Fenestron

[DakarNick]

Hi everyone. I went to Rotors of the Rockies yesterday at BJC since I had never been and wanted to see their hangar and aircraft. This was the first time I had seen an EC120 Colibri. Theirs is very nice!

 

What caught my attention is the fenestron. Not the 120's specifically, but fenestrons in general. Does anyone know what kind of CFM these produce? I know it depends on engine power output, wind conditions, effectiveness, etc. To me it just seems small, of course it does it's job.

 

I know the helicopter only has around 500 hp, how much of that is used for the fenestron?

 

Thanks

[Hedge36]

Just enough.

 

[DakarNick]

Just enough.

 

 

 

Yeah I guess! I am not sure if the blade area of a Fenestron and a two-blade TR would match, but it just seems so small. I wonder if they have to be geared up...

 

Oh yeah, I should have asked this in the tech forum. If a moderator wants to move it, please do!

[JDHelicopterPilot]

The blade area of an EC130 Fenestron is actualy more than what you get on an Astar. Hard to believe I know. However, the Astar tailrotor is more effective than a Fenestron. Then the Fenestron is also much more quiet and safer as well.

[Darren Hughes]

I studied this a little bit for the JAA ATPL(H). It seems that quite a significant amount of the thrust of the fenestron comes from the shape of the shroud heading into the blades and coming out. The shape of the shroud supplies a kind of venturi effect which increases the efficiency of the fan blades.

 

That's actually how they word it in some of their questions. Take from that what you will. Although most of what the JAA says doesn't hold much water with me cause they're a big bunch of bureaucrats. I've caught them out being wrong on several fundamental things in the past.

[JDHelicopterPilot]

If that is true about the shroud design creating more efficiency then they need it cause the traditional tail rotor is still more efficient.

[Darren Hughes]

The shroud also reduces loss of lift at the tips of the blades due to vortices increasing induced drag. That's what I think the FAA focus more on in their books. I don't know which gives the greater effect.

 

Yeah, I can't see any time in the near future a fenestron chopper competing with a AS350 for tail rotor authority at higher elevations.

 

Edit; on a side note, I wish the helicopter that is currently flying round in circles for the past 20 minutes above the motel6 in Lakelands, FL had a fenestron cause it's keeping me up!! Go away dammit, I've got exams in the morning!!

[dolphindriver]

I fly an aircraft with a Fenestron ( Coast Guard MH-65 dolphin/civ AS365 dauphin) and the fenestron is very efficient. I also have had the opportunity to fly the aircraft with two different types of engines. Fenestron performance increased dramatically with the new engines because there was obviously more power that could be applied but we also have a switch that increases our main rotor by 10 turns a minute which correlates to an increased amount of turns in the tail creating more lift. We have used them at pretty high altitudes in a hover as well.

 

it is a pretty good system but in the end all of the counter torque systems get the job done.

Edited November 6, 2008 by dolphindriver

[JDHelicopterPilot]

D-Driver, where can I get one of those switches? I'd like an extra 10 RPMs on a dime some times.

 

The EC130 fenestron works well but if not careful on a windy day it can get away and end up in an over torque. I have seen it happen several times. Better off making a right pedal turn on those days in the summer. Every design has positives and negitives. That is one for the EC130.

[dolphindriver]

D-Driver, where can I get one of those switches? I'd like an extra 10 RPMs on a dime some times.

 

The EC130 fenestron works well but if not careful on a windy day it can get away and end up in an over torque. I have seen it happen several times. Better off making a right pedal turn on those days in the summer. Every design has positives and negitives. That is one for the EC130.

 

 

Our switch came standard, I think you will have to ask the dealer, it might cost a bit more but probably not much more than a built in DVD player.

 

As for windy days and overtorques, we had the same problem with our old engines. But with the new engines we now have the power to back up the system and we have reduced our Uncommanded Left Yaw window dramatically.

[EC120AV8R]

JD, the EC120 is the same on windy days as well. I always feel more comfortable with a right pedal turn in the wind.

[JDHelicopterPilot]

I think the loss of effect the Fenenstron has is due to it not being open like a standard tail rotor is. I could be wrong though, that's my thinking. The Astar tail rotor is out in the open taking advantage of the main rotor downwash. Either way, since I didn't have a second engine or nice handy switch to give me another 10rpms, we had to be careful. Must be nice flying the MH-65.

 

There are somethings I miss about the EC130 but I am enjoying the A119.

[dolphindriver]

I think the loss of effect the Fenenstron has is due to it not being open like a standard tail rotor is. I could be wrong though, that's my thinking. The Astar tail rotor is out in the open taking advantage of the main rotor downwash.

 

Actually, the main rotor downwash is a hiderance, not a help on any tail rotor system as it breaks up the airflow. The NOTAR may be another story. However, I am curious if you use any of the following or something similar using your aircraft since you have a feneston. We live by it. Especially when our aircraft performance was similar to what you describe. With the new engines we have a wider margin before getting into it but it is still a concern. This is right out of our operator's manual.

 

Uncommanded Left Yaw (ULY) is a critical, low−speed aerodynamic flight characteristic which can result in a rapid, uncommanded yaw rate, that does not subside of its own accord, and if not immediately corrected, can result in loss of aircraft control. In other helicopters, this characteristic has been described as “loss of tail rotor effectiveness” or “LTE.” Previously in the HH−65, it was identified as “loss of Fenestron effectiveness” or “LFE.” Tests on the HH−65 and various helicopters have identified five major relative wind azimuth regions and/or aerodynamic factors that are conducive to ULY. ULY is generally the result of a combination of these factors. The following factors are illustrated in figure 6−1.

1. Weathercock Stability (120 − 240 relative wind):

All relative winds tend to cause the nose of the aircraft to weathervane into the windline. This characteristic is the result of the aerodynamic design of the fuselage and the vertical/lateral fins. The most significant aspect of tailwinds is that they can accelerate an established yaw rate. In the HH−65, this can be critical due to the large surface area (approximately 35 square feet) of the vertical fin.

2. Main Rotor/Tail Rotor Interaction (040 − 095 relative wind):

Relative winds from these areas can cause the main rotor vortices to shed directly onto the vertical fin/fenestron. These winds can potentially disturb the airflow through the fenestron causing variations in fenestron thrust for a given pedal position. Relative wind velocity is as important as the azimuth in directing the main rotor vortices into the fenestron. Regardless of helicopter type, the retreating blade side is most susceptible to this interaction. This area coincides with portions of the

HH−65 low airspeed pedal workload critical azimuth (figure 6−2).

3. Loss of Translational Lift (winds from all directions):

As the aircraft decelerates through translational lift, the increased power required to establish a hover requires a corresponding increase in right pedal. If pedal input is not coincident with collective application, a left yaw will occur. Translational lift,

while generally thought of as lift gained or lost during transitions from a hover to a forward flight regime or vice versa, can also impact power/anti−torque requirements during strictly hover flight. For example, if an aircraft is in a stable (into the wind or

crosswind) hover, it may have some degree of translational lift. If that same aircraft is allowed to drift with the wind, vice maintaining position, and the pilot makes no further control inputs, as translational lift is lost, the aircraft may both lose

altitude and/or yaw in the torque direction. Such an abrupt and unexpected loss of translational lift could contribute to initiate ULY.

4. Left Cross Winds (255 − 310 relative wind):

Since the intake of the fenestron is on the left side, relative winds from this region increase induced airflow through the fenestron, reducing tail rotor blade angle of attack and ultimately resulting in reduced tail rotor thrust for a given pedal position. This area coincides with the HH−65 low airspeed pedal margin critical azimuth (figure 6−3).

5. Collective Induced Yaw:

The relationship between collective application and aircraft yaw is a fundamental of rotary wing aerodynamics. Rapid,

positive collective application during an established left yaw, or while operating in any of the above regions may increase the possibility of encountering ULY.

 

Testing on the HH−65 has identified the following low airspeed critical azimuth regions for pedal workload and pedal margin.

1. Low Airspeed Pedal Workload Critical Azimuth(figure 6−2): Relative winds from these regions require frequent pedal movement due to yaw instabilities. Inattention to aircraft heading in these areas may allow yaw rates to build rapidly.

2. Low Airspeed Pedal Margin Critical Azimuth (figure6−3): With relative wind from 255 − 305 at velocities in excess of 30 knots, the pedal control margin is less than 1 in. Due to the reduced pedal control margin in this region, it may be difficult to establish a right yaw or arrest a left yaw.

 

HH−65 Flight Regimes Susceptible to ULY: Some maneuvers may involve a higher risk of encountering ULY. It is important to remember that most ULY incidents can be attributed to a combination of the previously discussed factors.

1. Low Speed Left Downwind turns: A low speed left downwind turn, particularly in high winds, may allow a number of ULY factors to influence the helicopter. Loss of translational lift, weathercock stability and main rotor/tail rotor interaction can combine to accelerate the yaw rate in these turns.

2. Hovering Pedal Turns, Tail Through Windline in High Winds: The large surface area of the HH−65’s vertical fin can be a tremendous yaw accelerant when passing through the wind line. Combined with the possibility of main rotor disc vortex interference, this could produce a substantial yaw rate.

3. Pilot Induced High Yaw Rates: Inattention or failure to make immediate pedal inputs to arrest an uncommanded left yaw rate can result in the establishment of high yaw rates. During AFCS off operations, the usual pedal feel is altered and may give pilots the perception that the pedals are not responding “normally.” Allowing high yaw rates to develop during AFCS off operation can make recovery more demanding. In addition, yaw rates will develop or accelerate more rapidly when operating at lighter gross weights.

4. Low Pedal Margin Maneuvers: When attempting any low speed maneuver with pedal control margin less than 1 in., there may not be sufficient right pedal remaining to arrest a left yaw once it is established.

5. Left Sideward Flight: In left sideward flight above 30 knots, high pedal workloads combined with possible low pedal control margin can result in ULY. Specific recovery procedures for ULY are published in section III of this flight manual. The best course of action to prevent ULY is to avoid the flight regimes and conditions conducive to ULY. However; if the ULY condition develops, the

following actions should be taken:

1. Immediate full right pedal to maximum deflection. Gradual input will not arrest increasing yaw rates and up to 300 of lag may be experienced between full pedal application and yaw stabilization.

2. Altitude/obstacles permitting, smoothly apply forward cyclic. This will achieve translational lift, which will reduce main rotor torque, thus reducing the left yaw rate. In addition, this will take advantage of the strong directional stability of the aircraft due to the large vertical fin and permanently offset lateral fins.

3. Altitude permitting, reduce collective. This will reduce main rotor torque and the resultant left yaw created by the torque effect. Lowering the collective will initiate or increase a descent rate. Pilots should recognize that if a large descent rate develops close to

the ground/water, the subsequent collective increase required to arrest the descent prior to ground/water contact may aggravate or re−initiate ULY.

4. NR HI switch−HI. Increasing main rotor RPM to 365 will significantly increase tail rotor effectiveness. HI NR should be selected prior to entering any flight regime where a ULY could become a factor. If dual pilot, copilot should select HI NR without request if

ULY is encountered.

[heli.pilot]

I fly an aircraft with a Fenestron ( Coast Guard MH-65 dolphin/civ AS365 dauphin) and the fenestron is very efficient. I also have had the opportunity to fly the aircraft with two different types of engines. Fenestron performance increased dramatically with the new engines because there was obviously more power that could be applied but we also have a switch that increases our main rotor by 10 turns a minute which correlates to an increased amount of turns in the tail creating more lift. We have used them at pretty high altitudes in a hover as well.

 

it is a pretty good system but in the end all of the counter torque systems get the job done.

 

 

D-Driver, where can I get one of those switches? I'd like an extra 10 RPMs on a dime some times.

 

The EC130 fenestron works well but if not careful on a windy day it can get away and end up in an over torque. I have seen it happen several times. Better off making a right pedal turn on those days in the summer. Every design has positives and negitives. That is one for the EC130.

 

 

I fly an R-22.... I'd like to place an order for 10 of those switches... Yes, they're all for the same aircraft...

[JDHelicopterPilot]

D-Driver, those which you point out are a factor for sure and have to keep in mind. Otherwise, a bad outcome can result. However, that is also true for a traditional tailrotor as well. Each aircraft is able to get LTE as you know. The B206 is a good example. The wind azimuths vary by helicopter design.

 

I was just trying to figure out why it is the Fenestron design is less effective than a traditional tailrotor. Any ideas? Excess drag as a result of many blades? Could the shroud?

 

What is your pedal position when in cruise flight? In the EC130 it was always left pedal in while in cruise flight. That made my legs hurt after a long day since of course it's not hydrualicly boosted either.

[dolphindriver]

I guess my whole point was that the Fenestron is not less effective. The fenestron directs flow over the blades which makes "cleaner" air for the t/r blades to go through which reduces angle of attack, which reduces power requirements. What makes it ineffective is the power available behind it just like a traditional tail rotor. You still need power to keep the blades turning. If you souped up a 206 you wouldn't have LTE problems either. Look at the OH-6, it has power for days and LTE is tough to get into. Like I said, I have flown the exact same airframe with powerful engines and not so powerful engines. With the poor performing engines, LTE, ULY, whatever you want to call it was a challenge as you could overtorque very quickly. With powerful engines the same problems in the same aircraft became almost non-existant. So it wasn't the fenestron, it was the power driving the fenestron. In fact we just changed from an 11 bladed t/r in the fenestron to a 10 blade system and the power requirements are pretty much the same.

 

When in cruise we are fairly neutral on the pedals with left pedal being a bit more forward than the right.

Edited November 7, 2008 by dolphindriver

[JDHelicopterPilot]

Makes sense to me. Thanks

[DakarNick]

Makes sense to me, also! Thanks! Very informative and helpful.

[CharyouTree]

However, that is also true for a traditional tailrotor as well. Each aircraft is able to get LTE as you know.

 

 

I'll jump in for FL Hooker here.... I think CH-47's and -46's are immune

 

 

Oddly enough, I haven't heard it talked about in Blackhawks, either. I assume it's possible, just not likely.

 

 

DD: I like ULY better than LTE. You can say yours like a word. (you-lee)

[clay]

i believe there is a video of a blackhawk getting into LTE, then rolling down a mountain if i remember correctly.

[JDHelicopterPilot]

Are you talking about the Mt. Hood rescue gone bad? I don't think that was LTE but a form of white out.

 

You got me on the CH-47s and 46s.

Edited November 8, 2008 by JDHelicopterPilot

[dolphindriver]

I'll jump in for FL Hooker here.... I think CH-47's and -46's are immune

 

 

Oddly enough, I haven't heard it talked about in Blackhawks, either. I assume it's possible, just not likely.

 

 

DD: I like ULY better than LTE. You can say yours like a word. (you-lee)

It is possible in any aircraft with a tail rotor. You just have to get into the right conditions. High altitude, high power etc. Again, it has everything to do with the power driving the aircraft. We used to land Apaches with a tail wind all the time because we didn't think it was possible to get into LTE. The same was felt in the Blackhawk community. That has since been proven wrong quite a few times.

 

The Blackhawk in that video get into self induced LTE. A bit of being disoriented because of the glare on the snow and a lack of horizon due to the snowt followed by a power pull at an already high power setting to get out of the situation. Nr droops and can't compensate causing LTE. A very bad day.

 

Charyoutree, that is exactly what we call it.