The forgotten rain effect
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The forgotten rain effect
Has anyone ever wondered what effect rain has on aircraft performance?
I’m not talking about the effect on braking performance during an RTO or landing, but rather on things like total weight, lift, aerodynamic drag, and energy/momentum losses.
If you calculate how much surface area a B747 has for example, then cover it with 1mm of water, how many litres of water would that equate to? I wouldn’t know how to do those calculations but I can imagine it being quite a lot.
The thin film of water must affect the aircraft’s boundary layer.
And how much momentum is lost as the raindrops pound the aircraft’s surface and instantly accelerate from a horizontal velocity of zero to anywhere from 130 kts to 300 kts?
I imagine that the rate of precipitation is an important consideration too.
Should V2 and Vref be adjusted?
I’ve searched around and have found no reference to these issues.
Any educated thoughts on this?
PS Could someone do the calculations for a B737-800? Thanks.
I’m not talking about the effect on braking performance during an RTO or landing, but rather on things like total weight, lift, aerodynamic drag, and energy/momentum losses.
If you calculate how much surface area a B747 has for example, then cover it with 1mm of water, how many litres of water would that equate to? I wouldn’t know how to do those calculations but I can imagine it being quite a lot.
The thin film of water must affect the aircraft’s boundary layer.
And how much momentum is lost as the raindrops pound the aircraft’s surface and instantly accelerate from a horizontal velocity of zero to anywhere from 130 kts to 300 kts?
I imagine that the rate of precipitation is an important consideration too.
Should V2 and Vref be adjusted?
I’ve searched around and have found no reference to these issues.
Any educated thoughts on this?
PS Could someone do the calculations for a B737-800? Thanks.
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I read a while ago (although I can't remember the reference) that heavy rain can add up to 2% in aircraft weight, and up 20% in drag!
As you would know...... since V2 and Vref are scheduled in accordance with the stall speed, which would increase in heavy rain, perhaps consideration should be given to raising these values.
Although I don't know of any SOPs that do.
Not very helpful.... I know.
As you would know...... since V2 and Vref are scheduled in accordance with the stall speed, which would increase in heavy rain, perhaps consideration should be given to raising these values.
Although I don't know of any SOPs that do.
Not very helpful.... I know.
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Not very relevant to airliners, but I read some time ago, that Rutan canard designs can be affected by even very light rain. If my memory is correct, the canard can stall.
Any more info would be appreciated.
Any more info would be appreciated.
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Noticed a Boeing publication some years ago (back in the 707 operational days) wherein it mentioned that 10 knots should be added to the approach speed during very heavy rain.
Suspect they were right.
Suspect they were right.
Prof. Airport Engineer
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Since waxing lyrical about him in the recent post in this forum -
http://www.pprune.org/forums/showthr...hreadid=108828
I just have to quote Davies on this:
I wonder if that is why Boeing had their advice about adding 10 knots - not so much for speed as to correct the ASI? Now the trick is to land firmly and pull up whilst still staying on the presumably flooded runway.
http://www.pprune.org/forums/showthr...hreadid=108828
I just have to quote Davies on this:
Heavy Rain
In or beneath a violent thunderstorm it is possible to suffer extremely heavy rain of 50 grams per cubic metre (~= 1 inch per minute) and this could cause engine flame out. This rate, however, is extremely rare and occurs only in an area where the aeroplane shouldn’t be anyway. Leaving this and other exceptionally heavy rates aside we come to normal very heavy rain of 2 grams per cubic metre (= 2 in. per hour). This has no significant effect on the aeroplane or its engines, except that the water might impact on the pitot head and cause temporary fluctuations of airspeed, in the underreading sense, even with the heaters on. Free water can exist in large quantities at high altitudes down to extremely low temperatures. This can cause ice-accretion at a rapid rate due to the naturally low temperature of the outside of the aeroplane. Depending on the efficiency of the rain dispersal system forward visibility can be significantly reduced.
In or beneath a violent thunderstorm it is possible to suffer extremely heavy rain of 50 grams per cubic metre (~= 1 inch per minute) and this could cause engine flame out. This rate, however, is extremely rare and occurs only in an area where the aeroplane shouldn’t be anyway. Leaving this and other exceptionally heavy rates aside we come to normal very heavy rain of 2 grams per cubic metre (= 2 in. per hour). This has no significant effect on the aeroplane or its engines, except that the water might impact on the pitot head and cause temporary fluctuations of airspeed, in the underreading sense, even with the heaters on. Free water can exist in large quantities at high altitudes down to extremely low temperatures. This can cause ice-accretion at a rapid rate due to the naturally low temperature of the outside of the aeroplane. Depending on the efficiency of the rain dispersal system forward visibility can be significantly reduced.
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The "Rutan problem" came about from their use of enhanced laminar flow aerofoil sections. The rain upset the boundary layer causing early transition at a lower Reynolds number. This caused a large reduction in lift and increase in drag, some Ezes could only just maintain level flight in moderate rain. Roncz subsequently worked on a much more tolerant section which largely eliminated that problem.
One of the main targets for enhanced laminar flow on airliners has been on engine nacelles, so I suspect there may be a rain factor on their performance.
One of the main targets for enhanced laminar flow on airliners has been on engine nacelles, so I suspect there may be a rain factor on their performance.
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THE EFFECT OF HEAVY RAIN.
The FAA sponsored a research programme to discover the effect that heavy rain has on aircraft performance. The results showed that:
1.- The impingement and rapid accumulation of water sufficiently distorts the shape of upper wing surface, albeit only temporalily, to diminish the total lift developed by up to 30% of its normal value.
2.- The impact of the rain on the aircraft and its lowered undercarriage increases drag to such an extent that it decreases the aircraft´s forward speed.
3.- When operating in such conditions a jet engine is slow to respond to rapid demends for power.
4.- The weight of the rain can momentarily adversely affect the aircraft weight.
Quote from "AIRCRAFT PERFORMANCE THEORY FOR PILOTS"
Author: P.J. Swatton. ISBN 0-632-05569-3
The FAA sponsored a research programme to discover the effect that heavy rain has on aircraft performance. The results showed that:
1.- The impingement and rapid accumulation of water sufficiently distorts the shape of upper wing surface, albeit only temporalily, to diminish the total lift developed by up to 30% of its normal value.
2.- The impact of the rain on the aircraft and its lowered undercarriage increases drag to such an extent that it decreases the aircraft´s forward speed.
3.- When operating in such conditions a jet engine is slow to respond to rapid demends for power.
4.- The weight of the rain can momentarily adversely affect the aircraft weight.
Quote from "AIRCRAFT PERFORMANCE THEORY FOR PILOTS"
Author: P.J. Swatton. ISBN 0-632-05569-3
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I think we all should be more interested in the effect that moderate to heavy rain can have on our engines during the normal idle setting during descent.
In the normal to high speed descent, if we encounter this level of rain, with the engines operating at idle, there is a marked increase in the percentage of water content ingested than there is during climb or cruise.
Enough to be a problem, can be!
At high speeds, with the engine at idle, in moderate/heavy rain, the N1 fan will only process a certain amount of air into the bypass/intake due directly to it's rotational speed(low).
The remaining excess volume diverts around the outside of the nacelle, but what of the rain droplets contained in that excess air volume?
Well, due to momentum effects, most of it continues straight into the intake, unable to change direction, as the air did.
The result being a very much higher percentage of water, with the resulting danger of a flameout, or surge.
Thats why it's good to unload the compressor a little in these situations by turning on engine anti-ice, or increasing the power(read RPM) a little, with some speed brake to compensate.
Cheers
In the normal to high speed descent, if we encounter this level of rain, with the engines operating at idle, there is a marked increase in the percentage of water content ingested than there is during climb or cruise.
Enough to be a problem, can be!
At high speeds, with the engine at idle, in moderate/heavy rain, the N1 fan will only process a certain amount of air into the bypass/intake due directly to it's rotational speed(low).
The remaining excess volume diverts around the outside of the nacelle, but what of the rain droplets contained in that excess air volume?
Well, due to momentum effects, most of it continues straight into the intake, unable to change direction, as the air did.
The result being a very much higher percentage of water, with the resulting danger of a flameout, or surge.
Thats why it's good to unload the compressor a little in these situations by turning on engine anti-ice, or increasing the power(read RPM) a little, with some speed brake to compensate.
Cheers
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Gliders do tend to come down fast in rain. The wings are sanded, waxed, polished and what not to get a mirror shine, enabling the flow to remain laminar quite far back, usually all the way to the turbulator strips and those seem to be 2/3rds to 3/4ths of the cord back most of the time.
Rain and kamikaze bugs are naturally more detrimental to a wing with that finish than to a wing which has a more coarse surface finish by design.
Competition pilots fit their aircraft with bug wipers, which can somewhat clean off the leading edges in flight. In the hand computer based glider performance software, you can often enter a compensation factor for the amount of bugs on the leading edge to get accurate performance figures.
I had the interesting experience of flying underneath what must have been a mini-CB which decided to more or less explode in my face (or perhaps rather on top of my head?) a few months ago. Right when turning final, of course. I had waves forming across the top of the canopy, ASI going up and down by 30 km/h (with a Vref of about 105 that's significant) and VS oscillating between -3 and +2... not something I care much to repeat! Well, saved wear on the boards at least!
Cheers,
Fred
Rain and kamikaze bugs are naturally more detrimental to a wing with that finish than to a wing which has a more coarse surface finish by design.
Competition pilots fit their aircraft with bug wipers, which can somewhat clean off the leading edges in flight. In the hand computer based glider performance software, you can often enter a compensation factor for the amount of bugs on the leading edge to get accurate performance figures.
I had the interesting experience of flying underneath what must have been a mini-CB which decided to more or less explode in my face (or perhaps rather on top of my head?) a few months ago. Right when turning final, of course. I had waves forming across the top of the canopy, ASI going up and down by 30 km/h (with a Vref of about 105 that's significant) and VS oscillating between -3 and +2... not something I care much to repeat! Well, saved wear on the boards at least!
Cheers,
Fred
There's a British single-seat Longeze derivative called a Goldwing. I've flown them a little, but never in rain - the aircraft carries a large and clearly visible placard stating "DO NOT FLY IN RAIN". The manual states that this is because of the rain breaking up the laminar flow on the mainplane and canard.
I've spoken to one pilot who did make this mistake and said he could just maintain level flight after passing through light rain with full power and full back-stick.
Some reasearch was done into this particular aircraft by Glasgow University in the mid 1980s. I believe that they concluded that although technically both mainplane and canard were designed for laminar flow, the real problem was the canard. I've seen a report from them that stated that a suitably rough paint finish on the entire canard would prevent this problem and allow flight in rain. I'm not aware of anybody who has ever felt the urge to prove this in flight-test, I certainly haven't.
G
I've spoken to one pilot who did make this mistake and said he could just maintain level flight after passing through light rain with full power and full back-stick.
Some reasearch was done into this particular aircraft by Glasgow University in the mid 1980s. I believe that they concluded that although technically both mainplane and canard were designed for laminar flow, the real problem was the canard. I've seen a report from them that stated that a suitably rough paint finish on the entire canard would prevent this problem and allow flight in rain. I'm not aware of anybody who has ever felt the urge to prove this in flight-test, I certainly haven't.
G
