1976 C-172M speed

Flyingfanatic

Flyingfanatic

Pre-takeoff checklist
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Flying Fanatic
All,

I regularly fly a 1976 C-172M. This airplane will only run about 95 KIAS at 2500-2550 RPM. It seems to be well rigged and flies straight.
It has a climb prop on it, but 95 KIAS seems way slow for that RPM. I've confirmed the airspeed is at least close by checking ground speeds over short distance and longer distances on a cross country.
The RPM gauge seems to be correct too just by my estimation.

Would a climb prop really affect the speed that much? I don't recall speeds of other 172's I've flown. The other one I fly is a 1997 model with a different engine.

Just wondering why this airplane is so slow. The book shows 110 kts for the same conditions. I know I shouldn't expect book numbers but
I can't imagine it is that far off.
 
That seems slow. The M model I fly does about 100 at 2500 without wheel pants. Probably the climb prop, you trade climb performance for top speed.
 
Gad, that is SLOW! I used to see 90 knots at the bottom of green arc on the tach, with 10 degrees of flaps deployed.
 
Do a check of your TAS on your next trip. If the airspeed indicator doesn't have a TAS converter, you can do this on an E6B or with an app. If the TAS is normal, you may have a leaky static system. If the TAS is slow, something else is going on.

Do you know what make and model of prop is installed on the airplane?
 
The first and easiest thing to check is the full-throttle static RPM with the brakes locked, as close to sea level and 59°F as possible, and compare it to the book. Or if you can't find a standard day at sea level, compare it to a stock 172M side by side.

---

Consider that a 172M with a climb prop compared to stock is analogous the difference between an S model 172 and R model 172.

The 180hp 172S and 160hp 172R have the same engine, but different propellers, with the "S" essentially having a climb prop and the "R" having a cruise prop.

The "S" model cruises 15 knots slower at the same RPM, or needs about 250RPM more to go the same speed. Likewise, the static RPM of the "S" is almost 250RPM higher.

(This might sound backward until you realize that the "S" can still cruise faster despite having a climb prop because it has a 300RPM higher redline.)
 
G-force said:
That seems slow. The M model I fly does about 100 at 2500 without wheel pants. Probably the climb prop, you trade climb performance for top speed.
Click to expand...
I don't feel it's much of a trade. Lol. It doesn't climb much better than others I've flown.
 
asicer said:
What OAT and altimeter? The KIAS/KTAS relationship changes as DA goes up.
Click to expand...
It was ISA +10 and 2500-3500 feet iirc. Checked against the grpundspeed, I don't think it's the AS I.
 
Harold Rutila said:
Do a check of your TAS on your next trip. If the airspeed indicator doesn't have a TAS converter, you can do this on an E6B or with an app. If the TAS is normal, you may have a leaky static system. If the TAS is slow, something else is going on.

Do you know what make and model of prop is installed on the airplane?
Click to expand...

I sure dont. I'll check it, but judging by our groundspeed,the ASI can't be off much.
 
The 172 is a pretty draggy airframe to begin with, but 95kts does seem a bit slow, even with a climb prop. I've flown an M and if IIRC it averages around 105, but it has a cruise prop. If you get a 15kt headwind, you might as well drive.
 
Two things: go here http://www.airweb.faa.gov/Regulator...3357f6c86257ea4004e8ae6/$FILE/3A12_Rev_84.pdf and scroll down to the 172M and look at the four different props the airplane was certified with. If the prop is not one of those, there must be an STC for the one on it. If there's no STC, you have an illegal airplane.

The static RPM ranges are given for each of the props. Use an optical tach checker and see if the tach is accurate, and see what max static RPM is. Tachs, by the way, age and their internal magnets lose their strength and they mostly start underreading. What you think is 2500 might actually be 2600 or more. In Canada we have to test that tach every year and it can't be more than 4% off in the middle of the cruise range. The law applies to the mechancial drag cup type tachs, so the electronic tachs are exempt. They don't underread when they get old, see?
 
The only other issue with it is hail damage from years ago. I'm curious if that is contributing to the slowness.
 
Flyingfanatic said:
The RPM gauge seems to be correct too just by my estimation.
Click to expand...

My first step would be to verify this with an optical tach. Check around - It is likely that a local AME has one you can borrow. It might cost you a box of doughnuts!

-Skip
 
asicer said:
Are you using the GPS 4 heading method when you check against GS?
Click to expand...
No, I haven't done that yet. This is just a comparison of GPS vs. IAS on a couple of cross country flights
 
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