Weak induction link.

Richard · #1 10-09-03, 01:30 AM

It's been a little over a year now. I've tore down my Skymaster to the metal. Replaced all wiring and 80% of all hard lines, softlines. I feel I know the 67 models pretty good by now, but I am by no means a professional Skymaster guy. I've noticed a weak link to the Skymaster though. Maybe I'm wrong, but the air filters seem a little small. The engines themselves only flow about 350cfm. This is giving it a 100 VME rating. So realistically at cruise maybe 275cfm. Even with this number in mind. The air filters still seem extremely small for that kind of flow. Most small diesel engines have larger effective service area than that. I've looked at the Bracket filters. They seem to have a good idea, but the counter for the large bracing is by large voids in the filter material. Second, the aux doors don't seem to make a perfect seal. Third, even in the newer Skymasters, they seem to have a few holes in the induction. On most engines it takes approx. 6oz of dirty to destroy the engine. Seems to me that VME could be increased and icing could be reduced by increasing the surface area of the filters. Not to mention the reduced risk of dust in the engine. Am I off?

WebMaster · #2 10-09-03, 09:58 AM

I think you are right on.
I had a filter get a little bit clogged, and engine performance went right down the toilet. Of course this was noticeable at take off, when performance was weak.

FRED-E · #3 10-09-03, 08:06 PM

Are we talking about the OLD PAPER filter or the newer SPONG filter ? The new spong type may handle more voluum.
Fred N358

Richard · #4 10-10-03, 12:23 AM

The new sponge or foam is better for volume (not avail on the front engine(at least I thought not)) but by the design of it. I would think that it would be not so good at traping the dirt. They also put such a LARGE and THICK basket on the thing that it takes away 30+% of the surface area. The naturally asperated models have more surface area on their filters than the turbo models. I don't know if their is anything we can do about this. Maybe that's why I'm so frustrated. My 1.9 liter diesel has a better flow rate than my plane..........

Formula used:

A=CID X RPM / 20839

360 X 3000 / 20839 = 52 inches

In order for the front air filter to be anywhere near 52 inches of surface it would have to be 4 inches tall! Bottom line. It's imposible to flow enough air through our stock filters.

FRED-E · #5 10-10-03, 04:43 PM

Richard

I don't understand the formula, is that 52 sq ins. Please explain.
Fred N358

Ernie Martin · #6 10-10-03, 07:29 PM

I'm very surprised by this thread. I saw it when it started and figured that someone more knowledgeable than me would step in and question the premise before it got too long. Bottom line is this: I don't agree that there is insufficient air-flow area.

I'm saying this without the benefit of calculations, but with lots of experience and some common sense. First, I spent 5 years modifying cars for racing. Have you seen the air filters on 500 cu. in., 6000 RPM engines? They're roughly the size of those on a Skymaster (based on the formula in the thread they'd have to be 3 times bigger!). Second, I'm a mechanical engineer (MS from Caltech) and I can't believe that in, what, 20+ years of building Skymasters, no one at Cessna noticed that the air filter was improperly designed. And there's plenty of room under the cowling, so it isn't lack of space. One final point: isn't this the first time we hear about this? In 5 years of following Skymasters in Peter's website and now Kevin's, no one (to my recollection) has ever brought this up.

Sorry to butt in without a more scientific argument, but I'm not buying.

Ernie

Richard · #7 10-11-03, 01:21 AM

Like I say. I'm not an expert, just a tinkerer. I'm an electrical engineer not a mechanical one. Just looking around at some of designs coming out for other aircraft and noticed they have alot more surface area. Researched some of the formulas that car manuf. are using for their filters and this is what I came up with. Don't know if it's correct or not. Glad someone knows more than me.

Equations say the engines need 52 (approx) sq inches of surface area. Maybe with all the pleets and such their is that much area. I just didn't dare cut the thing open to find out. Those things are expensive now a days.

Kevin McDole · #8 10-12-03, 01:55 AM

Richard,

I think the answer to all of this confusion lies in the definition of a few air filter words:

Peripheral Area: the area of the filter opening. For example, a filter that has a 3" x 10" opening would have a 30" peripheral area.

Surface Area: the area of the filter material - completely unfolded. Typical ratios of Surface Area to Peripheral Area can be in ranges like 5 to 1, up to 15 to 1. - meaning that as much as 15" of Surface Area could exist for each sq in of Peripheral Area.

Active Surface Area: this is the effective surface area of the filter, as corrected for the inefficiencies of the folds and other losses. The ratio of Active Surface Area to Peripheral Surface Area can be in ranges like 4 to 1, up to 5 to 1.

The formula you are using calulates the required Active Surface Area for a given displacement and a specific RPM.

One other thing to take into account in your formula is that “A” needs to me multiplied by 1.2 for paper filters, and 1.4 for foam filters.

Bottom line, if you measure the Peripheral Area of the filter, and multiply it by 4 (just a conservative Active Surface Area ratio), you'll see there's plenty of margin.