would such a machine have? lazy but very stable, like a Piper Cub?
b) probably less "sporty" handling, as compared to a 3-axis fully articulating rotor head with rudder, but would it be dangerous?

4) For Airframes, I've seen some welded 4130 steel tube type, but all of the aluminum frames seem to be bolted together.
a) 6061-T6 tubing has a very good strength to weight ratio: can it not be MIG or TIG welded to produce a very lightweight and strong airframe?

I know that there are a lot of tried and true designs out there, but I am just wondering about why some things are done or not done.

Thanks for any input!

Best Regards and God Bless!

Kugel

[Jazzenjohn]

I was one of the people who looked at the post but didn't reply. I can only say that I agree with everything Chuck said so I elected not to reply. Others may have done the same.

[KenSandyEggo]

Kugel, a friendly little hint on human psychology as to Forum's.......you should ask one or two questions at a time. Probably no one person has the answers to all your questions, so it means sorting and shuffling. Plus, it would take some time to answer, even though a lot of us blab on endlessly. You have a dozen-plus questions jammed in there. Start out with a couple or a group that's related and go from there. In other words, make it easy for people to answer. 12 questions in one lump is overwhelming, especially since they're basically not just yes or no answers. Some of your questions require several paragraphs to respond.

Which L.A.? Lower Alabama or Los Angeles or another one?

[Kugel]

I guess that was too many questions at once. I'll start over and start "smaller".

Lower Alabama! :)

Thanks and God Bless!

Kugel

[gyromike]

Kugel,

I changed the title of the thread to better reflect the content of your post.

You might get a better response. :)

[darrellwittke]

I love lists of questions so I'll try to steer you in the right direction. Most of this stuff can be found in the forum back logs.

1) Scissors effect. Ask Chuck B. or search it out in histories.

2) Don't know. More lift I think and better maneuverability. Suggest call Magni or Ernie B.

3) Cierva did it but better control/maneuverability without fixed head.

4) Welding can be done but not as robust a bolt together (ie. watch for cracks)

Good questions. Solve the scissors effect and you'll be set.

[spaceman spiff]

I like the bolted frames. Some if not most round tube frames use a cool bushing where the bolts go thru that has a flexible plastic tube liner. Gives quite a bit of vibration damping, and makes a very sturdy and somewhat flexable frame. good stuff IMO.

Can't help you with the blade questions.

[Vance]

[QUOTE=darrellwittke]
4) Welding can be done but not as robust a bolt together (ie. watch for cracks)

It has been my experience that a bolted joint is never as strong as a welded one. Most tube frame aircraft are welded for this reason. A gyroplane frame doesn't have to be very strong, so bolts are a reasonable way to go. A properly designed bolted joint can be strong, but generaly the bolted joints I have seen on gyroplanes are not designed to be particularly strong. It seems to be good enough for the aplication so unless lighter or stronger is the point, bolted is good enough. Cracks are only a problem on a badly designed joint or a poorly executed weld.

Thank you, Vance

[Kugel]

Thanks for all the input, guys! :)

I have found out, since I made the original post, that the main reason Aluminum is not MIG or TIG welded for Aircraft Frames is because it does indeed make a weak joint.

Example:

1) 6061-T6 Tubing has a 40kpsi yield strength and a 45kpsi tensile strength. The T6 is a solution heat treat and artificial aging spec, that give the tubing those properties.
2) When you MIG or weld it, you are MELTING the tubing in the joint. At the very least you are raising it above the annealing temperature. Therefore, the joint reverts to the annealed base 6061-0 specs...... which is a only 18kpsi tensile and 8 kpsi yield!! :eek:

There is a remedy, but it is too expensive and too complicated: you would have to weld up the entire frame, and then take it too an aluminum heat treat specialist (IF you could get them to do it, when they realized it was an Aircraft Frame) to have it whole thing solution heat treated and aged. Too much time, trouble and money for most of us and, probably unecessary.

I have also been looking into riveted/gusetted aluminum joints, too, and it is amazing how strong they are.

Best Regards and God Bless!

Kugel :)

[darrellwittke]

I thought I would attempt to explain what I meant in "scissors effect" Mr. Kugel. (Since I seem to garble almost everything Chuck B. has taught me on these gyro forums.) As you (and many others) have seen on the hub bar thread, the proper solution to isolating the inherent two per rev shake of the teetering rotor is to allow it to fly the path it must fly, as espoused by the aforementioned Mr. Beatty.

So when you add another rotor, you are compounding the effect, ending up with two rotors knocking each other about and inevitable stress cracks in the hub bars.

Boeing, with all of it's mighty resources, engineered a composite fiber hub to absorb these loads in its Bell 412 helicopter. Probaly out of reach of our recreationists abilities, however.

The solution, to my simple way of thinking, is to allow the two rotors to fly the path they must fly. This would mean two slider head types stacked one on top of another with some means of keeping them in 90 degree alignment or so to one another. Far easier said than done, (I think Skyguy was working along these lines.) but again, I digress.

4-bladed rotors do offer reported advantages (by both Boyette and Magni) of increased weight lifting and better maneuverability at that weight (also quicker spin-up?) but have never been produced because (to my knowledge) no one has efficiently solved the "bucking bar" effect.

(A bucking bar is used to flatten or shape rivets, functioning much the same as an anvil. The "scissors effect", as I recall now, is the effect of the RAF flexible mast and control rod arrangement working opposite of the probable intent of the designers.)

Sorry about any confusion my previous quick post may have caused. Sincerely, darrellwittke

[Rotor Rooter]

The Tri-Teetering Hub version (3-blades per rotor) of the Offset Teetering Rotor (http://www.unicopter.com/1127.html) might be viable for gyrocopters, if the flapping hinge offset was short. This rotor should 'theoretically' provide greater stability.
______________________

To date, its application to gyrocopters has not been considered. Perhaps some technical person may wish to evaluate it.

[skyguynca]

Thanks for all the input, guys! :)

I have found out, since I made the original post, that the main reason Aluminum is not MIG or TIG welded for Aircraft Frames is because it does indeed make a weak joint.

Example:

1) 6061-T6 Tubing has a 40kpsi yield strength and a 45kpsi tensile strength. The T6 is a solution heat treat and artificial aging spec, that give the tubing those properties.
2) When you MIG or weld it, you are MELTING the tubing in the joint. At the very least you are raising it above the annealing temperature. Therefore, the joint reverts to the annealed base 6061-0 specs...... which is a only 18kpsi tensile and 8 kpsi yield!! :eek:

There is a remedy, but it is too expensive and too complicated: you would have to weld up the entire frame, and then take it too an aluminum heat treat specialist (IF you could get them to do it, when they realized it was an Aircraft Frame) to have it whole thing solution heat treated and aged. Too much time, trouble and money for most of us and, probably unecessary.

I have also been looking into riveted/gusetted aluminum joints, too, and it is amazing how strong they are.

Best Regards and God Bless!

Kugel :)


Sorry but you are mistaken there. There are some guys who make Culver props, and VW redrives, Valley LLC. All their aircraft are welded 6061-t6 and can definitely show you that is in error. I have also built a biplane of tig''d 6061t6 and it still flies wonderfully with no problems in the airframe. 6061t6 as long as you only apply enough heat to weld (this is true of all welded metals) it will age back to t3 or t4 on its own in about 3 to 6 months depending on the storage temperature and as long as you did not over heat during welding. When I was doig the research on welding my airframe I talked with alot of professional welders, one guy at NASA (really nice group of engineers and they love this kind of stuff) and the guys at Valley LLC since they have been doing it. Oh and of course Classic Aero because Bert Howland has also been Tig'n airframes from 6061t6 that he sold as the H3 Pegasus and the H2A Honeybee aerobatic biplane http://members.cox.net/classic-aero/h2.htm (has competed in the IAC intermeadiate class) which is rated at +8 and -6 G's. I was afraid that welding round 5/8 .058 6061t6 tubing would fail at the welds and they more than proved it was fine and proven to work. After building my biplane I used to demonstrait its strength by standing between the cockpit and tail and jumping on it. I am 200lbs and it took it repeatedly with no failures. It was strong and light with no gussets just welded 6061t6 tubing.

You don't need to aneal or age or chemically treat after welding, there is no reason for it. I bought a book called perfomance welding also, it also says there is no need to do any of that unless you over heat the area during welding. I just made sure to make up some test pieces to perfect my setup before welding the actual airframe. I will be building a new aerobatic biplane this winter, it is the H2A Honeybee and it will be like all the others before it. A welded 6061t6 tig'd airframe.

[Kugel]

OK Skyguynca,

You obviously have personal experience in this area; and I am always willing to learn something new, especially from those with experience. Thanks for your input! :)

I do have a few questions, though:

1) What do you consider to be "overheating"? You said, "as long as you only apply enough heat to weld (this is true of all welded metals)". Are you implying that you are brazing, rather than welding? I mean, if you are "TIG welding", are you not melting the base aluminum tubing metal?

2) Can MIG be used, rather than TIG, to produce the T3/T4 joint results of which you spoke?

3) When you TIG'd your airframe, you did not use gussets in the joints. How much strength do you attribute to the shrink-fabric covering keeping everything in compression, rather that to the welding?

4) There is apparantly still a lot of outdated info out there on aluminum welding. All of what I could find indicated weakend joints (weaker than unwelded 6061-T6), from even high quality TIG welds (unless reheat-treated), or joint cracking problems on 6061-T6. What advances in materials and/or techniques have been made to reduce or eliminate these problems?

Sorry for all the questions, but our lives depend on getting it right..... the first time! :)

Thanks and God Bless!

Kugel