Blade lightening

[Jean Claude]

Since the length of the "standard" take-off throw is mainly due to the inertia of the rotor, it is surprising that industrial manufacturers have not yet tested lighter rotors.
Obviously the increased coning will increase the required undersling, increased vibration 2/rev, and transversal flapping angle.
But the long takeoff run is in my opinion a major handicap to the development of the recreational gyroplane.
The inertia of helicopter blades is justified by the pilot's reaction time in the event of an engine failure. But it is not necessary for a gyroplane

 

[Smack]

Thank you, Jean.
THAT is an interesting thought.
Do I understand the below correctly ?

To leave the ground, the gyroplane needs lift (some proposed 'cushion of air' awhile back).
The lift comes from 'airflow interacting with the rotor' and generally requires sufficient rotor RPM and airspeed.
A lighter rotor would spin-up more quickly and drain less power from the engine, so that now freed-up engine power would allow quicker acceleration to 'flying speed' and thus shorten the take-off run.
But....... have you not also stated that higher rotor RPM did not shorten the take-off run appreciably ?
Was that because turning the rotor to higher RPM drained too much power from the engine and thus it could not accelerate to flying speed quickly enough to take advantage of the higher rotor RPM ?
I think this must be the case, as Bensen used an independent engine for rotor RPM and supposedly reduced take-off run distance significantly.

 

[Jean Claude]

When we pre-launch at 0.5 times flight rpm (Ω), this means that the kinetic energy that must be acquired by the rotor during the takeoff run is:
Ec = ½ I Ω^2 (1- 0.5^2)
If our gyro has a rotor which rotates in flight at 360 rpm (ie 38 rad/s) and whose inertia I is 80 kg.m^2, then Ec = 43,000 Joules
If it takes us about 7 seconds to reach takeoff rpm, this means that it absorbs an average power of
43000 / 7 = 6200 watts just to overcome its inertia alone

By now replacing it with a rotor half as heavy of identical dimensions, it will then absorb the same average power but its inertia half as much will allow it to reach the same Rrpm while traveling half as long at the same forward speeds. . .
and therefore in four times less distance (since d = ½ acc t^2)
This is of course a very rough estimate since it would also be necessary to accelerate forward in half the time, which would require larger propeller thrust which is not.
So, it is more likely that instead of ⅟₄, the distance will be about ½ when the rotor inertia is ½

 

[rcflier]

Hmm, wouldn't lower inertia make it less forgiving in the flare when landing?

 

[Aerofoam]

There is probably a scale-able ratio regarding weight, length, chord, loading and speed.
What it is? I have not got a clue and my experience is in very low RE # stuff, I need to come up to speed (NO PUN INTENDED!!!!)
on the higher Reynolds number realm....
I can say as you scale down a rotor blade, it tends to have a wider chord and more negative incidence due to the unsteady state airflow
in the lower Reynolds # areas.
I have built a rotor that was too light and it would tend to lose RPM's in a turn and could just fall out of the sky like a tip stall.
The same rotor with tip weights flew fine. it also flew ok with more negative pitch and no tip weights, but was not as fast.
When flying at lower weight and more negative pitch, it performed well in the landing flare because it reacted faster due to the light weight and negative pitch. I believe lighter fat blades at negative pitch would achieve good performance, but would sacrifice speed and the receding blade stall would come much earlier.
Does this principle scale up? I don't know....
I do think the most promising area for short take off is in being able to initiate the takeoff procedure while moving and possibly de-pitching the rotor blades, so the airflow is through the disk from the very beginning and does not have to transition from being a "Fan", to being a wind driven airfoil.

On my current bucket list is an experiment with outboard control surfaces on the rotor blades (Like a Kavan).
I believe if the rotor blades were mounted in a normal fashion, but could be de-pitched with the outboard "elevons",
the prerotation phase could be done with the blades nuetral, and when the movement started, the flow through the disk would start right away,
then the neutral setting could be released and the blades would twist into flight mode with the flow already established.
Not a jump start, but a short take off.

Jim M was kind enough to give me some RAF blades and rotor head and I have been told by a couple people that those should never be flown, so they will make a good crash test dummy for the experiment once I add control surfaces and mount them on a heavy trailer......

 

[Jean Claude]

Hmm, wouldn't lower inertia make it less forgiving in the flare when landing?

The energy stored by the rotor inertia is only released if the rpm drops. But for rrpm to decrease, the load must decrease, i.e. after the flare.
Too late

 

[Jean Claude]

I have built a rotor that was too light and it would tend to lose RPM's in a turn and could just fall out of the sky like a tip stall.
The same rotor with tip weights flew fine. it also flew ok with more negative pitch and no tip weights, but was not as fast.
When flying at lower weight and more negative pitch, it performed well in the landing flare because it reacted faster due to the light weight and negative pitch. I believe lighter fat blades at negative pitch would achieve good performance, but would sacrifice speed and the receding blade stall would come much earlier.
Does this principle scale up? I don't know....

As you can see from these curves (here, for cylindrical section) there is a large drag discontinuity around Re ~ 3.10^5
Unfortunatly, for the blades of our gyrocopters, Re ~ 70* 100m/s* 200 mm ~ 1.4 10^6, while for a reduced 3 ft diameter model, Re ~ 70*40 *35 ~ 10^5 Just either side of the discontinuity
So we can't extrapolate the results observed on model flights to real gyroplanes.

 

[XXavier]

Hmm, wouldn't lower inertia make it less forgiving in the flare when landing?

Five years ago, in my ELA-07, I installed a set of Averso blades, that have more inertia than the original ELA blades. Since the change, I feel that my landings are smoother and easier... It's true that I almost always land with some engine power...

 

[Jean Claude]

I think the cause is not just the extra inertia. It's the underbalanced chord of Averso blades, which gives an elastic twist. This automatically increases the collective pitch during the flare overload.
Chance sometimes makes good things happen.

 

[Abid]

I think the cause is not just the extra inertia. It's the underbalanced chord of Averso blades, which gives an elastic twist. This automatically increases the collective pitch during the flare overload.
Chance sometimes makes good things happen.

Its only very slight underbalance on Averso. I like that feature actually. It does make it easier to handle landings.

 

[Jean Claude]

Xavier Averso told me that his blades were balanced at 30% of chord, whereas the Aerodynamic Center of the NACA 8H12 used is about 26%.
My in-flight measurements with him had suggested me, thanks to the steady rrpm increase measurement, that a load factor of 2 increased the blade tip angle by more than 0.5 degrees (rotor de 8.5 m, 210 kg.m2)
I understand that this makes landing a little easier, but it seems to me that a shorter take-off could be an greater advantage, and the lightening of the blades deserves to be tested. Just my opinion.

 

[Brent Drake]

In a lite rotor. It pre-rotates quicker and will brake quicker. But rides rougher in turbulence. You must not hesitate to apply power when doing touch and go because the RPM drops rapidly.
Heavy rotors are just the opposite. I prefer heavier blades.

 

[MikeBoyette]

Xavier Averso told me that his blades were balanced at 30% of chord, whereas the Aerodynamic Center of the NACA 8H12 used is about 26%.
My in-flight measurements with him had suggested me, thanks to the steady rrpm increase measurement, that a load factor of 2 increased the blade tip angle by more than 0.5 degrees (rotor de 8.5 m, 210 kg.m2)
I understand that this makes landing a little easier, but it seems to me that a shorter take-off could be a greater advantage, and the lightening of the blades deserves to be tested. Just my opinion.

This is the very reason Dragon Wings were made the way they were. They used an airfoil Dad developed from trial and error. Chuck Beaty eventually found a long retired airfoil that was used in fixed wings.

I have forgotten the NACA number. It was retired from use on fixed wings because it has a very abrupt stall. I believe that characteristic was to blame for at least one death during testing. It’s good characteristic of being very low drag and .25 cord wise balanced wasn’t enough to negate the stall risk.

Dad first carved the airfoil on a set of wood,steel, and fiberglass blades he made after having many conversations with Chuck on the phone. He needed something better than Bensen’s to lift his Continental powered Bensen. These were the perfect thing for it. They were 8 in cord. He made a couple of sets and the original set was on a duplicate machine of his in which his best friend did a PPO to his death in front of his 9 month pregnant wife. He lost interest for a few years after my sister was born.

In 1985-86 while moving we found another set of redwood/cold rolled steel spars he made and forgot he had. He decided once we moved to Chuck’s property to build another set these were the best yet. They were 24’ 8in cord and were so efficient we could fly both he and I @ 185lbs each on a 503 swinging a 60in prop. The blades were tried and loved by everyone that flew them. Then they were wrecked by a friend of ours who was very close to my pops committing homicide on him.

In 1990 my Dad suffered a back injury that prevented him to return doing what he had done all his life for a living. Thus he had to find another way to make a living. He built a set of blades using the same airfoil with a machined c spar brass tip weights. The skins were screwed to the spar on the leading edge and riveted on the trailing edge and featured no reflex (which because they were metal he and Chuck didn’t think they needed, later the reflex was added to make construction easier and there was an improvement in flight characteristics). These were not twisted as we had no means to do so at the time. We let anyone and everyone get them. Then got 6 gyro friends to put down deposits for start up money for an extrusion dye, glue, equipment, and several proprietary machines we built especially for the custom process. An oven designed by Dad with suggestions from Chuck. Dad and I built the first iteration of it. Dad and Lloyd made a spar twister. Plus many other things.

The end result was the lightest blade on the market for many years which was 38 lbs for a fully assembled 22’ set. With the tip weights they gave the inertia of a much heavier set. Later sets included the reflex along with tip weights ranging to 4” all the way to 12” for cruiser derivatives for single and two place machines.

These blades also are responsible for the decade plus success of the Mosquito Helicopter. Without the Dragon Wings being adapted for their use the blades they were using would have killed customers and their business. They unlike Dragon Wings delaminated over night at Bensen Days. Dwight developed his own take on DW’s only after dad sold the business got screwed and the scum bag stopped production and stiffed dad for over half of what he agreed to pay. Karma got that SOB and he died of undiagnosed prostate cancer at a very young age.