Fuel economy

[TomD]

Here is an article from AOPA

http://www.aopa.org/aircraft/articles/2 ... 25asi.html

For my M5 the formula works out to about 118 mph as the best speed/drag point.

I don't see any discussion of Manifold pressure or RPM in the article only drag vs speed; therefore, the Manifold pressure is whatever you can achieve at full throttle at altitude and ?? RPM.

Has anyone tried this out or have any comments?

Tom

[a64pilot]

Yes,
It's an established principle of aerodynamics. Arguing whether it "works" or not would make about as much sense as arguing whether 2 + 2 = 4 or not.
For my M-6 at gross weight, my "sweet" spot is around 12,000 or so. I assume the shorter winged Maules would be lower, except for the turbos, and I'm not sure where their "sweet spot" would be.
The sweet spot is easy to find, it's where your full throttle cruise will give you 1.3 times your VY in level un-accelerated flight. If you can exceed the speed, keep climbing until they converge.
Full throttle because your pumping losses are almost zero, and 1.3 times VY because it is the right corner of the "bucket" when the L/D ratio is plotted. Of course at those altitudes, you can lean the snot out of it and not hurt anything either.
Only works for traveling of course, and you have to take winds aloft into consideration.

[kakkenmc]

Thanks Tom,

I will try it. However, I have an MX7 with negative flaps. I'm not sure of the implications of that regarding the formula you found since climb and cruise have different wings. Or, for that matter, I'm not even sure what flap setting my manual assumes for best glide speed.

Kennon

[a64pilot]

kakkenmc,
Your shorter winged Maule with the neg. flaps will have a lower altitude, probably. There is an altitude where the neg. flap setting actually slow you down. For Me that's around 12,000. The effectiveness of reflexing the flaps is greatest when down low, flying at a light weight and a high power setting.

[TomD]

I guess the other part of the equation is if I climb until I reach the sweet spot then I am spending a fair amount of time in a relatively rich setting and slow airspeed (good drag performance but high fuel burn).

Does anyone have a "rule of thumb" on the length of the trip vs climbing to altitude.

For example if I climb to 12k and fly at 118 mph for best fuel economy how long would the trip have to be to pay for the climb from, let's say sea level, to 12k. I know you get payback on the descent on the other end but is it a two hour trip, three hour trip, or what. It makes no sense to power up to 12k if you are only going on a 45 minute trip, for example.

TD

[benflyn]

In Maule's performance spec's is this referred to as optimum altitude?

The bottom line in the chart.

http://www.mauleflight.com/faq/M5.html

[a64pilot]

In Maule's performance spec's is this referred to as optimum altitude?

The bottom line in the chart.

http://www.mauleflight.com/faq/M5.html

I don't think so, at least not for the speed part, that is usually around 7500 or so. Max range will be at a different altitude than the altitude for speed, and all altitudes will vary according to density altitude and gross weight. Therefore it makes no sense to qoute an altitude, because it's not constant. I believe what Maule is saying is those numbers can be made, but only under perfect conditions.

[a64pilot]

Once you are at an altitude where 75% power cannot be made even at full throttle, why not lean it some? Remember your cruise climbing, not hanging it on the prop so cyl head temp should not be a problem.
Once manifold pressure is below 25", I lean to 100 ROP, but I climb at 2300, once at cruise, I lean to 50 ROP, still 2300 if I'm in a hurry, 2200 if not. Just my technique, that doesn't make it right.
Tom, what you ask is complicated. I've seen programs that compute such things, and of course all the numbers change as fuel is burned off and wx changes etc. The Apache Longbow's computer is constantly computing performace numbers, weight and balance etc., so it's possible. I assume the airliners do too. I have a flight planning program that will compare altitudes, but you have to initially tell it climb speed, fuel burn rate etc. it's as accurate as the info you give it.
I will climb if travelling, but I'm also known to climb in the summer until the OAT is below 60, and heck with the fuel.

[maules.com]

As Lyc say's that 55% power gives best range and 45% power gives best time in the air, would one not climb to the 55% altitude for full throttle and cruise at that altitude. The 55% can be read from the Lyc operating book which takes temperature, manifold pressure and rpm into account.
Maybe this altitude correlates with the 1.31 times Vy of 90mph speed at peak egt. for economy.

[Flyin'Dutch']

I am no mathematician by any stretch but the 45 and 55% are range and performance figures not those of best fuel/speed conversion.

Norris states, according to the AOPA article, that the most efficient way to use any aeroplane is to fly it at a speed equal to 1.31*Vy

If you use more power you are burning too much fuel for not enough speed gain and if you use less you are going less fast along than you could by using a smaller incremental increase of fuel which would give you more speed.

So what matters is the speed.

Fly with the power setting required to achieve that speed (and the wider open the throttle and the slower the prop rpm you will be using the engine more efficiently too) and lean appropriately.

The higher you are able to climb by maintaining that indicated airspeed, the faster you will go as TAS will increase by about 2% per 1000 ft.

Obviously you will need to take into consideration, head and tailwinds, distances to fly (but on the whole it pays to climb), airspace (especially for us European boys), icing and oxygen requirements.

I thought it was a very interesting article and will certainly consider using this information on further touring trips.

[a64pilot]

Maybe this altitude correlates with the 1.31 times Vy of 90mph speed at peak egt. for economy.

Completely different things of course, The 1.3 times Vy is an aerodynamic thing and 55% power a powerplant thing but I wouldn't be too surprised if they don't occur close to each other in the big motored Maules. I don't have the charts with me, but I want to think that a normally asperated engine is only able to produce about 60% power at 12000 or so, reduce RPM a little for cruise and your all over 55%. That's all from memory and probably just wishful thinking Of course if it were a much smaller engine, then the altitude where full throttle would give you a level flight speed of 1.3 times Vy may be at an altitude where more power was being made than would allow for leaning

[benflyn]

Is there a simple rule of thumb for determining optimum altitude,

like the 50-70 rule for take off 50% of the runway behind you, your

airspeed needs to be 70% or greater to have a successful take off.

[a64pilot]

The altitude that gives you 1.3 times Vy at full throttle and at your cruise RPM and while leaned out is your "optimum" altitude. You'll just have to go fly and see. If I remember right, mine was 9 to 11 thousand without VG's and 10 to 12 with. It's indicated airspeed of course.
As long as your close, and it won't be a night and day difference. Playing the winds aloft will in my opinion yield better results than just picking an altitude to fly. It's another tool in the box to use, like lean of peak, some swear by it and some think it's a waste of time.

[210TC]

With all of your numbers what was the net effect of vg's at high altitude?

[a64pilot]

With all of your numbers what was the net effect of vg's at high altitude?

I haven't gone high, not above say 16,000 or so. The VG's seem to increase lift and drag. I think I know what your asking, and my answer would be that the VG's will allow a higher altitude to be achieved than without VG's. On my airplane they seemed to move my best altitude upwards by 2000 ft., my NA engine just runs out of steam much above 12,000, so I don't usually go there.
Being that you have an "altitude" engine, but short wings, I think VG's would be a big improvement for you for high altitude work.