Donaldson Air Filters

[PT20J]

It seems that a lot of folks on BeechTalk have switched from Brackett to Donaldson air filters. It avoids the sticky Brackett goo and some are claiming higher MAP on takeoff and better filtration. Any pireps from Mooney owners?

[EricJ]

1 hour ago, PT20J said:

It seems that a lot of folks on BeechTalk have switched from Brackett to Donaldson air filters. It avoids the sticky Brackett goo and some are claiming higher MAP on takeoff and better filtration. Any pireps from Mooney owners?

I switched about a year and a half ago just because I was beyond tired of the sticky goo from the Brackett.   I think it also may have been a contributing factor to fuel-servo issues, since that crap can go in the ram air tubes and has nowhere to go to get out.   FWIW, everything around the air box stays nice and clean now, and the filter is easy to handle and inspect and clean.   And because you don't need to change it every year, it doesn't seem like it really costs any more to own.   I'm still on the same filter and it still looks great.

I did have to adjust idle a little bit, which suggested it was breathing a little easier, but that may have been coincidental.  It always seems to me that there's enough operational variance due to temperature, pressure, humidity, etc., that any improvement due to the filter is probably in the noise to where I haven't noticed it, but I haven't been looking, either.

 

[PT20J]

27 minutes ago, EricJ said:

I switched about a year and a half ago just because I was beyond tired of the sticky goo from the Brackett.   I think it also may have been a contributing factor to fuel-servo issues, since that crap can go in the ram air tubes and has nowhere to go to get out.   FWIW, everything around the air box stays nice and clean now, and the filter is easy to handle and inspect and clean.   And because you don't need to change it every year, it doesn't seem like it really costs any more to own.   I'm still on the same filter and it still looks great.

I did have to adjust idle a little bit, which suggested it was breathing a little easier, but that may have been coincidental.  It always seems to me that there's enough operational variance due to temperature, pressure, humidity, etc., that any improvement due to the filter is probably in the noise to where I haven't noticed it, but I haven't been looking, either.

 

What did you use to remove the goo from the intake duct?

[EricJ]

4 minutes ago, PT20J said:

What did you use to remove the goo from the intact duct?

I think mineral spirits or some similar solvent.   

[DCarlton]

Got a recommended link to buy the filters for the F?  What about K&N?

https://www.knots2u.net/challenger-air-filter-high-performance-cp-1176/

https://www.donaldsonaerospace-defense.com/library/files/documents/pdfs/007217.pdf

[Marc_B]

I've had a Donaldson air filter and no complaints so far for my M20K.  I previously came across the comparison @DCarlton linked above.  I usually take advertising with a grain of salt, but at least their results support my choice in filter.  FWIW, my silicon trends on Blackstone are lower than universal averages...but not sure how that correlates or not?

[EricJ]

That Donaldson link is interesting.  About twenty years ago when I was very actively racing cars and helping friends with their sand rails, etc., there was a lot of discussion and controversy around cotton gauze (e.g., K&N) filters, paper, foam, using no filter, etc.     A buddy with a sand rail that spent a lot of time in the Imperial Sand Dunes was observing a ton of dust on the interior of his intake past his K&N filter.   About that time people were figuring out that you needed to run a foam cylinder over the top of the K&N to reduce the dust, so he started doing that.   There was less dust, but still a ton of dust getting past the filter.   People were doing dyno runs with back to back filter changes, no filter, etc., and for power production no filter tended to be best, then a K&N, then a paper filter, but the differences were typically very small.

So I had a digital microscope, which was a Big Deal back then, and took pics of a K&N and a paper filter, and a shop towel.   I never had any desire to use an automotive K&N after that, because it was obvious that the reason it made a little more power was because it didn't filter very well, there were huge holes in it everywhere.   You could hold one up to a light and see them really easily, and it was clearly the reason the sand rail guys were not getting decent filtering.    Some of them switched to paper filters and after that if there was dust in the intake it was because there was a leak somewhere.

The pics in the Donaldson document linked above reminded me of all this, because I'd come to essentially the same conclusion back then.

FWIW, I have zero experience with the K&N filters used on airplanes and I don't know how they compare to the automotive/aftermarket filters we were using twenty years ago.

Twenty-year-old automotive K&N microscope pics:

http://ericjacobsen.org/sho/KN_60x_tl3.jpg     Top-lit.
http://ericjacobsen.org/sho/KN_60x_bl.jpg      Same, but back-lit.

Paper filter, same magnification.   Couldn't back-light it because nothing came through:

http://ericjacobsen.org/sho/Paper_60x.jpg

Blue shop towel, just for fun, same magnification.   The holes are a lot smaller than the K&N:

http://ericjacobsen.org/sho/Shoptowel_60x.jpg

Anyway, for me the take-away was that it's hard to beat a paper filter for actual filtration.    We don't do dyno comparisons on airplanes, so there's no social status to be had by the extra 2hp you might make (or not make) with a less effective filter.   

[A64Pilot]

Silicone is the indicator for dirt, but comparing to others is irrelevant you may just operate in a cleaner environment than average.

Now if you had before and after numbers and operated the same then that might be meaningful.

I’ve got some experience with filters, mostly K&N, pretty much by definition you can’t get both better filtration and higher flow, that goes for any filter, oil, hydraulic, fuel and air.

On edit, as posted above the K&N is actually a poor filter, better than nothing but for a recip engine it’s not so good, a turbine is more tolerant for dirt and less tolerant for restriction, Thrush long ago went K&N on their turbines, our competitor went with large Donaldson truck filters and suffered significant performance loss from that, but remember turbines move huge volumes of air and are more dirt tolerant.

On the AH-64A we went with K&N filters for the Air pac airconditioner for desert use, originally no filter so the K&N was better than nothing.

From a dirt motorcycle experience oil soaked foam actually filters pretty well, but again I can almost promise you that you don’t get less restriction (higher MP) AND better filtration, better as defined as trapping smaller particles.

 

[Pinecone]

The deal with the K&N is it filters better with a coat of dust to fill up the holes.  But that increases the air loss.

One friend of mine just used a new paper filter for every race weekend.  His testing was it flowed almost as much while filtering better, but as it got used and dust loaded, the flow went down a good bit.

[A64Pilot]

7 minutes ago, Pinecone said:

The deal with the K&N is it filters better with a coat of dust to fill up the holes.  But that increases the air loss.

One friend of mine just used a new paper filter for every race weekend.  His testing was it flowed almost as much while filtering better, but as it got used and dust loaded, the flow went down a good bit.

Yes, the best way to operate a K&N is to once it starts to look dry, to leave the dirt there and re-oil the filter, it then will filter better but as you note flow decreases.

TANSTAAFL

‘Oddly enough back from my motorcycle drag racing days from porting and polishing Kawasaki Z-1 heads, the K&N cone filter flowed slightly better than an open velocity stack on a flow bench, only way I can explain that is the velocity stack must have not been designed well.

[A64Pilot]

Oh, and you actually can get better filtration and better flow, by fitting significantly larger as in area filter, but as we can’t it’s one or the other.

‘Aircraft are a lot like boats, most of the time we operate is pretty clean environments, unless you fly off of a dirt strip or something it’s not that big of a deal usually.

[GeeBee]

1 hour ago, A64Pilot said:

I’ve got some experience with filters, mostly K&N, pretty much by definition you can’t get both better filtration and higher flow, that goes for any filter, oil, hydraulic, fuel and air.

And that folks is the bottom line with element filters. You only get one choose wisely.

 

[DCarlton]

Without consideration to flow, are the original bracket filters a good filter ?  Haven’t found a Donaldson for the F yet.  

[Marc_B]

2 hours ago, A64Pilot said:

Now if you had before and after numbers and operated the same then that might be meaningful.

Unfortunately it came with a Donaldson filter and I replaced with the same when the time came for replacement.

[A64Pilot]

1 hour ago, DCarlton said:

Without consideration to flow, are the original bracket filters a good filter ?  Haven’t found a Donaldson for the F yet.  

I haven’t seen a bad filter, I’m not a fan of K&N as others have posted when you take one out of your car for cleaning you notice dirt on the clean side of the filter in the air box, and you don’t with an OEM paper one, but does that mean it’s bad in an airplane? I don’t know.

My belief is if you don’t see prop erosion your in a pretty clean environment.

Every airplane that has carburetor heat that I’ve seen bypasses the filter when the heat is on and of course if you have ram air and use it, your bypassing the filter too.

I think people like me that fly off of grass strips probably need a filter more than most, I don’t go full throttle until I pass through 20 kts, yet I’m getting significant prop erosion from the Fl sand, so I know I’m getting sand in my air filter, I’m using the brackett because that’s what’s there and have no plans on changing. I don’t see dust on the clean side.

People that do oil analysis will get a better idea from silicone readings, if you do and silicone is high I’d first look for leaks and if you can’t find any sure pop for a paper filter, it’s likely there is a reason why just about every auto and truck farm tractor etc made comes with one.

I’ve wondered how can they work when wet? I’ve assumed without looking it’s not paper but some fiber or something?

[Fly Boomer]

On 12/23/2023 at 11:36 AM, EricJ said:

So I had a digital microscope, which was a Big Deal back then, and took pics of a K&N and a paper filter, and a shop towel.   I never had any desire to use an automotive K&N after that, because it was obvious that the reason it made a little more power was because it didn't filter very well, there were huge holes in it everywhere.   You could hold one up to a light and see them really easily, and it was clearly the reason the sand rail guys were not getting decent filtering.    Some of them switched to paper filters and after that if there was dust in the intake it was because there was a leak somewhere.

I’m suspicious of any filter that flows more air (increasing manifold pressure).

[A64Pilot]

40 minutes ago, Fly Boomer said:

I’m suspicious of any filter that flows more air (increasing manifold pressure).

Do an experiment, take your filter out and run it up on clean pavement, note your MP, then install the filter, repeat run-up.

The difference in MP as long as your filter is clean is difficult to determine it’s so slight, so how can a K&N or any other filter add any significant MP if no filter at all doesn’t?

Older J models had ram air, which of course both bypassed the filter and was in fact ram air, the performance difference was so slight Mooney discontinued the ram air, and Mooney wasn’t the type of company to decrease performance which of course is cruise speed.

So how can just a filter make a significant difference if no filter plus ram air wasn’t worth it?

 

[Fly Boomer]

9 minutes ago, A64Pilot said:

Do an experiment, take your filter out and run it up on clean pavement, note your MP, then install the filter, repeat run-up.

The difference in MP as long as your filter is clean is difficult to determine it’s so slight, so how can a K&N or any other filter add any significant MP if no filter at all doesn’t?

Older J models had ram air, which of course both bypassed the filter and was in fact ram air, the performance difference was so slight Mooney discontinued the ram air, and Mooney wasn’t the type of company to decrease performance which of course is cruise speed.

So how can just a filter make a significant difference if no filter plus ram air wasn’t worth it?

 

If the “filter” makes no difference, it’s not filtering.

[Pinecone]

I think his point was, the difference in not significant and may be too small to see on your MP gauge.

[DCarlton]

This discussion reminds me of the debates as to whether masks were/are effective in preventing the spread of viral respiratory infections.  Paper, cloth, N95...  yes no.  Same problem; different intake system.