Gained 2 knots at cruise

[OneTwoSierra]

I just added the nose gear to cowl intersection fairing that I originally was going to leave off (have flown for a year without it). This added an extra 2 knots. (10.5K feet density altitude, about 65% power). I was surprised.

 

[fodrv7]

Intersection Drag

Intersection Drag can be very significant.

Have a look at these pics of Sptifires. The intersection fairing is huge and it isn't there for show.


Pete.

 

[Mike S]

intersection drag

A very good source of info here is Mike Arnolds video "why does it go so fast", about his record setting AR 5.

213 MPH on 65 hp.

Mike

 

[AlexPeterson]

Regarding ascertaining a two knot speed increase, how did you establish that? I'm not suggesting the increase is not real, it is just that there are so many variables. A slight change in temperature, density altitude, mixture, rpm, weight, bug splats, instrument interpretation, climb/descent, etc. can easily be more than two knots. I have found the process of speed data collection to be very, very difficult, requiring many flights, and even then I'm not sure. Kevin Horton may have some thoughts if he sees this thread.

An interesting comment on wing/fuselage intersection fairings - a number of years ago someone with a computational fluid dynamics software package digitized an RV6A, and tried numerous different configurations of wing root fairings. None resulted in a measurably speed increase on that aircraft. I recall vaguely that they understood why, but I don't recall the details.

 

[pierre smith]

Fairings.....

Allow me to chime in a little here. Years ago I built a Cassutt F-1 racer and managed 230 MPH at 3700 RPM with a 100 HP Continental. I was a slow F-1 since others were running 250 or better (1974). I bought a lot of articles about speed/fairings, etc. and one thing I learned was that a 90 degree intersection, or thereabouts, like the wing fuselage junction on the RV6,7,9 are best suited to a simple gap fairing the way Van does it.

The Spitfire and also the P-51 would have had an acute angle at the wing root/fuselage junction and a lot of drag, were it not for that bodacious fillet. Mooneys, Bonanzas and a few others have that big root fairing for very good reason.

John Roncz was consulted by Burt Rutan concerning the aerodynamics of the Voyager and if you look closely, it ended up with 90 deg or greater junctions at the wing/boom/fuselage junctions and no fillets/fairings. Guys have added radiused fairings to the RV6/7 series and LOST speed.

On the A model 6/7/9 the acute angle at the lower main gear leg junction is reduced by the generous intersection fairing radius and less drag results. Same is true of the upper intersection at the gear leg/fuselage junction,
Regards,

 

[pierre smith]

Addendum

Here's the link to a great article....over 213 MPH on 65 HP! Lotsa drag reduction info too.

http://www.ar-5.com/index.html

 

[OneTwoSierra]

Regarding ascertaining a two knot speed increase, how did you establish that? I'm not suggesting the increase is not real, it is just that there are so many variables. A slight change in temperature, density altitude, mixture, rpm, weight, bug splats, instrument interpretation, climb/descent, etc. can easily be more than two knots. I have found the process of speed data collection to be very, very difficult, requiring many flights, and even then I'm not sure. Kevin Horton may have some thoughts if he sees this thread.

An interesting comment on wing/fuselage intersection fairings - a number of years ago someone with a computational fluid dynamics software package digitized an RV6A, and tried numerous different configurations of wing root fairings. None resulted in a measurably speed increase on that aircraft. I recall vaguely that they understood why, but I don't recall the details.

I have a dynon (which has a TAS indicator) and a grand rapids EIS (which has a digital tach and a digital OAT). I have about 116 hours in my 9 now, most of which is going places. With my RV, I'm now 2 hours from my birthplace instead of 8, and 2 hours form beaches and salt water fishing. Since the beginning, I established my cruise settings at 2550 RPM (at altitude) burning just under 8 GPH. According to my memory, the charts in my engine manual says this is right around 65% power. Plugging in the OAT into my Dynon, always gave me 150 TAS. I used this because the numbers are nice and round - easy for my simple mind.

Now that same rpm setting yields 152 knots. This is how I established it. I'm always looking to learn, so if you think this is erroneous, please let me know.