True Speed Help Please

AndyRV7 · #1 07-16-2010, 09:42 AM

Hi,

I am scratching my head a bit these days trying to figure out the real speed of an RV7. I was hoping someone could help.

I flew my transition time with Jan Bussell recently. It was my first flight in a 6/7 (Can't recommend him enough!). His plane cruised at 185mph and I was surprised because I thought it would be closer to 200 with 180HP engine. It turns out that every plane I have inquired about or flown in since Jan's has been slower. On the order of 170-178 mph versus his 185.

I realize there are a lot of variables like weather, prop type power setting, altitude, and weight, but for the sake of argument, these numbers seem to be the true or real-world cruise speed of these particular planes. I didn't get the impression that these slower speeds were at an economy power setting, yet the planes are a full 19-21 knots slower than Van's numbers say they should be.

So I am trying to get a better handle on what kind of speed I could expect from an RV-7. Maybe my expectations are too high??

Thanks. Andy

Ron Lee · #2 07-16-2010, 10:05 AM

Last time I checked I was at or higher than the Vans official spec number. I do not cruise at that setting (fixed pitch prop) but can match his numbers.

Brantel · #3 07-16-2010, 10:22 AM

I am real close to Van's numbers and I do not have my fairings slicked up yet and do not have lower intersection fairings on yet. I get ~ 170KTAS @ 8000ft wide open throttle now with rough glass and no lower intersection fairings solo. I have a carbed O-360 with a FP Sensenich prop.

You have to compare apples to apples to get a good comparison.

Was it IAS or TAS, 75% power or ??, solo vs gross weight, what prop, what eng., etc, etc...

sprucemoose · #4 07-16-2010, 10:32 AM

Andy,

Van's published performance numbers are accurate, and have been verified time and time again by professional testing, not to mention almost 7,000 completed aircraft. Even allowing for slight differences due to props, constructions quality, etc. the published numbers are right on the money.

The industry standard for measuring cruise performance is 75% power at 8,000 feet, leaned for best power. In a normally aspirated engine this is wide open throttle, and probably at or near redline. If your testing conditions were not these, then your numbers are not valid for and apples to apples comparison. If you have the opportunity, fly again in that same airplane and fly it in the above conditions and see what you get.

For example, my airplane will consistently true out at 173 kts (+/- 1) at those conditions. However, I flight plan for a more fuel efficient 160 kts. So, what would you say is the cruise speed of my airplane?

AndyRV7 · #5 07-16-2010, 11:38 AM

Thanks for the insight. I hesitated to even post this because there ARE so many variables. I was figuring cruise to be 75% power like Van's publishes, but I guess that didn't have to be the power setting that was used by the people quoting me the performance of their plane. I just figured if someone was trying to sell a plane and told someone how fast it was, they wouldn't be telling that person what the economy speed was. Incidentally, one plane was a Catto 2 blade fixed. The other was a 3 blade constant speed. Jan has a fixed 2 blade Sens on his.

I have figured that 190mph was a safely conservative performance margin from the 200mph Van's quotes. That works out to about 165 knots. So if I could find a plane that "cruises" at 165kts, I would be happy. When I started to see these planes coming in 10-15 knots slower I was confused and figured I ask some other opinions.

I do agree that a good test would be to take another ride in a plane and see what the actual power setting and speed was. I am still learning!!

Thanks for the responses.

Andy

Bavafa · #6 07-16-2010, 12:10 PM

I had not been pushing my plane at higher power setting since the breaking period and really didn't know my top speed, but consistently cruised at 187 mph or 163-164 knots.
A couple of nights ago, I was calibrating my TAS and decided to go to WOP at 25 square and at 5000 feet, my TAS was 177 knots or roughly 204 mph. I thought it would be faster but I am happy with that. I am going to try it at 8000' tomorrow.

Bubblehead · #7 07-16-2010, 12:24 PM

Last night in my -8, 8500 ft, hot, hot day (I don't have an OAT probe) 2500/22" (WOT) leaned to best power I got 159 kts or 183 mph. This was determined using the 4-leg GPS ground speed method.

I've thought I had a slow -8 but maybe mine is in line with some of the other numbers. I will fly this weekend and go to 2700 rpm and see what speed I get.

I found an easy way to get the average of the 4 legs easily. Pick what you think the average will be. Last night I guessed 150 kts. As you do the 4 legs just keep track of a running total of how much above or below the guess is. After the 4th leg divide the + or - by 4 then add or subtrack it from the 150.

I've found you can do the calculation in your head.

Example:

DATUM 150
N 165 +15
E 160 +10 (subtotal = +25)
S 150 0 (subtotal = +25)
W 161 11 (subtotal = +36)
+36/4 = 9
ave 159 159 (150 + 9)

Looks like Brantel and Ron Lee are kicking a** with their planes.

AndyRV7 · #8 07-16-2010, 12:33 PM

I'm getting smarter by the minute.

Just to be a little clearer too, I do not fly a CSP plane, so when people speak to me about pressure and RPM, I have no idea what they are talking about. I usually just try to remember the numbers and go home and ask someone else what that meant!!

rvbuilder2002 · #9 07-16-2010, 12:52 PM

Quote:

Originally Posted by AndyRV7

Thanks for the insight. I hesitated to even post this because there ARE so many variables. I was figuring cruise to be 75% power like Van's publishes, but I guess that didn't have to be the power setting that was used by the people quoting me the performance of their plane. I just figured if someone was trying to sell a plane and told someone how fast it was, they wouldn't be telling that person what the economy speed was. Incidentally, one plane was a Catto 2 blade fixed. The other was a 3 blade constant speed. Jan has a fixed 2 blade Sens on his.

The majority of the problem is because of two things...

#1
A large protion of the pilot community does not know what power conditions (MP, RPM, and Mixture) actualy produce 75% power.

#2
A large portion of the RV fleet with fixed pitch propellers are running with a pitch that will only produce 75% power at low altitudes. It is the higher altitudes (8000 ft +) and 75% power that produce the True airspeeds quoted by Vans.

AndyRV7 · #10 07-16-2010, 12:56 PM

Quote:

Originally Posted by rvbuilder2002

#2
A large portion of the RV fleet with fixed pitch propellers are running with a pitch that will only produce 75% power at low altitudes. It is the higher altitudes (8000 ft +) and 75% power that produce the True airspeeds quoted by Vans.

Now THIS is exactly what I was afraid of. If so, then you could truly be in a plane that tops out at substantially less than the capability of the plane.