8,000 foot speed test for 7A

[Webb]

I ran the speed comparisons to Van's numbers for a RV7A with a 180 hp engine with a CS prop. At 55% and 75% power at 8,000 feet, I used several different combinations, check in 2 directions, leaned to between 75 and 100 degrees ROP, used several different MP and RPM combinations that will give you the percent of power, and averaged the numbers.

What I learned:

My plane is about 8-9 mph slower than Van's numbers at these power settings. Hmmm....I looked at the Van's website and blew up the 7A photos. The difference. I have things hanging on the plane they don't unless I can't see them). Steps, a VOR/GS antenna, ELT antenna, GPS puck, MB antenna, wing tie down rings, and second comm antenna. Since I built a cross country traveler IFR traveler, I'm satsified with the numbers.

For those that are asking how to build a faster plane, everything you have hanging out will cost you about 1 mph. Perhaps this would be a fairly decent rule of thumb.

The second thing I learned. Higher RPM for the same power setting will cost you more in fuel for the same speed. Want to run 55% on economy cruise - then crank the prop back and add more MP. The prop set at 2,100 rpm gave me about 24 miles per gallon, when set at 2,400, I was burning more gas. This doesn't make sense to me since you are producing 55% power, however the fuel flow meter says different. I'll check this one out again. And of course, YMMV.

Last thing, because of examining the Mothership's pictures of their 7A, an experience tip comes from this. For you nose draggers, always leave the tail tie down ring in. Van's had theirs in but I could not see the one for the wings. Back in my 172 days during a flair, not once but twice I managed to barely touch the rear tie down ring on the runway due to wind shear. Had it not been there, the tail would not have survived quite as well.

[andrew phillips]

I was disappointed in my numbers which I got yesterday. I do have a heavy bird (1159) and was very close to gross weight. I have the steps and 2 antennas hanging out. I was at 7500' indicated (7200' pressure alt) so I don't know how much affect that has but here is what I got for my 7a (no pants or gear leg fairings), c/s 180 hp. I was running 30 degrees LOP to get the associated fuel flows.

TAS/A/S(MPH) RPM MAP FUEL % power
174/163 2450 23 10.5 75
167/154 2350 22 9.5 65
160/147 2250 21 8.5 55

[Geico266]

I would think those numbers are good running LOP and a heavy airplane, and no leg fairings or wheel pants. Try ROP.

[Ron Lee]

Andrew, adding the wheel pants and fairings will add about 15 mph (plus or minus)

[Jamie]

Originally Posted by andrew phillips

I was disappointed in my numbers which I got yesterday. I do have a heavy bird (1159) and was very close to gross weight. I have the steps and 2 antennas hanging out. I was at 7500' indicated (7200' pressure alt) so I don't know how much affect that has but here is what I got for my 7a (no pants or gear leg fairings), c/s 180 hp. I was running 30 degrees LOP to get the associated fuel flows.

TAS/A/S(MPH) RPM MAP FUEL % power
174/163 2450 23 10.5 75
167/154 2350 22 9.5 65
160/147 2250 21 8.5 55

Your numbers are pretty much meaningless without wheelpants and gear leg fairings. Those things will really surprise you!

Those look like really odd cruise settings. You really shouldn't be getting down around 2250RPM unless you're cruising along with a Cessna or something and you're running something like 15".

My typical cruise setting for long trips (meaning loaded down) is 21", 2450 RPM. That gets me 162kts TAS, 8.6gph running ROP at 9500ft.

Lightly loaded, wide open, 23 inches, 2680RPM yields 179KTS TAS.

Also, doing two way runs is not a good way to measure TAS. The best thing is to go to Kevin Horton's very nice RV site and download the 4-way gps run spreadsheet. It only needs three legs to calculate TAS but it uses the fourth to calculate measurement error. Very nice!

[hevansrv7a]

I do leave my tail ring in, but not for protection from the ground. If you run a string from the back of the main tire's contact patch to the bottom of the rudder you will see that the ring will not protect it. I wish it did.

[Bryan Wood]

Webb,

Did you make your own pitot tube per the plans? My indicated airspeeds were originally off and then after some experimenting with the bend on the tube came in line with other RV's.

[Bob Brown]

Originally Posted by Bryan Wood

Webb,

Did you make your own pitot tube per the plans? My indicated airspeeds were originally off and then after some experimenting with the bend on the tube came in line with other RV's.

What did you do? How did you change the bend or measure it relative to the airflow or wing?

[Mike S]

Hey Bob, how is the 10 project going???

[Bob Brown]

The RV-10 project is going well, mating the mid and fwd fuselage pieces this weekend, tail kit is done, lots of subprojects are done. You're welcome to come up and visit anytime, we'll find a place for ya!