Wheel Pants Installed - Speed 203.86 mph

Vans specs out the RV-8 max speed with 180hp at 75% power at 204mph.

N37DD came in at 203.86 this morning. (177 KTAS) Thats with no upper intersection fairings or with strut close out at wheel pants.

Catto was turning up at 2720 rpm. Power was set just rich of LOP but not at it. If I had spent time and set 100 ROP, the numbers might have been slightly better.

Also, I believe the GRT Sport EFIS TAS read out is accurate. It provides inflight wind vector/speed plus a headwind or wind component. When the aircraft is flown directly into the wind or directly away from it and the component is added or subtracted to ground speed the result agrees within a knot of the GRT TAS read out. This is using the GRT internal GPS or the the Garmin 295 ground speed which is always in agreement with GRT.

Another confirmation of GRT TAS number is with the an old Sanderson SC-3B computer. When 18C OAT is lined up over 8500' pressure altitude, 151 KIAS converts to 178 KTAS on the scale. Which of course brings N37DD in at 204.83 mph. That's using GRT IAS, if I used Vans steam gage IAS, TAS is about 4 knots faster, but common sense says EFIS is more accurate than steam. Each has its own pitot tube but share static pressure.

I admit to a compulsive disorder when it comes to performance numbers.

I flew N707DD, the -7A with the Subaru engines (2 of them) for some 400 hours thinking I had built a crooked airplane, its top speed was some 15 knots below Vans numbers for 180 hp. All that changed when I finally tossed in the towel on Subby and installed a BPE Lycoming engine - the airplane met Vans numbers easily - it was not crooked after all.

Total RV-8 time to date - 23.1 hours with 65 landings and still much to learn. On final now I say "take it lower, take it lower, take it lower...." in an effort to flare lower than has been the habit. It is beginning to sink in - had a couple decent landing today although they are wheel landings not 3 pointers.

Great #s

Good on you, David. I'm kind of a numbers guy also and bet you'll get more knots with the other fairings. I think I got 3 more with the intersection fairings.

I'm making my first trip ( via SWA) to KOSH so can cross it off the B-list... See you there perhaps?

Love to see your posts,

Jerry

David, tell us more adout your set up. What is your engine? Lycoming or other? Roller lifters? Fuel injection or carb? Also , the pitch and diameter on your prop? What is your weight? As Jerry said, you will likely get more with the intersection fairings in place. The 8 is an incredible plane.

Also, I believe the GRT Sport EFIS TAS read out is accurate. It provides inflight wind vector/speed plus a headwind or wind component. When the aircraft is flown directly into the wind or directly away from it and the component is added or subtracted to ground speed the result agrees within a knot of the GRT TAS read out.

Click to expand...

Mornin' Dave,
Still need to fly some 3-leg calibration runs. Best I know, the GRT is merely subtracting computed TAS from GPS groundspeed to display wind speed. The GS is always valid, but if TAS is inaccurate, the GRT will merely display whatever equally inaccurate wind speed is required to make up the total.

Method and the handy calculator spreadsheet at the NTPS site, or at Kevin Horton's site.

brad walton said:

David, tell us more adout your set up. What is your engine? Lycoming or other? Roller lifters? Fuel injection or carb? Also , the pitch and diameter on your prop? What is your weight? As Jerry said, you will likely get more with the intersection fairings in place. The 8 is an incredible plane.

Click to expand...

Brad,

The engine is a Superior IO-360X assembled by Allen Barrett at BPE in Tulsa. It has Superior roller tappets and horizontal fuel injection by AFP. The prop is a 3 blade Catto 68"D by 74" pitch.

Empty weight is 1029 but top of the wings has been painted since so it probably comes in at about 1035 right now.

The speed numbers are about what they should be for this engine and prop.
Craig Catto designed the prop for 205 mph at 2750 at 75% power. With close out fairings at the wheel pants and upper intersection fairings installed it will be there. That's the objective for now.

To make it go faster will require additional drag reduction and/or more power. But for now I am satisfied with meeting Vans numbers.

DanH said:

Mornin' Dave,
Still need to fly some 3-leg calibration runs. Best I know, the GRT is merely subtracting computed TAS from GPS groundspeed to display wind speed. The GS is always valid, but if TAS is inaccurate, the GRT will merely display whatever equally inaccurate wind speed is required to make up the total.

Method and the handy calculator spreadsheet at the NTPS site, or at Kevin Horton's site.

Click to expand...

Hi Dan,

The math required to calculate TAS from cardinal heading ground speed is very complex and if the input is not accurate due to turbulence or other factors, the result won't be worth much. If I find some calm air today I will do 3 runs as per the procedure and see what the spread sheet method computes.

I've done it in the past with questionable results. The recordings must be made precisely on cardinal headings and in very stable air.

I have found the old method of OAT, Pressure Altitude and IAS on a slide computer to be accurate. That's how it was done in the good old days before electronic computers. Navigators used it before GPS and Loran.

It also seems reasonable to assume a ground speed into a direct head wind and another in the opposite direction would be an accurate determination. Granted, the wind vector is created by the device being checked but ground speed does not lie, plus 20 one way and minus 20 the other is about as good as it gets. The average is TAS. The device creating the vector is being verified if nothing else.

GRT

DanH said:

Mornin' Dave,
Still need to fly some 3-leg calibration runs. Best I know, the GRT is merely subtracting computed TAS from GPS groundspeed to display wind speed. The GS is always valid, but if TAS is inaccurate, the GRT will merely display whatever equally inaccurate wind speed is required to make up the total.

Method and the handy calculator spreadsheet at the NTPS site, or at Kevin Horton's site.

Click to expand...

GRT has an internal TAS calibration utility, but you must drill into the menu to find it. Then you fly the heading/speed until it tells you to turn etc. it then applies a correction factor to that unit for that airspeed, you need to repeat for each unit installed and do this at several different airspeeds.
Tim

DanH said:

Mornin' Dave,
Still need to fly some 3-leg calibration runs. Best I know, the GRT is merely subtracting computed TAS from GPS groundspeed to display wind speed. The GS is always valid, but if TAS is inaccurate, the GRT will merely display whatever equally inaccurate wind speed is required to make up the total.

Method and the handy calculator spreadsheet at the NTPS site, or at Kevin Horton's site.

Click to expand...

Hi Dan,

Took off at 6:30 this morning in perfectly calm air and recorded 3 cardinal heading ground speed runs using the auto pilot.

Here's what the REA TAS computer came up with.
http://www.reacomp.com/true_airspeed/index.html

This was at relatively low speed.

#1 REA TAS 133....GRT TAS 135...(GS N=113 W=125 S=152)
#2 REA TAS 133....GRT TAS 135...(GS S=152 E=143 N=114)

Then ran the GRT TAS calibration procedure which came up with a -1.2 correction factor at IAS of 125.2.

#3 REA TAS 133....GRT TAS 135...(GS N=113 W=128 S=153)

It looks like GRT is running 2-3 knots fast on TAS and the calibration factor was not applied, I need to run it again, could be I did not "save" the data.

Do the flight again but at high airspeed. If the error changes then you have a static position problem and the GRT calibration is close enough