Slow (relative) 7A

[7DeltaLima]

OK, I admit it. I seem to have a slow 7A relative to what I see others post as their numbers.

I'm flying a 7A with about 310 hours running a IO360-A1B6 (dual mags) swinging a 3 blade MT prop. The plane flys like a dream, always has. Smooth as can be. Doesn't have a heavy wing, ball centered, flys hands off. My point, the plane appears to be built straight.

I went out one last time tonight and did one more run just hopeing that a miracle had happened and I'd gained 10 knts. before confessing I'm slow.

My numbers tonight and they are consistant with past runs:

trk 90 - gps 169 kts
trk 360 - gps 146 kts
trk 270 - gps 167 kts
trk 180 - gps 192 kts

which using GPS spreadsheet yields 169.3 knots avg speed with .3 std dev. I see others consistently posting above 180 kts avg.,

RPM 2650 MP 24.5 WOT Den Alt 8000 leaned to best power ~13 gph Ind Alt 6810 & baro 29.98 1140lbs empty + 230 me + ~30gal fuel

I've read the posts about the MT 3 blade being 3-5 knts slower and I'd probably trade that for the smoothness.

My questions.... where do I start to try to improve the speeds - what/how do I check. Is it possible/probable it's mostly attributable to the 3 blade prop? What else should I look at? If the fairings were mis-aligned, wouldn't it be indicated by the plane not flying straight? I know there are other RV's running the 3 blade MT, what kind of numbers are you seeing in real life?

Thanks for any suggestions.
Doug

[lorne green]

try cutting off the front gear!




(Just kidding). nearly 170kt is not bad at all.

[Kevin Horton]

How well aligned are the flaps and ailerons with the wing? Is it possible that they could be angled a bit down?

Where does the leading edge of the elevator counterweight sit with respect to the HS when at max cruise? (I'm looking for a symptom of misrigged flaps or ailerons).

What is the height of your inlets in the cowling, measured at the aft end of the fibreglas? If you have margin in your cooling, try making a temporary reduction in size of the cowl inlet to see the effect.

What do you have for external antennae?

Did you fly the aircraft without fairings, get the rigging sorted out, then add fairings incrementally? Or did you have all fairings on from the start? If the latter, maybe you have some fairings misrigged in one direction, and the tendency is cancelled out by other fairings misrigged in the other direction.

What is the rate of climb at VY? Measure the rate of climb with a stop watch against the altimeter - don't use the VSI. Describe the altitude and temperature at which you did this test. I'm looking for a way to confirm the engine isn't down on power.

Have you calibrated your tachometer? How does the MP gauge compare to the actual barometric pressure on the ground (not the altimeter setting)?

Have another builder inspect the aircraft, looking for anything unusual that could increase drag.

[dan]

Originally Posted by 7DeltaLima

RPM 2650 MP 24.5 WOT Den Alt 8000 leaned to best power ~13 gph Ind Alt 6810 & baro 29.98 1140lbs empty + 230 me + ~30gal fuel

"Best power" is always an ambiguous thing when anybody other than Walter Atkinson mentions it. Simple solution...try leaning to peak indicated airspeed. It may not recover all the speed you want, but it probably will produce another 1-2 knots. Also, close your vents.

[dan]

Also check your timing and make sure it's where you want it. I believe on your engine, stock timing is 20 BTDC.

[7DeltaLima]

Kevin, 1st thanks for the questions as they provide a place to start.

How well aligned are the flaps and ailerons with the wing? Is it possible that they could be angled a bit down?

Flying straight and level the left aileron is down below the wing tip what appears to be ~1/2 inch when viewed from the left seat. The right aileron is up correspondingly. I've always assumed this was just wing tip mis-alignment but sounds like this may be the place to start.

Where does the leading edge of the elevator counterweight sit with respect to the HS when at max cruise? (I'm looking for a symptom of misrigged flaps or ailerons).

Don't know. I'll need to verify.

What is the height of your inlets in the cowling, measured at the aft end of the fibreglas? If you have margin in your cooling, try making a temporary reduction in size of the cowl inlet to see the effect.

I don't have much cooling margin with the A1B6 so this would be one of the last places to work on but will review.

What do you have for external antennae?

One bent whip and one stub for transponder on the belly. Archer in the wingtip for Nav.

Did you fly the aircraft without fairings, get the rigging sorted out, then add fairings incrementally? Or did you have all fairings on from the start? If the latter, maybe you have some fairings misrigged in one direction, and the tendency is cancelled out by other fairings misrigged in the other direction.

I first flew with no fairings. After about 10-12 hours I added gear leg fairings to help with cooling and finally added the wheel pants after 20-30 hours. Nothing really stands out as a problem in doing so. In hindsight, what I don't know though is if the ailerons aligned with the wing tips without any fairings.???

What is the rate of climb at VY? Measure the rate of climb with a stop watch against the altimeter - don't use the VSI. Describe the altitude and temperature at which you did this test. I'm looking for a way to confirm the engine isn't down on power.

I can't lay my hands on recent data but here is a data point from initial tests when the plane had ~12 hours on it. The plane had gear leg fairings on the mains, no wheel pants and nothing on the nose leg.

110 kias starting below 2000, timer started passing through 2000.
2-3000 :32
3-4000 1:06
4-5000 1:48

95 Kias
2-3000 :29
3-4000 :58
4-5000 1:33

85 kias
2500 - 3500 :30
35-4500 1:03
45-5500 1:37
55-6500 2:16
65-7500 2:56

Have you calibrated your tachometer? How does the MP gauge compare to the actual barometric pressure on the ground (not the altimeter setting)?

I just verified the tach. I had the prop dynamically balanced (didn't do much with the MT as it was very good to start with) Tuesday and the Tach is accurate. Can't specifically say MP vs baro but I believe I'm seeing >28 mp on takeoff.

Have another builder inspect the aircraft, looking for anything unusual that could increase drag.[/quote]

Thanks again for taking the time to respond. Looks like I need to investigate the aileron to wing tip as the first step.

Doug 7A
~310 hours

[7DeltaLima]

Originally Posted by dan

"Best power" is always an ambiguous thing when anybody other than Walter Atkinson mentions it. Simple solution...try leaning to peak indicated airspeed. It may not recover all the speed you want, but it probably will produce another 1-2 knots. Also, close your vents.

Dan, that's actually what I did for best power. I'd read a previous post or somewhere on your site to do that. I leveled off, got everything trimmed, hit AP for heading and alt then spent a few minutes tweaking the mixture using airspeed as metric. Didn't close the vents though.

Thanks,
Doug 7A

[hecilopter]

Check this as well as this is the one place you can get a lot of extra drag if you have leaks. Also make sure you have glassed in the inlet ducts on the top of the cowling. When they are first installed, there is a pass thru between the inside of the duct and the outside of the cowling which is a serious leak. By glassing these openings closed it helps with speed and cooling.

Also, I noted something strange when setting my ailerons. Be sure your elevator is clamped neutral before adjusting ailerons to center. If you watch your ailerons when you move the stick fore and aft, the ailerons actually move up/down a little! If you set your ailerons with the elevator UP or DOWN, they won't be the same at neutral. Just an observation.

Good luck!

BTW, my -7A with Superior 0-360 180 hp, carbed, mags, Hartzell blended airfoil prop and unpainted with all intersection fairings and pants trues about 179 - 180 kts (WOT and 2650 rpm). My ailerons are perfectly straight in level cruise as well as the elevator counterweights in relation to the HS.

[Kevin Horton]

Originally Posted by 7DeltaLima

110 kias starting below 2000, timer started passing through 2000.
2-3000 :32
3-4000 1:06
4-5000 1:48

95 Kias
2-3000 :29
3-4000 :58
4-5000 1:33

85 kias
2500 - 3500 :30
35-4500 1:03
45-5500 1:37
55-6500 2:16
65-7500 2:56

Any idea what the weight and OAT were for these tests?

[Michael White]

Doug,
Reality check time...

169.3 knots = 195 mph.
Van's posted cruise speed for a 180 HP RV-7A solo (from their web page): 198 mph.
Van's solo weight (posted, Hartzel 2 blade CS): 1,400 lbs.
Your solo weight (calculated from your original post): 1,550 lbs.

So even if the MT costs you 3-5 mph, and your extra solo weight costs you a couple more, you're still within 2% of "book numbers". Why do you think you're slow? Sounds more like you're "average" to me.