Need a little help figuring something out

[Darren S]

I am considering buying an RV-7. I went for a flight with the owner today but the cruise speed seems quite a bit down from what I expected and from the Van's site.

The engine is an IO-360, Hartzell C/S prop (I don't know the diameter), 38 gallons of fuel, no baggage, pilot/copilot weight 350 lbs.

Overcast ceiling at 4000 ft, OAT 6 degrees C, slightly humid, rain showers had just passed.

Airport altitude: 900 ft. Cruise altitude : 3000 ft. Winds 13 knots at ground level. RPM set at 2350, MP set at 21", fuel burn about 9 gallons/hour, CHT fine, EGT 1320 range for all four. Ground speed as verified by the GPS 140-150 mph. Climb at takeoff is 1500 + fpm.

This ground speed reading was taken when the IAS was pretty much the same as the GS reading, which to me means the winds aren't affecting performance too much.

Now, the Van's site says that the cruise speed at 8000 feet is more in the 180 - 200 range.

Now don't flame me ok..... but the higher we fly the more efficient the engine runs because we can lean more and thinner air means the plane flies faster ?? Is that right ?

Do these cruise numbers seem unusually low to you ?

Thanks for your help. I really like the plane but keep thinking that the slower cruise speed may mean that the engine isn't putting out as much power as it should.

Darren

[Jamie]

If that is mph, then yes, that is low.

At that altitude, those settings and that altitude I would be about 153kts or so.

These are my typical cruise settings and the numbers I use to flight plan. All assume 8500ft DA. Numbers verified by 4-way GPS runs.

Balls to the wall (who does this anyway except for bragging rights?): 179kts
21.5" 2400RPM 165kts TAS
20.0 2380RPM 153kts TAS

[Darren S]

Yes the readings are mph, not Knots. I wish they were knots. I just kept telling myself that it's the air density, or the upper winds. I'm making excuses.

On Sunday I'm going flying again. I think I'll climb up high this time and see the WOT readings. Maybe the Airspeed gauge is reading low. Again, more excuses

Darren

[David-aviator]

Originally Posted by Darren S

I am considering buying an RV-7. I went for a flight with the owner today but the cruise speed seems quite a bit down from what I expected and from the Van's site.

The engine is an IO-360, Hartzell C/S prop (I don't know the diameter), 38 gallons of fuel, no baggage, pilot/copilot weight 350 lbs.

Overcast ceiling at 4000 ft, OAT 6 degrees C, slightly humid, rain showers had just passed.

Airport altitude: 900 ft. Cruise altitude : 3000 ft. Winds 13 knots at ground level. RPM set at 2350, MP set at 21", fuel burn about 9 gallons/hour, CHT fine, EGT 1320 range for all four. Ground speed as verified by the GPS 140-150 mph. Climb at takeoff is 1500 + fpm.

This ground speed reading was taken when the IAS was pretty much the same as the GS reading, which to me means the winds aren't affecting performance too much.

Now, the Van's site says that the cruise speed at 8000 feet is more in the 180 - 200 range.

Now don't flame me ok..... but the higher we fly the more efficient the engine runs because we can lean more and thinner air means the plane flies faster ?? Is that right ?

Do these cruise numbers seem unusually low to you ?

Thanks for your help. I really like the plane but keep thinking that the slower cruise speed may mean that the engine isn't putting out as much power as it should.

Darren

9 GPH at MP of 21"?

At 21", I am burning less than 7 GPH when leaned out with the EGT's over 1400. Did the demo pilot lean the engine at all? As is, the numbers don't make much sense.

[Darren S]

Hey David,

Maybe you are on to something. Most of the EGT's were 1320'ish. I was told not to let them climb above 1350. One hit 1366 and then he richened the mixture a little bit. I didn't know better so I just went with it.

So you say lean EGT's to 1400 ? This will certainly bring the fuel burn down, but will this bring the airspeed up ? I want to see the compression numbers from the last annual. The engine was a new Superior IO-360. It only has 227 hours on it. There has got to be a reason the plane is so slow. I'm going to keep digging.

Empty weight 1112 lbs. So it's not super heavy.

Thanks for the help.

Darren

[Ironflight]

A couple of thoughts for you Darren,

Wheel pants, or no wheel pants? Pants and fairings are good for about 14 - 18 knots!

Second, if the IAS was roughly equal to GS, then you had some winds, because your TAS was going to be higher than IAS at most altitudes. A few knots difference in TAS is a few knots. If no wheel pants, then add those numbers, and you are getting CLOSER to RV speeds.

Also, talking about absolute EGT values is meaningless - they depend on the distance between the probes and the exhaust port on the cylinder, so you can't really use one airplane's numbers on another, unless they are drilled identically (rare). You should be able to lean to peak and run there, at least for a little while, to do speed runs.

I agree that the airplane should be faster - but do take the time to computer TAS (my whiz wheel is buried somewhere...) and make sure you are computing everything properly to get a good comparison.

[szicree]

Originally Posted by Darren S

...the higher we fly the more efficient the engine runs because we can lean more...

More altitude means less air. With less air, we lean to maintain correct fuel/air mix. We then have less total fuel/air mix in the motor, so less power with same throttle setting.

Still, that sounds slow.

[L.Adamson]

Originally Posted by Darren S

Hey David,

Maybe you are on to something. Most of the EGT's were 1320'ish. I was told not to let them climb above 1350. One hit 1366 and then he richened the mixture a little bit. I didn't know better so I just went with it.

So you say lean EGT's to 1400 ? This will certainly bring the fuel burn down, but will this bring the airspeed up ? I want to see the compression numbers from the last annual. The engine was a new Superior IO-360. It only has 227 hours on it. There has got to be a reason the plane is so slow. I'm going to keep digging.

Empty weight 1112 lbs. So it's not super heavy.

Thanks for the help.

Darren

My engine is running fast & smooth, when my one and only EGT is around 1325 to 1350 (cyl. #3). My plane also checked in at 1172 lbs. In fact, it's as fast or faster than some other lighter weights I fly with. I'll often be looking at GPS ground speeds of 165 to 185 knots. The second GPS is set to mph, and was 197 yesterday. Has been to 214 with tail winds. As to yesterday's flight, I didn't get around to figuring winds or TAS. I don't have electronic instrumentation that does most of it, as some do.

edit: addtion ---- flight between 8500 & 9500' msl.

L.Adamson --- RV6A/ Lyc 180/ Hartzell CS

[Ron Lee]

Go to around 8000 feet DENSITY ALTITUDE and check GPS groundspeeds full throttle.

[Darren S]

Thanks for the help fellas,

Here's what my next plan is. Oh by the way, yes the plane has wheel pants and gear farings.

I suspect that the engine isn't being leaned properly. Reading some old posts last night, it was suggested that once cruise attitude is established then lean slowly till the engine runs rough and then richen back slightly.

I will climb up to approximately 8000 ft., establish wings level and hold altitude. Run 2350 rpm and 21 " MP, or there abouts. I'm going to try the lean till rough and then richen back idea and see where that gets me. Also, I'll fly a square to try and negate any winds.

My thought is that if the EGT probes aren't mounted correctly then the readings may not be as helpful.

150 - 160 knots should be where I'm at, not 150 - 160 mph.

Thanks to all for the input,

Darren