Performance and Fuel

[ddnebert]

After over 30 hours of flight, I have been puzzled by the somewhat invariable rate of fuel consumption for less-than-ideal performance. I have to admit that I have a little different version of the -10 than most:

• O-540-B4B5 (235 HP, low compression, carbureted, 2575 top rpm)
• Hartzell 'traditional' matched prop and hub

Checking in with the Hartzell folks they calculated a 6% efficiency drop in my propeller compared to the blended airfoil that Vans sells, could cause it to fly about 5 knots slower for the same power output. I have no idea what the fuel rate difference is in carb vs FI for the 540 but would anticipate, likewise, that my version would be less efficient at metering fuel to the engine.

In my flight test 130nm triangle at 8500ft in August, I was seeing top TAS at around 155kts with full throttle and RPM at 2500, leaned appropriately. The test flight revealed a 15.5 gph thirst. Talking with operators of a 182 at a local airport that also has an O-540, they would budget 15 gph for flight planning. That's a far cry from what I was anticipating...

If I seek an economy cruise, after studying the power curve charts from Lycoming for this model of engine, it should be in the range of 2200 and 20 inches, but then my TAS is about 129kts at 13 gph. At a more pleasant sounding 2400 and 21, I am getting around 140kts, but still see 14 gph. I see a disturbing trend here where gph = Vkts/10

My indicated fuel pressure is typically in the neighborhood of 125psi, which I don't believe (so says the EFIS) and that is purely the mechanical pump.

So, what do you all recommend I try in order to improve my performance and fuel consumption (other than replacing the prop and engine)? Certainly, getting the wheel pants on might help a little. Could the carburetor need adjustment or overhaul? Other suggestions?

Doug.

 

[Radomir]

Where do we start?

First, what were your expectations? What you are not mentioning is the position of the red knob.. it makes a big difference in fuel flow Unfortunately, the numbers you posted w/o knowing how far ROP you were do not have a whole lot of meaning..

Having said that, if you don't have all the fairings etc on I am not sure that collecting performance data is in order.

IMHO, I don't think there's anything wrong with your carb (though anything's possible)..

 

[ddnebert]

Expectations

I was hoping for something in the 10-12 gph at a reasonable speed (over 150Kts) based on other postings. I have lever controls rather than red knobs, but have been leaning to ~1500F max EGT.

Doug.

 

[pierre smith]

Doug, wheelpants don't help a little...

....they help near 18 MPH! They added 16 MPH to my -6A and improved ROC by 200 FPM combined with upper and lower intersection fairings and landing gear fairings. It wouldn't surprise me if you see 170 knots then.

Best,

 

[w1curtis]

I was hoping for something in the 10-12 gph at a reasonable speed (over 150Kts) based on other postings. I have lever controls rather than red knobs, but have been leaning to ~1500F max EGT.

Doug.

Doug,

Absolute EGTs are not particularly useful. What you want to determine is how close each cylinder is to peak EGT. And if on the rich side or the lean side. Look at www.gami.com and download the GAMI lean test procedure. Then at least others can compare their apples to your apples.

 

[Kyle Boatright]

After over 30 hours of flight, I have been puzzled by the somewhat invariable rate of fuel consumption for less-than-ideal performance.

<<<snip>>>

Certainly, getting the wheel pants on might help a little.

Doug.

The portion of your post I quoted and bolded is the explanation. Unfaired wheels, tires, and brakes are very draggy, particularly for anything faster than a Cub or Champ.

Install the fairings and let us know how much your plane's performance increases.

 

[RV10inOz]

Well, where do we start...........and can we stay in knots everyone

1. As Pierre has pointed out, spats and fairings will mean around 15+ knots

2. A carby engine is not likely to behave LOP so you need to run carefully, at say 8500' full throttle and 2300-2400 RPM you should be fine running at peak egt, but be careful down lower than 7500.

3. A ROP engine run will for me be 16+GPH at altitude, WOT and 23-2400 RPM so your fuel burn figures are not unusual even though you are 235HP.

4. When leaning and if you can get LOP your plug gaps must be spot on, 16-18 thou and no more than 20, otherwise they start misbehaving. ROP nobody will notice.

5. TAS..... well I learned a heap a year or so back, research my posts if you like. What is the TAS figure you quote based upon, the actual GPS calculated speed by flying the box? Or is it TAS from a Dynon/AFS etc?

The reason I ask that is the STATIC PRESSURE on RV's can be an interesting thing that not many folk realise is in error. When we first flew our -10 I was convinced we had the slowest RV on the planet, until I found the IAS and hence TAS display was out by 9 knots due to static error. I could write a few pages on the experimenting I did until we got the error down to 1 knot. This also has an effect on your altimeter and encoder/transponder so it is very important you get it right. Especially if you are flying IFR, but anyway it matters when in an ATC environment more so than just wandering around in class G.

So if you want some more help there feel free to drop me a line!

 

[Steve Barnes]

Apples and Oranges, but...

My I.O.540 C4B5 with 10:1 pistons always get a consistent economical fuel flow, which still amazes me. At 2100 and 19" I always get 8.8 gph. I always lean to my fuel totalizer, now that I know where to set it. It's just a little lean of peak. At 21 square I'm right at 10 gph per hour. The speeds I get are irrelavent because I have a different plane. Another number is, at full power/full rich at sea level, the fuel burn is about 26.5 gph.

Steve Barnes "The Builders Coach"

 

[elippse]

Let's say that a typical LOP cruise will be at an SFC of 0.43 to 0.45; that means, with gasoline at 5.9 lb/gal, your engine will be putting out about 13.4 hp/gal/hr. or 0.0746 gal/hr/hp. See if you can get comparitive results using these coefficients. Also, a 6% more efficient prop will be like having 6% more hp which will increase your speed 2%.

 

[ddnebert]

Responses

Thanks for all your input and ideas. Here are some thoughts in response:

a 6% more efficient prop will be like having 6% more hp which will increase your speed 2%.

That was the conclusion I came to after settling on this prop instead of the blended airfoil version.

At 2100 and 19" I always get 8.8 gph.

These are the kinds of cruise numbers I'd heard of but am so far from. Also, if I flew at 2100 and 19" I'd have a TAS, at height, of around 120kts.

What is the TAS figure you quote based upon, the actual GPS calculated speed by flying the box? Or is it TAS from a Dynon/AFS etc?

The TAS is calculated by the EFIS with GPS input and GS. It even shows me effective wind direction and velocity.

... spats and fairings will mean around 15+ knots

OK, I'll get to work on those promptly!

So, is the procedure the same for leaning a carb engine (at height) as for fuel-injected? Basically, lean to reach peak temperatures and then a little further to see if they drop?

According to the Lycoming Operations Guide:

For cruise powers where best power mixture is allowed, slowly lean the mixture from full rich to maximum power. Best power mixture operation provides the most miles per hour for a given power setting. ... For engines equipped with controllable pitch propellers, lean until a slight increase of airspeed is noted. ...If the pilot attempts to go leaner than peak EGT (with fuel injection only), the power decreases rapidly as fuel flow decreases. ...

... Slowly lean the mixture until engine operation becomes rough or until engine power rapidly diminishes as noted by an undesirable decrease in airspeed. When either condition occurs, enrich the mixture sufficiently to obtain an evenly firing engine or to regain most of the lost airspeed or engine RPM. Some engine power and airspeed must be sacrificed to gain a best economy mixture setting.

OK, so what is "Peak EGT" as a rough range of values? The Lycoming Guide does not spell it out other than saying "...peak EGT is right at the edge of best economy mixture, and is our only practical point of reference in the best economy mixture range."

Doug.

 

[dlloyd3]

Peak EGT values

Doug,
Peak EGTs will range from around 1450 to 1550 typically, but this will depend on a number of factors such as individual probe location, altitude, OAT, humidity, RPM, MP, etc. Each cylinder will reach it's own peak EGT individually as the mixture is leaned. They will not be the same nor reach their peak at exactly the same time.

 

[Bill.Peyton]

Doug,
Since you have a carburated engine I would not even attempt a Lean of Peak operation. Running at peak egt anything under around 8K feet and over 75% power is not a good idea. By peak egt, I mean leaning until the hottest cylinder reaches it's peak. I have been running my engine that way through TBO on the first engine and 800 hours on the second engine. It works just fine. I run 50 deg rich of peak on all other occasions.
I normally see anywhere from 1470 to 1520 egt on the peak cylinder.
Bill

 

[ddnebert]

Power at altitude

I am actually losing power at higher altitudes, so even though my throttle may be full forward I would only be getting perhaps 65% at 10,000 feet, right? My EFIS does not indicate % power so I'll refer to the Lycoming graphs for my engine:

At 8500 feet at 2575 Max RPM I should see no more than 20.25" due to the thinness of the air, and get 170 HP / 235 HP = 72% power

This suggests that I can be full throttle, should indicate around 20" and be pulling no more than 72% power.

Am I reading these correctly?

 

[Bill.Peyton]

Yes, you are reading the charts correctly. That's why you can run at peak around 8000 feet or higher, you will not ever be making full power.

Remember, in thinner air the airplane has less drag, so your optimum speed will occur at an altitude where your engine can still make 75% power, but the drag is reduced. Also to remember is the prop has thinner air to bite into along with everything else.

I know this will stir discussion but... The altitude and power referred to in the chart is at "standard temperature" not indicated. Most of the tables will show the standard temp. at any given altitude. When flying, you should actually adjust for temp and pressure to determine your real power setting and determining your real % power output according to the Lycoming chart.
Bill

 

[RV10inOz]

Yep and remember the Vans cowls and some of the other fancy ones generate almost an inch or more extra MP so when you compare notes with your mate in his 182 remember you are getting a ram air effect he is not.

DB