What is the seaplane lip?

[170C]

Used to slow down Green planes so red ones can keep up

[Bruce Fenstermacher]

Is it possible the larger seaplane lip is intended to keep water spray from getting to the firewall?

Probably not. I did not realize until I got my second 170 what a difference the lower lip makes and my observations have panned out with anecdotal evidence over the years with an occasional anomaly.

My first 170 had a early cowl with the internal pressure box. It was essentially the '52 set up. It ran hot like most 170s. Remember most 170s where produced '52 and later. They all had a 3/4" lip. My second 170A runs cold. It does not have the full 3" lip it should have, it's been modified closer to 2". When I tried to figure out why my second 170 ran so cold was when I started to discover that most 170s with the larger lip run cold and most of them are '48 and 170A.

I believe the larger "Seaplane" lip was an effort to draw more cooling air through the cowl since the engine would most certainly be operated at a higher power setting yet because of drag the airframe would be moving slower through the air.

[DaveF]

Thanks Bruce! My '54 Avcon runs high CHTs and I'm considering extending the 3/4" lip. Sounds like that could give me some relief fairly easily. I'd happily give up a few mph at the top end to bring my CHTs down.

When you say "runs hot" or "runs cold", do you mean CHT? Oil temp?

[blueldr]

I would assume it would be oil temperature since CHT instrumentation on a C-170 is all "after market" equipment.

[Bruce Fenstermacher]

Oil temperature is the only temperature gauge I have. If your pulling more air through the cowl you have to lower the CHT as well.

My current airplane with the 2"+ lip barely gets over 200 on the warmest days when a '52 with the shorter lip but otherwise the same would be 225 degrees.

If I were you I'd try a temporary extension from 3/4 to say 2" and see if it makes a difference.

[minton]

Higher cruise power settings/longer take offer power settings == higher CHT's. That's the reason for the 3 inch lip. It induces a higher negative pressure behind the lip== more airflow though engine compartment. Has nothing to do with protecting the firewall. If you go back and forth between you might consider installing nut plates on the 3 inch lip.

[GAHorn]

The "seaplane" lip was intended to create a greater low-pressure area to facilitate increased airflow through the cowling for increased engine cooling.
The seaplane operates at higher power settings in conjunction with lower airspeeds, which created higher engine temps. The "seaplane" lip was intended to alleviate that somewhat.

[Bruce Fenstermacher]

Well I think between Gahorn, Minton and my posts, we pretty much have this covered, saying the same thing three different ways.

[GAHorn]

The seaplane lip does not make much difference in the reports I've received. After consideration of all the possible variables, I don't think it's possible to predict what the effect will have on any particular airplane. A greater/more predictable improvement will likely be realized by tuning up the cooling baffles, air boxes, and exhaust flanges/risers to improve cooling and remove heat sources. IMO.
In addition, any increase in lip size and/or air flow thru the cowl by such mod will actually introduce additional drag. I would not choose that route at all.

[Green Bean]

Water taxiing and takeoffs requires relatively high power settings for long periods and this is often the most critical condition for cooling systems for float aircraft.

Getting air out of the cowling is often the factor limiting how much air can be pushed through the engine and how well it is cooled. Increased duct size (scooping more air through the engine) may be necessary. The shape of the outlet of the cowls controls how effectively air is sucked out of the cowling and is arguably the single most important aspect of cowling design.

A rule of thumb, is the cowl outlet area should be at least 3 times the combined area of all the cowl inlets. The small lip added to the rear of the cowls gives a large improvement to pressure differentials and engine cooling, later aircraft use cowl flaps for this purpose.

The pressure difference between the low pressure outlet area of the cowls and the high pressure inlet areas
controls the amount of air flowing through the engine.

So the simple change that Cessna did for getting improved engine cooling for the 170 on floats, was to increase the size of the Lip on the lower cowling.

[minton]

Thanks Bruce! My '54 Avcon runs high CHTs and I'm considering extending the 3/4" lip. Sounds like that could give me some relief fairly easily. I'd happily give up a few mph at the top end to bring my CHTs down.

When you say "runs hot" or "runs cold", do you mean CHT? Oil temp?

We all run across "Gotcha's"

Your cure should be a larger lip. I'd also check gasoline grade, mag timing, baffling fit and give a look at your oil cooler plumbing (See my story below)

Mine could have been huge and costly.

After buying my damaged C-170B/0-360 project and 5 years later before test flying a big problem was noted with hooking up the oil cooler lines to the rear case, 3 threaded holes in the rear case and two holes in the oil cooler. Two options will work and one will not (Being two will push oil through the cooler and into the case as return oil) and one will not circulate hot oil through the cooler at all but act as a return to return, No circulation!. I found mine in the return to return condition. Luckily I had mine torn down for overhaul and new crankshaft so no harm/no foul as the engine was never run in that configuration but the last bozo's did for some 200+ hours. Imagine the oil temp!! The parts manual is vague at best until you establish the pressure port location.

Check your engine to firewall for metal bushings and engine stringer (4) condition (cracks or intergranular corrosion) just for grins .

[dowser]

Does anyone have information on the lip? Part number, size, material, drawing, etc.?

Edit: My question is really directed to Joel -- did you locate the documentation you referred to above?

I would also be very interested in drawings for this 3" lip.
I have a 1953 170B with del-air 180hp conversion on floats.
Oil temps are consistently between 170-195 depending on OAT.
However, I can put the CHT's thru the roof on climb if I do not lessen my angle of attack.
I have installed an EDM700 monitor and my CHT readings are accurate.
Baffling is good as well as fuel grade, etc... The O-360-A2G has approx 550 hrs.
CHTs at 2450RPM cruise are between 360F and 410F depending on cylinder.

Obviously modifying the air intake is limited so I would be interested in exploring the 'lip vacuum' theory.

[Bruce Fenstermacher]

Not much to it. Extend your 3/4" lip to 3". Look at a '48 or A model.

[minton]

Sounds about what I've seen on my C-170B/0-360-A1A Del-Air. Wouldn't worry but be sure to keep it in your scan! Seaplane drivers have to deal with it most of the time.