A question regarding a Sikorsky experimental craft

[Dave_Jackson]

Sikorsky produced an experimental helicopter 30 or 40 years ago that had a unique tail-rotor It articulated 90-deg. from being an anti-torque tail-rotor, to a pusher propeller for forward thrust. I don't believe that it was ever flown.

Does anyone know it's model number or other means of finding it?

Thanks.

Dave

[toolguy]

Dave,

It was a highly modified S61 that had a pivoting tail rotor gear box mechanism that could swing 90 degrees form normal position into a pusher prop. I have a picture of it, but no means to scan it. My guess is that the flapping loads on the spindles and stops was pretty high in the pusher mode, but it never caught on. :}

[NickLappos]

Dave,
It was flown extensively back in about 1970, as a modification to the experimental S-61F test bed aircraft, which was flown to explore all kinds of high speed concepts, such as jet compounding, lifting wings. The "swing-tail" was a moderate success, but its speed gain was modenst, and its weight and cost never justified itself, I think.

[Dave_Jackson]

toolguy and Nick,

Thanks for the model number and information.

Dave

[NorthSeaTiger]

Here is a link to a PDF which lists a few experimental Sikorsky machines inc photos.

NST

http://www.sikorskyarchives.com/newsindex/News%20Sept%2000%20Experimental.pdf

[Graviman]

Nick, is the swing-tail concept ever likely to reappear? At high speed i imagine X2 uses a rudder surface, while in hover using MR differential collective or RPM (depending on control strategy for autos). I'm considering that grey area where MR yaw may not be responsive enough, but rudder is still ineffective.

Mart

[Dave_Jackson]

Mart,

The XH-59A ABC uses differential collective for yaw control during slow-speed powered flight. Above 80 kts the differential has been phased out and the rudders have taken over.

The vertical tails is swept back and the rudders are not effective enough during autorotation. They proposed inverting the vertical tails.

These inverted tails can be seen on the proposed X2 configurations. [US design patents ~ D524,227, D524,228, D524,229, D524,230, D524,718 and D526,269]


Nick,

I take back the years of saying bad things about the tail-rotor.

Long live the tail-rotor :). Or the nose-rotor, or both. :ooh:


____________________________
A Teaser

http://www.unicopter.com/0424-A.gif

Anyone want to guess how the transition from hover to cruise is done? :confused:

[IFMU]

On paper?

-- IFMU

[Dave_Jackson]

IFMU,

Congratulations. Your guess was far better than all the others. :ok:


Actually, it's a de-winged S-72 (http://en.wikipedia.org/wiki/Sikorsky_S-72) on Spanish Fly. In other words, there is a possibility of 'getting it up'.


Dave

[Graviman]

Again i get the strong impression that Sikorsky have probably tried everything that might fly, in order to select the ones that work. :ok: The use of compressed air driven virtual X-wings is pretty amazing (unless compressor goes phut).


Dave, transition is simple: The counterotating rotor, omitted for clarity in "Detail A", provides a moment torque for the advancing blades. The retreating blades, omitted for clarity in "Detail A", are feathered to minimise their impact on airflow. ;)

Actually, i am not sure what to make of X-wing concepts. Perhaps when you introduce all the aerodynamic devices required to stop blade flex diverging, as Nr reduces, X-wing is the next logical step. The advantage over ABC is reducing the mass of one rotor and the lack of of reverse flow over feathered aerofoils. I have the odd feeling that the penalties would end up nullifying the advantages though. Reverse flow aerofoils will have vicious stall characteristics, due to the sharp edge. In theory the design could just rely on active blade twist.

Probably one of those ideas that floats around until another idea facilitates its introduction...

Mart

[Dave_Jackson]

Mart,

The problem is that the S-72 was a 'test bed' and it never flew with the X-Wing (http://www.dfrc.nasa.gov/gallery/photo/X-Wing/HTML/EC86-33555-2.html). In addition, both of the
Boeing_X-50 (http://en.wikipedia.org/wiki/Boeing_X-50)'s, which have similar blades/wings, crashed.

Your concern about a reverse velocity airfoil is valid. Modifications to the blade's profile, which are done to improve lift from the reverse velocity, must not result in an equal or greater lessening of the lift from the forward velocity.

This is for a single rotor craft. Plus, these blades do not experience reverse velocity.


This is the basic idea.
It was previously discussed on PPRuNe last April. (http://www.pprune.org/forums/showthread.php?t=220726)

http://www.unicopter.com/1369-2.gif

Before you laugh and hang-up, consider;

The airfoil profiles of the S-72 and X-50 are far from being optimal. Where as, the profile of this Traversing Twist Rotor is a conventional NACA 00xx,
This Traversing Twist Rotor can be made today, by many different means.
Therefore the two primary concerns are;

How to produce this Traversing Twist Rotor so that it is light and strong and etc. etc. etc.; so that it is practical.
During transition, when the blade is at 90-degrees azimuth, it will produce a significantly reduced lift. Lateral symmetry dictates that the lift of the blade at 90-degree azimuth must also be reduced. Therefore, the blades at 180 and 360-degrees must carry a large percentage of the load.
The idea is being fleshed out at Rotor X Wing (http://www.unicopter.com/B463.html) and so far it looks like a 'can do'. :ok:


Dave

[Graviman]

Dave,

My main concern here remains durability of the blade from fatigue failure - i speak from bitter experience about design for fatigue! It can really catch you out where you least expected it.

To work you MUST keep the stresses below the endurance limit, which generally impies low strain (ie movement). Independant root and tip twists the blade nowhere near as much as this concept. Convince me about the reliability of that system first.

Besides, i still don't buy the aerodynamics. You will get stall either side of the zero vel circle, and leakage through it. Ok i know helo rotors stall well above fixed wing due to vortex shedding etc, but i just don't see the aerodynamics working in practice. Besides as the rotor goes around the azimuth, you are asking it to undergo torture as it tries to follow the airflow. I'm sure you have some novel contruction ideas to enable the blade to do this, but the next risk is then blade flexural stability.

I really do just see this concept as carrying too many risks.

Mart

[Dave_Jackson]

Mart,My main concern here remains durability of the blade from fatigue failure No problem. The craft has quadruple redundancy.
It has 4 blades. :O


Some Marketing;

The Large Chord & Low Tip Speed (http://www.unicopter.com/1090.html) rotor means that the blades are reasonably thick at the spar.
During cruise the blades are parked at 45-deg off the primary co-ordinates. The airflow past the blades will therefore be at 45-deg to the blade's chord. This means that a NACA 0017 profile with a chord of 2 feet will appear to the air as a NACA 0012 profile with a chord of 2.82 feet. This means that an even thicker spar can be implemented.
Assume that the blade's cutout ratio (http://www.unicopter.com/B263.html#Cutout_Ratio) is 0.1 and that the tip speed ratio (http://www.unicopter.com/B263.html#Tip_Speed_Ratio) of 1.0 occurs at 100 kts. This means that the Traversing Ideal Twist is not active during slow speed hover nor during moderate to fast cruise.More concerns requested.

Dave

[JohnDixson]

Actually, the swing tail was installed on 325Y, the experimental SH-3 ship, Nick, not the 61F. Was one of those projects looking forward to the Sikorsky AAFSS design as I recall. Picture of it on the back wall of the Pilots Office in Stratford.

[Graviman]

Dave,

My concern with the "traversing ideal twist" is that during some part of the flight you are asking the blade to twist 180 degrees back on itself.

Being curious about plastic response to fatigue i have just managed to break a "shatter resistant" ruler. The load case? I cycled the ruler by fanning it at high speed. Although not immediately, it broke neatly without any initial indication of strain in the area of fracture. In fact it nearly broke in a second position, as if to prove my point.

There is just no way a blade could be asked to do this for more than 1000 hours without failure. This would be >100000 reversals, depending on flight regime, cracks would be inevitable.

Mart

[Dave_Jackson]

Mart,

The following is just two ideas out of perhaps dozens.

In both cases, consider that the spar is a tapered cylindrical tube, which is constructed primarily from pultruded unidirectional carbon thread. The spar can have an extremely high strength since its diameter and wall thickness are not a serious concern [ref. my previous posting].

Next consider that the blade's outer skin and webs, which might consist of may adjacent or many adjoining segments.

They could be adjacent segments that have a 2 or 3 fixed position magnetic actuation between them. The control of the segment-segment relationship could be controlled by a small CPU at each segment, which reads a stream of address&instructions from the primary CPU. The small on-blade weight would be concentrated in the leading edge to give balance about the 25% of chord.
They could be adjoining segments where the skin of the complete blade consists of a elastomer and carbon cloth. Embedded in the elastomer sheet is a unidirectional mat of prepreg carbon tow. The elastomer is curred. Then this flexible 'cloth' is laid into the blade's final mold and the epoxy in the carbon tow is heat or ultraviolet cured. The carbon tow will be in the coardwise direction thereby giving high skin strength and yet allowing twist about the feathering axis.A side issue is that if this rotor generates vibration, the vibration can be curred 'at source' since there is physical control at every element of every blade.

The spewing out of wild ideas, without the concern of being ridiculed, might represent a 'Wikinomic moment'.


Just :yuk: some ideas.

Dave

[Graviman]

Yeah, but if you disregard cost and reliability anything is possible, Dave. Don't forget that part of an engineer's job is to filter out ideas which will just take too much development to make them feasible. Part of this is that the system must work 100% reliably for the durability target of the product, or at least have failsafe modes with simple repairability.

Honestly i just don't think the aerodynamics would justify the design investment for the "traversing ideal twist" system. The main reason is that the blade stall and leakage at the zero vel circle stop any advantage. You may as well go for a foil section which is a fn(radius) and optimise the root and tip twist. Reversable foil at the centre and conventional at the tip. This is better use of your advance control systems.

I suspect this sort of conversation occurs all the time in the Sikorsky/Schweizer design offices. I imagine that the compromise accepted for high speed flight was the weight penalty associated with a second rotor. The possible aerodynamic interference would then be reduced with CFD. The point is that the solution will go the design life distance.

Mart

[Dave_Jackson]

Mart,

Over the next few days I'll try to work out what the lift would be on the retreating side during translation (or is it 'transformation' :) ) when the lift is at its least.

If the lift is very little then the Traversing Ideal-Twist System (TITS) (http://www.unicopter.com/B456.html) could be 'tits up', and not in the desired way.



Conceptualizing and Engineering:

The best helicopter of its time was developed by a conceptualist and an engineer. Flettner was constantly raising wild ideas and Hohememser was technically shooting down most of them .


Dave

[Dave_Jackson]

Mart,

What do you mean by "foil section which is a fn(radius)"?

It looks like there should be sufficient lift during transformations between a rotating rotor and an X-wing. http://www.unicopter.com/1354.html


Dave

[Graviman]

Dave, i just mean that the blade has a revesable foil near the root and a conventional foil near the tip. Since the root can have very good pitch control there is little risk of stalling the sharp leading edge.

Mart

[Dave_Jackson]

Mart,

Not sure if you mean "reversible" as in a physical reorientation of the airfoil or "reversible" as in identical leading and trailing edges. However, either way will have its own set of problems.

After review, my previous statement "It looks like there should be sufficient lift during transformations between a rotating rotor and an X-wing." ain't quite as valid.


There are days when Nick's 'Free Lunch' looks very elusive. :{

Dave

[Graviman]

Dave, sorry i meant reversible as in an aerofoil which produces lift with air from either direction. This is the least risky approach, particularly since it is only at the centre. The free lunch is just something that aircraft can never really enjoy, no matter what techniques are used to throw air at the ground.

Actually ground vehicles fair little better. I laugh that my car has a Cd of 0.3 to 0.4, since this indicates that the aerodynamicists aren't even trying. I also find it amazing that the calculations prove that my current heavy equipment project requires <100kW to attain its top speed from air resistance, but >600kW to overcome tyre rolling resistance. :ugh:

What is the transport industry plan regarding CO2? Simple it will just keep pumping it out like there is no tomorrow, and rely on marketing. Fortunately CO2 is fairly easy to capture (using a compressor), and can be pumped into the ground for absorbtion - this will rely on political will power. So we're all screwed then... :rolleyes:

Mart