Performance numbers

[Bubblehead]

As much as I like my RV-8, it seems slow compared to others. I'm not formation qualified so have not flown formation to compare with others, and have not taken any trips, even short ones, with other RV flyers. Looking at the Vans website at performance numbers, or looking at some performance numbers in the forums, makes me believe my RV is, well, slow!

Today I went up and collected data to see what my actual airspeed was. I used my Garmin 496 to record ground speed at the four cardinal directions at altitudes from 5500 to 10,500 feet. I do not have an OAT indicator, so just used pressure altitude, not density altitude.

The weather was not the best for this test. There was about .8 cloud cover and a warm day, so there may have been thermals and downdrafts. There was a very strong wind - gusting to 30 kts at the surface. If the wind was steady it would come out in the averaging of speeds but with the gusts and changing weather there is probably some error.

I used WOT amd 2500 rpm, and converted mp and rpm to %pwr using an equation derived from the "rule of 48." The plane is a 2001 licensed RV-8 with a 180 hp IO-360, standard wing tips and wheel pants. Compressions were fine at the November condition instpection. It flies well and the ball stays centered with very little help from the rudder, so I think the plane was in coordinated flight during the data runs. The prop is a Hartzell constant speed prop recommended by Vans for the -8. Exhaust is a standard Vetterman 4 into 2 system for a horizontal draft engine. Fuel was 3/4 full and I was solo, so gross weight was about 1500 lbs.

75% power and 7000 feet gave TAS of 180 mph. This seems about 20 mph low for a 180 hp RV-8.

Any ideas? Here's a picture of the plane.

[Alan Carroll]

Could be a lot of things, but for starters I'd focus on getting better test measurements. Very hard (maybe impossible) to do in strong, gusting winds.

Since you don't have OAT you might try going out early in the morning (cooler and calmer), climbing to 1500 or so, and pushing all the levers full forward. Fly several minutes on each heading so airspeed can stabilize. In light winds you can just average the 4 groundspeeds; stronger winds you'll need a more sophisticated approach (see Kevin Horton's website for example). You should see something around 210 mph if you're in line with Van's numbers.

From the photo the only thing noticeable is that the exhaust hangs down pretty steeply and looks like the lower cowl outlet has been trimmed forward of the firewall. These things will add drag, but not 20 mph worth.

[rvbuilder2002]

Originally Posted by Alan Carroll

Could be a lot of things, but for starters I'd focus on getting better test measurements. Very hard (maybe impossible) to do in strong, gusting winds.

I agree with Alan.

You first must start with test conditions that can give you valid data.
Even light turb. can have a very measurable effect on speed. You need to let the airplane run for a minute or two while being flown very smoothly (level) to get good data points.

It would also be good to get an air temp so you can work to a density altitude. So that later if you start making any changes to try and improve your speed, you have a real bench mark to compare too.

[elippse]

First off, most of the planes I have tested have OATs that are in error, and I've found, that, at least here on the west coast, forecast temps are probably as good as you'll get! Forecast winds aloft are also quite good most of the time, so use forecast winds corrected from true to magnetic, and fly into and with the wind and get your GPS groundspeeds, then average them and you'll have your TAS. Even if the wind direction is off by as much as 90 degrees, if the wind is less than 20 mph, your error will be less than 1 mph! Your plane does seem slow to me, for Jim Smith's 150 HP RV-6 with its three-blade ELIPPSE prop averaged 192.5 mph TAS from three runs at 7000' dalt, 2730 rpm.

[Aden Rich]

Agree with the above, do more tests.
If these prove to be true and your plane is slow here is what I would check.
Timing on the engine-may not be making the horsepower.
Angle of incidence on the wing per Vans instructions
Angle of incidence on the tail- your elevators should be in trail in flight with you in the plane and say full fuel. A Harmon Rocket guy was getting his rear end beat all over the sky by a 200hp RV-4. Found the tail alignment was way off. He got 17mph out of the airplane after it was fixed.
Get a wing jig and check that the ailerons are in trail along with the flaps. The trailing edges should all line up when the stick is centered.
Make sure the engine RPM is true. If the rpm is not what it says it is this could slow you down. We had a certified UMA RPM that was off by 178rpm. You can get one of those tachs you place on the dash and it picks up prop RPM.
Check wheel pant alignment as wheel as landing gear leg fairing alignment.

[Bubblehead]

Lots of good info and suggestions. I'll try for a calm morning this week. Accurate OAT is important for developing numbers vs density altitude and checking IAS against TAS. For the purposes of determining if this plane is slow I don't think it's important. The TAS at all altitudes was about 20 mph low. I do agree completely that long term I need to be able to convert to density altitude.

Good suggestions on aileron trim, although in flight the plane flights true hands off and visually from the cockpit both ailerons look like they are in line with the flaps. Aden - not sure what you mean by getting a wing jig. Can you amplify your comment?

The exhaust comment was very good. Both pipes hang down into the breeze. I ordered a set of 4 into 1 pipes from AWI last week. They should be ready late this week so I can put them on over the weekend. I'll let you all know what those pipes do to the speed. I will post a picture tonight of the exhaust pipes exiting the cowling and the lower cowl outlet.

While I have the cowling off I'll check timing and compressions.

Engine RPM - I have one of those hand held tachs. I will take it with me when I fly today.

Tail incidence is another great idea. I have manual trim so it is tough to determine where my trim tab is while in flight but if I loosen the shoulder straps I should be able to see it ok.

[rvbuilder2002]

Originally Posted by Bubblehead

Tail incidence is another great idea. I have manual trim so it is tough to determine where my trim tab is while in flight but if I loosen the shoulder straps I should be able to see it ok.

No need to see the trim tab...just look to see if the elevator counterbalance weight arms are aligned with the ends of the stabilizer.

[Aden Rich]

Van's supplied a wing jig or airfoil outline with all wing kits on the side of the kit box. I maded mine from the material the box comprised of. It's a outline of the airfoil with the aileron included so you can see if your aileron is in trail with the chord line of the wing. Remember, your flaps and airlerons can be in line at the trailing edge but that does not mean they are in trail with the chord line. I have seen some airlerons that have "hung" nose down into the airstream. Plane flys fine but this will cause drag and lose of airspeed. A RV that flys hands off still can be out of rigging, just has the rig set to take out the turns. Does the airplane show a tendency to fly out of balance at slower airspeeds? Once it gets to equiulibrium it can fly hands off but still be dragging control surfaces through the air. This will cause drap but all of the control surfaces will "look" ok. My dad worked on a RV-8 with 180 and C/S prop that got beat by all of the other similar powered RV's. It was way out of rig at slower speeds but flew fine at 180mph. I hope you get it dialed in because 20mph of drag is costing a lot in expesive AVgas. Good luck, Aden.

[Bubblehead]

Aden, rvbuilder2002; thanks for the additional information. I will check that out next time I am at the hanger andfly. I did fly yesterday. My wife and I flew over Cedar Rapids, Iowa City, and down the Mississippi a ways looking at the flood waters. Cedar Rapids and Iowa City look dry but I'm sure there is lots of damage that we could not see. At least one bridge was down in Cedar Rapids. Lots of flooding along the Mississippi though.

Here are a couple of pictures of the exhaust pipes and cooling air outlet on my -8. I've asked Vetterman's about the uneveness of the pipes and they did not seem to think it was unusually. The plane was licensed in 2001 and I'm sure Larry has tweaked things since then. Regardless, I'm going to install a new 4 into 1, hopefully next weekend.