Stick skills v airmanship

[fdr]

Originally Posted by aa777888

How far do MRB bits fly?

The rest of your post was quite extensive, thank you, but already evident to me with two exceptions:


That's extraordinary. I would not have thought even a not-off-the-shelf R22 could do that. I am curious about the rest of this story. What were the modifications?

That is something I had not considered. In your professional opinion, from the video evidence, do you feel that there were such over-speed events during the flight in question?

Originally Posted by aa777888

How far do MRB bits fly?

A good question.

For the safety case for research flying, I was asked that and came up with a rough estimate, without having to go test out my maths. The MRB and TRB are going to come off with a fair amount of surface area and modest inertia and will develop their very own lift in most but not all cases. Occasionally it will be pure ballistics, in most there will be some lift related to the rotation of the part, A Magnus effect will usually occur to some extent. So we plotted pore ballistic estimates given worst-case and expected altitudes and used also the assumption of rotational lift to give a wind drift outcome. In the end, Magnus effect is more pronounced at altitude, down low, ballistic will control. Other bits were not of as much concern, but we looked at lightweight panels, not a factor but have large spread, and high-density stuff, which is going to be ballistic. In the end, for a couple of the flights that ended up with a particular corridor that was approved, for the area stuff, there was a lot of open country with no one about. Not Mojave level, but not far away.
Down in the weeds, a rotor blade coming off is going to give debris out a long way, its coming off at a tip speed of around 230m/sec, from about 3m from the ground, so it will drop to the ground in around 0.78 seconds assuming no lufting going on. so, thats around a touchdown at 200m from the shaft, and then it's still got inertia, and is going to ping off further. Add another 50% of the first distance for stamps.... expect your MR blades within 300m of the wreck flaming datum. The TR is going to be a parabolic arc potentially but is going to lose energy quickly, it has relatively low density, so, it will still come down short of the 200m and will tend to ricochet less. Anyone with a better answer, I'm all ears.

Originally Posted by aa777888

That's extraordinary. I would not have thought even a not-off-the-shelf R22 could do that. I am curious about the rest of this story. What were the modifications?

The out of the box R-22 has a very good TR authority. First up,

WARNING

please note that anyone flying at high speed sideways or rearwards in a helicopter near the ground needs to have a good reason to do so, the chances of a successful autorotation are unquantified by the OEM other than for forward flight, per the H-V curve in the Performance section of the POH.

Going right, your out of the box TR is good to around 26-28kts, and that is dependent on limit cases, but I was flying worst-case CH and weight, and high DA... you may get different outcomes, but there is a limit to the TR authority. Going to the left, there is also a point that you will run out of nerve.

Part 2 to follow shortly... got a flight to do...

§ 27.87 Height-speed envelope.(a) If there is any combination of height andforward speed (including hover) under which a safe landing cannot be made under the applicable power failure condition in paragraph (b) of this section, a limiting height-speed envelope must be established (including all pertinent information) for that condition, throughout the ranges of -
(1) Altitude, from standard sea level conditions to the maximum altitude capability of the rotorcraft, or 7000 feet density altitude, whichever is less; and
(2) Weight, from the maximum weight at sea level to the weight selected by the applicant for each altitude covered by paragraph (a)(1) of this section. For helicopters, the weight at altitudes above sea level may not be less than the maximum weight or the highest weight allowing hovering out-of-ground effect, whichever is lower.

(b) The applicable power failure conditions are -
(1) For single-engine helicopters, full autorotation;

So, going rapidly backwards, or sideways, there is no H-V curve, y'all are on y'own.

§ 27.143 Controllability and maneuverability.

(a) The rotorcraft must be safely controllable and maneuverable -
(1) During steady flight; and
(2) During any maneuver appropriate to the type, including -
(i) Takeoff;
(ii) Climb;
(iii) Level flight;
(iv) Turning flight;
(v) Autorotation;
(vi) Landing (power on and power off); and
(vii) Recovery to power-on flight from a balked autorotative approach.

(b) The margin of cyclic control must allow satisfactory roll and pitch control at VNE with -
(1) Critical weight
(2) Critical center of gravity;
(3) Critical rotor r.p.m.; and
(4) Power off (except for helicopters demonstrating compliance with paragraph (f) of this section) and power on.

(c) Wind velocities from zero to at least 17 knots, from all azimuths, must be established in which the rotorcraft can be operated without loss of control on or near the ground in any maneuver appropriate to the type (such as crosswind takeoffs, sideward flight, and rearward flight) -

(1) With altitude, from standard sea level conditions to the maximum takeoff and landing altitude capability of the rotorcraft or 7000 feet density altitude, whichever is less; with -
(i) Critical Weight;
(ii) Critical center of gravity;
(iii) Critical rotor r.p.m.;

(2) For takeoff and landing altitudes above 7000 feet density altitude with -
(i) Weight selected by the applicant;
(ii) Critical center of gravity; and
(iii) Critical rotor r.p.m.

(d) Wind velocities from zero to at least 17 knots, from all azimuths, must be established in which the rotorcraft can be operated without loss of control out-of-ground-effect, with -
(1) Weight selected by the applicant;
(2) Critical center of gravity;
(3) Rotor r.p.m. selected by the applicant; and
(4) Altitude, from standard sea level conditions to the maximum takeoff and landing altitude capability of the rotorcraft.

(e) The rotorcraft, after (1) failure of one engine in the case of multiengine rotorcraft that meet Transport Category A engine isolation requirements, or (2) complete engine failure in the case of other rotorcraft, must be controllable over the range of speeds and altitudes for which certification is requested when such power failure occurs with maximum continuous power and critical weight. No corrective action time delay for any condition following power failure may be less than -
(i) For the cruise condition, one second, or normal pilot reaction time (whichever is greater); and
(ii) For any other condition, normal pilot reaction time.

(f) For helicopters for which a VNE (power-off) is established under § 27.1505(c), compliance must be demonstrated with the following requirements with critical weight, critical center of gravity, and critical rotor r.p.m.:
(1) The helicopter must be safely slowed to VNE (power-off), without exceptional pilot skill, after the last operating engine is made inoperative at power-on VNE.
(2) At a speed of 1.1 VNE (power-off), the margin of cyclic control must allow satisfactory roll and pitch control with power off.

You are assured of TR authority per the certification up to 17kts from all azimuths.... with CG and RPM worst cases within the weight/height and envelope chosen by the OEM. [side note... the (f)(2) case is very unpleasant to fly from my experience. Even with a parachute on. If your POH states a high-speed limit for being in auto, that is there for good reasons.]

Originally Posted by aa777888

That is something I had not considered. In your professional opinion, from the video evidence, do you feel that there were such over-speed events during the flight in question?

Hmmm.

The audio of this is rubbish, presumably a 44K sampling rate but that could be wrong. My sampling of that to use to get a useable sample also sucks, but is not as bad as all that, it is only a 44K sampling rate though. The spectra is shown below. The first one is more or less the whole video audio output sampled. There are obvious dopplers happening, no surprise there. There is also some acoustic far-field aspect variation, which is not very high. In the near field, the exhaust note and TR are the most significant signal sources, in the far-field, the TR is still a big factor, the MR less so but determinable as a signature and the most significant noise source is the BVI or rotor slap. In far-field, the exhaust is adding little to the spectra, but it can still be isolated. That comes from my reading and my testing, including flyover, etc, but I will gladly defer to any acoustics DER input and will be set straight on my thoughts. I near field testing there is quite a prominent lobeing of the noise SPL that was measured (in my testing at least) coming from the exhaust and from the aspect to the TR disc.

Anyway, as this chopper picks up and pirouettes to the right, the spectra shift noticeably. There is no doppler of note, it appears to be a blade rate harmonic,

NML MIN 101% (404 RPM 6.68Hz)
(99% 398 RPM 6.63Hz on appropriate tachos)
MNL MAX 102% (408 RPM 6.8Hz)
ENG MAX105% (2800 RPM 46.66Hz)
108% (432 RPM 7.2Hz)

The TR and MR have 2 blades, so reinforcement lines will occur at 2 times shaft rate.
A reinforcement will occur for the MR and engine at around 6.73x. The engine is also a 6 cyl, so there is a reinforcement as well due to exhaust impulse and the power pulse, half that as it is a 4 stroke.

Anyhooo, the sampling sucks, and which spectral line we are picking up is questionable, but it is showing a marked rise where there would be no major doppler from the source towards the recorder/observer. So, best guess is, yup, that engine will have had a rough ride as would the MR bearings. The engine would show characteristic markings from a really good Overspeed, the coning bearings could show some dust around the outside of the bearing to the head hole, but would not necessarily show damage from an overspeed. A sound lab could do a much better job of analysis, but I would be happier not to ride or rent that machine without inspection. The rise is seen in the second spectrogram below. I would say it is possible to probable that the aggressive rotation to the right through unloading the left pedal gives the rise that may have beaten the governor, just as the left mitt of the pilot can do at any time.

The owner may know, but the pilot should know what he got. Down low though, his eyes are outside or should be, so... maybe not. When I was doing testing, the engine, performance, attitude, MRB tensile loads, pitch link loads, TR flapping, MR flapping, and control positions were being recorded. And as stated previously, I wouldn't start the engine without Nomex, gloves, and an MSA Gallet or Alpha dome. It takes too much effort to put Humpty Dumpty back together when you screw up.

sorry.