Are the sonex performance numbers correct?

[planeolbob]

Here are some photos of the trip data that Charlie is talking about:

https://www.flickr.com/photos/50734158@ ... 6106765302

https://www.flickr.com/photos/50734158@ ... 4901408992

https://www.flickr.com/photos/50734158@ ... 0790558074

https://www.flickr.com/photos/50734158@ ... 7293832558

[markschaible]

Hello SonexBuilders.net!

I want to chime-in on this thread regarding the accuracy of our published performance numbers. Like my recent Building Costs post, I will archive this to the Sonex Aircraft, LLC Posts at: viewforum.php?f=41

I'll also post to the Factory Posts forum another article on the subject written by our Technical Communications Manager, Kerry Fores back in December, 2010.

As with build times and costs the phrase, “your mileage may vary” definitely applies here. There are many variables in performance data gathering, which is why proper flight test is such an extensive process. There are also many variables in equipment, particularly those built and operated in the field by customers. Our job as the aircraft manufacturer is to measure data using standard methodology in our standard equipment and publish it in standard terms so that there is an apples-to-apples comparison to the published data for all other aircraft using the same aviation-standard methods and terms.

Wheel Pants and Gear Leg Fairings:
You need them! An unfaired 4.00x5 wheel and tire requires approximately 30hp to shove through the atmosphere at 130mph. An unfaired rod section of the main landing gear leg creates as much drag at cruise speeds as an entire Sonex wing panel. Think of it this way: flying with no gear leg fairings essentially gives your Sonex the drag profile of a Sonex biplane. Want to drive John Monnett crazy? Show him a Sonex Aircraft with no wheelpants or gear leg fairings. Now, we all know that builders have their own reasons for conducting some or all flights without these fairings, but you should know that their absence will significantly reduce performance in cruise AND climb.

RPM:
Several RPM’s were discussed in this thread, which obviously has an impact on cruise speed. Remember that our AeroVee and Jabiru engines and propellers were designed to run all-day in a higher RPM range than your typical certified GA engine. Our published cruise speeds and fuel burn data for the AeroVee was measured at 3200 RPM and our published Jabiru 3300 data is measured at 2850 RPM. When it comes to fuel burn data, make sure you are properly leaning the engine to attain optimum fuel burn and power (note that our Jabiru 3300 data is collected using the AeroInjector/AeroCarb — you probably won’t have luck duplicating our numbers with the BING carb since you have no control over it’s mixture).

Atmospheric Conditions:
A HUGE variable! Note in our graph at: http://sonexaircraft.com/ads/fuel/index.html (yes, we know some of the info is dated — we’re working to update this page for our new web site, which is coming soon!) that the Competive Comparisons graph gives density altitudes for the 8,000 foot data points, which aren’t exactly 8,000 feet. This is a variable that is difficult to control in flight test due to OAT and non-standard lapse rates. Our procedure when flying these speed measurement sorties is to set the altimeter to 29.92 regardless of ATIS setting in order to fly the target altitude as a pressure altitude setting. We record OAT as quoted by ATIS on the ground and we record OAT at testing altitude. On the ground, we use the good ‘ole E6B to convert all the data to standard day conditions and adiabatic lapse rate. In the era of GPS we also cross-reference our ground speeds from the GPS with our data. We also use instrument-derived TAS numbers as a cross-reference, but we don’t rely on either GPS or Instrument TAS as our real number, as those devices are subject to their own variables. Oh, also, make sure your ASI has been calibrated!

Heading and Time On-Heading:
Also important to note that we conduct these flights using a rectangular or triangle course, averaging the data for each heading to negate winds. Data is not measured until we have been stabilized on-course for several minutes as-well. This is to ensure that the aircraft has attained it’s stabilized speed at that power setting, and that the prop has unloaded, providing the optimum cruise pitch. Remember that our wood or wood-core composite coated props have flexibility and therefore change pitch slightly according to how they are being loaded, which is why they are better all-around performers. Here at Sonex, we call this stabilized cruise condition as being “on the step” or “on plane” as-in floatplane terminology.

Props:
All of our data was collected using the standard Sensenich prop blade profiles, pitches and diameters specified for each engine and airframe combination ( http://www.sonexaircraft.com/kits/props.html ). Blade profile does make a difference, as does other variables between manufacturers, and with some manufacturers, variables from one prop to another. Just because you have a 54x44 prop from “Brand X” does not mean that you have a prop that will match the characteristics of the Sensenich 54x44 with the JV5 blade profile (W54JV5L-44).

Hopefully that should give you some insight into our methods for deriving our published performance numbers. As you can see, a lot of effort and discipline goes into collecting and analyzing these numbers, making sure that they are represented on a level playing field for comparison with other aircraft designs. Our daily experience (and yours) is going to vary from day-to-day, and even from one air mass to another within a single flight, and it will also vary from one airplane to the next based on prop, engine, fairings, maintenance and cleanliness of the aircraft, the list goes on...

Very Best Regards,
-Mark
--
Mark Schaible
General Manager
Sonex Aircraft, LLC
phone: 920-231-8297
fax: 920-426-8333
http://www.SonexAircraft.com
http://www.AeroConversions.com

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[kevinh]

Hi Mark,

Thanks for your great reply to my question. All of your comments (and the various replies from others) very much match all the variables and testing caveats that I'm familiar with from Van's land. They have all the same issues of testing variablity 'in the field' etc... My takeaway from this thread before your post was "yep, numbers are around what I'd expect and it seems like Sonex's published numbers are correct." Your post confirms that and adds important context.

I know that when I was doing the first phase 1 on my RV I had to spend many hours tuning fairing installation, running early-morning GPS triangles at 8K' for my TAS spreadsheet to reach the numbers Van's published. I won't mind at all if a similar level of fun flights of tuning is needed on my Waiex. ;-)

I'm looking forward to receiving my Waiex kit and starting building in June! Onwards and upwards! I'm also really glad everyone here has made such a nice forum.

Kevin

[Bryan Cotton]

A couple of my buddies built an RV7A down in Florida. Initially it was the slowest one in captivity and they were dissapointed. After adding all the gear leg and pant fairings, working through the cooling installs, and breaking in it was a lot better. They have a bunch of hours on it now. It is a phenomenon common to all types.

[planeolbob]

Hi everyone,
Remember, those numbers I posted were average ground speed. Not true air speed. At 3000 to 3100 rpm I show 120 mph +/- depending on the Temp and weight. Right on the posted numbers Sonex publishes.

At the end of this album I show the break in of my rebuilt AeroVee. Note the airspeed numbers and RPM. Almost identical to the published numbers.

Bob Mika

https://www.flickr.com/photos/53052604@ ... 039505910/