Clean air? Rotor wash? Good beginner helicopter reference?

[WildBillKelsoe]

Hi,

Being a proud owner of Huey (sim), I often refer back to the manual for new terms. I'm dedicating this thread to help as Huey specific learning reference.

 

Basically, I'll put a term and some of you who understand what it means can explain in lay language what it means. A plus would be having an animation or a pictorial reference embedded in the post to help visualize better where applicable.

 

 

This should be a good reference to those new pilots and seasoned heli pilots to better understand why the helicopter behaves in such a way.

 

 

Ok, off to the first term:

 

CLEAN AIR:

 

Is this the air that is not inside the rotor disc but incidental on the plane of rotation, and if so, do headwinds count as clean air or just plain still air not in my own rotor disc?

[Home Fries]

Great Idea for a topic, Wild Bill.

 

My understanding of clean air is exactly how you describe it. And it does include headwinds, but is only a factor if groundspeed is a concern.

 

Here's a quick rundown on the wind triangle for the "Peter Pilot":

 

In these examples, assume that we are at sea level at standard temperature and pressure. This means that True Airspeed (TAS) equals Indicated Airspeed (IAS), so the terms are interchangeable.

 

Case 1: On a zero wind day, what we see on our airspeed indicator (IAS) and our compass (heading) is what we get for a track and groundspeed (IAS=GS and Hdg=Trk).

 

Case 2: With an offset headwind as shown above, the Huey is pushed over ground in the direction of the wind, so our airspeed indicator and compass would show exactly the same as it did in Case 1, but our track and groundspeed would be offset as shown in the diagram.

 

 

Now to answer the question:

 

Just like a fixed wing aircraft likes to take off and land into the wind because it reduces the groundspeed required to land at any given indicated airspeed, a helicopter can enter and exit Effective Translational Lift (ETL) speeds at a lower groundspeed if flying into the wind. ETL is all about horizontal airflow over the rotor (i.e. clean air).

 

If groundspeed is not a concern (i.e. you're not trying to hover over a patch of ground), then the headwind acts like the rest of the body of mass, and your airspeed indicator won't show any difference than you would expect in a zero wind day.

Edited April 20, 2015 by Home Fries

[slowhand]

Great post both of you:thumbup:

[Roadrunner]

nice idea mate, you might want to link each topic in the first post to have a table of content?

[Flagrum]

Just an idea: we can now use the DCS wiki again. A project like this is probably best to be kept there where the information can be easier structured and restructured when new knowledge comes up.

[WildBillKelsoe]

Once we covered everything here, I transfer to wiki. Yes, once we sort them out I'll index in my first post by post number URL.

 

Addendum picture to Home Fries case 1

[WildBillKelsoe]

Next up, the 3 T's:

 

* Effective Translational Lift

* Transverse Flow effect

* Translating Tendency

[WildBillKelsoe]

Torque:

 

 

(Why is it required and how to set it in the Huey?)

Edited April 26, 2015 by WildBillKelsoe

[Xtrasensory]

Translating tendency:

A counterclockwise rotating helicopter wants to drift to the right due to the tailrotor to compensate for torque. This is countered by a little left cyclic to tilt the mainrotor system. Some helicopters also have design features to help compensate for it. Like a slightly tilted mast for the main rotor. Clockwise turning helicopters tend to slight left. Coaxial helicopters will not have this phenomena.

 

ETL

At around 20 knots (+/- depending on helicopter) the helicopter leaves the turbulent air caused by vortexes. Air is clean and comes in more horizontal, thus decreasing the induced flow and drag and increasing the effeciency of the rotor (same angle of incidence with a greater angle of attack). Less power (collective) is required for the same amount of lift.

 

Transverse flow

When the helicopter picks up forward speed, the air will come in more horizontally at the front of the disc and it will have a greater downward velocity in the aft of the disc. Thus creating a greater induced flow and smaller angle of attack at the back of the rotor and a smaller inducednflow and great angle of attack in the front.

Due to gyroscopical precession you will notice a right roling tendency in ccw rotating helicopters

[Xtrasensory]

I dont see torque as required for flight, its more of a side effect. The rotor creates drag as a side effect of lift ,which tries to slow the Nr, the engine has to create more power to maintain the Nr. This will increase the torque on the drive shafts and in the gearboxes. Helicopter turbines are limited by EGT limits, Ng limits or fuel flow limits. The torque is limitedby the gearboxes (it will not actually limit it, but break when the torque is to great).

Depending on altitude and temp you may either hit the egt, Ng or fuel flow limit of the engine before you hit the torque limit.

So the collective increases your pitch, increasing the lift, increasing the drag. Rotor speed will decrease, engine has to create more power to maintain Nr and the torque will increase.

Edited April 27, 2015 by Xtrasensory

[Xtrasensory]

If people are interested in helicopter aerodynamics, try looking up the fm 3-04 203 fundementals of flight.

Or the book: fundementals of helicopter flight.

[WildBillKelsoe]

If people are interested in helicopter aerodynamics, try looking up the fm 3-04 203 fundementals of flight.

Or the book: fundementals of helicopter flight.

 

 

Thanks for your answer Xtra.

[WildBillKelsoe]

Ok, next up:

 

FAT (FREE AIR TEMPERATURE)

OAT (OUTSIDE AIR TEMPERATURE)

[Xtrasensory]

I dont think that there is a difference between the two. Both are measuring static air temperature I believe.

Not 100% sure.

[mvsgas]

Have you read the FAA Helicopter Flying Handbook?

[WildBillKelsoe]

No, but it would defeat the purpose of this thread.