Dave_Jackson

The purpose of this posting is to place the following invention in the public domain, so that it is freely available to all.


In addition, any comments or critique will be appreciated.
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Potential Features;
. ` . ~ An advance ratio far beyond 1 (mu >> 1).
. ` . ~ Stopped rotor.
. ` . ~ High lift/drag ratios in high-speed forward flight and in hover.

Quick Explanation;

. ` . The rotor incorporates ideal twist. This ideal twist traverses the span of the blade as the blade rotates through the retreating side of the disk. The center of ideal twist moves so that it is always concentric with the location on the blade of zero forward airflow.


Additional Material:
. ` . ~ Overview of the invention; http://www.UniCopter.com/1369.html
. ` . ~ Construction of Simplistic Blade http://www.UniCopter.com/1504.html

Graviman

Dave, not pushing home your symmetry arguement then? I thought i was quite reasonable in that last thread, and no engineering point is ever won easy - there is normally just too much at stake...

I don't think the retreating rotor will ever offer the ideal downwash distribution. My main concern with this (we discussed this before?) is that the aerofoil near the zero velocity circle will be stalled since it is pulling a very high AOA. This means that there can be no pressure gradient, so air must "leak" back through this region. That's why retreating feathering just makes intuitive sense to me, naturally combined with counterrotation.

Besides you are asking the blade to twist 180' 5-10 times a second, and this just sounds like a fatigue knightmare to me. Your control system complexity goes well up over simply just needing to control tip and root independantly, since you have to determine exactly where the blade reverses. This entire system has to be designed for large movements 5-10 times a second. I won't mention the FMEA risks associated with failure of any of these components, but it is not good.

Having finally understood the nature of the aerodynamically induced eigenmodes in the blade (thanks Nick), i can see this as a dynamic knightmare too. You are asking a blade to undergo major changes in shape while passing through air that just may not be going in the direction you think right. If the vibrations are cured by the tube-spar design, this still invites the aeroflexure structural divergence demon...

Mart

Dave_Jackson

Mart,

Perhaps I did not make it clear enough that the Advanced Method, mentioned on the linked web pages, will not work. At least, it will not work in its present form.

Any discussion or critique that is focused on 1/ the overriding concept itself, and 2/ the Simplistic Method of implementing it, will be appreciated.

Dave

Graviman

1)Isn't that what i just gave?
2)There isn't one, that was my point.

Mart

Dave_Jackson

Mart,

All proposals to date for reverse velocity utilization have entailed moving the blades through the air in their reverse direction, to my knowledge. This involves an airfoil profile that is compromised, due to the need to provide lift from air that will flow from both directions.

The intent of the 'Traversing Ideal Twist.' concept is to utilize the reverse airflow, however, do it with a conventional airfoil profile, due to the ability of having the air only flowing in one direction across it. IMO, to do this will require a blade that is capable of providing an extremely large twist. In addition, this twist must take place at the intersection of the blade's azimuth and the edge of the reverse velocity region.

There are two methods under consideration for accomplishing this;

Quote:

Your control system complexity goes well up over simply just needing to control tip and root independently ...........

Yes. The so-called 'Advanced Method' proposes to use tip and root control to twist the blade.

Quote:

...........since you have to determine exactly where the blade reverses.

No. The so-called 'Simplistic Method' proposes to use the aerodynamic forces to select and 'flip' the appropriate blade segments.


You are correct in that the frequency of twisting will be high (perhaps too high). However these blades are intended for very fast aircraft and this entails 'slow speed - large chord rotors', plus a continual reduction of the rotor speed as the forward speed of the craft increases. Note that the 'flipping of the blade elements at the tip will be a lot slower than those at the root, since the advance ratio must be 1 before the tip elements must 'flip'.
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Having said all this, the 'Simplistic Method' may not be quite as simple as was previously envisioned. The proposed method for setting the pitch angles of each segment for forward and reverse flow ain't going to work as planned.

Graviman

Dave,

Ok so this will switch in at a particular speed, but you are still talking about flexing the blade 180' a very large number of times during the life of the vehicle, hence my fatigue worries. The dynamics may be helped by the reduction in frequency, but remember my main concern is that the air will not be going in the direction you might want. There are still potential causes for divergence, if not flutter.

In the zero velocity circle, the air is being asked to flow over an aerofoil from a region of low pressure to a region of high pressure. This is the cause of turbulence in conventional aerofoils (laminar flow designs too). Either side of the zero velocity circle the aerofoil will effectively be pulling an extremely high pitch. You are assuming in this design that the air will all be travelling uniformly downwards, so the aerofoil has to simply minimise it's profile. In practice i really can see air leaking backwards through the disk in this region, so the local AOA could be anything. The aerofoil will be at best stalled.

In a torsionally flexible blade i can see the blade following the proflie you want, using trailing edge flaps. The problem is you need to retrim these every cycle for the inverted flight portion. This means you have to consider independant root and tip control anyway. Combine that with the need to provide control/damping against chaotic flow near the zero velocity circle suggests you have to adopt the mechanical root/tip system from the outset.

Put simply the system is too complex. Better to accept that the retreating portion cannot produce enough lift and minimise its impact on the flow. This is where counterrotators win. On the other hand build a test rig and prove me wrong!

Mart

slowrotor

Dave,
Some work has been done on using circulation control (blown airfoil) instead of twisting the whole blade they can adjust the airflow. You may have looked at the idea. I think they were working on fixed blade controllable props for airplanes. Gets very high CL also by blowing.
slowrotor

Dave_Jackson

Mart,

You are talking about flexing the blade 180º and fatigue worries. Perhaps I have not presented the so-called 'Simplistic Method' clearly enough. The information on the web page http://www.unicopter.com/1504.html has now been improved.

Slowrotor,

Thanks for mentioning circulation control. It is defiantly a candidate for reverse velocity utilization, however its main feature appears to be improved lift, particularly when related to retreating blade stall. I wonder how good its L/D ratio is, particularly when applied to high-speed rotorcraft.

Sikorsky has at least one patent for circulation control. They proposed using in on the X-wing. However, I don't believe that they ever put the X-wing on the craft. In addition, the craft was a compound one with wings.

Regarding your interests, I wonder if a variation of circulation control, combined with the earlier discussion of using compressed air, could provide a Very Short Take-Off and Landing fixed-wing aircraft?

This is not quite what we were discussing, but .... http://www.mastermindtoys.com/store/brand.asp?brand=47

Dave

Graviman

OK Dave, i see where your coming from. Demanding 180' from an elastomeric bearing is still more than is generally possible (we assume +/- 15' from suspension components - Westlands weren't hiring). In order for each segment to find it's equilibrium at the required pitch (or local AOA) in forward and rearward flow, the spar has to remain at a contant pitch while the elastomeric stiffness changes. You may find yourself using nylon bushes with a compressed air centering spring. Mechanical spings and an axial spar tensioner would be more reliable, if fiddlier to implement (cost). There is much scope for various eigenmodes, so damping must be considered (elastomeric still has a high Q factor).

You will also need to allow more than +/- 90' in the event of engine failure, so the poor test pilot can autorotate (assuming he isn't already hiding under his desk, refusing to be coaxed out). In fact due to the complex nature of this control system you are going to have to throw this machine at as many circumstances as you can. The whole point of testing is to figure out what the designer didn't - there will be many such "variables" with this design.

My real worry here would be the steps between aerofoil segments, particularly since flow over a rotor blade has a large spanwise component over much of its azimuth. These will each produce a vortex, which may help with flow attachment or trigger it's seperation. There will also be leakage between the segments, and any practical attempt to cover gets back to the fatigue concerns. I have already mentioned my conviction about flow close to the zero velocity circle - i really just do not see it behaving the way it's "supposed to".

Again, i still feel it is a too complex solution to a problem that can be overcome by other means. Independant root/tip control will benefit the advancing blade, by optimising the downwash distribution in various flight regimes. Since you are convinced about the benefit of conterotating machines (in whichever guise) then why obsess with producing lift from the retreating portion? As long as you can keep a high control system torsional stiffness against reverse flow divergence, with a 50% max chord (aerofoil operating at optimum AOA due to IRAT anyway). Even with compromised aerodynamics, i really don't see the overall rotor system efficiency being any lower.

Still, build a test rig and prove me wrong!

Do you have any comparisons of Cd/Cl curves for reverse flow aerofoils against say 63-012 or 65-012? Although AOA will be limited, i really can't see them being so different as to require this extreme a solution...

Not using compressed air, but...

Multirole crop spraying and grass cutting machine

Mart

slowrotor

Dave,
I don't think a blown fixed wing will have any lift at zero or near zero airspeed. So the only way to achieve super STOL is with powered lift.
At least thats my thought.
slowrotor

Graviman

Dave,

What news on the rotor blade technology you were developing? I ask since i am interested, and the basis of the idea is sound enough. Independant Root&Tip control is the way to go for future heli design, since efficiency (ie power/fuel requirement) is the helicopters main weakness. I just wasn't convinced about the durability of the concept in this thread (part of my job is to approve designs for durability/reliability).

Mart

PS: I'm hoping you didn't take offence at my remarks on the electric helicopter thread. It was intended more as a friendly spur, than an unfriendly aspersion.

Dave_Jackson

Mart,Since you ask, here is the personal answer.

Researching and conceptualizing different ways to improve rotorcraft is an enjoyable challenge.

For me, the problem comes about at the next level when the detail engineering and the building must take place. I started, built, and then at 47 sold a manufacturing company, because the challenge was won and the work became boring. Back then I could conceptualize an automated machine and let the engineering department detail it and the plant build it. In 'retirement' there is little interest in doing that which I did not have to do at work.

With limited motivation, the construction of the CNC workstation proceeds.
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On the lighter side, here is the original version of your previously mentioned Multirole crop spraying and grass cutting machine

http://www.unicopter.com/Temporary/cyclogiro.jpg

Dave

Graviman

Well, it's great to hear you're still persueing it. Trust me, i know well how demotivating long term objectives/projects can be. I don't know your circumstances, but it strikes me that you are best off thinking about your ideas as a private research project. You can actually do a pHd this way with the Open University for example - don't think you even need a degree to start with! Lets face it there is enough literary research on that web site of yours. Maybe "Advances in Helicopter Aerodynamics by Blade Twist Control of Intermeshing Rotors".

I still feel you would do well off getting a simple 3D cad package, in particular to layout the hub complexities of two swash plates. Maybe just a simple wind/smoke tunnel and computer high speed film system. You could lay down the control system that this type of heli would need in various flight regimes. I would be very suprised if an equipment grant wasn't available. This means you don't get bogged down developing an entire machine, but can provide fantastic guidance for those who want to. You could probably entice the local academic establishments to help you out - let's face it they'll have the best equipment. So you have to spend a day a week in their tunnel - as long as you are not spending your own money, right?

Forgive my enthusiasm, but i genuinely feel there is a lot of mileage in this. Lets face it there are enough charlatans out there that seem to get by...

Mart

Is this classified as an Interthreshing machine?