Why is my 6A so slow?

Cadstat · #1 07-06-2011, 11:10 AM

My slider 6A doesn't deliver the numbers I was hoping for. I'm only 170-172 mph @ 8k tops. I'm the third owner. The plane has over 30 'stick on' applique decals of flames as you can see in the photo. They have a slight texture that you can feel. I can't believe they are slowing me down but who know? What else can I do to improve speed.
Its a IO-320 160 hp once turbo'd with 745 hours, 2600 rpms max. Bendix mags. Good compression and doesn't use oil. Hartzel C/S, 1175 pounds empty.

ChiefPilot · #2 07-06-2011, 11:27 AM

Are the gear leg fairings & wheel pants properly aligned? A tech counselor told me a similar tale once and fixed the issue by re-aligning those fairings. I filed that tidbit away for future reference...

sprucemoose · #3 07-06-2011, 11:36 AM

John,

Some more data would be useful in diagnosing. What MAP/RPM/ fuel flow are you seeing? How are you leaning? Is 8K pressure or density alt? How are you measuring the speed? ASI, True ASI, GPS 3 way run verified?

gasman · #4 07-06-2011, 11:44 AM

Quote:

Originally Posted by Cadstat

I'm only 170-172 mph @ 8k tops.

Its a IO-320 160 hp once turbo'd with 745 hours, 2600 rpms max. Bendix mags. Good compression and doesn't use oil. Hartzel C/S, 1175 pounds empty.

Looking at the above numbers...... let's take 8K and make 8K DA. and then convert 172 indicated into TAS...... now we are going 194 MPH. Not bad for a motor that will only turn 2600rpm.

To produce 75% power at 8K da, you will need about 22' of MP. You need to find out why your motor will not produce 2700 rpm. Timing. Cam timing, flat cam?

Good luck.

Jamie · #5 07-06-2011, 12:03 PM

Quote:

Originally Posted by gasman

To produce 75% power at 8K da, you will need about 22' of MP. You need to find out why your motor will not produce 2700 rpm. Timing. Cam timing, flat cam?.

...add to that list -- bad governor?

Cadstat · #6 07-06-2011, 12:03 PM

The speed is by GPS as well as the altitude. And the numbers have been the same over many test checks with two GPS.
Timing unknown. The previous owner said the rpm is limited by the governer.
I do get a 350 rpm drop each side on the mag check. Plugs are brand new.
Shows 22 Manifold pressure @8k, rpm doesn't change the speed. 2200-2400 no change in airspeed. No fuel flow but digital EGT at 1300.

FresnoR · #7 07-06-2011, 12:10 PM

Quote:

Originally Posted by Cadstat

The plane has over 30 'stick on' applique decals of flames as you can see in the photo.

Strange, the flames should speed you up. Maybe because they are decals and not paint, only the commitment of painted flames can give you those extra few knots.

Have you tried racing stripes?

sglynn · #8 07-06-2011, 12:14 PM

Also, you can't go by GPS speed because that is relative to earth, not relative to the local wind you are flying in. What's important is air speed. Above it was suggested to use TAS adjusted for altitude and temperature from indicated. You may be doing better than you think.

Alan Carroll · #9 07-06-2011, 12:27 PM

John,

How exactly did you do the GPS check? You should go out and fly three (or four) legs at 90? to each other (in smooth air and carefully holding altitude), and correct for the wind using one of the formulas linked on Kevin Horton's web page:

http://www.kilohotel.com/rv8/phplinks/index.php?&PID=49

If the speed is indeed low, there might be more than one cause. I'd start with getting the mags checked or overhauled; 350 rpm is way too much drop. It also sounds like either the prop or the governor needs adjustment; you should be able to turn 2700. While you're at it check to make sure the throttle and mixture cables are adjusted correctly.

From there I'd look at alignment of the gear fairings and control surfaces. The airplane is also very heavy for an RV-6A which will affect speed a bit, but I'd only expect this to be on the order of a couple of knots.

Sig600 · #10 07-06-2011, 12:40 PM

Quote:

Originally Posted by Cadstat

The speed is by GPS as well as the altitude. And the numbers have been the same over many test checks with two GPS.
Timing unknown. The previous owner said the rpm is limited by the governer.
I do get a 350 rpm drop each side on the mag check. Plugs are brand new.
Shows 22 Manifold pressure @8k, rpm doesn't change the speed. 2200-2400 no change in airspeed. No fuel flow but digital EGT at 1300.

This is definatley not normal.

As was mentioned before too, gear leg fairings dragging you sideways will cause a lot of drag as well. The stickers, while you may get some turbulent flow it wouldn't be 15-20 mph worth of drag. I'd start with the mag drop, that is way out of tolerance.