Downwash question

[memories of px]

Can anybody tell me if there would be any rotor downwash from a pair of stationary Apaches. They were waiting at the threshold, as i came in to land in a PA28, there was a strong wind blowing in the direction from the Apache's towards my landing point, and i experienced turbulence and windshear. I'm not sure if this would be due to the Apaches, or just the strong wind conditions, any thoughts would be greatly appreciated.

[Sloppy Link]

Depends if they’re holding ground taxi power sitting on the brakes. Even then, the symptoms you describe seem a bit severe.

[[email protected]]

If they were on the ground, the downwash would be negligible - if they were in the hover then it might have been an issue.

The turbulence and windshear would be due to the strong wind you mention.

[currawong]

Not necessarily downwash, but turbulence, yes.

Anything upwind, such as a tree or building will give you turbulence.

Wind turbines are well known for their turbulence and they are deriving thrust, not imparting it.

And don't overlook short finals/ flare/ float is the most vulnerable phase of flight.

[memories of px]

Many thanks, they are big machines, and I think you are all correct, it was turbulence, ATC maybe should have held them further back the taxiway with those wind conditions.

[[email protected]]

A 200' wind turbine side on isn't the same as a 60' rotor edge on and a helicopter that is designed to fly through the air with a minimum of drag is unlikely to generate a great deal of turbulence when at flat pitch on the ground.

[Flying Bull]

Well,
even if the collective is flat pitch - if one of the pilots is lazy on the pedals, the tail rotor can produce quite a bit of wind.
I blew a (rotten) wooden fence with the power of a fenestron ;-)

[Davey Emcee]

Well,
even if the collective is flat pitch - if one of the pilots is lazy on the pedals, the tail rotor can produce quite a bit of wind.
I blew a (rotten) wooden fence with the power of a fenestron ;-)

Presumably you were "lazy on the pedals" !!

[rottenjohn]

Well,
even if the collective is flat pitch - if one of the pilots is lazy on the pedals, the tail rotor can produce quite a bit of wind.
I blew a (rotten) wooden fence with the power of a fenestron ;-)

should only be one pilot on the pedals, and why did you have so much pedal in to do that while you are sitting on the ground?

[Flying Bull]

Presumably you were "lazy on the pedals" !!

Well, not really, I wanted to turn (wheeled helicopter)

@ rottenjohn - if someone stands on the brakes its a little bit different then normal - depending on the pedal design and your seating position, you need to stretch the leg/toes to get proper braking action.
Assume it is at the holing point of a runway, you´re normally turning a little bit to reach that position - hence uneven braking/pedal position is a possibility

[[email protected]]

But again, that is a rotor side on, doing what is designed for - producing thrust - not one edge on at flat pitch.

[Flying Bull]

But again, that is a rotor side on, doing what is designed for - producing thrust - not one edge on at flat pitch.

The tail rotor produces thrust if you put pedal in, flat pitch on the collective or not.
Have seen someone blown away passing to close to the tail - lucky for him, he was on the right side. Was an ground accident to happen....

[212man]

The tail rotor produces thrust if you put pedal in, flat pitch on the collective or not.
Have seen someone blown away passing to close to the tail - lucky for him, he was on the right side. Was an ground accident to happen....
I think that’s what Crab was saying.

in a Super Puma you can roll the aircraft on its side with too much right pedal!

[[email protected]]

Quite. You blew away a fence because your TR was producing thrust - the OP was about turbulence from a MR at flat pitch:ok:

[Flying Bull]

Quite. You blew away a fence because your TR was producing thrust - the OP was about turbulence from a MR at flat pitch:ok:

A plank driver asked, wether it is possible to have turbulence due to nearby helicopters.
Not understanding all the factors - he assumes, it must have been downwash.
I just added another possibility - which might not spring into mind helicopter pilots operating smaler helicopters - but heavy "birds" ;-) ahh helicopters need a lot of anti torque - which is produced by the tail rotor.
Even a slight foot forward, which won´t move or turn the helicopter will produce quite a lot of wind from the big fan at the back.
Understandable now?

[[email protected]]

Even a slight foot forward, which won´t move or turn the helicopter will produce quite a lot of wind from the big fan at the back.
Understandable now? I quite understand what you are trying to 'mansplain' to another helicopter pilot BUT a wheeled helicopter like an Apache will turn with very little pedal if the tailwheel is unlocked and, since they appear to have been waiting to line up on the runway and would ground taxi to do that, it is extremely unlikely that the tailwheel would be locked again and even more unlikely that they would sit with pedal applied and the tailwheel locked.

Yes, a TR will produce thrust on the ground but in the scenario posted by the FW pilot - the turbulence he experienced would not have been due to the helicopters.

[Flying Bull]

Would you bet your life on that?
May be they were already light on the Wheels, ready to line up immediately after the landing aircraft?
Thus making „a little bit of wind“?

[Sloppy Link]

I refer to my earlier answer but even then it would only be about 10% Tq above MPOG. Symptoms still seem too severe for the AH to have a demonstrable effect.

[[email protected]]

May be they were already light on the Wheels, ready to line up immediately after the landing aircraft? And maybe the turbulence pixies had eaten beans for dinner;)

[griffothefog]

I call bull**** on the helicopter turbulence ������

[megan]

Can anybody tell me if there would be any rotor downwash from a pair of stationary Apaches. They were waiting at the threshold, as i came in to land in a PA28, there was a strong wind blowing in the direction from the Apache's towards my landing point, and i experienced turbulence and windshear. I'm not sure if this would be due to the Apaches, or just the strong wind conditions, any thoughts would be greatly appreciated.Difficult to say as we we were not there. Some considerations, helicopters produce a down wash even at flat pitch, sufficient if you're on wheels to initiate and sustain taxi, the Apache manual suggests only pulling power to taxi if use of the rotor disc is insufficient. The very presence of an obstacle upwind is going to cause turbulence, more so if it is a helicopter modifying the airflow with its input via down wash. I don't think some here appreciate the handling qualities of light fixed wing aircraft such as your Cherokee. Congratulations on the airmanship in considering the issue, exercise extreme caution when helicopters are in the vicinity of your fixed wing landing or take off area.

[Sloppy Link]

Difficult to say as we we were not there. Some considerations, helicopters produce a down wash even at flat pitch, sufficient if you're on wheels to initiate and sustain taxi, the Apache manual suggests only pulling power to taxi if use of the rotor disc is insufficient. The very presence of an obstacle upwind is going to cause turbulence, more so if it is a helicopter modifying the airflow with its input via down wash. I don't think some here appreciate the handling qualities of light fixed wing aircraft such as your Cherokee. Congratulations on the airmanship in considering the issue, exercise extreme caution when helicopters are in the vicinity of your fixed wing landing or take off area.
Tosh. Always pull power to ground taxi, I say again about 10% Tq above MPOG. If you don’t, you cause stress on the strap packs.

[LRP]

Tosh. Always pull power to ground taxi, I say again about 10% Tq above MPOG. If you don’t, you cause stress on the strap packs.


Yep. At the higher DA's we found 28-30% to be much easier on the straps (A models).

[JimEli]

IMHO, best case would have the 2 Apaches at flat pitch creating some possible turbulence in the immediate area downwind of them. Worst case, with the Apaches holding near taxi power (20-24% Q as recommended by the RFM), they would be producing ~16 mph of turbulent wind (see analysis below). Ultimately, it’s always left to the PIC to accept or decline a clearance.

Assume 75% power to hover and using 24% for taxi = ~1/3 of hover thrust. Assuming a GWt of 15000, means each Apache would create about 4950 lbs. of thrust. Therefore, the downwash velocity (at the disc) is SQRT((GWt)/(DA*2*rho)), or SQRT(4950/1809*2*0.002378), at SL. So, each Apache is making 24 f/s, or 16mph of wind.

[Jimmy.]

Just asking (I really don't know): there is a linear relation between torque and thrust? Beside the direct physics and math involved, one idea occured to me: some torque is used to drive the powertrain and as it is increased part of this extra torque is used to drive the TR, so things seems to be a little more complex.

[JimEli]

Just asking (I really don't know): there is a linear relation between torque and thrust? Beside the direct physics and math involved, one idea occured to me: some torque is used to drive the powertrain and as it is increased part of this extra torque is used to drive the TR, so things seems to be a little more complex.

In my overly simplified analysis, I use 75% torque for hover power. That value would also include the power to drive tail rotor, accessories and other losses. My guess is flat pitch torque would be roughly 15%. For simplification, I make the assumption its linear, however, it’s certainly not linear from 0. Also, flat pitch doesn’t necessarily imply 0 degree angle of attack, especially considering blade twist.

[Jimmy.]

I've understood your simplifications, but my lack of knowledge is on the linearity of the torque/thrust relation from 15% to 75% Q. Also, at any torque from 15% (operational NR, I don't know if the Apache works at 100, 105, 107%...) the torque required to drive the MGB and accessories (assuming constant generators loads etc) is the same, but the torque used to drive the TR increases (and again I don't know the relation between them).

[JimEli]

I've understood your simplifications, but my lack of knowledge is on the linearity of the torque/thrust relation from 15% to 75% Q. Also, at any torque from 15% (operational NR, I don't know if the Apache works at 100, 105, 107%...) the torque required to drive the MGB and accessories (assuming constant generators loads etc) is the same, but the torque used to drive the TR increases (and again I don't know the relation between them).

Theoretically, at a hover, assuming all constants are constant, it’s a linear relationship. However, in the theory, we ignore many quirks due to real life…

[Jimmy.]

Consulting a graphic (Indicated Torque Required to Hover IGE, 105%NR, AC Off, Wh = 10ft and AI Off) on a RFM, 500ft AD and Aircraft Gross Wheight of 17,000lb and 26,000lb, I found 50%Q and 80%Q, respectively. If the relation is linear, in this helicopter at these conditions, 30%Q correspond to 9,000lbs of thrust, or 1%Q to 300lb.
So, 17,000lb should require 56%Q, and we didn't consider the torque needed to drive the powertrain...

[megan]

Tosh. Always pull power to ground taxi, I say again about 10% Tq above MPOG. If you don’t, you cause stress on the strap packs. Added for completeness Sloppy & LRP
8.25 TAXI
CAUTION
• Excessive cyclic displacement with low power settings will result in droop stop pounding.
• If forward cyclic inputs appear excessive while taxiing, increase collective as necessary. Appropriate collective setting is a function of cyclic displacement and surface conditions.
• Excessive forward cyclic displacement with low power settings will result in high strap pack loads.

[JimEli]

Consulting a graphic (Indicated Torque Required to Hover IGE, 105%NR, AC Off, Wh = 10ft and AI Off) on a RFM, 500ft AD and Aircraft Gross Wheight of 17,000lb and 26,000lb, I found 50%Q and 80%Q, respectively. If the relation is linear, in this helicopter at these conditions, 30%Q correspond to 9,000lbs of thrust, or 1%Q to 300lb.
So, 17,000lb should require 56%Q, and we didn't consider the torque needed to drive the powertrain...

I would think 26K is outside the optimized hover region for the Apache. Quirky things happen there. L/D should be more linear in the central region where hover occurs for most helos.

[Jimmy.]

I would think 26K is outside the optimized hover region for the Apache. Quirky things happen there. L/D should be more linear in the central region where hover occurs for most helos.
As I don't have access to an Apache's RFM, I used a S-92A one, in order to try to understand what you are affirming, choosing torques and weights as round as possible. I've tried HOGE and things got worse.
On a thread about LTE a long time ago, the discussion about the relation between loss of NR (due overpitching) and torque increase (added to the loss of NR on the TR) emerged and someone, I think Nick Lappos, explained why it occurs.
So, you applied a linear relation between torque and thrust on your calculations and my question was why it is linear.
Reviewing you post and trying to understand, another question arised: if the helicopter is on ground, the amount of thrust generated by the MR shouldn't be related just to the power/pitch/torque (collective position...) applied instead of torque and takeoff weight? If the helicopter isn't in the air and the torque setting is well below the required for hover, why the weight on wheels should matter to determine the thrust being produced?

[[email protected]]

Having re-read the OP, memories of px states that the Apaches were holding at the threshold - did he mean near the threshold, because the distance from the landing area to the holding point would be significant in determining any turbulence downwind of the helicopters.

However, I remain pretty convinced that the wind-shear and turbulence experienced by memories of px is far more likely to be caused by normal wind gusts - he noted that there was a strong wind and such gusts and lulls are commonplace in those conditions.

[[email protected]]

Jimmy - the faster you spin the TR, the less the Tq required so if the Nr reduces, the Tq increases.

At MPOG (min pitch on the ground) there will be very little thrust produced by the MR.

[212man]

On a thread about LTE a long time ago, the discussion about the relation between loss of NR (due overpitching) and torque increase (added to the loss of NR on the TR) emerged and someone, I think Nick Lappos, explained why it occurs.

In basic terms it's because for any given weight a fixed power will be required to hover (in identical conditions), and as power is a measure of 'rate of work done' it basically means Power = Torque x Nr, with Torque being the 'work' and Nr being the 'rate'. One goes up or down, the other goes down or up.

Reviewing you post and trying to understand, another question arised: if the helicopter is on ground, the amount of thrust generated by the MR shouldn't be related just to the power/pitch/torque (collective position...) applied instead of torque and takeoff weight?

Not sure I follow what you are saying here, but the MR thrust at MPOG will not be affected by the aircraft weight at all.

[Jimmy.]

Crab and 212, we are on the same page, thanks. I brought the Torque/NR example because it was new to me at that time and was explained in few lines, as you did above.
Any light on the Torque/Thrust relation?

[JimEli]

For my back-of-the-napkin calculation, I call this linear.
https://cimg0.ibsrv.net/gimg/pprune.org-vbulletin/500x368/ld_c2400ae8ab09699546fc504e5f8dbef02b4d2c11.png

[Jimmy.]

For my back-of-the-napkin calculation, I call this linear.
https://cimg0.ibsrv.net/gimg/pprune.org-vbulletin/500x368/ld_c2400ae8ab09699546fc504e5f8dbef02b4d2c11.png
Jim, I'm not challenging you. I clearly don't have your background, but your post made me curious about something new to me, and asked for a short explanation. Also, as a non native English speaker, maybe "linear" could be well replaced by "direct" on my question.
But, as PPRuNe isn't a classroom, you don't have the obligation to explain until I understand.
Thanks anyway.

[[email protected]]

Jimmy - my take on it is that since the coefficients of both lift and drag - shown on the axes of Jim Elis graph - change with AoA, and since the V squared doesn't change lifting to the hover, then thrust will increase as CL increases and Tq will increase to over come the rise in CD.

[Jimmy.]

Jimmy - my take on it is that since the coefficients of both lift and drag - shown on the axes of Jim Elis graph - change with AoA, and since the V squared doesn't change lifting to the hover, then thrust will increase as CL increases and Tq will increase to over come the rise in CD.
Got it, thank you.