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K & N filters in heavy rain?

pierre smith

pierre smith

Well Known Member
When we departed KEYW last Friday, I pointed out the Alt air knob to my buddy, not knowing how heavy or how long, we'd be in rain, nor do I know how much water the K & N filters in our RV's can handle.

As it turned out, we never needed alt air but I'm still curious as to how much rain they can endure before either A) collapsing, or B) closing off the air supply to the engine.

Ideas/information anyone?

Thanks,
 
Pierre,
I sent the tech contact @ K&N an email regarding performance in rain. Let's see what they say Monday.
I too was concerned when I flew through some moderate rain a while back. I did notice a substantial decrease in airspeed in the rain (~8kts). This probably due to both airflow over lifting surfaces and high humidity air going into the engine.
 
Good Question

Pierre, Glad you had a good time in Key West.

I don't know the answer to your question but I will relate the following.

While doing IFR training (C-172) in the 70's I was in rain so heavy that I couldn't believe the engine could keep running. It was like several fireman with large hoses were flooding the aircraft/windscreen. The airport (Mansfield, Oh) we were shooting approaches at didn't have weather radar at the time but after we told them of the downpour they contacted KCLE who confirmed we were in the middle of a heavy thunderstorm. I suspect that the RV style filters can perform the same or better than 70's vintage air filters but judging from my experience I don't really want to find out.
 
Pierre - I'm not sure if the -10's alternate air is the same as my -6A's. If it is the same, closing it might only reduce a bit of the amount of rain coming into the FAB. While most airflow would be directed around the sides of the opening, I'm not sure what percent of the accompanying rain droplets would go with the air vs going straight into the FAB inlet. I suspect most snow flakes would follow the airflow, but rain I'm doubting would.

Good question! (DanH, are you in on this one?)
 
Interesting Question

On a couple of occasions, I've flown my normally aspirated 9 thru some of the "red" areas painted by NEXRAD. While the noise in the cockpit was significant, I never noticed a drop in RPM. On the other hand, when I've encountered precip at the freezing level, even in a shallow layer, it has quickly lead to a fall-off in RPM. While this was solved by immediate application of carb heat (shutting off the air box), there's also a corresponding drop in power as I decrease fuel flow to match airflow. Never good on a climbout.
More worrisome for us tip-up drivers is the seal on the front edge of the canopy. If water is dripping on my feet, it's the radios I'm worried about and not the engine. While the seal has been holding well in recent years, I still have my 396 always ready as a backup.
Terry, CFI
RV9A N323TP
 
David Z said:
Look at a float plane prop and you'll see why that's a bad idea.
Click to expand...

I know nothing about floatplane props. Water splash erodes metal or merely removes paint? And why is it different from heavy rain?
 
AlexPeterson said:
Pierre - I'm not sure if the -10's alternate air is the same as my -6A's.
Good question! (DanH, are you in on this one?)
Click to expand...

The 10's alt air is in the middle of the air filter, on the bottom of the FAB and activating it doesn't close off the front of the FAB, like our -6A did. It just opens a big, round hole that's centered under the air filter.

Dan, I may just do the water hose trick but how do I relate that to either moderate, or heavy rain?

Best,
 
DanH said:
I know nothing about floatplane props. Water splash erodes metal or merely removes paint? And why is it different from heavy rain?
Click to expand...


The airflow and vortex patterns in front of the prop at low speed (think taxiing with too much power, or beginning of take-off) suck large water drops into the bottom of the prop arc. They pit and open up the leading edge of the prop. Normally it's just small burrs, but if it's left to get bad, it looks like the metal is peeling apart.

The best picture I could find shows a prop that's already been repaired. The blade tips are no longer nice and round. They're angled off where a mechanic has filed away the damaged part.
http://www.airliners.net/photo/Pacific-Coastal-Airlines/De-Havilland-Canada/2012559/L/&sid=422f5b32e95a2f8a57e5aba05ba6b409
 
I have flown through some pretty heavy rain a couple of times and have never had the engine give any indication of a problem. I did, however, have to have my MT propeller repainted. I am much more conservative about rain these days. :rolleyes:
 
With all due respect, why fly in moderate to heavy rain or for that matter a red radar return in a single engine airplane? Moderate to heavy rain can lead to bigger problems quickly. When getting paid to fly in such conditions the Ops manual for a jet advised caution and circumnavigate red returns.
 
pierre smith said:
Dan, I may just do the water hose trick but how do I relate that to either moderate, or heavy rain?
Click to expand...

Apparently if the the paint stays on it is like moderate rain. If the paint comes off it is like heavy rain. If it eats metal you have landed on a lake ;)
 
Not sure how much this helps as I do not have any exact numbers, but I experience a situation when I believed my filter sucked in enough rain to effect engine performance. My 6a has an electronic fuel injection system that uses a manifold pressure sensor (along with air tempature and rpm) to calculate the fuel delivery. My filter and air box were the standard units provided by Vans for a carbureted plane at that time. I was flying in some pretty heavy rain and noticed the rpm started to smoothly drop, as if I was closing the throttle. I do not remember for sure, but I believe I lost around 150-250 rpm pretty quickly. I was able to regain most of the loss by going WOT, but the plane was certainly not performing like it was WOT. Since the EFI uses manifold pressure and the loss in power was very smooth (like slowly closing the throttle) I sumized the filter was getting water logged. After the filter dried out never I had that problem again. I am in the process of installing the same system on a new engine and have decided to build a new air box with an alternate opening that bypassed the filter all together because of that incident.
 
Saber25 said:
With all due respect, why fly in moderate to heavy rain or for that matter a red radar return in a single engine airplane? .
Click to expand...

Hans, the radar returns on the weather channel only showed mostly green with a few yellow patches, also confirmed by my 496 in flight.

Like you. I wouldn't even consider penetrating red areas....matter -of-fact, I was concerned about the yellow patches but I learned a valuable lesson...that a small bit of yellow is not really a problem. We were in a well-equipped airplane, with two IFR rated guys up front.

Best,
 
I would avoid red returns if at all possible. My biggest concern however would be what is producing the rain. If this is convective activity then you need to be very careful. Lots of bad things can happen flying through or under thunderstorms and they can change quite rapidly. You as the pilot can't predict the change so if you choose to fly through or under or even near convective activity then your simply rolling the dice. Delta 191 in Dallas picked up what looked like a very small convective rain shower on final. A much smaller aircraft flew under it 1 minute before they did without issue. 1 minute later that small shower slammed a 400,000 lbs aircraft into the ground even though they still had plenty of energy and actually impacted the ground at 180 knots. They simply could not overcome the huge downdraft in the microburst. The smaller aircraft just seconds ahead reported a nice ride and no problems!
If I were flying a well equipped IFR aircraft through a warm front type of rain activity and there were yellow areas it would not give me a great concern. If on the other hand it was a front with convective activity a yellow return would scare the blank out of me! The delay in weather reporting could mean that yellow return is something very different when you get there. That does not even include what might happen with a lighting strike.
I remember looking at the weather on the day Pierre was flying and I think it was just a big warm front so I would have had no problems flying the route he did.

George
 
Although flying into radar reds in a light plane is not fun and no one does it on purpose (hopefully!!!!), I think it is a great idea to know all there is to know about the behavior of the engine when flying in such very wet conditions.
that should give an idea of what to expect in case it ever happens. It can only help!
My 2 cents.
 
gongreg said:
....... I think it is a great idea to know all there is to know about the behavior of the engine when flying in such very wet conditions.....
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Agree, and that was the point of the water hose suggestion; one test is worth 50 opinions. If spraying the prop is a bad idea, there are surely ways to rig a "water spray introduction device" directly to the air intake.

Maybe this....a length of vinyl tubing routed in via the cowl outlet, with a short length of crimped brass tube stuck in the end as a spray nozzle. Tuck the brass end under the rubber flap seal at the airbox entry. Calibrate water flow to some rough standard before installation, as in some degree of valve opening equals some quantity of water in a given time.

9v9j.jpg


My own airbox was designed with an automatic alternate air door downstream of the K&N filter. It opens at a differential pressure of 3" H2O or less (0.2 Hg) and takes in hot air from the lower cowl volume. When it opens it lights an LED on my switch panel to let me know. I do not fly IFR, and with this system I don't worry about the level of rain which still allows easy VFR. If the filter becomes saturated with rain (or even ice) I expect a small power loss and a light to tell me why.
 
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I wonder if a one off test would cause all that damage to the prop. Maybe the life of a hard working float equiped DHC Beaver is real hard on the prop, but spraying it once for the purpose of this test... should it cause all that erosion, if any?
 
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I was coming back from Canada one summer when at 8000 I was in the soup and it was raining pretty heavy. Everything was going along fine until I started to see my indicated airspeed dropping. It went from 145 Knots at cruise to as low as 105, all with the same power setting. At this point I was starting to worry and was about to pull the Alt Air figuring the air was being blocked by the rain, but since it is only resettable on the ground I instead ask ATC for higher and after about 10 minutes that was granted and I climbed up to 11,000 and then was out of the rain. It then took about 20 minutes for the filter to dry out and my airspeed took about 20 minutes to slowly recover back to 145 Kts.
 
Here is the response from KN filters. It sounds like they have a product to address this. I haven't looked yet at which one and how to obtain


Bill

We do test our filters in 1.5 inches of water. But we don’t actually test them in actual weather conditions. As long as the entire filter isn’t submerged you should be fine. The only time in which an air filter will collapse is when someone puts our filter directly onto a turbo or they do not have a large enough filter for their engine. (Custom builds)

All K&N systems which relocate the filter outside of the engine compartment include a K&N Drycharger filter wrap. This filter wrap is water repellant up to 1 year, and must be used with the K&N system when the filter is moved outside the engine compartment, or vehicle/engine damage may occur.

If you decide on using a water bypass anyway, please be aware that it will void the K&N Million Mile Warranty on the intake system.

If you have additional questions, please reply to this email or contact our Customer Support department at 1(800)858-3333.

Thanks for writing,
 
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I have a K&N filter in my motorcycle and I have to wrap the filter in plastic bag when I wash the bike otherwise it will not start till the filter dries. I have been surprised that even when I try to avoid the filter box, it will have this effect unless I completed protect it. Having said that, I was caught in a very heavy rain once and did not notice any drop in RPM or performance, only chipped paint on my leading edge and wheel pants.
 
gongreg said:
I wonder if a one off test would cause all that damage to the prop. Maybe the life of a hard working float equiped DHC Beaver is real hard on the prop, but spraying it once for the purpose of this test... should it cause all that erosion, if any?
Click to expand...

If I were doing the test on an airplane I ownd, I'd do what Dan H suggests. Spray water directly into the intake, behind the prop.

No, a one off test isn't going to cause huge damage to the prop, but it is vastly accelerated wear. Think about it this way, a 72" prop spinning 1500rpm has the prop tips going 321mph. Do your runup at 1700rpm and they're doing almost half the speed of sound. Water is pretty hard when it hits that fast.

Also, spraying a garden hose on low into a prop is a lot more water than flying through heavy rain. It will fill a 4' diameter dog bath a lot quicker than a thunder storm.
 
Great!

Thanks for posting K&N's response, BTW.

Y'know, I've been giving this subject some thought and have never read about these filters collapsing or having an engine quit because of rain. The water will only impact the front of the filter in rain and the rest of the air can go around and enter the sides and back side of the filter, so even if the front is pretty drenched, there's a lot more area available.

Best,
 
pierre smith said:
Thanks for posting K&N's response, BTW.

Y'know, I've been giving this subject some thought and have never read about these filters collapsing or having an engine quit because of rain. The water will only impact the front of the filter in rain and the rest of the air can go around and enter the sides and back side of the filter, so even if the front is pretty drenched, there's a lot more area available.

Best,
Click to expand...

The 2 place RV with the IO360 cowl though has a "relatively" small flat square filter with a frontal area that would be directly impacted during rain penetration.
 
Saber25 said:
With all due respect, why fly in moderate to heavy rain or for that matter a red radar return in a single engine airplane? Moderate to heavy rain can lead to bigger problems quickly. When getting paid to fly in such conditions the Ops manual for a jet advised caution and circumnavigate red returns.
Click to expand...

I agree with you for the most part. But, having extensively used radar, XM weather and ADSB weather, the "RED" returns are all different.

I have flown through XM and ADSB red returns and never had a drop of water hit the airplane. That's because they are time sensitive. Radar, I believe it.

Rest assured I'm not advocating flying "through" real heavy rain, but as we used to say flying in real bad weather, "one peek is worth a thousand sweeps".

Look out the windows!
 
?The 2 place RV with the IO360 cowl though has a "relatively" small flat square filter with a frontal area that would be directly impacted during rain penetration.?
This is part of why I went with Rod Bowers setup and well the forced air. I did not really care for Vans IO setup
 
One would imagine that with the IO360 horizontal induction little water will hit the air filter since the filter profile (face) is not in the direct airstream. Would that be a fair assumption?
 
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