Airgap inside rotor blades?

[MadMuz]

Just out of interest, what does the airgap inside of the rotor blade do? I am guessing it is to reduce the weight (chordwise) at the back (trailing) edge of the blade to achieve the centre of balance (chordwise) at about 25%?

[gyroo]

Hi Muz,It"s impossible for a solid aluminium airfoil extrusion to be noseweighted to get it to balance at 1/4 chord. There"s no substance on Earth dense enough to use as a balance rod to get such a blade to balance at quarter chord. The airgap in extrusions doesn"t serve any purpose other than being an empty void. Blades like AKs demonstrate convincingly that hollow extrusions are plenty stiff enough, so there"s absolutely no need whatsoever for the blades to be solid. And, obviously, solid extruded blades would weigh heaps for no valid reason.I can"t be bothered to do the math at how long it would take solid aluminium blades to change plane, but I suspect they"d be extremely sluggish in response.Mark R

[tonydenton]

my head hurts just to think that some one can do maths that can calculate how long it would take for solid ally blades to change plane. ??? ??? ??? ??? ??? ??? ???

[Tim]

Muz, I agree with Mark. I don"t have the weight/kg of solid 6061 Aluminium, but rest assured you probably wouldn"t be able to lift them. McCutcheon"s and Patroney"s would seem like feather weights in comparison. Even if you packed the leading edge with lead you still would not be able to effect a 25% or better chord wise balance. Being long and limber they would also be very flexible and "flutter" as soon as they were fast enough to create some lift, and destroy themselves in short order. Propeller blades are short enough and torsionally rigid enough to resist fluttering.Another problem would be the total mass that (as Mark alluded to) would be heavy to twist with the joystick, as well as it would add considerable extra weight to the gyro that would reduce the payload capacity (fuel & pax).

[MadMuz]

Fair enough, just bored and thinking too much for a change/as usual

[Tim]

Thinking is good!

[MadMuz]

That"s what I thought

[birdy]

Solid alu extrusions would help if you hada htl issue.

[Tim]

I think eagles would only be a minor nuisance also.

[MadMuz]

With solid rotors, probably trees, bridge pylons etc would only feel like a little bump..... would be like a pair of axes swinging around

[blakemorem]

I had a close call with and eagle a while back, he passed a few feet over the rotor and i still ponder what would have happened. Best not think about it. I have always understood hollow extrusions are more flexible and solid tends to crack easier. I have seen this theory applied to a high tensile shaft and it worked never cracked again. may be we have an engineer that could tell us more about it.Mike.

[gyroo]

Hi Mike,I"m not aware of any helicopter accident statistics that show any loss of machine from a rotor strike with a bird, although there are a few that have resulted in loss of control when large birds have crashed through the bubble windscreen and incapacitated the pilot. In 1987 a large brown bird of some description was disintegrated by the rotor of a Hughes 300 I was flying at 2000 feet above Taggerty here in Vic. All I saw with absolutely no time to react was a brown-grey blur with spread wings coming at the rotor, then for maybe less than a tenth of a second on the right front I saw what I thought were loose feathers and a couple of larger blurs as I simultaneously sensed a sort of a slight thudding down into the stick and couple of momentary lateral wobbles of the rotor hub before everything settled to normal. I had to set down at Reefton anyway (about 20 ks away) and when I did I noticed blood streaks on one blade at about 3/4 span, plus some blood spots on the rear angled stabilizer. There was no blade damage. I figured the slight wobble I sensed in the head came from the blade that was hit being dragged back slightly on its damper and being slightly out of phase for 2 or 3 revolutions before it righted itself.Based on this personal experience and of accident statistics generally, I doubt that a bird strike into a gyro rotor would cause any problems whatsoever. However - a bird strike right into your face in an open framer would cause real injury and a likely loss of control, I guess.Cheers,Mark R

[blakemorem]

Thanks for the confidante Mark as I was worried about this issue. A CFI also told me similar like won"t hurt your rotor. I guess if you look at Newtons law of Energy is Mass times velocity then the rotor wins but it must be one hell of an impact. And your right it happens so quick you have no reaction. I also encountered a flock of Pelicans at about 9 o"clockon take off closing in like guided missiles. Fortunately I gained quick altitude while veering to right and it was a near miss. I"m always looking for birds now. Problem with Eagles they hover around at substantial altitude and hard to see. We aren"t the only ones to enjoy thermals, just got to be vigilant I guess.Mike.

[MadMuz]

I had the misfortune of being hit in the chest by a galah over the escarpment of lake George, it felt like copping a whack from a cricket bat and scared the poop out of me, I"ll tell you.... I immediately put the wire back from the keel to the cheek plates once I returned, I saw where the galah came from, the wire would have deflected it (or cut it in half) but it would have hurt less. I can imagine one bird, probably even a pelican getting hit by the rotors at flying speed shouldn"t damage the blades unless the pilot is really, really unlucky.... I have heard of pilots on here saying they have trimmed small trees and branches with rotors on outlandings and such, a small tree firmly embedded in the ground would offer far more resistance than a bird which is in free flight so the initial impact would be substancial, but the bird would be thrown clear post haste... limiting the potential for damage. Because our rotors have a generally pointy solid=ish leading edge, thin in construction, heavy with a lot of inertia and a constant shallow angle of attack.... hitting the biggest bird in exactly the right (wrong) place would be hard to achieve, and if it were to happen, the bird would more likely be deflected by the rotor disk, or if one blade were hit only, the bird would be instantly gone, the rotor hit would undoubtedly be rather upset, but would quickly resume its path, as long as no structural damage was done...... I would be not so sure if one encountered a flock of birds hitting the rotors one after one... striking rotors while the blades are upset.... like a lawnmower can mow over the odd clump of grass.... but if the grass is contantly clumpy, maybe different story... if you catch my drift.The thing I would be more worried about would be a bird or 2 missing the rotors and me and hitting the prop..... bigger the bird, more risk of prop damage and if a prop blade comes off, which direction will it go? and will the motor get ripped out if a prop blade parts company at decent revs!It probably sounds pedantic, but after the galah hit me, I fitted those black plastic "roo whistles" on the front of my machine, they are supposed to give wildlife a noise thy can locate and therefore head away from.... they work on the road with roos, I never had another birdstrike after fitting them, so maybe they work?