Heli down

[iChris]

tangential velocity

You’re R22 trained… You know all this.

 

http://youtu.be/ljuNEv2ofHo

Edited December 4, 2013 by iChris

[pilot#476398]

You want me to watch a 7 minute video?,....ah just forget it!

[Wally]

Kinda tangential to the subject, I read some of the file (actually everything I could find) in the NTSB file on CEN11FA599, one of the documents described that accident's flight after the engine stopped. The failed auto was described (as I recall) as rolling off to the retreating blade, pitching nose down as the aircraft trajectory arced ever more vertical, yawing 360 at least once before impact, corkscrewing into the crash site.

That follows, somewhat, the textbook description of a retreating blade stall as pitching up, rolling to the retreating blade, or both.

But, with an NR dropping out of flyable, un-powered, wouldn't it seem reasonable that both pitch and roll would continue loading the disk, increasing the angle of attack on the decelerating blades, exaggerated by increasing coning... until impact, which might be sideways or even backwards.

 

NR is LIFE!

[iChris]

Yes, that is true. But, retreating blade stall can also occur at ANY speed, under low RRPM.

 

 

The correct statement would be:

 

Blade stall can occur at ANY speed.

 

Retreating blade stall is a specific type of stall with specific characteristics that occurs under specific operating conditions, not just any.

 

I was told that no placards, charts, or POH's talk about this. Why? Because they expect the pilot to ALWAYS be in the green on RRPM.

 

In fact, most of your aircraft’s speed and rotor limitations are based on design factors like power available, retreating blade stall, compressibility, stability, and vibration effects. These factors are also covered in certification under §27.1505 and §27.1509

 

The rotor tip speed is often selected as a compromise between the effects of retreating blade stall and compressibility. Low tip speeds have the advantage of low noise and good hovering performance. High tip speeds have the advantage of low rotor and drive system weight and high stored energy for autorotative entries and flares.

 

Therefore, there is a limited range of acceptable tip speeds (figure below), which become smaller as the helicopters speed increases. Compressibility effects on the advancing blade and stall effects on the retreating blade combine to restrict the maximum forward speed of the helicopter. However, take note, factors like power available, stability, or vibration issues can limit VNE and rotor speed before the effects of compressibility or retreating blade stall ever occur. This is the case with the R22.

 

To avoid advanced-tip compressibility it’s generally accepted that the “Advanced-tip Mach number” be no more than about 0.92. It’s also generally accepted that the “Advance ratio” should not exceed 0.5 to avoid retreating blade stall. In other words, with an Advance ratio of 0.5, the maximum forward speed should be no faster than 50% of the rotors tip speed in order to avoid retreating blade stall.

 

Advance tip Mach number - M_at = (ΩR + V)/c_S

Advance ratio - μ = V/ΩR

Tip Speed (ft/sec) = (Rotor Radius(ft) x RPM)/9.55

 

As an example:

 

The R22 [102kts (172.15 ft/sec), Sea level, tip speed @ 699 ft/sec, Speed of sound (c_S) = 1116.9 ft/sec]

 

R22 (current): Advance tip Mach number is 0.77 and the Advance ratio is 0.24.

 

From those numbers you can see other factors like power available, stability; vibration, etc. limit the R22’s VNE.

 

Using an upgraded “Advance ratio” of 0.35, the R22’s current tip speed of 699 ft/sec would allow for an increase of its current VNE, if the airframe and components could withstand it and the power plant could deliver the required power.

 

Follow the slanted μ 0.35 line down to our new Advance tip Mach number of 0.84. Then go straight down to obtain the new maximum forward speed, new VNE 145kts.

 

 

 

Edited December 8, 2013 by iChris

[Joe_P148]

Two days ago. Old news.

 

What a D 1 C K! Have a little respect.

[iChris]

I was told that "no" placards, charts, or POH's talk about this. Why? Because they expect the pilot to ALWAYS be in the green on RRPM.

This information is included in some RFMs. You'll find this type of info in most Military RFMs and Sikorsky has done a good job with their RFMs in the past. Some current manuals lack technical data.

 

 

 

Edited December 9, 2013 by iChris

[WolftalonID]

Ok..iChris.....there are great inputs from you from time to time, then there are the ones that baffle Einstein, lol

 

So here is a link for dummies...I mean Helicopter pilots! Haha. I had to google it myself so i could get it.

 

http://www.dummies.com/how-to/content/calculating-tangential-velocity-on-a-curve.html

Edited December 10, 2013 by WolftalonID

[500E]

Air Accidents Investigation: S9/2013 - Eurocopter EC135 T2+, G-SPAO

 

Intrim report

[JDHelicopterPilot]

Ok, back on track here. The AAIB found the following thus far:

 

There was 95kg of fuel onboard: they did't state which tanks that fuel came from. That could make a difference.

 

What really got me was the aircraft struck the roof with high vertical speed, little forward speed and NO main rotor or tail rotor RPM.

 

This accident has me very interested. There are a number of causes I can think of. However, those I think of mean the pilot would have had complete disreguard for the checklists, Emergency Procedures very poor ADM.

[Hotdogs]

Loud popping noises heard by ground witnesses leads me to believe that it could have been an engine malfunction e.g. compressor stalls or a attempted engine relight, but it could be anything with amount of rotating objects on a helo.

 

What is concerning about that is the aircraft has two engines. One failing is unlikely, two failing is extremely rare, and lastly with at least one engine they should have be able to maintain some sort of power-on landing profile to a sliding landing. Extremely Odd.

 

http://www.nbcnews.com/id/53855786/ns/business-stocks_and_economy/#.UrNANZETHwI

 

Next question I want to know was if the pilot was on goggles and when was his last aided night auto.

[JDHelicopterPilot]

A lot of twin drivers don't do auto-rotations as part of their training. This is more common that you may think or I agree with.

 

EuroCopter has been notified by Bond (operator in Europe ) of a problem with the EC135 supply tank fuel indicator not displaying proper fuel level. This is a concern. There is now a check for operators to complete to verify the supply tank probes are working properly. Bond discovered problems with two of their EC135s. Right now EuroCopter is not saying this is related to the accident. The result of this problem is the fuel caution and warning lights may come on when not expected but should be followed as per the checklist despite the fuel indication.

 

It should also be noted, with the main tank empty and supply tanks full, there is just under 30 min fuel remaining. Thus, most pilots will not fly into that and will not use the fuel in the supply tanks. I for one will allow my main to run dry but will use ALL of my supply fuel as reserve fuel even though there is more than 20 min. To me it makes sense as an added safety buffer but also makes fuel planning a little easier.

[Nearly Retired]

I've got about 2,500 hours in Bo-105s which have pretty much the same type of fuel system (big main tank plus individual supply tanks to feed either engine). I'd fly the main right down to zero but I *never* used any fuel out of the supplies. It was sometimes funny to watch the expressions of knowledgeable passengers as they'd see the main tank gauge drop right down to zero. Even I admit that it's unnerving. Then we'd land and the main needle would come back up a bit. Seems it didn't read all that accurately at the 10 degree nose-down cabin attitude that the Bolkow cruised at.

 

You fly a Bolkow by your watch, man. And if you see that supply tank needle moving, you done really screwed up.

[iChris]

 

The result of this problem is the fuel caution and warning lights may come on when not expected but should be followed as per the checklist despite the fuel indication.

 

It should also be noted, with the main tank empty and supply tanks full, there is just under 30 min fuel remaining. Thus, most pilots will not fly into that and will not use the fuel in the supply tanks. I for one will allow my main to run dry but will use ALL of my supply fuel as reserve fuel even though there is more than 20 min. To me it makes sense as an added safety buffer but also makes fuel planning a little easier.

 

That's correct... Some have pointed to the fuel sensor problems at Bond; however, the the low fuel warning, separate stand alone system, would have still operated. Consequently, at that point (“low fuel” alert) they would have had 8-10 minutes to land.

 

According to the fuel system functionality tests performed, the fuel sensors that measure the fuel levels in the supply tanks of the aircraft operated by Bond incorrectly indicated the available fuel in the supply tanks. The reason is under investigation.

 

Since the sensors in the supply tanks of this aircraft did not work properly, the amber “caution” signal linked to the sensors was not triggered. However, the red “low fuel” alert still worked correctly because it operates on a completely decoupled system.

 

The red “low fuel” warning is triggered when there are 32 kilograms of fuel in supply tank 1 and 28 kilograms of fuel in supply tank 2. When this occurs, the pilot has 8-10 minutes left to land, regardless of the conditions and as dictated in the flight manual.

 

Tests have confirmed that the affected fuel content probes generate erroneously high signals if contamination with water occurs. In each case an incorrect frequency is being transmitted to the indicating system. That leads to an erroneously high indication of fuel quantity and prevents the amber FUEL caution light from operating. The red LOW FUEL 1 & 2 warning lights continue to operate correctly.

 

Fuel system functionality tests subsequently performed by two other EC135 operators in Europe have revealed possible similar supply-tank fuel gauging errors on some aircraft.

 

The fuel tank system of the EC135 consists of one main tank and two supply tanks. The main tank is the reservoir that continuously feeds the two supply tanks, which in turn feed the engines located on the right and left sides of the aircraft. Each supply tank feeds its own engine. This “feeding” is a continuous process. The supply tanks therefore remain full until the main tank is empty, at which point the fuel levels inside the supply tanks begin to decrease.

 

Operators are advised to refuel as soon as the supply tank “reserve” is tapped – meaning that the main tank is empty.

 

Two sensors – one located at the front of the tank, and one located at the back, measure the fuel levels in the main tank. This system ensures that the fuel level of the main tank is measurable regardless of the aircraft’s tilt.

 

The fuel levels of the two supply tanks are measured by one sensor each.

The design of the sensors in the main tank is the same as the design of the sensors in the supply tanks, the only difference being the size.

 

 

Edited December 22, 2013 by iChris

[iChris]

EC-135

 

 

ALERT SERVICE BULLETIN

 

EMERGENCY AIRWORTHINESS DIRECTIVE - AD No.: 2013-0307-E

Edited December 22, 2013 by iChris

[JDHelicopterPilot]

EuroCopter made temp changes to the RFM. In short, they added a note to the Emergency Procedures to remind the pilot to not ignor the warning light despite fuel level indication.

 

There is a reason why the caution and warning lights are independent.