Transverse flow effect- lateral

[StanFoster]

I was just out enjoying my Turbinator and was paying attention to how transverse flow effect causes a slight roll to the right as I went from a hover through translational lift. My first helicopter lesson I was especially watching my instructor go from a hover to forward flight, and the subtle left stick he subconsciously applied to counter transverse flow. I was aware that transverse flow doesn't care which way the helicopter is going, but had never observed it while going from a hover to left or right sideways flight............................................ So todays flight I hovered then proceeded with right sidewards flight. I was careful observing the cyclic input that was soon going to be needed. Sure enough, while flying to the right, I needed forward cyclic to keep the nose from rising. For those that don't understand this, what ever the direction the rotor is moving, the leading edge of the rotor disc sees more horizontal moving air and will have a higher AOA than the rear of the rotor disc. The air above the rotor and going down through has had more time to be accelerated downward and this causes the rear of the disc to have a smaller angle of attack. More lift in front/less lift in the rear- causes the rotot to tilt 90 degrees later because of aerodynamic precession. My rightward flight caused the disc to raise at the front end of the cabin, causing me to apply a little forward cyclic............................................ ...........Of course I had to go do a leftward flight. This time as expected, the nose dropped and it took a little back cyclic to raise it................................................ ............I know you do not need to know this to fly a helicopter, as your reflexes just take care of these subtle movements necessary to make the proper corrections. Many helicopter pilots probably don't care, just fly the thing they will say..................................But for me, I find this stuff continually fascinating and it never gets boring flying this machine. In my opinion, if flying helicopters AND trying to understand why and how they fly ever gets boring to me, I have a BIG problem! Stan

[Doug Riley]

I understood (or maybe misunderstood) that an attempt to counteract this phenom was the reason for delta-3 offset in the flapping or teeter hinges of helos.

[gyroman]

I did not know that Stan.. thanks for educating the non aviation masses... thats interesting, I could visualize what you were describing.

[bryancobb]

Stan,
Look at the "20" point on this chart.
This is a very crude chart showing cyclic stick position versus speed. The top of the circle represents your direction, be it sideways, forward, or even backward.

At "20" the cyclic is displaced LEFT and corroborates what you detected.

[Paul Salmon, MD]

I know a helo pilot with thousands of hours who had a job interview. He sat down and the first subject discussed was "Explain transverse flow effect". He couldn't, and the job interview was over. He was never ill prepared for a job interview again. He studied all the possible subjects he could be tested over- Transverse flow effect, Delta hinge design, etc.
By the way, the delta hinge produces a greater degree of change of angle of attack for a given amount of blade teetering, so will offset pitch links.

Hey Stan, got a question for you to think about. Why does the last few pounds of weight when you approach max gross, or the few pounds of (god forbid) being over gross cause such a significant increase in power requirements?

[birdy]

At "20" the cyclic is displaced LEFT and corroborates what you detected.

I know im just a SCG, but i cant figure why the stick position at 100 knots is more left than at 90kts??
All the dots are in a logicl pattern, cept the 100kt one.

[bryancobb]

I would also say that decreased lift on the advancing half of the rotor disc due to the effects of compressibility would also be a factor. The tips of the advancing blade is probably into the transonic range above 100 KTS.

[C. Beaty]

From Gessow and Myers; Aerodynamics of the Helicopter:

[Rotor Rooter]

Stan, the following link presents a good description of transverse flow effect.
Transverse Flow Effect

It should be considered in conjunction with Bryan's image.


You may also want to relate them to this drag bucket.



Dave

[birdy]

Birdy- My understanding...................
So, wot part dose differential lift play in this picture?
I woulda thought the stick woulda kept go'n forward n rite as speed increased, to counter the unequal lift.

[birdy]

I know, but the TRV is alined with the TPP, so you compensate with the stick.

[Paul Salmon, MD]

Hey Stan,
As the max gross weight is approached (or exceeded), The amount of coning can become significantly increased. This "narrows" the width of the rotor disc, effectively producing less lifting surface. (decreases the area of the rotor disc) Your absolutely correct that the vortices on the end of the rotors increase with the increased angle of attack that is used with increasing weight as well, additionally the increased AOA created more drag as well as you stated.
Excess coning not only decreases the area of the rotor disc, it also decreases the airspeed along the blade most importantly at the blade tips. Let's say your flying a a/c with a governor that is going to hold your blade rpm at 500 rpm. If you rotor disc is 25 ft, then you rotor speed at the tip is about 446 mph, let say the rotor disc width decreases to 23 feet then at 500 rpm your tip speed is only about 410 mph. That may not seem like much but remember that as you double the speed across a given airfoil at the same AOA the lift increases by 4 fold, not 2 fold. If you halve the speed then the airfoil is producing 1/4 the amount of lift, so you have an exponential change in lift either up or down. So using the above example, all things being equal the amount of lift reduction due to the airspeed reduction of about 8% would produce about a 19% reduction in lift.
So at high gross weights coning is increased, this reduces lift for the 3 reasons discussed, so you have to pull more collective that increases the AOA which again reduces lift, etc. So you end up with a significant amount of power required for that last 50 lbs or so when you get near or exceed max gross weight.
What you will usually notice is flight is an increase in top speed as you burn off fuel/weight for a given manifold pressure setting (piston) or torque setting (turbine). The "exponential importance" of the weight reduction as you burn off fuel sort of mirrors the "exponential importance" of the lift reduction caused by the excessive weight in the first place. In other words, the first 50 lbs burned off will increase your performance more than the next 50 lbs.
Hope all that rambling made sense, let me know what you think.

Got another question for you if your interested, this ones makes you think a little too- What's the advantage of having wheels on a blackhawk, what's the disadvantages? There is a few of both.

I happen to think that the more pilots understand the details as to why something occurs concerning performance, they are better able to maximize the performance and make better decisions.

Keep in touch,
Paul Salmon

[birdy]

All good info Paul.

Im happy that i still know i know buggerall.
Leaves plenty of room for learning.

[Paul Salmon, MD]

Since I have time now, I'll answer my own question-

Advantages outnumber the disadvantages. They include the ability to get the aircraft above ETL with less power than it takes if you have skids. The wheels are supporting the weight of the helo and the amount of rotor thrust required to get it moving across the ground in quite small (depending on the surface your rolling on- pavement versus sand versus mud, etc.) compared to a helo with skids. Think of it this way, let's say you want to move your car forward on level pavement. If you were required to pick up the car and carry it forward down the street it's going to take several thousand pounds of force/lift to accomplish that. However if the car is on wheels it can be moved with only 100 lbs of force or so, and moved down the street. A helo with skids is required to pick up most of the weight before you can get it to slide forward for a run-on take-off, or all of the weight if hovering, obviously that's going take a lot more force that rolling it on wheels. So the same helicopter on wheels could be overloaded to a greater degree than the same helo on skids and still fly. (I'm not advocating overloading the aircraft, but if somebodies shooting at me, and I'm out of ammo, and my weight will be the difference between being overloaded and not, we're going to be overgross, scoot over dude!!!)
Additionally, since it takes less rotor thrust to get it moving forward, there is less rotor wash, there is less FOD blown up by the rotors and you have less tendency to produce a "sand storm, snow storm, dust storm" and you literally fly out of your own smaller dust cloud, etc. sooner, so there's less chance of becoming disoriented with loss of visual clues and crash the aircraft. Obviously less sand blown up into the main rotor and tail rotor produces less surface wear. It also clogs up air filters to less of a degree.

Disadvantages? You could have a flat, HA!!! And a little greater tendency to develop ground resonance. Generally a higher cost maintaining the struts, wheels, tires, etc.


Paul

[Paul Salmon, MD]

I can honestly say that the thought of securing the ground handling wheels on my old brantly with a roll of duct tape and rolling it across the ground on take-off crossed my mind on more than one occasion. Wasn't brave (or stupid) enough to try it though!!!!!

Paul