Wind direction and max performance take-off

[SquirrelFlight]

When I flew my checkride, one thing I didn't do too well on was keeping the nose of the R22 into the wind. We had done an confined approach into a ditch, so had to do a max performance takeoff to get out of it. Out of sloppiness, I let the nose swing a little to the right as we came up, and ended up without enough lift to get over the lip of the ditch. The examiner then demonstrated that, by turning back into the wind, there was lift to spare.

 

My question is why is it that this makes a difference? That is, what is it about the aerodynamics that gives the extra lift? I don't think it's a form of ETL doing it - according to RHC, ETL is from the extra flow across the disk, so it shouldn't matter what direction the aircraft below is pointed. Does the helicopter act as a lifting body, so that the extra wind over the body provides the lift?

[PhotoFlyer]

Hovering crosswind takes more pedal. More pedal takes more power. More power for the tail means less to the main rotor, and because of that, less pitch (collective) before you will droop the rotor RPM. Sooooo, since you were crosswind, and needed more pedal than hovering into the wind, you didn't have enough power to make it over the obstacle.

[SquirrelFlight]

As I understand it, only left pedal (in an R22) requires more power, as it is directly countering the torque from the main rotor. When you add right pedal, you are "un-countering" the main rotor torque, which requires less power at the tail, so you have more available for the main. I was angled to the right in my checkride.

[PhotoFlyer]

Yawing right, and right crosswind are different. A crosswind, left or right, requires more power than a headwind. If you were continuously yawing then you would be correct, but to stop the yaw (you did try to stop it eventually, didn't you?) and hover or try to turn into the wind would require more power.

[Darren Hughes]

You may also have been getting some main rotor disk interference by allowing the nose to rotate right. THat will also eat up some power as the tail rotor is operating in turbulant air.

[SquirrelFlight]

Yawing right, and right crosswind are different. A crosswind, left or right, requires more power than a headwind. If you were continuously yawing then you would be correct, but to stop the yaw (you did try to stop it eventually, didn't you?) and hover or try to turn into the wind would require more power.

 

I think I see what you're saying. What causes the extra need for power? As long as the main rotor is tilted into the wind to compensate for drift, why should it matter which direction the helicopter is pointed?

 

My first guess was that the extra power is needed to counter the weather-vaning tendency of the helicopter, but if that were the case, then we'd be in the situation where we are for maintaining a yaw: the weather-vaning tendency causes a torque in one direction, I apply right pedal to adjust for it (to hold my cross angle to the wind); right pedal required less power from the main rotor, etc. So, this isn't it.

 

What am I missing here? What causes the need for extra power?

[PhotoFlyer]

Left X-Wind: With this wind the helicopter requires more power because of main rotor vortex interference, tail rotor settling with power, or a combination of both. The weathervaning tendency counters yaw in this situation, but does not overcome yaw and additional power is needed to maintain heading.

 

Right X-Wing: Two things are happening here.

 

1: The right x-wind increases the tail rotors inflow (same as induced flow) and reduces angle of attack. Because of that reduction more pedal is needed, which means more power is needed.

 

2: The wind is pushing the tail left (nose right), which is the same direction as main rotor torque, so that requires more pedal to overcome the combined force of weathervaning and torque.

 

Not much more to it then that.

[SquirrelFlight]

Okay, I think I've got it now.

 

Thanks!

[Mikemv]

Squirrelflight and all, any time you are out of trim(increased drag), more power is required. When you begin a take-off, you tilt the rotor system and decrease the quanity of the lift vector so even in trim more power is required. Think of a 4' x8' x 1/2" sheet of ply wood at a stand still. And just for the discussion say that it takes X amount of power to keep it in place(hover). Now start moving it forward (1/2" side going forward)against the air(friction/drag). How much surface area is there against the air? Now get it a little out of trim(moving it a few degrees left or right of its desired direction and let the air act/drag against so much more area). More power required to continue but so much more drag.

Many of you have flown with me and know my pet peev is you not keeping the nose straight/aligned during take-off(from ground to a hover). Directional control about the mast is the easiest of all controls to learn but most often minimal attention is given to this as a pilot progresses with training. Do not feel bad as I see this in pre-hire check rides and just watching seasoned pilots in operations around airports. Focus and concentrate on the simplest things and bring your skills to a level of perfection.

I hope that you passed your check ride and continue to grow in your knowledge and experience. Good, safe flying wishes for you, Mike

[SquirrelFlight]

Squirrelflight and all, any time you are out of trim(increased drag), more power is required. When you begin a take-off, you tilt the rotor system and decrease the quanity of the lift vector so even in trim more power is required. Think of a 4' x8' x 1/2" sheet of ply wood at a stand still. And just for the discussion say that it takes X amount of power to keep it in place(hover). Now start moving it forward (1/2" side going forward)against the air(friction/drag). How much surface area is there against the air? Now get it a little out of trim(moving it a few degrees left or right of its desired direction and let the air act/drag against so much more area). More power required to continue but so much more drag.

Many of you have flown with me and know my pet peev is you not keeping the nose straight/aligned during take-off(from ground to a hover). Directional control about the mast is the easiest of all controls to learn but most often minimal attention is given to this as a pilot progresses with training. Do not feel bad as I see this in pre-hire check rides and just watching seasoned pilots in operations around airports. Focus and concentrate on the simplest things and bring your skills to a level of perfection.

I hope that you passed your check ride and continue to grow in your knowledge and experience. Good, safe flying wishes for you, Mike

 

Thie situation I described, though, wasn't in a take-off run, it was in a maximum performance vertical take-off. The disk is tilted into the wind to prevent drift, but there isn't any movement of the ship itself, so no induced drag.

 

I think the other explanation is the correct one: more power is required to hold the ship out of trim, leaving less for the main rotor. I did fine second time around - this effect was demonstrated to me, so I have no problems at all believing it. It's just that I come from an academic background which included some aerodynamics and was wondering what the explanation is for this. Which I believe I've gotten - thanks all!

[Mikemv]

Thie situation I described, though, wasn't in a take-off run, it was in a maximum performance vertical take-off. The disk is tilted into the wind to prevent drift, but there isn't any movement of the ship itself, so no induced drag.

 

I think the other explanation is the correct one: more power is required to hold the ship out of trim, leaving less for the main rotor. I did fine second time around - this effect was demonstrated to me, so I have no problems at all believing it. It's just that I come from an academic background which included some aerodynamics and was wondering what the explanation is for this. Which I believe I've gotten - thanks all!

 

Dear Squirrelflight, I see that you have picked up on the many good points offered in the posts. My response was not only limited to a Take Off Run vs. Vertical Ascent! Think about this, "Does it require more power to hover into the wind or X-wind? A zero airspeed Take-off is just hovering with a climb. If it takes more power to hover x-wind, is this any different than a zero airspeed climb when you are out of trim. Yes, it does take more pedal/power to hold the aircraft out of trim, right?

 

Also, you say vertical climb, no forward run but if you have air or wind passing over the fuselage and thru the rotor system is there a difference? Is 10mph forward movement on a no wind day any different that a vertical climb with 10mph wind? (think tilting of the main rotor lift vector for either situation.

 

It is good that you want to understand all of this. Continue to put all of the pieces together. I have read some of your other post and know that you are very intelligent, Mike

[Linc]

Consider that the the profile of the nose of the robbie (even a partial profile) may provide more flat-plate drag than the entire tailboom, and at a larger moment than the rear of the fuselage (and if the doors were off...). Attempting to maintain your misaligned heading in the takeoff profiled the nose into the wind enough that it was acting as the largest lever on the aircraft, requiring more left pedal to maintain your x-wind heading than a heading into the wind would have required.

Edited June 21, 2008 by Linc

[SquirrelFlight]

I had a hard time getting to sleep last night and started thinking about this again. The claim has been asserted that it requires more power to hover x-wind than in trim, but without explanation as to why.

 

First of all, it is clear that being in trim has the least aerodynamic drag (less area in the profile seen by the wind), so we need to look at the mechanics of what happens when turned out of the wind.

 

In the case where you turn left out of the wind, it is pretty clear: the turn to the left and holding the turn to the left required left pedal input, which increases the thrust on the tail rotor, which takes power from the main rotor, meaning you need to add power to maintain you altitude. Like I said, pretty clear.

 

In the case where you're turning right out of the wind, things get strange. You add right pedal to turn out of the wind and hold it to maintain the heading. However, right pedal *decreases* the power produced by the tail rotor, meaning more power goes back into the main rotor. To maintain the turn, you're using, not countering the torque from the main rotor, which now has more power available to it. This is the one that confuses me. Less power at the tail rotor, more power at the main rotor, why is there an increase in power needed at all? Like I said, I know it's needed, I've experienced it, and in conditions where LTE was not a factor (thinking in terms of other aerodynamics at the tail rotor). I just want to know *why*. What is it I'm missing?

 

Whoa!!! I think I just got it! Like right now as I'm writing this. I scrolled up to see if there was something I missed in some of the other posts. Here goes: The increased drag of the non-trim profile (the wind sees more of the helicopter's profile) required more tilt of the main rotor to maintain zero movement over the ground. That is, for less drag (in trim), small main rotor tilt; for more drag (out of trim) more tilt. More tilt means more resultant in the horizontal, less in the vertical, meaning more power to maintain lift. So, in prinicple, if I had added a little left cyclic to drift *with* the wind (that is return the main rotor thrust to a vertical vector - not that I would have done so, the vertical take-off was to get out of a confined area), then that would have also provided me with the power I would have needed.

 

I still think that, for the reasons given above, that there is an asymmetry between being left out of trim and right out of trim - both require more power, but right-out-of-trim required less additional power than being left-out-of-trim.... does this sound reasonable?

[captkirkyota]

All that you just said makes sense, plus, in my understanding, there is a point in which the torque of the main rotor is not enough to swing the tail around, hence the right pedal inputs does put the tail rotor in a position past neutral and does begin to produce thrust going with the direction torque wants to turn the heli, thus requiring effort or power output from the T. R. ( at least as much as I've thought about this , which is not a lot, seems to me that that does happen. ) In my understanding, the reason for needing negative angle ability for the tail rotor to produce thrust in the same direction as the torque wants to turn the heli is because just like in the weights and balances figures, and certain lb at a distance from a point creates a moment much greater than the actual weight, and since the force of wind out at the tail is at such a great distance away from the axis of rotation, ( the MR shaft ) that can produce enough of a force, even at low amounts of force from wind, to overpower the power of the torque. Kinda like how if I scoot in close enough on a teeter totter, I can achieve an even balance with my 2 boys out at the far end who only weigh 84 lbs together, when I'm 188.

IDK, I don't think too much about this kind of stuff, I'm just trying not to forget all that I've been learning so I can get done with my commercial stuff.

[Gomer Pylot]

The tail rotor takes power no matter where it is set. It takes the least amount of power when it is at zero pitch, but the power it uses increases whenever the pitch increases, in either direction. In general, a wind from the right will require more pitch, and more power, because the tail rotor has to counter yaw from both the weathervaning tendency and transmission torque. A wind from the left requires only countering weathervaning tendency. The weathervaning tendency can be very strong, and I've lifted a 206 from the deck up to an OGE hover with my feet on the floor, in a 25kt or so wind.

[Linc]

Squirrelflight,

 

If you were in a left x-wind situation (nose to the right), the wind would've been blowing the aircraft to the right rear, so wouldn't a right neutraling cyclic have allowed you to drift with the wind, not left cyclic (more left cyclic)? But then, if you were aware enough to drift with the wind to recover power available, why wouldn't you just put your nose into the wind?

 

Gomer,

 

Do different aircraft have different tendencies for weathervaning?

 

I've only flown the 206 and the 406, so my experience is limited otherwise. Even in the 206-series, having the doors off can change the weathervaning tendency in different quadrants. Which is why I hinted that if the doors were off of the robbie, then it increases the parasitic drag when the wind can grab at the opening, changing the power requirement to maintain a certain position. Considering that it is summertime in most locations (especially my current location), it is normal to assume that many aircraft capable of operating without their doors will do so for the comfort of the pilots.

[Gomer Pylot]

Yes, different aircraft have different characteristics, but they will all weathervane to some extent, and all take more power to hover in a crosswind, whichever way the wind is. The tailrotor can require a significant percentage of the available power, and if you think you're going to be power limited, you need to consider putting the nose into the wind. I've seen up to 20% difference in the torque required between a 90 degree crosswind and nose into the wind.