Transverse Flow Effect and ETL

[Iridus]

So I'm a little confused on the difference between TFE and ETL. What is the difference between these and why are they important?

[WolftalonID]

Transverse flow effect or otherwise known as inflow roll effect. Its when the helicopter is moving in any direction,typically forward, and as the movement approaches 11-15 kts of speed the high angle of attack of the rotor needed to create hover power lift is now gaining more lift from this forward movement. Its not just producing inflow or induced flow air speed, but now its also moving along through the airmass and relative wind is hitting it square on as well as from above due to the rotation.

 

This makes the tip of the rotor lift up and due to gyroscopic procession, it also rolls the helicopter to the right.

 

Most CFIs cannot say all this in the three point 7 seconds it takes to happen so we call out..."going through ETL". However its also inherently wrong to say that.

 

ETL happens just after TFE as the entire rotor disk moves along past that "wall" of TFE as subsequently you tend to start climing if there is not any further control inputs.

 

Think about the flow of areodynamics in this order.

 

Induced flow (hover power)

TFE ( begining forward takeoff movement)

ETL ( just after that little warble on take off)

TL,Translational lift,( when fully in the lift effect of forward flight)

TT,Translational Trust, happens just about 65-70 knots when the tail rotor is moved just above the disk as the nose tips down in flight.

 

5 basic steps each along the take off profile up into full forward flight.

 

Its really easy to show with a white board and a toy heli but I have no white board at the moment!

Edited February 3, 2016 by WolftalonID

[ridethisbike]

WolftalonID, you have a PM.

 

The funny thing with ETL and Transverse flow... transverse flow effect (right roll), in every book I've seen, is described as happening before the effects of ETL (the nose up), but every time I show a student what happens when you don't "push through" ETL, the right roll always happens last. Never quite understood that one.

[WolftalonID]

Well in an R-22 consider the cyclic input for hover.

 

From neutral position we hold it slightly left of center to combat translating tendency and also slightly back. So its back and left.

 

They say to combat that right roll as you go through inflow roll to push fwd and left.

 

When infact we are already left with the cyclic to start with we just push fwd through it. If you delay the push it will raise the nose followed by the roll because areodynamic lift at the front of the disk is happening first, then it processes around to the left and lifts(rolls) the helicopter to the right.

 

Once we are in a steady cruise fwd flight configuration our cyclic input is much further fwd than that of hover and also we are now more right, nearly centered considering left to right cyclic displacement position.

[Eric Hunt]

Remember that "the disk" is the tip path plane, not actually where the blades are - the blades are not flying flat, they are coned up. In the hover, both are up above the horizontal by the same amount, and the TPP is horizontal. Air flows like a big funnel from above and gets pushed down through the disk. Induced flow is at its highest, and is equal over the disk.

 

So, when you poke the stick forward, the TPP and "the disk" is tilted forward to generate forward thrust. The front blade has come down a bit towards the horizontal, and the back blade has moved upward. The front blade is moving into clean air, which hasn't yet started to be induced downward, so it is a bit more efficient. By the time the back blade has moved with the machine to the same spot, the airflow has already started to get sucked downwards, and because the forward movement is now coming from above this back blade, it is seeing more induced flow and is less efficient.

 

Phase lag (loosely described as gyroscopic precession) means that the lower lift on the back blade is seen as a loss of lift around 90 degrees later, on the right side, causing a right roll.

 

However, as soon as the aircraft starts to move forward, you have an advancing blade getting more relative airflow and a retreating blade getting less raf. This again causes a different lift across the disk, causing the disk to flap back away from the direction of movement. If the student doesn't add forward cyclic to overcome this, the nose will pitch up, the forward movement stops, and he will find himself going backwards. This is why you might not get to see the inflow roll until well after the flapback has been stopped.

[WolftalonID]

Love that break down. Never brought into the consideration the left side loosing lift but it would be..good thoughts Eric.

[iChris]

So I'm a little confused on the difference between TFE and ETL. What is the difference between these and why are they important?

 

As the helicopter transitions from hover into forward flight, a horizontal component of velocity, inflow, enters the rotor system. Since the rotor system is required to produce both lift and thrust, the rotor disk must tilt forward relative to the oncoming inflow. Under these conditions, the symmetry of the flow through the rotor, common in the hover, is lost due to the rotor tilt. Consequently, the tilt of the rotor disk with respect to the mass inflow, causes a reduction in the induced velocity and an unequal induced velocity distribution, fore and aft across the rotor disk.

 

The mass inflow that is the source of the reduction in induced velocity and the resulting reduction in power required is what pilots refer to as translational left. The point in time were the overall reduction of induced velocity and power required are first noticed, as a climb tendency, is refer to by pilots as effective translational left. From a technical perspective, the climb tendency is the result of the excess power available. Even though this climbing behavior is referred to as effective translational lift, that term is a misnomer, because unless significant acceleration is involved, rotor lift equals helicopter weight and the helicopter climbs by virtue of the excess power available not from excess lift generation. In addition, the unequal induced velocity distribution, fore and aft across the rotor disk, is the source of the right roll tendency which pilots refer to as transverse flow effect.

 

The overall reduction in the induced velocity and the unequal induced velocity distribution are important because they help define the control inputs required by the pilot in order to transition into and maintain steady state forward flight.

 

The induced velocity reduction and unequal induced velocity distribution resulting in a right roll tendency begins with rotor tilt; however, the right roll tendency trails behind due to the rotors inherent phase lag. In the early stages, the roll tendency is not noticeable, as pilots subconsciously compensate by applying some lateral cyclic as the stick is moved forward.

 

The total flow effects in forward flight are more complicated since you still need to deal with the things like the rotor vortex, tangential velocities at the rotor blades, tip speed ratio, and accounting for the asymmetrical lift between the advancing and retreating blade.

 

It is interesting to note, the R22 has a 18º delta three angle designed into the upper swashplate to reduce the right roll at low and high forward airspeeds. . ( Link: R-22 ROTOR SYSTEM ~ Posting by Frank Robinson on pprune.org ~ Nov. 30, 2000 (Post #183) )

 

“This tendency to automatically go into a climb first becomes noticeable at forward speeds as low as 10 or 20 knots and is known as "translational lift" or sometimes "transitional lift". It can be very helpful in getting heavily loaded helicopters into the air. Translational lift is simply the result of the collective pitch being in an out-of-trim position for level flight.”

 

Ray Prouty, Helicopter Aerodynamics

 

 

 

 

 

Edited February 5, 2016 by iChris

[WolftalonID]

I love these diagrams! I have a few copies that are just about identicle. I have used them in grounds to teach with. I find that trying to explain this alone is just about impossible, but showing these diagrams the students mind unlocks.

 

My only change is the use of collective position during take off. I went away from the old school way of lowering the collective to prevent balloning during take off, to keeping it set at hover pitch and using cyclic to accelerate. Pushing through to 40kts to keep the ship 5-10 feet then adjusting cyclic to a 60kts attitude for climb out.

 

Take offs were smoother, TFE was adjusted for more consistantly, pedals and cyclic movements were reduced. My students learning curve was improved, and take off profiles became easier to do consistsntly.

 

As always iChris, a thorough explanation.

[iChris]

My only change is the use of collective position during take off. I went away from the old school way of lowering the collective to prevent ballooning during take off, to keeping it set at hover pitch and using cyclic to accelerate. Pushing through to 40kts to keep the ship 5-10 feet then adjusting cyclic to a 60kts attitude for climb out.

 

 

You’re correct, that is the preferred technique. Moreover, its been in the FAA’s Basic Helicopter Handbook since 1973, 43 year ago, no mention of lowering the collector. Good thing you escaped that system of training.

Basic Helicopter Handbook AC 61-3A 1973 (pg. 77)

 

As you stated, that’s the most effective way to accelerate into forward flight using the excess power available which is exactly what was mentioned in Ray Prouty's quote about the collector being out of trim with respect to the power required.

 

Translational lift is simply the result of the collective pitch being in an out-of-trim position for level flight.” Ray Prouty, Helicopter Aerodynamics

 

Edited February 8, 2016 by iChris

[FlyAF]

Holy sh*t yall are smart as hell, im a student pilot and this is one the coolest things I've found/read on any topic. Just wanted to say thank you