Translational Lift

[slick1537]

Reading now, and not flying yet so excuse me if this question is silly. Translational lift will decrease the relative inflow, thus requiring the pilot to lower the collective to decrease the angle of attack. There for there is less drag, less requirement for power, and the helicopter flys more efficiently. Can it then be said that flying into the wind is more efficient that flying with no wind? Thanks guys!

[sactown77]

Yup. when the induced flow or relative inflow is reduced (less horizontal) due to forward motion or wind in a stationary hover, the AoA is increased, meaning more lift for a given collective setting. sounds to me like you got it!

[AndrewT]

Can it then be said that flying into the wind is more efficient that flying with no wind? Thanks guys!

 

 

yes, thats why you take off/land into the wind so you reach ETL faster on take off and stay in it longer on landing

[flyby_heli]

Translational lift will decrease the relative inflow, thus requiring the pilot to lower the collective to decrease the angle of attack.

If the pilot wants to maintain the same state of flight (same altitude and airspeed), yes. However, if translational lift is gained on take-off where your goal is to accelerate and climb, you should not reduce collective as this would result in a reduction in lift and therefore a lower climbrate/slower acceleration.

[Gomer Pylot]

No, flying in wind is not more efficient. The helicopter has no idea whether the wind is blowing, except at a hover, where you may be in translational lift with no groundspeed.

 

You may need to reduce power somewhat during a normal takeoff to stay out of the avoid area of the height/velocity diagram. If you allow the aircraft to climb too quickly, you will be too high at too low an airspeed, and thus be inside the knee of the avoid region. You can bring the power back in during the climb, after you've achieved sufficient airspeed. This is for a single-engine helicopter, and a performance class 2 takeoff in a twin is done differently.

[Sparker]

No, flying in wind is not more efficient. The helicopter has no idea whether the wind is blowing, except at a hover, where you may be in translational lift with no groundspeed.

 

You may need to reduce power somewhat during a normal takeoff to stay out of the avoid area of the height/velocity diagram. If you allow the aircraft to climb too quickly, you will be too high at too low an airspeed, and thus be inside the knee of the avoid region. You can bring the power back in during the climb, after you've achieved sufficient airspeed. This is for a single-engine helicopter, and a performance class 2 takeoff in a twin is done differently.

 

The helicopter has no idea, but the rotor blades do....

[james28]

i can't believe you just said that ^

[Sparker]

"At about 15 to 35 knots, the helicopter begins to benefit from translational lift, which is the additional lift obtained through airspeed because of the increased efficiency of the rotor system. This means that while it may take 90 percent of a helicopters available power to fly at 20 knots, it may take only 80 percent to fly at 45 knots, and 65 percent to fly at 120 knots."

-Learning to Fly Helicopters, R Randall Padfield

 

 

So... even if the helicopter does not know it is windy the rotors do. (it was a joke)

 

Anyway my point is that if you are flying straight into a 40 knot wind at 20 knots, the net is 60 knots airspeed on the rotors, putting you into translational lift, if you are flying 20 knots with no head wind.... no translational lift. The rotors don't care where the airflow comes from...

[Eric Hunt]

Sparker says:"if you are flying straight into a 40 knot wind at 20 knots, the net is 60 knots airspeed on the rotors, putting you into translational lift, if you are flying 20 knots with no head wind.... no translational lift"

 

 

Umm... Sparker, if you are indicating 20 kt while flying into a 40 kt wind, you are going backwards over the ground at 20 kt, but with 20 kt worth of forward airflow going over the blades. Twenty knots is twenty knots - the ASI sees exactly what the rotor disc gets, but only if you are travelling forwards. The ASi doesn't know if it's going into a headwind or not, and if you are going sideways or backwards, the ASI doesn't show Jack.

 

But if you are flying into a 40 kt wind and have 20 kt groundspeed, your ASI will show 60 kt, not 20.

Edited March 6, 2007 by Eric Hunt

[Sparker]

Sparker says:"if you are flying straight into a 40 knot wind at 20 knots, the net is 60 knots airspeed on the rotors, putting you into translational lift, if you are flying 20 knots with no head wind.... no translational lift"

Umm... Sparker, if you are indicating 20 kt while flying into a 40 kt wind, you are going backwards over the ground at 20 kt, but with 20 kt worth of forward airflow going over the blades. Twenty knots is twenty knots - the ASI sees exactly what the rotor disc gets. The ASi doesn't know if it's going into a headwind or not.

 

But if you are flying into a 40 kt wind and have 20 kt groundspeed, your ASI will show 60 kt, not 20.

 

I was refering to ground speed... the point being that IAS is the net of head wind and groundspeed... I never refered to indicated speed.

[Linc]

You guys can fly a 300 or R22 in 40 knot winds? Or is this just for the sake of conversation?

 

I've always learned that translational lift was 5-24 knots with ETL being approximately 15-24 knots. Granted, this may all be based on UH-1s in the U.S. Army, or possibly the TH-55A.

 

Although it seems to bear out in my current aircraft as well.

[Pogue]

You guys can fly a 300 or R22 in 40 knot winds? Or is this just for the sake of conversation?

I think it's more for conversation. I've flown in 15kt with gusts to 25 or so in an R22 (with a CFI, please don't SFAR me to death here...) Kind of a lot of workload, but there was enough tail rotor authority to do a 360 pedal turn at hover.

[Gomer Pylot]

The rotor blades have no idea if there is wind, either. All they know is airspeed, whether there is wind or not. On the ground, in a hover, there will be additional lift from a strong wind, granted. But the rotor blades have no idea whether that's from wind or from movement of the aircraft. The point is that wind does not make flying more efficient. It does help with takeoffs and landings, permitting the use of less power because of the airspeed it gives at zero groundspeed. After ETL, though, it's less than useless, because it reduces range. Any round trip will take longer in a wind, no matter which way it's blowing, and the time increases with windspeed.

[Sparker]

The rotor blades have no idea if there is wind, either. All they know is airspeed, whether there is wind or not. On the ground, in a hover, there will be additional lift from a strong wind, granted. But the rotor blades have no idea whether that's from wind or from movement of the aircraft. The point is that wind does not make flying more efficient. It does help with takeoffs and landings, permitting the use of less power because of the airspeed it gives at zero groundspeed. After ETL, though, it's less than useless, because it reduces range. Any round trip will take longer in a wind, no matter which way it's blowing, and the time increases with windspeed.

 

Completely agreed...

 

I don't think I am the best at explaining this... but my whole point was that the helicopter doesn't give a crap where the airspeed comes from, even if it is from wind, but if used correctly the pilot can benefit from wind and reach ETL faster and stay in it longer at slower ground speeds.

Edited March 6, 2007 by Sparker

[Witch]

What I find interesting is that in a hover on a calm day, the upscope stick is about halfway up, and on a windy day, the upscope stick is less than halfway up. In fact, I have to push the upscope stick down to keep from going up. Now on a calm day, if I push the upscope stick down, I go down, and the ground comes up. But if I go forward and pull up on the upscope stick, I go up fast, and if I push down on the upscope stick a small bit, I go faster but not go up until I pull back on the go fast stick which eventually makes me go not so fast.

 

OK, I'm confused now.

 

Later

[Goldy]

You guys can fly a 300 or R22 in 40 knot winds? Or is this just for the sake of conversation?

 

 

30 Knot gusting in an R22, not horrible but I wouldnt recommend it. Last 2 feet off the ground is where it can get interesting..I think you could do 40knot winds safely in a 300...for sure in an R44.

 

By the way, the limits in the R22 POH are the standards that they have to test against for certification...they are not maximums or reflect the limits of the bird in any way. The real issues with the R22 in gusting winds is insuring you dont get into a neg g situation.

[slick1537]

So basically what I should assume then is that when flying into the wind, the inflow angle is lower, creating a larger angle of attack, however the parasite drag is going to be greater than the extra efficiency you gained anyway?

Edited March 7, 2007 by slick1537

[Sparker]

So basically what I should assume then is that when flying into the wind, the inflow angle is lower, creating a larger angle of attack, however the parasite drag is going to be greater than the extra efficiency you gained anyway?

 

I think parasite drag will mainly effect top speed, and ETL will help you climb better, allowing you to reduce power somewhat, or maintain the same power and climb. I'm not sure if ETL gains you much in the way of airspeed. Maybe it does allow some air speed because you have more power reserve. not sure...

[Eric Hunt]

Sorry about this longish post, but have a read thru it to refresh the mind - there are some misconceptions about.

 

Go look at your basic aerody handbook, at the chart which shows "Power required to overcome drag" against "Airspeed".

 

There are three individual lines on the chart, which are (power required to overcome)rotor profile drag, induced drag, and parasite drag.

 

Profile drag starts a little bit up the Power (Y) axis, and is almost a straight line, climbing slightly with airspeed (X axis). This drag doesn't change much.

 

Induced drag starts the highest up the Y axis, stays steady for about 5 -10 knots, then drops down like the RHS of a bell curve - but never reaches the X axis. Thus the power needed to overcome the induced drag is highest in the hover, but with forward airflow, the induced flow and induced drag drops dramatically, and you need less power to fly slowly than you need to hover. When ground effect is taken into consideration, the curve changes a little at the 0 airspeed bit - it is lower than for a free air hover, goes up slightly as you "move off the cushion" and joins the free air hover line, before dropping in the usual fashion. This means you can hover IGE with a certain power, but when you start to move forward, you will sink a bit and / or need more power, until you get the boost from translational lift, the power needed drops off, and you have either more power to climb away, or you use less power to fly level.

 

Parasite drag starts at almost 0 on the Y axis, and then goes up in a parabola, as it is directly associated with the square of the speed, and this is usually the limiting factor on forward speed. It is also why the faster machines have retractable gear and minimal drag-causing protruberances.

 

Then there is a fourth line on the chart, which is the sum total of all the others.

 

This one starts off like the IGE part of the Induced Drag line, going up a little, then dropping away like the bell, but it rounds out at about 35 kt and then curves up like the parasite drag line and disappears top right.

 

The minimum power section of the curve is from about 15 kt to about 40 kt, where a little change in airspeed doesn't need much more power, and the line for "Power available" from the engine is the furthest away, giving you the biggest margins. But the ASI is hopelessly inaccurate below 30-35 kt, so you will never really know if you are flying an accurate speed or not when trickling along just in translational.

 

As altitude and temperature go up, the line from the total climbs the Y axis, and the Power Available line drops down. Eventually it will cut into the top of the line, and you will have enough power for an IGE hover, but not enough to stay off the ground when you move forward. That is where the "Chickenhawk" takeoff appears, skidding along the ground until the curve drops back below the Power Required line, and you can fly away, as long as you stay above that speed.