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Darren S

Well Known Member
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
 
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
 
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
 
Darren S said:
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
Click to expand...

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.
 
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
 
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.
 
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Darren S said:
...the higher we fly the more efficient the engine runs because we can lean more...
Click to expand...

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.
 
Darren S said:
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
Click to expand...

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
 
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
 
You're still not making 75% at 21"...

...Darren. Van's numbers are for 75% so run it WOT, should be near 23" and 2350 or thereabouts. Lean to peak and don't be so concerned about absolute values of EGT's, as Paul pointed out.

Best,
 
Ok Pierre, I hear ya. I'm new to the plane so I want to clarify something. Sorry if it's seems obvious to others that have the same set up.

You say run Wide Open Throttle and check speed numbers. WOT should be about what, 2700 RPM and 27 " MP ? Thereby making 75% power a setting of 2350 RPM and 23 " MP ?

Sounds right ? Maybe this guy is just babying the dang plane :)

Thanks,

Darren
 
Ballparking Numbers

Here's a rule of thumb for you in estimating performance:

1) Maximum manifold pressure decreases about an inch for each 1000 feet of altitude above MSL. (normally aspirated and no ram effect)

2) True airspeed increases over indicated airspeed about 2 percent for each thousand feet above MSL.

So for example: The altimeter setting is 30.10. You are at 8000 MSL with an indicated airspeed of 140 knots. The highest manifold pressure you should see is around 22 inches, and your TAS should be about 162 knots.

Don
 
Yes and no...

As Don pointed out, all the MP you can get at a DA of 8000' is something over 22" and don't run it 2700 RPM, but 2300-2400 and you should be very close to 75%. While you're at it, do a 4 way GPS run...North, West, South and east and record your groundspeeds, then average them for a quick double-check. There are more sophisticated spreadheets and all that, but for now, it'll be close enough.

At low altitudes, I get 29.2" at 800 MSL, WOT, with a clean air filter. If you can't get near 29" and the barometric pressure is around 30", then you may well have a dirty air filter....an easy way to keep tabs on your filter condition,

Best,
 
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Please excuse me for being blunt, but a guy would have to be CRAZY to buy an airplane built by Joe Shmoe in his garage without first having a thorough inspection by an expert. I know it's a "kit" but there are still so many places to screw things up that you can't afford not to have it looked at. I'd pay particular attention to firewall forward stuff, since Vans gives little guidance there.
 
There is a program called "True Airspeed Calculator" that makes it easy to find what your RV is really doing. It uses a 3 or 4-way heading using GPS to calc speeds that x's out wind effects.

Find it here: http://www.reacomp.com/true_airspeed/index.html

For testing, I would not go lean of peak, just maybe a tad rich. Otherwise you can cut the power a bit giving a lesser speed.
 
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