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Slow RV-7?

N

NateZ

Member
Been reading on threads that people are getting mid-170kts cruise at certain configs around 8/9 thousand feet (2600rpm or so) with 180 hp and hartzell cs props.

I'm flying a (did not build this airplane) recently completed RV-7, Mattituck TMX360 185hp with a hartzell scimitar prop and the absolute best I can get out of the plane is 165 kts TAS at 7000 feet full trottle and 2400 rpm with the ram air open. Slower (>160kts) at 8500 through 9500. Higher RPM's seemed to slow the plane. The plane is rigged well. There are two blade type transponder antennals and two comm antennas on the belly. Its a sliding canopy if that makes a difference too. Should I be seeing more speed? I didn't catch the OAT at altitude when I was flying today but it was a pretty cool and dry south florida winter day.

These TAS numbers were read from the Dynon Skyview displays. Possible that these are off? I need to fly again and compare to what my GNS530 says.

*edit - all the testing was done at WOT at all three altitudes.

Nate
 
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Dan beat me to it...

I used Doug Gray's Excel worksheet found at the bottom (last entry) of this page. I see that the NTPS page also mentions Doug Gray's method.

You will want to fly constant indicated airspeed for all the runs. Log OAT and pressure altitude so you can calculate true airspeed derived from pitot/static (and Dynon info) for each run. If your system is calibrated precisely, the Excel calculator and the derived TAS numbers will be the same. If your pitot/static system and Dynon are in good order, you can get close. I'm 2 knots off (EFIS reads low) at 180 KTAS.

For what it's worth, at around 8500' density altitude, 2400 rpm & WOT with 180HP Superior clone IO-360 and Hartzell B/A prop, I would expect to get about 165 KTAS burning 8.8 gph or so, lean of peak.
 
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You need to know if you have any error for sure. You should be a little faster, but actually aren't off too far from most others. Also depends on how much leaning you're doing.
I think for RV-7's mine is more towards the upper end of the scale. If I want to burn 8.5 gph at 8500' I'll be about 175kts maybe a couple more. If I want to burn some fuel, I can push the numbers up. I have a 2 kt error above 185K TAS also, but mines Dynon reads high.

Here's an example. 2600rpm, 24.8"mp, 13.4gph shows 188kt True...but GPS is 186.3. If I push my prop to 2700, it doesn't really go any faster.

aviation2012015.jpg
 
Airplane was leaned to best power in all instances. Seems like i'm off 5-10 kts at least, and ground speeds seem to coincide with TAS/winds so could it really be that far off?
I see your indicated 170, mine indicates in the 135-140 range when I'm trying to push it.

At 7000 feet im not at 24 inches of MP, 22.5 at best with the ram air open at WOT... maybe thats part of the problem. Suggestions?

Nate
 
Without a actual test, it is hard to know what your error is. The wind readings can be off, your OAT indication can have a significant effect on your True airspeed also.

I have a very efficient(but unfiltered) air box, so my MP readings seem to be higher than others also.
 
Lars said:
For what it's worth, at around 8500' density altitude, 2400 rpm & WOT with 180HP Superior clone IO-360 and Hartzell B/A prop, I would expect to get about 165 KTAS burning 8.8 gph or so, lean of peak.
Click to expand...


I'd say Im closest to these numbers, however ROP best power, closer to 10gph.


Your right, I'll do the test to atleast get a base line and go from there. Thanks for the info guys.

Nate
 
Nate...I get about same speeds as you..about 165 kts true rop, 160 lop. I typically fly in cruise at 2400 rpm. I suppose if I push the prop up to 2600 rpm I would get more speed.
 
NateZ said:
I'd say Im closest to these numbers, however ROP best power, closer to 10gph.


Your right, I'll do the test to atleast get a base line and go from there. Thanks for the info guys.

Nate
Click to expand...

At best power mixture I'd be on the far side of 170, but I'm cheap enough to sacrifice a few knots :)

Yes, check your airspeed indication. I was using non-stock static ports at first. Testing showed my true airspeed indication was 7 knots low at 180 knots. Installing stock Van's static port rivets made my RV-7 magically 5 knots faster :)
 
We were dog slow until we put all the wheel and gear fairings on our 7A. Picked up over 10 kts. But we still did not make the vans TAS numbers. Did the GPS airspeed cal and we were slow by 4 to 6 kts depending where you were in speed. We just add four kts now to what the Dynon says for true speed and ignore the difference at lower speeds.
 
Double check your static port location AND shape. A friend has a -9A with the port located one bulkhead too far aft, plus it's an aftermarket, raised port, not the one Van recommends.

According to the GPS, the airplane is a lot faster than indicated, so it might just be your pitot-static system.

As others have pointed out, do a three-way NTPs run.

Best,
 
I have a RV7 with 3 blade Catto prop.Engine is an ECI kit with high comp. pistons , cold air ind. and LASAR ignition, Last weekend flying at 7500 ft with autopilot on, smooth air ram air open . Leaned for max RPM of 2740 my four heading speed was 208 mph at 12.2 gph . Leaned LOP RPM of 2600 my four heading speed was 194.6 mph at 9.4 gph on two GPS. I only have 17 hrs on aircraft unpainted.
 
My RV7 has 30hrs total.
While flying with 24"/2400rpm @ 5500ft (OAT aprox 23 Celsius... yes... pretty hot here!) my reading is 148KIAS and 160TAS aprox.
My empty weight is about 530kgf, and i weight 95kgf (i know i'm heavy!) i was carrying about 130 liters os avgas... so i supposed my gross weight was about 720kgf.
A Friend who has a RV-7 too, but 25 kgfs lighter than mine has better readings.
My plane is equipped with a XIO-360 M1B with Hartzell C/S Blended Airfoil.
 
You should be able to get faster than 165ktas at WOT.

Where is you OAT probe and is it really accurate.


Run the 3 way GPS tracks and do the worksheet.

Just using GPS ground speed by itself is useless unless you can assure you have ZERO wind, which you can't.

If able you can fly formation with someone that's got an accurate A/S indicator and compare.

Running about 50-100 ROP will give you just about your best power.

Let us know what you come up with.
 
We did the worksheet, appears to be off 1.5-2 kts TAS. Indicating 138/139 during the runs. It is a dual skyview, dual adhrs, dual OAT probe setup, I can pull up both adhrs to compare them in flight and they show exactly the same readings. The OAT's are located about 6 inches below the horizontal stab on each side directly below were the adhrs are mounted on a shelf underneath the stab fairing. 165 KTS TAS looks like thats all she will do at 6500 (Lose about a 1kt st 7500, and another at 8500) This is wide open, 2450 rpm, 24.1 (at 6500') in mp. leaned to 75* ROP. It was me and the planes owner (335lbs) no baggage and 25 gallons of fuel give or take. We have a full classic aero interior. I will say that I don't remember what the OAT was, but it was a bit of a cooler day so maybe it will loosen up a bit when it gets warmer.

Flying in formation is the next step I suppose but I don't think we are going to sweat it.
 
If that is a AWI 4 into 1 it is aiming 90* , might cut it off at 45*. Make changes one at a time so you know what works .
Tom
 
Bummer about your speed, I think you're leaving some speed and efficiency on the table. That exhaust turndown is costing you for sure, how much I don't know. But cylinder shapes are some of the worst. You also may have some backpressure on the exhaust system costing you power. The angle of the picture might be deceiving how much the protrusion into the airstream it is.

I used 36 gals during 4.5hrs and covered over 700nm of travel this past weekend. Here I had the power pushed up a bit racing the setting sun coming home.
cae755d2abfde5fc087bcab520df2eb4_zpsbb21d79b.jpg
 
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