"Modern" wooden gyro rotorblades

[Vance]

Hello Rob, Why extruded? From what alloy?

Thank you, Vance

[C. Beaty]

Originally Posted by gyromike

Chuck,

Were the blades for your 3-bladed rotor constructed like the Bell 47 blades?

Mike, below is a scan of the spar section used for the 3-blade rotor.

It was pretty much a copy of Bell-47 construction except ¼ chord balance was accomplished by an airfoil shaped LE lead bar nailed to the spar with heat-treated wire brads.

The afterbody was vertical grain balsa sawn from planks and shaped on a homemade profiling machine.

Straight grained birch has about ½ the tensile strength of 6061 aluminum based on cross section area so twice as much is required.

I don’t remember materials cost but it wasn’t very much. However, if my labor was worth $1/hour, these were very expensive blades.

If you look closely at the video of that 3-blader in flight, you’ll notice there was zero stick shake.

[Bob]

Vance :
Monty the guy I got the plans from made them and has flown them for years says they are more forgiveing than lighter blades
in that they are slower reacting, which in my opinion the short coupled gyros need. they havn't came appart on him
and I am sure they won't on me , I bet my life on it !
Bob......

[Bob]

C.Beaty / Vance
I realise the idea of building Rotorblades goes against the grain of some folk, but wooden rotor blades have been used for years its nothing new by any means ! ballanceing them however is alot harder than it first looks.
My idea of makeing wider blades i.e. 10" instead of 8" cord causes problems in the ballance portion of the blades.
in one respect it makes them easier to ballance at 1/4 cord because there is more Lighter section behind the centerline than there would have been , so less weight will be needed to bring them into ballance, but even at best these blades will be hard to ballance
even if I put in a 1/8"x2"x12' steel bar as ballance weight...(as far foward as I can get it )
so if we asume 1/4 cord is the best place to ballance a rotor blade, that leaves only one method of ballanceing them right ? like the lead weight hainging off the leading edge or nothing
as wooden blades will take alot more weight to ballance than composit blades...
I understand why the blade is ballanced infront of the half way point on a rotor blade ... but why as far foward as 1/4 cord ?
that just makes ballanceing that much more dificult !...
is it because the blades can become "Squirley" or not track as they should ?
... more than likely its just a "Rule of thumb " adapted ages ago and every one still uses it because it works so well<Grin>
nothing wrong with that at all, i just like to know Why !
thanks !
Bob........

[mceagle]

Bob, a blade balanced tail heavy will be unstable and react the wrong way to gusts or air pockets. Effectively, the blade is tryind to turn over and go tail first. This leads to severe flutter which can destroy the blade in a flash.

[Ga6riel]

Originally Posted by Vance

Hello Rob, Why extruded? From what alloy?

Thank you, Vance

Hello Vance
Extruded provides a product with consistent engineering properties, ie weight thinkness mechanical properties.

It is possible to make dies that produce an exact profile
Nose weight can be incorporated as a slip in inbetween the webs
The surface can be polished to improver performance
there are no mechanical joints to inspect in the blades themselves
I know of blades extruded from 6061 and 6063 alluminium

[C. Beaty]

Bob, here is something you can prove for yourself.

Cut a strip of flashing aluminum, say 3/4” wide x 12” long and make an arbor using a 1” long 6-32 machine screw and nut located at dead center. Spin this in your 3/8” electric drill and it will flutter violently.

Then make a leading edge noseweight, located as near to the leading edge as you can without the hole breaking through, using a ¼” long 6-32 screw and nut located at ~80% radius. It no longer flutters.

With a lump noseweight, spanwise location is critical. Located at the tip, your rotor will flutter inboard, located at midspan, it will flutter outboard.

When a real rotor flutters, you fall out of the sky at best; at worst, a shower of toothpicks.

An airfoil, including a flat strip of aluminum, has its aerodynamic center at ¼ chord. Aerodynamic forces act as though they were applied at ¼ chord.

If an airfoil is stiff enough, it is sometime possible to get by without ¼ chord balance. Propeller blades are short and stiff. Airplane wings, if metal skinned, are often stiff enough. A Cessna 150 manages with a single wing strut. Fabric skinned wings require “V” struts to prevent flutter. At 300 mph, most likely both would flutter.

[Ron Iaconis]

After what Chuck Beaty has just described, it's sure makes so much common sence to go extruded aluminum spar and aluminum skin.

[Doug Riley]

Yes, the 1/4-chord idea is not a rule of thumb, it's a rule of nature. The aerodynamic forces on the trailing-edge protion and the leading-edge portion happen to balance out at this point on a well-designed airfoil. On a badly-designed and/or a flexible airfoil, the aerodynamic balance point shifts around as angle of attack changes. This causes no end of trouble.

It was the downfall of Rogallo hang gliders.

[Bob]

Ahhh good info Thanks guys !

are there any objections of using a 10" wide cord rotor blade on a light gyro ? If they are ballanced correctly as far as I know they should work fine ... most rotorblades are 8" or less in cord
the original blades I had were 7.5" in cord .
i believe the added lift in this thin air up here will give me the edge I have been needing at least according to gyro-calc it works
but I am leary of the flap line in the program... not knowing that much about blade flap I am hesitent to try it .
if i calculate my old rotor blades on gyro-calc it shows my gyro not being able to leave the ground... and so far that has proven true for the most part...but the flap angle are a fairly steep line
in which you are well above the flap angle at normal flight...
with the 10" wide cord blades the flap angle is slot broder
and protrudes into the flight envelope a bit....
I believe it means that it Can Flap at say 30mph not that it will.... but it is possable for the blades to do so... and that isn't good in my book... but sense normal flight speed will never go below 40mph it should well be acceptable
.....
though I have never thought of my gyro as a High altitude flyer
that is what I am actually trying to build here because of the thin atmosphier here... it may well be able to reach 10,000ft ASL when completed... or perhaps even higher.
....
I have to give this ballance thing some serious thought my pre concieved notions seamed to be all wet. so time for some resurch
thanks for the help !
Bob.......

[brett s]

You might want to research "aspect ratio" before you start increasing the chord...there's a reason most have smaller chords.

[Doug Riley]

Yes, but in this case it's not aspect ratio.

As Chuck Beaty has pointed out over and over again, the wider the blade (other things being equal) the more the rotor resists cyclic pitch changes. In our teetering rotors, the whole rotor assembly has to "feather" relative to its own orbital plane, twice per rev. The wider the blades (that is, the higher their moments of inertia about the feathering axes), the more they fight this constant quick feathering.

As the blades fight back, stick shake and 2/rev vibes are a result.

[C. Beaty]

A good many people are confused by lift, Bob, so you’re not alone.

No matter how good or how bad a rotor or wing, with enough power, it will fly.

If your flying machine weighs 500# AUW and flies, the lifting device is producing 500# of lift.

The real question is how much power is required to produce 500# of lift? If 100 hp is required and you have 50 hp available, it will never leave the ground.

Increasing chord or adding more blades does not produce more lift but may reduce the power required to make them spin at flight speed.

Rotor blade tips are typically moving at ~300 mph so a good bit of power is consumed in moving them through the air, particularly with draggy airfoils and external noseweights.

More blade area, for the same lift, lowers the tip speed and reduces power consumption of what is called profile drag power.

Lift varies as the square of airspeed. If you double the chord, the blades only need to turn 70.7% as fast for the same lift (1/square root of 2)

The power required to drag the blade profiles through the air varies as the cube of tip speed; double the tip speed and 8x as much power is required.

Double the chord so the blades run at 70.7% of the previous tip speed and the profile power is only .707³ = .354 of its former value except you’ve also doubled the chord; .354 x 2 = .707, right back where you started.

Doubling the chord may reduce blade profile power to 70.7% of its former value but you’ve added weight and profile power is only a part of total rotor drag.

The other part of rotor drag is called induced drag, the drag due to lift. The power expended in producing downward acceleration of air mass. That depends only on disc loading and isn’t affected by blade area or profile drag.

So, back to the first question; will more blade chord increase lift? The answer is no but it might reduce rotor power enough to enable the machine to leave the ground.

[Bob]

C.Beaty:

So what youe saying is stick with the standard 8" blade cord and increase the disk diamator if anything Correct ?
....
well I already did that , I went from 22ft rotor diam to 26' rotor diam. with the adition of a 4ft hub bar and it seamed to make alot of diference, infact it did get off the ground finally under its own power.
Increaseing drag is something I cannot afford to do as I am working with a mear 40hp Rotex so it sounds like i better stick with the 8" cord blades and stay away from the 10 " cord blades due to the added drag. I just doubt I have the hp to push any more drag than I have already.
if I procede as I originally planned with makeing 12 ft 8" wide blades that will give me a 28' rotor diam. I feel as with as much help as the 4 ft hub bar made in helping the gyro get off the ground that this would probly be the best bet in the long run.
I am still desideing on what to do as you can tell, but I will construct my own blades soon hopefully I will start construction next month ....
so I need to make up my mind soon on which way to go ...
Without a doubt in my mind I can say with the 24ft rotors the machine would fly , it was only my own short comeings that caused the crash last year , as the machine did leave the ground
under power... I do Not know it it would have sustained it however, but I suspect it would have easily.. as it gained air speed extreamily fast when the wheels left the ground ...like it should have.
....
the more I read and hear about the subject the more I think stick with the 8" cord and just increase the length another 2 ft
on the rotor blades and this will more than cure the lift
problems.
.....
I was aware of increaseing the airfoil size would increase the drag but was not awair that the added lift could be cancled by the drag , in that it slows down the rotors spin... Gyro-calc planly stated that the rotor RPM would drop by almost 100 rpm
I thought this a good thing, but perhaps its not if you figure the lift is due to the speed of the blades through the air.
so , Yah it makes sense ! thank you for your input !
.....
with all this information that you guys have provided I can safely say that there is only one way to go here... so there isn't much desideing to be done now... thank you !
now all I have to deside to do is make the blades 11 ft or 12 ft long <grin>
Knowing My pridicument and altitude do you fell'as think it better to go with a longer rotor blade or should I just stick with what I had at 26ft diam ?
I am guessing , but I think most would say longer is better
to a point.... I don't want the things to touch the ground and they might if I go 12 footers, 11 footers should just barely clear the ground by inches .

thank you all for the help you've given ! even those that i havn't mentioned directly your coments have been noted ! and I thank you !

C ya '
Bob........

[Al_Hammer]

As Chuck Beaty has pointed out over and over again, the wider the blade (other things being equal) the more the rotor resists cyclic pitch changes. In our teetering rotors, the whole rotor assembly has to "feather" relative to its own orbital plane, twice per rev. The wider the blades (that is, the higher their moments of inertia about the feathering axes), the more they fight this constant quick feathering.

As the blades fight back, stick shake and 2/rev vibes are a result.

I think we can agree that the blades feather with respect to the flapping plane, but I'm having trouble accepting Chuck's statement that the blades will "resist" being feathered.