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DO YOU PLAN FOR TAS CHANGES FOR RAIN


Danb

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I flew from Shreveport La to Wilmington De yesterday with about 75 minutes of reserve fuel. With the winds worse than forecast  and numerous storms along the way I noted true airspeed for differing types of rain. Conditions 11,000 feet, TAS 185 providing about 6hr 45 min flight.

No rain 185

Mist 183

Light rain 180

Moderate 177

Heavy 172 or so

These differences in TAS on this trip would make a fuel stop necessary.
 

WHAT IF ANY IS YOUR DECREASE IN TAS FOR VARYING RAIN CONDITIONS 
 

 

 

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22 minutes ago, Browncbr1 said:

I don’t stretch my fuel load that thin if there is any weather requiring deviations.  

Not the point, I’m trying to ascertain what folks figure if anything for effects of rain on speed.

BTW, I landed with 24 gallons of fuel, after 1150 miles of flight,

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1 hour ago, Danb said:

Not the point, I’m trying to ascertain what folks figure if anything for effects of rain on speed.

BTW, I landed with 24 gallons of fuel, after 1150 miles of flight,

I expect a decrease in performance. I’ve never calculated it, but if I saw the kind of performance decreases you did, I’m sure I would have noticed. 13kts is a lot...makes me curious about your engine data at the same time. I wonder if water ingestion was affecting power...

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Are you sure rain was causing the performance hit? It could just be the downflow caused by the participation. Whenever you are flying in a sinking air, you are essentially climbing to maintain altitude. 

If rain in and of itself was the issue, it would be included in the performance charts. 

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19 minutes ago, N201MKTurbo said:

Are you sure rain was causing the performance hit? It could just be the downflow caused by the participation. Whenever you are flying in a sinking air, you are essentially climbing to maintain altitude. 

If rain in and of itself was the issue, it would be included in the performance charts. 

Absolutely many articles written on performance effects of differing rain droplets, affecting weight, airfoil,drag  basically all aspects of drag lift weight etc. The various articles I read became complicated mathematical formulas, I’m concerned about performance limitations of the Mooney wing in relation to varying  types of rain.

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Not as much to notice in M20J, I think around 5kts as my first guess, surely way bellow 10kts threshold where one start panicking

I guess some of it has to do with wings and air sinking when rain drops (most noticeable on the top half to cloud base), I flew two aircrafts with rain issues: DA40 (manual says don’t get surprised but huge perf loss, especially on takeoff distance) and a motor-glider SF25C (manual does say NO rain flying or wet wings takeoff), I did get a loss of aerodynamics in both, in the later I was basically going down not able to maintain level and did not take much for carburettor to say he did not like it 

In the other hand, I have a feeling that it is going faster in rain as you get visual effects on window and slightly more bumpy, as fast as I am concerned the Mooney will just punch through but it may help you finding some water leaks on sealed doors, if you fix them you may get better sound isolation :D

Obviously, on 1150nm flying mission Coriolis effect matters as much as rain :lol:
 

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3 hours ago, Danb said:

Not the point, I’m trying to ascertain what folks figure if anything for effects of rain on speed.

BTW, I landed with 24 gallons of fuel, after 1150 miles of flight,

Sorry to comment in a way that doesn’t convey my thinking.   Yes, there are speed differences in precipitation...  but there are equal and greater differences for things that are unplanned like vectoring, deviations, altitude changes, holds, turbulence, mountain wave, actual winds that are quite different from published winds, etc.   or like the other day when my gps signal was jammed and I had to fly VORs.   

So, to me, flying through some precipitation is a tiny variable that isn’t worth calculating because I plan a conservative fuel load to ensure it doesn’t need to be considered anyway.  Plus, type of precipitation varies hugely.   I’ve been in hard rain that is accompanied by lift that added 10-20ktas...  I’ve also been in light drizzle that included so much sink that I lost 20 kts and everything in between.   Usually, my gross weight affects TAS more than anything and even then, the actual winds are a bigger factor.  

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20 hours ago, Danb said:

I flew from Shreveport La to Wilmington De yesterday with about 75 minutes of reserve fuel. With the winds worse than forecast  and numerous storms along the way I noted true airspeed for differing types of rain. Conditions 11,000 feet, TAS 185 providing about 6hr 45 min flight.

No rain 185

Mist 183

Light rain 180

Moderate 177

Heavy 172 or so

These differences in TAS on this trip would make a fuel stop necessary.
 

WHAT IF ANY IS YOUR DECREASE IN TAS FOR VARYING RAIN CONDITIONS 
 

 

 

Those numbers look about comparable to the speed loss in knots in a J as well in stable rain. When there’s instability, I’ve seen 20-30 knots loss of speed when you combine rain and down drafts. On the other hand, you usually won’t be continuously in rain in when it’s unstable. Might be in rain a few minutes with a big hit on speed but then be out of it.

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I have not planned a different speed in rain... but...

When you see how much water is flowing across the wings in torrential downpours....

All that rain weight has to get accelerated from 0 - 185 TAS...

100% rh has some effect on power as well... less O2 available, more heat absorbed by the H20...

Excess water shouldn’t be an issue as the Post-modern intake system should be separating the water out... 
 

aerodynamics is an interesting concern... the water adheres to the wing...  may have an effect on the boundary layer...
 

There are interesting impact affects of rain... it can chip the paint off...   and water flows around the speed brakes when deployed...
 

PP thoughts only, not an aerodynamicist....

Best regards,

-a-

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I verified the effects of rain several times flying a jet with a sensitive AoA. Go into a shower level flight, A/P on, auto thrust on, yep AoA increases, come out decreases. It happened with building, steady and dying storms. We all know humidity decreases air density (those H2O molecules have to go somewhere) so it is only natural that AoA has to increase, which means drag increases. Power requirement to maintain level flight increases. A slight increase in power for a given throttle position would be expected in fog or mist (aka water injection) both for turbines and recipes, heavier than that however and you might see a power decrease. 13 knots difference however is likely more down draft than humidification.

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Well, did you see the same change in ground speed or not? Airspeed is a measure of ram air pressure.  Humid air is less dense and therefore the pressure is lower before the air ever gets into the pitot. Your TAS indicator does not adjust for humidity, but the difference is real. In short, make the air humid and your gauge will indicate a lower airspeed, both indicated and true.  So I would be more concerned if there was a big ground speed change, because that is a better measure of whether and how much the aircraft actually slowed down, as opposed to simply a change in indicated ram air pressure.

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I’ve noted all sorts of variations in TAS by flight. Temp, precip, in and out of cloud and bumps will  rattle it a bit.  Most of the time I’m noticing it when it’s a few kts lower than expected and I’m racking my brain about everything that could be wrong with my gear rigging engine etc to be three knots less.  Then it works it’s way back. 
 

Practically I plan for 3-4kts less than what I should be planning.  And all my fuel planning is conservative - I set my reserves at 120min plus alternate or closest VMC if it’s further away than the legal alternate.  

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I have never checked the airspeed loss due to rain but I know that it happens.  When I was in VietNam and doing artillery calibration if we were shooting into dense clouds we used a timed fuse because the dense cloud would set off an impact fuse.  

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I first notice a loss of airspeed with the slightest amount of turbulence. Anything less than perfectly smooth air results in a loss of airspeed. Therefore  I wonder how much of a loss of airspeed from rain is really due to disturbed air. I can only remember experiencing only very light status drizzle, without much vertical development and still in very smooth air. I can't ever recall moderate or heavy precipitation, that requires much more vertical development,  without some degree of disturbed air and more often with significant uncomfortable turbulence that I'am likely already slowing down for anyway when it includes big bumps - at least till I am out of it.  I 've landed short many times for such conditions and just descending down below 10K and closer to the bases often is much smoother ride as well. 

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In cold front rain, one tend to trade 1/ good visibility and smooth ride in sinking air near the ground base but lot of ground turbulences (either terrain on high wind wind or rain hot ground) for 2/ bad visibility and very turbulent up/down ride near a convective cloud base, none of these choices will help with higher TAS planning but all has to do with “turbulent air” conditions rather than “wet air”, I tend to plan for min altitude TAS and add few 10kts to the headwinds irrespective if it is wet or not

In warm fronts rain, I don’t think there is anything noticeable in terms of performance loss at least on Mooney airframes...

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14 hours ago, jlunseth said:

Well, did you see the same change in ground speed or not? Airspeed is a measure of ram air pressure.  Humid air is less dense and therefore the pressure is lower before the air ever gets into the pitot. Your TAS indicator does not adjust for humidity, but the difference is real. In short, make the air humid and your gauge will indicate a lower airspeed, both indicated and true.  So I would be more concerned if there was a big ground speed change, because that is a better measure of whether and how much the aircraft actually slowed down, as opposed to simply a change in indicated ram air pressure.

Yes

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