Maximum Endurance and How Does A Tach Work?

[aerospacepilot]

Hey,
I am flying in a flying club that charges their aircraft on tach time. I think it is a great idea, afterall, maintenance and fuel burn is directly proportional to tach time. Also, I find I fly much safer when I am being charged on tach time. But I am curious how I can minimize being charged.

How does the tachometer work? Is tach time directly related to RPM's? How is 1 hour of tach time calculated (for say a C-172)?

What speed should I fly at to minimize tach hours and maximize flight hours. My first thought was best L/D, but then I realized that endurance is proportional to Cl^1.5/Cd. How would tach time be related to endurance? Same question with what altitude should I fly at. I know endurance is proportional to density^.5, so max endurance comes from flight at sea level, but is the same true to minimize tach hours?

I am just looking for things I can do to minimize my costs. I know the general concensus would be to pull the power back, but I am interested in specific airspeeds, RPM's, and altitudes (for a C-172) that would minimize tach hours?
Thanks.

[fish314]

I think that your best bet is flying at the max endurance speed, since that is the speed which will allow you to stay aloft the longest with the lowest fuel burn (and therefore I would assume the lowest RPM/throttle setting although that might not be EXACTLY the case).

But if you were trying to actually accomplish anything with that time aloft (like get from point a to point b), then you are probably best off just flying the best range airspeed. In other words, I think that whatever you gained in tach time vs. real time by flying slower you probably would loose out on by the added time that the trip took.

Of course, keep in mind that I'm basically pulling this squarely out of my rear. Haven't actually done any math on it to back it up or anything, but it seems like jumping through hoops to try to be really efficient on tach time would probably just not be worth it.

[tgrayson]

Originally Posted by aerospacepilot

but I am interested in specific airspeeds, RPM's, and altitudes (for a C-172) that would minimize tach hours?

I agree with fish. If you were flying an a/c with a constant speed prop, you could use the lowest RPM setting for a particular percent power, but you don't have that luxury in a C172

You need to choose the lowest RPM that will keep you in level flight. The airspeed will be whatever it will be, but it would likely be in the 60 knot range. Not a pleasant way to drive around.

As for altitudes, endurance is maximized at low altitudes. Power required for every airspeed rises with altitude, because the TAS increases. And power required is Drag * Velocity.

[trafficinsight]

It's pointless, really. The tach hourmeter is usually calibrated to a normal cruise RPM, some are calibrated at full power RPM, but the difference really is miniscule. The best thing you can do is pull it back to a normal cruise power setting and you'll find that that's really the most efficient use of tach time. Lower power settings give slower airspeeds and the flight takes longer.

Take a tach calibrated at 2600 rpm = 15,600 revolutions in .1 (6 minutes). That gives you 120 knots, say, and your 100nm flight takes 50 minutes or .8 on the tach.

Run that engine at 2400 rpm and it takes 6.5 minutes to turn the same 15,600 revolutions, but it only gives you 110 knots and your flight takes 55 minutes... or .9 on the tach .

Run that engine at 2200rpm and you get that 15,600 revs at 7 minutes, and you only get 100 knots, 1.0 on the tach.

[ElyJs]

My understanding is that Cessna Tachs are calibrated based on cruise RPM 2400. Slower on the tach then that and you will get charged less per hour faster you will get charged more. The reason people charge on the tach is to make sure renters give ample warm up time. Also most everyone does there maintenance on tach. Why wouldn't you? It runs slower!

-Jason

[aerospacepilot]

Thanks. I am trying to build 50-60 hours, so I don't mind if it takes slightly longer to get from A to B. If I am alone, I just want to build the time and save a little money. Thanks for the insight. Instead of cruising around at 2500rpm's, I will probably be cruising at 2200rpm's. Thanks.

If anyone knows the exact (or close) RPM setting that the tach is set to in a C-172, could you please let me know?

[mhcasey]

Originally Posted by aerospacepilot

Thanks. I am trying to build 50-60 hours, so I don't mind if it takes slightly longer to get from A to B. If I am alone, I just want to build the time and save a little money. Thanks for the insight. Instead of cruising around at 2500rpm's, I will probably be cruising at 2200rpm's. Thanks.

If anyone knows the exact (or close) RPM setting that the tach is set to in a C-172, could you please let me know?

I held that philosophy once too, and found I was splitting hairs. You will not save much at all, and you will get less and less barbecue at slow airspeeds, as bbq is directly proportional to distance which is directly proportional to airspeed.

That said...the C172's I've flown have not seemed too eager to fly at 2500rpm. I'd compromise at 2300-2400.

[nosehair]

Aerospace pilot, fly it at the bottom of the green arc on the tach.


Isn't it amazing how some folks are warning you how you lose speed and therefore will take you longer so it doesn't save you anything? They still don't get it that your mission is to acumulate time, not distance?

Climb at low rpm until you reach the highest altitude you can enroute, then power back more below green arc and descend the rest of the way.

Over time, you will save a buck or two; and you will learn a great deal about energy management.

[trafficinsight]

Originally Posted by nosehair

Aerospace pilot, fly it at the bottom of the green arc on the tach.


Isn't it amazing how some folks are warning you how you lose speed and therefore will take you longer so it doesn't save you anything? They still don't get it that your mission is to acumulate time, not distance?

Climb at low rpm until you reach the highest altitude you can enroute, then power back more below green arc and descend the rest of the way.

Over time, you will save a buck or two; and you will learn a great deal about energy management.

umm, it seemed to me like he was asking how to minimize cost... if you want to build time fly everywhere at 1900rpm with a notch of flaps down.
To minimize cost get there as fast as you can... 172 wise anyway... In my experience the Arrow is the cheapest way to get anywhere.

[nosehair]

Originally Posted by aerospacepilot

What speed should I fly at to minimize tach hours and maximize flight hours.

...or, "how can I stay in the air the longest at the lowest rpm?"

I am not trying to go anywhere, I am building flight time.

or, if I go at 2400 rpm and get there in 1 hour, and get charged 1 hour, but if I go at 2100 rpm and get there in 1.2 hours but get .9 on the tach, and get charged .9, but get to log 1.2 actual flight time...that's what he is attempting to find out.

I used to do that - back when it was common to charge on the tachometer - before Hobbs meters showed up in trainers.

Actually, sometimes, I would climb the little Cub up to 6 or 7 thousand feet, at full power, but at lower than cruise rpm, over an old abandoned airport in Florida, and pull the mixture, shut down the engine, and glide down to a landing on the old airport. I'd get maybe .3 on the climb, but actually fly .4 or .5.

Yeah, it's nickle-and-dime, ...but when that's all ya got, it worked for me.

[trafficinsight]

Originally Posted by nosehair

...or, "how can I stay in the air the longest at the lowest rpm?"

I am not trying to go anywhere, I am building flight time.

or, if I go at 2400 rpm and get there in 1 hour, and get charged 1 hour, but if I go at 2100 rpm and get there in 1.2 hours but get .9 on the tach, and get charged .9, but get to log 1.2 actual flight time...that's what he is attempting to find out.

I used to do that - back when it was common to charge on the tachometer - before Hobbs meters showed up in trainers.

Actually, sometimes, I would climb the little Cub up to 6 or 7 thousand feet, at full power, but at lower than cruise rpm, over an old abandoned airport in Florida, and pull the mixture, shut down the engine, and glide down to a landing on the old airport. I'd get maybe .3 on the climb, but actually fly .4 or .5.

Yeah, it's nickle-and-dime, ...but when that's all ya got, it worked for me.

That works

What I was attempting to show was that pulling it back a lot doesn't make a whole lot of difference, and usually means you end up paying more, not less.

[TonyC]

Originally Posted by aerospacepilot

I am just looking for things I can do to minimize my costs. I know the general concensus would be to pull the power back, but I am interested in specific airspeeds, RPM's, and altitudes (for a C-172) that would minimize tach hours?
Thanks.

If you could push up the power and get 25 Tach hours a day, would you only log 24?


If all you're concerned about is cheap hours, use a good pen. Pay for an hour, log 2. If your conscience bothers you, fly 1, pay for 2, and log three. It's all about hours, right?



I'd recommend you stop worrying about the slight difference between what your tach says and what your watch says, and start thinking about building EXPERIENCE instead. Concentrate on becoming a better pilot, not just a better resume.



Just my 2¢ worth.




.

[amorris311]

good two cents mr. c!