Rotor Blades

[3 D]

I have a question regarding the design of rotor blades and would be most grateful of any comments.. .. .I have been wondering why we don't use hollow rotor blades?. .. .The benefits of this, I thought, would be:. .. .If the blades were hollow with a hole at the root open to atmosphere the air inside the blade would be forced towards the tip of the blade as it rotates. This air could then be bled out at the tip to do such things as help delay mach crit, reduce tip vortices and re-energize the boundary layer. . .. .I will be the first to admit that I know very little about the complexities of aerodynamics so I guess there are reasons as to why this simple idea won't work I was just wondering if someone could help me to understand why.. .. .The only drawback I could think of is the reduction in the strength of the blade, maybe this would outweigh any the benefits?. .. . <img border="0" title="" alt="[Confused]" src="confused.gif" />

[Lu Zuckerman]

To: 3D. .. .Some rotorblades do use a hollow spar and the trailing edge (pockets) are attached to the spar while others have the skin wrapped around the spar and the trailing edge is then bonded in some way to make a complete airfoil.. .. .Sikorsky on some of their rotorblades seals the hollow spar at both ends and pumps nitrogen gas into the spar. Attached to the inboard end of the spar is a pressure indicator, which constantly monitors the internal pressure. If the monitoring device indicates a pressure drop it could indicate that the spar has suffered a crack or that there is an internal leak in the sealing elements at each end of the spar.. .. .The monitoring device provides a visual indication and can only be monitored with the blades at rest. There was some talk about using atomic radiation in that if there were sufficient internal pressure the atomic element would be retracted. If the pressure decreased the radiation element would protrude and there were sensors on the helicopter that would sense the radiation and give the pilot a warning that there was a leak allowing him to take measures to land and inspect the blade(s). .. .Sometimes the system can be fooled. Several years ago a Sikorsky S-64 crashed when it lost part of a rotorblade. The accident analysis indicated that there had been some rework on the spar and when the blade was reassembled and the pockets were installed some sealant covered the rework. A crack developed but gas did not escape due to the sealant. The crack propagated to the point that the spar failed.. .. .Some helicopter designs employ a hollow blade and send pressurized air through the blades causing them to rotate. This is a tip jet design and there was one design that not only used the tip jet principle it also ejected air under pressure across the span of the blade increasing lift.. . . . <small>[ 17 March 2002, 23:44: Message edited by: Lu Zuckerman ]</small>

[slgrossman]

3D,. .. .Actually, most, if not all, metal and composite rotor blades now in use do have hollow spars. I suspect your idea may have been considered, but perhaps the air pressure generated in this manner was insufficient to provide the desired effect. With the exception of your idea about reducing tip vortices, I think the inboard section of the blades is where you'd need to apply this "free bleed air." Seem to me you'd lose much of the pressure head on the return trip from the tip back toward the root.

[Flight Safety]

3D, you said.... .. .If the blades were hollow with a hole at the root open to atmosphere the air inside the blade would be forced towards the tip of the blade as it rotates. This air could then be bled out at the tip to do such things as help delay mach crit, reduce tip vortices and re-energize the boundary layer.. .. .I've thought of this too. One of the main drawbacks to a blown surface boundary layer control system, has been the air pump requirements for the blown air. I've also thought that the the main rotor itself, with hollow blade spars, would provide the perfect air pumping mechanism for such an air supply.

[widgeon]

Lu I could be wrong but I heard the CH124 ( Canajun sea king ) had in flight BIM indication.

[Dave Jackson]

What an intriguing concept. The following is offered as additional fodder for discussion. . .. .Might the air be used to delay retreating blade stall?. .For this, would the optimum location to discharge the air be on the aft upper surface of the tip? Will the use of discreet ports work or should it be a porous surface, to give a more even discharge? Might the roughness of this porous surface disturb the boundary layer?. .. .An addendum to your idea:. .. .If the air discharge is only required at a specific azimuth and centrifugal effect will not provide sufficient air, then the following might be considered. Provided a valve on the blade root's incoming port. The valve will cycle at the same rate as the RRPM, opening and closing at specific azimuths. This will cause the 'hollow' airfoil or spar to act as a tuned exhaust and thereby deliver pulses of air to the discharge location.. .. .An addendum to the addendum:. .. .Could pulsed ejection of air from the blade aerodynamically (as opposed to mechanically) change the angle of attack and thereby be used as a means of Higher Harmonic Control?. . . . <small>[ 18 March 2002, 02:53: Message edited by: Dave Jackson ]</small>

[Lu Zuckerman]

To: widgeon. .. .The in-flight BIM system on the Canadian Sea King might be an offshoot of the atomic radiation design I described above in which there are pick-ups on the stationary star that sense the decrease in pressure. It may be a proximity sensor.

[rotorque]

3D,. .. .Good to see you enjoy thinking of things like this. . .The idea is plausible if we fulfill two main criteria. The weight(overall mass)of the air inside the blade and the RPM of the rotor system it is contained in. In practice, there is not enough of either to make a usefull application.. .. .In actual fact, I would go as far as to say that there would be more advantage in using the low pressure at the tip of the upper surface to draw air through the blade, as this force is most likely to be larger than the one you subscribe to.. .. .Food for thought.

[leading edge]

The S61N fitted with Long Spar blades has a cockpit BIM indication and a BIM test that can be periodically checked in flight.. .. .BIM warnings in flight are comparatively rare but obviously necessitate a diversion to check the actual indicators on the blade roots.. .. .Very occasionally, a false BIM warning can occur if the aircraft has been running with one or both engines at idle with the blades stopped, I seem to remember it was due to heat on the indicator with the wind in a certain relative direction.. .. .In 2000 hours of S61 flying, I don't think I ever had a real in flight BIM warning.. .. .The old girl really was (and still maybe) the Queen of the skies.. .. .LE

[RW-1]

In flight BIM on the CH-53E has always been around.. .. .No element physicaly pushed out of BIM indicator (as I remember), a small amount of an radioisotope would be released as the pressure dropped. Of course, you still had the visual sight clues on them as well.. .. .On the upper fuse is a detector, which when sensing the isotope, would light the light when a threshold has been reached.. .. .I've been nearly out to the carrier a number of time and have had in flight BIM lights. Nothing that has caused me worry at those times.

[ShyTorque]

Prior to the present fibre composite blades, the military Puma had hollow alloy blade spars which were pressurised with nitrogen (Wessex similar?). In the event of a blade crack, the spar depressurised and set off the BIM caption. We had a lot of false captions. One day, my instructor decided to fly 10 minutes back to base with a BIM caption on as he thought it was probably spurious. In fact it wasn't and the spar had only one third of intact metal remaining across the crack. He took a few days off after discovering what he had just escaped.... <img border="0" title="" alt="[Eek!]" src="eek.gif" />

[3 D]

Well thanks very much to all that have replied.. .. .At the risk of going on about this topic a few more points I have pondered over:. .. .The feature almost certainly wouldn't be efficient enough on a small cross section blade, due to the points rotorque mentioned mass of air, rotor rpm and large drag on the flow of the air through a narrow void. But I wonder on a large blade if the blade could be made almost hollow (we could still have a spar and other supports inside so long as they didn't restrict the span wise flow of air) is this still the case?. .. .There are none of the drawbacks such as the complicated mechanics pipe work and seals etc. asscociated with the system Lu mentioned (tip jet propulsion) . .. .Would the friction of the air passing through the blade mean that any more power would be required to turn the rotor? Or do we indeed get something for nothing?. .. .How much could boundary layer blowing (if its called that) help us improve the performance or rotor blades particularly in the areas as mention previously of retreating blade stall, tip vortices and advancing blade stall?. .. .Has anyone tried this idea with modern materials?. .. .If i filed a patent could i sell it and pay for my I.R. ? <img border="0" title="" alt="[Big Grin]" src="biggrin.gif" />

[Dave Jackson]

3 D. .. .This post follows the 'air down the blade' theme of your thread but differs in the application of this air.. .. .Vibration is a problem to helicopters and a reasonable amount of research is currently being conducted in an attempt to reduce it. This effort is referred to as Higher Harmonic Control, Individual Blade Control and Active Blade Control. It appears that all or most approaches have a mechanical device move a tab or change the shape of the trailing edge.. .. .I, in ignorance, believe that the time required to activate the mechanical device, which will change the angle of attack, which will change the lift of the blade (and then return it all) Is TOO long for anything higher then the lowest harmonic(s). A pulsed discharge of air immediately changes the angle of attack without having to overcome the blade's inertia about it pitch axis. And in addition, nothing has to be done to return the angle of attack to is original attitude.. .. .rotorque has mentioned the probable limitation of being able to supply a sufficient amount of air.. .A tuned exhaust is used on two-stroke engines to increase the compression ratio. It is a simple concept and only requires the opening and closing of a single port. The frequency of the pulses is only dependent on the length of the tube. . .________. .. .I am very concerned about rotor-to-rotor vibration on my two intermeshing helicopter projects, so if you patent the idea, please give me rights of use. <img border="0" title="" alt="[Smile]" src="smile.gif" />