Helicopter rotor blade scintillation

#1
Hi guys,

I was reading an unclassified MoD document pertaining to glowing helicopter blades in dusty environments, particularly during take off/landing.

Have any of you personally observed the phenomenon?

Do you guys have any pictures of it? I'm particularly interested to know if it also affects the tail rotor and other rotors, like turboprops.

Do you have any anecdotes of at what RPM you observed HRBS to occur, and on which helicopters (actually I just need the rotor diameters to calculate the tip velocities it starts at)?

Some dumb questions but when the phenomenon occurs, does the air smell any different? Do you find you discharge static electricity on objects more readily when in the rotor downwash?

I'm designing a laboratory test rig to replicate HRBS in a convenient setting and wanted to know if I need to be aiming all the way for sonic blade tip velocities, or can start seeing the phenomenon at say 100m/s.

Yeah I've tried asking the contact in the document but haven't gotten a response yet.
 
#6
I find it a very interesting clue that HRBS disappears when helicopters are landed.

Does anyone know if helicopter pilots tend to reduce the lift of their landed helicopters by their decreasing their collective, or by reducing rotor RPM?

Any personal accounts however trivial they might seem are most welcome (here or by PM).

Thanks.

P.S. If the research proposal I am submitting to the MoD gets funded I vow to make a sizeable donation to a nominated source (which we will vote on) in support of our servicemen/women.
 
#7
IndependentBoffin said:
I find it a very interesting clue that HRBS disappears when helicopters are landed.

Does anyone know if helicopter pilots tend to reduce the lift of their landed helicopters by their decreasing their collective, or by reducing rotor RPM?

Any personal accounts however trivial they might seem are most welcome (here or by PM).

Thanks.

P.S. If the research proposal I am submitting to the MoD gets funded I vow to make a sizeable donation to a nominated source (which we will vote on) in support of our servicemen/women.
Doesn't seem to know much about aircraft for a boffin........
Obviously not realising that when an aircraft has landed it is grounded/earthed... Doh
 
#9
Whats the differance between sintilation and St Elmos fire as I thought they were the same thing,ie visible static electrical discharge
 
#11
Bravo_Bravo said:
Scintillation only appars on helo blades, AIUI.
Got you, then I have seen somthing like it on a Scout flying under a Q-nimbus in Ballykelly the weather was crap and he tried to fly under it along the coast to get back to Aldergrove, when it returned the disc was glowing and we thought it was St Elmos, hadn't heard of sintilation then, I also saw it in El Adam on an RAF Wirlwind of 1564 SAR flight, Is it weather induced as on both occasions there were Q-nimbs about and the weather was really awfull
 
#12
Could be interesting to see if there is a corrolation between blade design and effect, i.e. effect of metal v composite, leading edge strip material, rotor hub design (material composition of hub).

Other factors may be under carriage design (wheels v skids) which may affect grounding resistance, particle size and ambient humidity.

Looks like a fun thing to investigate, although if it isn't causing any problems why bother...just take photos and admire the sparkly lights.

S_R
 
#13
qualitycrab said:
IndependentBoffin said:
I find it a very interesting clue that HRBS disappears when helicopters are landed.

Does anyone know if helicopter pilots tend to reduce the lift of their landed helicopters by their decreasing their collective, or by reducing rotor RPM?

Any personal accounts however trivial they might seem are most welcome (here or by PM).

Thanks.

P.S. If the research proposal I am submitting to the MoD gets funded I vow to make a sizeable donation to a nominated source (which we will vote on) in support of our servicemen/women.
Doesn't seem to know much about aircraft for a boffin........
Obviously not realising that when an aircraft has landed it is grounded/earthed... Doh
Thanks for your constructive criticism but my line of reasoning is that there are two things which happen when a helicopter has landed and its ramp goes down: lift drops (which may be due to collective or RPM decrease) and the helicopter is electrically grounded.

You are assuming that electrostatic discharge is the cause of HRBS.

There is no definitive consensus in the scientific community as to the cause.

If collective is decreased but RPM is the same when landed then it is a strong indicator or electrical discharge being the cause of HRBS. If RPM is decreased then it suggests that blade erosion, triboluminescence or chemoluminescence may also be at work.
 
#14
Sympathetic_Reaction said:
Could be interesting to see if there is a corrolation between blade design and effect, i.e. effect of metal v composite, leading edge strip material, rotor hub design (material composition of hub).

Other factors may be under carriage design (wheels v skids) which may affect grounding resistance, particle size and ambient humidity.

Looks like a fun thing to investigate, although if it isn't causing any problems why bother...just take photos and admire the sparkly lights.

S_R
AIUI it is making helicopters easier to locate and target for insurgents.
 
#15
codbutt said:
Ultimately they reduce RPM and angle of attack (collective pitch) but the collective pitch is reduced first.
Can you say if the helicopter's RPM is above the speed at which HRBS is observed to occur at the stage of takeoff/landing in these images:



In this photo as people are near the helicopter I'm guessing that the pilot has reduced RPM to idling and the collective to the minimum so that there is minimal downwash.



In the last photo as the helicopter is just about to take off I'm guessing the pilot had RPM to max and was slowly increasing collective, but I could be wrong.

Or is all this really quite dependent on the pilot's style?

Thanks.
 
#16
tropper66 said:
Whats the differance between sintilation and St Elmos fire as I thought they were the same thing,ie visible static electrical discharge
HRBS is noted to be wide band (c.f. my original link: "Wide band emission but unknown if detected by infrared sensors.")

Whereas for St. Elmo's fire:
"The nitrogen and oxygen in the Earth's atmosphere causes St. Elmo's fire to fluoresce with blue or violet light; this is similar to the mechanism that causes neon lights to glow."

Therefore even if both are caused by static electrical discharge there must be a consistent explanation for what is causing light emission at other EM frequencies to produce wide band visible light, instead of just a blue/violet glow.
 
#17
It's been a while since I did my rotordynamics, but I think the majority of aircraft (both rotor and prop driven) tend to run at a nominal RPM and use the collective pitch to provide most of the thrust. Keeping the RPM constant has some benefits for vibration (keeps it constant and in known bounds). Obviously there are RPM changes but they have a minimal effect on the thrust when compared to collective pitch.

If landing somewhere hostile I would be surprised if the pilot throttled back the RPM, as it does take time (due to momentum of the engines/blades) to bring it back up to a point where you can apply thrust through the rotors, I would expect the top photo to be at operating RPM, or close to it, with minimal pitch.

Also note that some helicopters (naval mainly) can apply a level of negative thrust by applying negative collective pitch, may be interesting - may not.

If you want a good book in rotor aerodynamics then I can recommend the following book:

http://www.amazon.co.uk/dp/047023394X/?tag=armrumser-21

Which is basically the rotordynamics course given by Dr. Simon Newman at Southampton uni. probably one of the brightest blokes I've met (I did the MEng Aero/Astro course there in the late 90's) for helicopter dynamics. He's cone a joint book which seems slightly more basic as well, but came out after I left Uni so ahve never read it.

S_R

p.s. as I said I'm a bit rusty, been 4 years since stopped flying on twirlybirds and nearly 10 since Uni.
 
#19
Thanks for the info sympathetic_reaction. I did helicopter dynamics during my UG course but like you time has addled the memory somewhat. I can't recall being taught stuff on actual piloting procedures (it was all very theoretical), but I can follow your logic that RPM is ideally kept constant to give the pilot low latency in the event of an emergency takeoff being necessary.
 
#20
IndependentBoffin said:
Thanks for the info sympathetic_reaction. I did helicopter dynamics during my UG course but like you time has addled the memory somewhat. I can't recall being taught stuff on actual piloting procedures (it was all very theoretical), but I can follow your logic that RPM is ideally kept constant to give the pilot low latency in the event of an emergency takeoff being necessary.
Just read the reference and noticed it says it occurs on takeoff and landing. During landing there is a large change in pitch during the last phase as the pilot will flare to reduce downwards velocity and finally land, this can sometimes (depending on the pilot) produce a larger rate of change of pitch and on take-off.

Tended to find the Naval SAR pilots the best for smooth landings, but the AAC ones for a good speedy takeoff....the RAF tended to be in the crewroom drinking tea.

S_R
 
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