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Downwash question

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Downwash question

Old 24th Nov 2019, 04:05
  #21 (permalink)  
 
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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.
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Old 24th Nov 2019, 11:47
  #22 (permalink)  
 
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Originally Posted by megan View Post
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 donít, you cause stress on the strap packs.
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Old 24th Nov 2019, 15:31
  #23 (permalink)  
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Originally Posted by Sloppy Link View Post

Tosh. Always pull power to ground taxi, I say again about 10% Tq above MPOG. If you donít, 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).
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Old 24th Nov 2019, 20:43
  #24 (permalink)  
 
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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, itís 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.
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Old 24th Nov 2019, 22:01
  #25 (permalink)  
 
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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.
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Old 24th Nov 2019, 22:28
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Originally Posted by Jimmy. View Post
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, itís certainly not linear from 0. Also, flat pitch doesnít necessarily imply 0 degree angle of attack, especially considering blade twist.

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Old 24th Nov 2019, 22:53
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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).
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Old 25th Nov 2019, 00:19
  #28 (permalink)  
 
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Originally Posted by Jimmy. View Post
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, itís a linear relationship. However, in the theory, we ignore many quirks due to real lifeÖ
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Old 25th Nov 2019, 00:23
  #29 (permalink)  
 
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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...
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Old 25th Nov 2019, 01:41
  #30 (permalink)  
 
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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.
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Old 25th Nov 2019, 04:02
  #31 (permalink)  
 
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Originally Posted by Jimmy. View Post
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.


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Old 25th Nov 2019, 04:26
  #32 (permalink)  
 
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Originally Posted by JimEli View Post
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?
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Old 25th Nov 2019, 10:22
  #33 (permalink)  
 
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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.
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Old 25th Nov 2019, 10:25
  #34 (permalink)  
 
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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.
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Old 25th Nov 2019, 10:46
  #35 (permalink)  
 
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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.
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Old 25th Nov 2019, 12:59
  #36 (permalink)  
 
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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?
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Old 25th Nov 2019, 13:07
  #37 (permalink)  
 
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For my back-of-the-napkin calculation, I call this linear.
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Old 25th Nov 2019, 13:22
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Originally Posted by JimEli View Post
For my back-of-the-napkin calculation, I call this linear.
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.
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Old 25th Nov 2019, 21:07
  #39 (permalink)  
 
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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.
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Old 26th Nov 2019, 02:58
  #40 (permalink)  
 
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Originally Posted by [email protected] View Post
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.
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