The Cessna 120-140 Association Forum

I need some ideas on what might be an issue. I went out to the airport yesterday to do some touch and go's. I aborted the take-off because I didn't feel the engine was making full power. Here are the details as best I can relate them.
Airport elevation: 4500
Density altitude: 5050
Outside air temp: 70°F
Winds calm
Relative humidity 100% Intermittently sprinkling but really just misting mostly.
Left tank 7/8; right tank full
Pilot: 200lbs.
Just had an annual 1 month ago and the compression's were 79,79,76,79. Air filter was replaced.
I have a cruise prop.

Here at E38 we have a crossing runway at mid field. On a hot dry day, I normally lift off just before I reach the crossing runway. In fact, I try hard to do this because it is a bit of a bump and that's a bad time to hit a bump.
So during my run-up, The mag check and carb heat was fine and the engine sounded perfect. She fired right up after a couple of revolution, so I have no indications of anything amiss.

Yesterday, I took the runway on the roll and applied full power. I admittedly wasn't paying as much attention to the airspeed indicator as I usually do, because I was really concentrating on how the plane was tracking down the runway. Long story, but basically I was trying to see if I could stay perfectly on the center-line since I wasn't happy with my last results. I did glance at the oil pressure once as I recall. As I rolled towards the crossing runway, I lifted the tail-wheel as normal. As I started getting closer to the center, I instinctively started pulling back on the yoke but the plane wasn't ready to fly yet. I didn't want to take my eyes off the runway because of the bump I was about to hit. But I remember thinking that the wet pavement was probably the culprit. Perhaps the water on the wings. So the bump kind of launched me ever so lightly into the air, but then when I settled back down and had the plane under control, I looked at the RPM's. I was at 2200. I had my hand on the throttle, so I know she was full forward. I kept the plane centered and kept trying to lift off but after about another 100 to 150 ft. I started to worry something was wrong. I aborted and taxi'd back to the ramp. I did another run-up and the mags were fine. I thought about things for a bit and decided I would try again but this time with just a small amount of carb heat and slightly lean. I used the settings that gave me the highest RPM at static. About 2200. Everything on the second attempt went about identical to the first. I taxi'd back to my hanger so I could check things out. I looked at the throttle cable and verified it was on the stops at full powers. I pulled the prop through and verified compression on all cylinders and it felt pretty normal and even. No obstruction at the air intake. Brakes are not sticking.

So, here I am wondering what's wrong, if anything! What else should I check before an attempt on a dry day because that is all I can see that is different? Sorry that I did not look at the airspeed indicator. My brain was going in a lot of different direction thinking of a mechanical issue, but I'm not seeing it. HELP! BTW, I will look in the logbooks and see what the static RPM was at annual, but my thinking now is that 2200 is in spec. but i'll check on that.

C85 or O-200?

If you have 2200 RPM static then you should see a lot more than that on your take-off run. The higher your airspeed the lower the effective pitch of the prop is, so the engine should accelerate when the plane accelerates.

That said, you're not going to get full power at DA 5500. At 2200 RPM you're down to about 65% power. Factor in the 3% loss per 1000 feet and you're close to 55%. For a C85 that's about 46 hp...

Leaning with a little carb heat may be the problem. One counteracts the other. Also, as the engine and exhaust heat up the carb heat increases and richens the mixture. If you have a Stromberg as the rpm increases the engine starves for air, with the mixture applied.

6298 wrote: ↑Mon Jun 18, 2018 4:06 pm Leaning with a little carb heat may be the problem.

Yes! I missed that. Apply the carb heat before your takeoff run, but take it off, then lean for maximum RPM, but only of you get an increase, just before you release the brakes.

I think you were just on the "hairy edge" with environmental , etc.

"100% humidity, "misty" - smaller droplets of water easier/quicker to solidify with cooling effect of venturi. Possibly not enough to restrict air flow but not getting full benefit of combustion due to heat required( 80 calories per gram of water) to melt droplets. I know, I know, kinda far out! Be surprised if normal operations don't return on dry day. 2c

8170 wrote: ↑Mon Jun 18, 2018 1:27 pmC85 or O-200?

Sorry for the omission. C85

6298 wrote: ↑Mon Jun 18, 2018 4:06 pm Leaning with a little carb heat may be the problem. One counteracts the other. Also, as the engine and exhaust heat up the carb heat increases and richens the mixture. If you have a Stromberg as the rpm increases the engine starves for air, with the mixture applied.

I only leaned and applied carb heat on the second attempt. I wanted to rule out ice. The results were identical to the first run. I re-read the POH and it states that 2200 is within range for the metal prop. I also went back and looked in the logbooks and could not find the static RPM on the last annual noted.

Because i have never flown this plane in anything other than very dry conditions, I'm really wondering if the 100% humidity was causing the issue. I thought higher humidity increased power. Would the drag from the wheels and the wet wing surfaces cause this? Definately a learning experience for me if this is the case...

Moisture in the air degrades performance. First, the water vapor displaces some of the oxygen, and second, humid air is less dense than dry air. Double whammy...

Your static RPM should not vary much in response to changing conditions. All the things that reduce engine output also tend to decrease the "bite" of the prop, and vice versa: colder, dryer air increases power output and also increases propeller load.

6643 wrote: ↑Tue Jun 19, 2018 3:21 pm Moisture in the air degrades performance. First, the water vapor displaces some of the oxygen, and second, humid air is less dense than dry air. Double whammy...

You are spot on, and I had it backwards. Yes, of course humid air is less dense. This is no doubt the cause. It pays to keep this fresh in ones mind, but since almost all of my flying is in dry desert conditions. I admit I didn't foresee this happening and should have been prepared. Lesson learned!