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Slow Poke 7-A

G

garrys

Active Member
Compared to the speeds others say they're getting, my plane seems awfully slow. Mine is a 7A, weighing in at 1,010 with full wheel and leg fairings. The engine is a new Aerosport IO360 with 9.2:1 pistons, roller lifters, LightSpeed ignition. Should put out about 190 HP. In search of speed I fitted Sam James Holy Cowl and plenum. The prop is a fixed pitch Catto 3 blade 66" Diameter by 76" pitch.

At 2450 RPM at 2,000 ft, I'm seeing 155 Kts (180 MPH). Wide open which is 2700 RPM I see 162 Kts (190 MPH).

From what I gather, everybody else is cruising at 170 Kts (200 MPH) or better, and their top speed is 185 Kts (215 MPH).

Any ideas why I'm so slow?
 
garrys said:
Compared to the speeds others say they're getting, my plane seems awfully slow...
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I apologize if this is insulting, but how are you determining your speed?
 
garrys said:
At 2450 RPM at 2,000 ft, I'm seeing 155 Kts (180 MPH). Wide open which is 2700 RPM I see 162 Kts (190 MPH).

From what I gather, everybody else is cruising at 170 Kts (200 MPH) or better, and their top speed is 185 Kts (215 MPH).

Any ideas why I'm so slow?
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I'm curious too, if these are indicated or true airspeeds...If they are indicated speeds, you are doing great, but that depends on your altitude too...
 
slow airspeed

I would suggest that you try to borrow another propeller of a different size. My experience tells me that a longer (72") two blade propeller should yield a improved high speed performance. Also a variable pitch prop would allow you the best take off and cruise performance, however at a much higher cost. I personally have had the best performance with an Aero Composites 74" prop on my RV8.
Dick Martin
RV8 N233M
the fast one
 
Weight?

How did you get it so light? My -7A was 1050# sans fairings with IO-360 and reasonably lightweight panel.
 
I did my test flying today at 2,000 ft altitude and determined my airspeed by a combination of Pitot tube indicated airspeed and GPS Groundspeed......in both directions.

Garry S
 
did you have any throttle left at 2,000 ft when you reached 2,700rpm? How about at 8,000 ft density? What's max RPM you can get there? can you overspeed it? are you underpitched?
 
A 66X76 Catto is probably underpitched for your application. I have a 66X76 on my 6 with a box stock Aerosport/ Superior carbed O-360. I can easily hit 2800 RPM or more at altitude.

Try some 4 way speed runs at WOT and 8K and give us the numbers. That's a more realistic way to test.

FWIW- 155 kts @ 2450RPM down low doesn't sound too bad at all.
 
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Definitely

n468ac said:
I like to see you goto 8000' DA and go 2700 rpm. Then give us your IAS and TAS.
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You absolutely have to go to 7500-8000'. That's where Van gets the numbers from, and with mixture leaned. Do a four way or three way run writing down GPS numbers on each leg for a more accurate speed. Someone I respect told me once that the gain is around 2 MPH/per thousand feet but I can't document that.

Regards,
Pierre
 
pierre smith said:
around 2 MPH/per thousand feet but I can't document that.

Regards,
Pierre
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Take indicated airspeed X .02 and you will get the gain per thousand feet. It will come in around 3 mph per 1000'. (using mph anyway) This is one of the steps to roughly figure true airspeed actually. If you were to then take the result and multiply it times the altitude (example 7.5 for 7500') you would come up with the amount to add to indicated to get true. At least it will get you very close.

Blue Skies,
Bryan
 
The airspeed indicator should not be assumed to be reliable until you've done some calibration tests using GPS. As already suggested, if the numbers you're reporting are IAS then you're probably about where you should be. I'm guessing that 2000' this time of year in Florida is probably around 4000 density altitude?. If so, 155 KIAS is 167 KTAS, close to what I expect to see under similar conditions with my RV-8. Van's numbers are all reported as TAS.

When you do your 4-way tests (N, E, S, W), make sure to allow plenty of time for the speed to stabilize on each heading. This can take several minutes to happen, and you need to hold altitude very carefully (?20 feet or better). Record the stabilized GPS ground speed (also remember to look out for other airplanes!). If winds are light you can get an approximate TAS by just averaging the four numbers. A more accurate calculation can be made using one of several calculators, for example this one: http://reacomp.com/true_airspeed/. Avoid doing tests when the wind is strong, or when there's turbulence.

The highest speeds come at low altitude with throttle wide open, because you're making the maximum possible power. You'll also be burning the maximum fuel, in the neighborhood of 15 gph. You probably won't be able to get numbers that are directly comparable to Van's maximum during the summer though, due to the high density altitude. I think Van's max speeds are measured at 1000' DA or less. The other reference number is measured at 8000 density altitude, which is approximately the maximum altitude at which you can make 75% power. This is Van's "cruise". It is slower than the maximum speed down low, but fuel burn is less frightening (for this test you should lean for best power).
 
I agree with going to 8K. That'll make a lot of difference.

But, there is another point: propeller.

3 blade propeller is good for take-off roll and climb performance
2 blade propeller is good for reaching faster cruise speeds

An example of airplanes that I 've flown:

PA34-II (twin non-turbo engine, 2 blade) is about 5 kt faster than PA34-III (twin turbo engine, 3 blades). And -III is a bit more powerfull than -II. Difference is the take-off and climb. And it's a huge difference.
 
Seneca reference isn't directly comparable to the RV, because Catto's props are pretty darn efficient. HOWEVER, these guys are right, 8000' is where you should be testing at.
 
8000' DA...this time of year, 8000' MSL is typically WAY higher than 8000' DA.

Imho, best to bracket some tests at various altitudes, noting altimeter setting and OAT.
 
8000 DA

I've found that an easy way to find 8000 DA is to use the E6B function on the GPS (I think all the recent Garmin units have this). If its warm, go to 6000 MSL, note the OAT, punch the numbers into the GPS and see what DA it gives. Then adjust altitude up or down based on the calculated DA, enter the new MSL altitude and OAT, and recalculate. Usually it only takes one iteration to get within a few hundred feet of target, which is close enough.

Using this method I've found I get consistent TAS results at different times of year, usually within 1-2 knots.
 
How did this end? I have a similar situation. I believe my RV is reasonably fast. Would like to know how to get the EFIS KTAS to read correctly...

Going from memory here so I may be off by 1-2 knots

at 8000'
3 way calculated airspeed via GPS tracks is 160 KTs WOT, leaned, 2600 RPM
GRT Sport EFIS indicates 139 TAS
Analog airspeed gauge (reading in miles on the same pitot/static system) is 184mph

So I believe the airplane is going 160 knots, why does the EFIS TAS typically display 135-140 TAS?

GRT tech tells me that the GPS speed readout and the TAS always use the same units. This rules out one displaying in mph and the other knots but the difference is 15% which is exactly the difference between miles and knots. Makes me wonder.

Any ideas what I'm doing wrong? Could it be displaying KIAS? (even though is says the units are "KTAS")

Bevan
 
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Okay, I'll assume the GPS is correct, that you've "averaged" properly to get 160 KTAS. (e.g., average the ground speed flying directly into the wind, and directly with the wind, to get TAS)
Your analog indicator must be wrong. Changing MPH to knots gives 160 KIAS, but at 8000' (what was the density altitude, btw?) the indicated speed has to be noticeably lower than TAS.
Your GRT looks correct if it is KIAS. Are you reading the main airspeed linear indicator? IIRC it cannot be made to read TAS (safety issue, for landing at high altitude airports). However, your GRT should compute TAS from the data (requires OAT sensor to work) and show it just as a digital number, somewhere.
 
GRT TAS calibrate function

You will find a TAS calibration function in the menus if you dig around. You have to fly the headings given and maintain altitude, but it will compute the TAS correction and apply it to the EFIS, and you need to do each unit if more than one.
Tim Andres.
 
Static ports without the 'bumps' (flush with the skin) will often cause the ASI to show IAS to be as much as 10 kts lower than reality. Easy check: Assuming you fly from a field where it's safe to do this, set your altimeter at field elevation prior to flight, then take off & do a high speed pass (cruise speed or faster) down on the deck. If your altimeter indicates lower than reality, you've probably got a static port issue.

If you still think it's slower than 'book', I'd note that you're leaving 100 rpm unused. 75% at 8K feet requires the same rpm as full power at sea level. Check your Lyc owner's manual.

Charlie
 
Hi Bevan, not at all sure this is your issue, but you have presented a puzzle for sure. I note the Sport Setup and Config Guide says the following:

Upper Left Corner Box (Ground Speed / True Airspeed)
Select the data to be displayed in the upper left corner of the PFD. Groundspeed is recommended. If two display units are installed, Groundspeed is displayed on the map page of the MFD, so display TAS here.

So I was wondering about your data source. Is is possible you are reading (as Bob alluded to) IAS instead of TAS, assuming you are getting your groundspeed reading off the PFD? Or do you have a two screen setup and can get both simultaneously. Or perhaps you have a separate GPS you are getting groundspeed from.

I assume your analog ASI is correctable for TAS to get the 184 mph number.
 
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Throttle cable

6 Gun said:
Make sure your throttle is opening all the way.
Bob
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At the carburetor end of the cable, insure that it is actually touching the stop at the wide open throttle position. I found one on an RV9A condition inspection that was not, made the adjustment and the owner found out he got a faster plane out of his condition inspection.
 
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