Fast cruise in a Cessna 152
 

Is there any harm in cruising a Cessna 152 at 100 knots apart from burning up a lot of fuel? I was taught to cruise at 90 but a year or so ago an instructor told me to speed up to 100 to "save time"?

Does anyone know what the fuel efficiency difference is?

Molesworth 1, you have just redefined 'oxymoron.'

I suggest consulting the POH (or googling for a POH if you don't have one to hand). It will answer everything you've asked.

NR - Ok I'll rename it to "Faster than normal cruise in a Cessna 152" :cool:

POH says VNO is 108. So it's fine and obviously in the green arc.

At sea level we have 4 hours endurance at 65% power and just under 3.5 hours at 75% power. So how do you know when you are using 75% power?

RPM and density altitude. Again, there's a power setting table in the POH :ok:

If you look in the Performance section of the POH (probably chapter 5) you'll find a table linking altitude, temperature and engine RPM to percentage power. For example, at 2000ft and standard temperature 2400 RPM will give you 75% power, and 101 kts IAS.

Yes, I see it. Hmm.. interesting. So if you're on a relatively short trip and you're paying by the hour and not for the fuel, then it makes no sense trundling along at 90 knots?

Indeed! At our club you pay by the Tacho hour, or the actual flight time, whichever is higher...the trick is to run the engine as fast as possible without getting the tacho time over the actual flight time. It's a fine balancing act :ok:

No wonder the flying club in no hurry to show me the POH!

Molesworth, please do yourself a favour and read the manual from cover to cover. But to get back to your question, here's the powersetting table from a C152 POH (check if it matches your aircraft before using!):

http://www.vc10.net/div/C152_powersettings.jpg

If you're going cross-country find the row for the pressure altitude that you'll be flying at and the column for the temperature. Then choose a %power from the '%BHP' column and use the numbers to the right of that. Set that RPM when settled in the cruise and keep it at that number. Keep in mind that you'll need the engine at 'Recommended lean' i.a.w. POH chapter four to get the fuel flow numbers mentioned here, and even then add a percentage to your trip fuel for safety's sake (5-7% would be my recommendation but more fuel is always better as long as you're within the W&B envelope).

Example: For 2000 feet on a standard day, 66% gives you 96 KTAS at 2300 rpm.

J. Thanks - I discovered this page all by myself and not rocket science to work it out!

Methinks you have missed my point. If I'm not on a flight of more than 2 hours there is little point in maximising fuel efficiency. Flying is expensive enough as it is without having to worry about saving a couple of quid off the flying club's fuel bill. They are making good money from me as it is!

SoCal App: You don't get much joy at flying clubs this side of the pond with a request to look at the POH - or any other aircraft documents for that matter. It's a case of sign the tech log - don't take it away - fill it in when you get back. Any problem with the a/c tell us and we'll write it on a sticky note for the maint team.

There is no legal requirement to carry the POH, even when travelling to another country - and I know of no one who does. Please explain - AR(R)OW check?

True, it's not rocket science, and I may have tried to write a 'dummies guide to....' when that was not called for. But as pointed out before a good read of the POH would have answered your question and personally I think it is compulsory reading before setting foot in an aircraft. If the club doesn't let you, then hire somewhere else.

Going back to your original question, indeed you might just as well cruise at 100 kt instead of 90 without doing harm to the aircraft. As you say it is easier to not maximise your fuel efficiency. Two things though: 1. the power settings in that table are those at which, or between which you can/may cruise. That is useful information which defines your playing field. 2. Operating an aircraft efficiently shows good airmanship. That's never a bad thing. You never know when you might find yourself in a situation where you'll need those last drops of fuel and then you'll be glad you saved them.

Edit: Over here the POH is part of the ship's papers, you are required by law to have it on board. I don't have time now to refresh my memory as I cannot remember if that is EU law or Dutch law. Someone with more knowledge than me of UK air law will surely answer that question.

Yes, of course J - I take your point. For me, conserving fuel in an emergency situation would involve flying slowly with a lean a mixture as possible. This is borne out in the table, but goes without saying even without reference to any table.

The table is interesting all the same - thanks!

Stay at 90kts. Simple reason, it makes the maths easy. 90kts is 1.5nm/minute, 100kts is 1.6667Nm/Min - which apart from being infinately harder to work out "at a glance" - isn't enough to make a difference in the great grand scheme of things.

And flying schools - contruary to your belief - don't make a huge ammount of money from you (Unless you're on an integrated course). Margins are very slim...

Just having a re-read of Jeremy Pratt's Air Law book.

He does indeed stress familiarity with the POH for safely and legal reasons. It's strange that flying club's like (and often require) you to read their Standing
Order book but you are never ever offered a full POH. Usually all they show you (if you are persistent) is a ring binder/plastic folder with landing/takeoff and W&B tables.

I thought a 152 had just two throttle settings - open and closed, there is so little difference in between.

How do you do your weight & balance calculation without it? How about calculating required runway length? I like to bring it with me. Better make sure you bring it with you if go to Germany (fast cruise or not) as it has to be onboard the aircraft there.

Anyway, sounds as if a flight is something which you just need to get over and done with in a hurry. Don't you enjoy it?

edmj

I think you will find that most flight school ops dont fit in on the weight and balance data for a 152? and looking at the size of some of the occupants who get in there! not surprised :ugh:

Is there such a thing as a high speed cruise in a 152 :E i thought they only did slow speed to slow medium speed ;)

Pace

Weight & balance sheets are available, generally, as are takeoff and landing tables. Do you work out weight and balance every time you fly? According to the sheets you are outside of limits if you take off in a 152 with two people and a full tank. That would rule out practically every student and instructor flight.

Yes, I do enjoy my flying, thank you. Sometimes I fly much slower than 90 knots, sometimes I fly faster. Sometimes I need that extra speed so that I can get to a destination that otherwise would be outside my budget.

I will remember to bring the POH with me if I should ever get as far as your country - doubtful as that might be in a Cessna 152!

Hands up all those who did their PPLs in the UK in a Cessna 152 and ever laid their eyes on a full POH!

These are about as rare as a Shakespeare first folio. The type is all wonky like it just came off the trial run for the 15th century(?) Gutenberg printer. Reads somewhat like the instructions for operating a washing machine. Did you know for example, that the proper way to ditch a 152 was to descend at 55 knots and land with a level attitude? And that the way to stop a spin is simply to let go of the controls? And if you find yourself in IMC, then fly with your feet?