c172 sp cruise speed

[wood73]

Hi
I'm currently training in a c172sp.
Everywhere I read the cruise speed is 124kts @ 75% power, however I have been taught to fly at 100 kts, which equates to 2300 rpm.
I'm guessing there is a good reason for this i.e. fuel economy or engine life, but just wondering what speed other people cruise at in a similar aircraft.

[O_K_]

75% power is the maximum recommended power in cruice, and is a fairly high power setting. It puts you well above Va, and very close to Vno. Controll inputs should be gentle and small at these speeds, and as you mentioned the fuel consumption will be high.

Power and speed depneds very much on the purpose of flight. If you are just flying around to gather hours for your PPL, or to show off the birds view of the area to some friends 100kts (lower end of green arc on the RPMs) is perfect. Fuel consumption is lower so the cost per hour in the air is lower, and you are closer or at (depending on weight) to the Va, so there is less chance of overstressing the airframe if any sudden or abrupt controll input is needed.

If the purpose is to get from A-B then more speed can be nice, but it will cost you (or your club) in fuel, which eventually will hit the hourly rates.

[27/09]

O_K_75% power is the maximum recommended power in cruice, and is a fairly high power setting. It puts you well above Va, and very close to Vno.

I'm not so sure about some of what you say. Who has made the max cruise at 75% power recommendation?

The Lycoming engines in the C172 (either O320 or IO360) so far as I know according to Mr Lycomings book are good for Full throttle 2700 RPM all day. i.e. 100% power. I wouldn't want to pay the fuel bill mind you.

75% is a pretty normal power setting.

There's absolutely nothing wrong with cruising above Va. In fact most light aircraft I can think of cruise above Va. The wording if I recall correctly is no full and abrupt control inputs above Va. This still means normal control inputs are OK.

However as you allude to, the speed/power setting being used by the OP is most likely to do with saving money, i.e.fuel.

wood73

The 124 kts you mention is probably true airspeed (TAS) not indicated (IAS). Only at sea level on a standard day are TAS and IAS the same.

For a 75% power setting most non turbo charged light aircraft will achieve their best TAS at around 6000 to 8000 feet. On a standard day an IAS of 100 kts at 8000 feet will give a TAS of about 113 KTAS. For your 124 KTAS you would need to indicate 110 KIAS

[ChickenHouse]

Who has made the max cruise at 75% power recommendation?

Cessna did.

POH SECTION 5
Maximum cruise is normally limited to 75% power.

[A and C]

For simplicity and SOP flying clubs normally use a standard power setting for the teaching of the normal flying. This setting is used regardless of conditions to simplify things for the student and in the early stages of flying this practice is no bad thing but sticking to " flying club standard " is not good post PPL.


A long look in the flight manual will give you a range of power settings and the RPM to set to get the % power setting for differing altitudes.

Along with this are graphs to show the fuel consumption and speed you can expect from the settings.

This is not something that can be covered by posts on an Internet forum but I hope that mine and the very useful posts above will point you in the right direction to find the information in the flight manual that you need.

[Cows getting bigger]

100kts? They're just trying to save fuel, which isn't a bad thing.

A 172SP feels best at about 115-120kts (this being a bit broad brush but representative of a standard UK day at about 3000ft).

[piperboy84]

I'd hazard a guess and say the reason for the 75% power for cruise is that is about the highest power precentate Lycoming recommends leaning at , so 75% and under while cruizing allows to lean right back for best engine performance and economy.