What am I missing?

[johnr9q@yahoo.com]

Noel: Can you be more specific. Do you think the Aerovee is the best VW conversion? I wasn't referring to the Shade Tree conversions just the larger companies like Revmaster, Great Plains, Aero, and Hummel. From my limited ability to make judgements as to what is better, it seems like Great plains has a good idea in putting a speed reducer on their engine. Others tout more robust crankshaft and bearings. Does a longer stroke in some of the engines bring the maximum torque at a lower RPM?

[johnr9q@yahoo.com]

seanmb: Again, maybe I am missing something but I thought that the shorter prop was required to preclude the prop from going supersonic. And, again, maybe I am missing something, a shorter prop isn't as good as a longer prop.

[Rynoth]

seanmb: Again, maybe I am missing something but I thought that the shorter prop was required to preclude the prop from going supersonic. And, again, maybe I am missing something, a shorter prop isn't as good as a longer prop.

Aeroconversions tests dozens of props for the Aerovee application, and I'm certain that prop efficiency is of primary concern (as well as ground clearance.) The specified props are likely picked for their economy, reliability and efficiency. Doing a simple circumference/RPM calculation tells me that tip speed won't reach the sound barrier until well above 4000 RPM, as seanmb mentioned.

Sure, an infinitely long propeller offers the best efficiencies, but for practical purposes, a 54" prop on an Aerovee powered Sonex seems to do the trick, as evidenced by the number of them flying.

[Fastcapy]

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[Fastcapy]

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[seanmb]

seanmb: Again, maybe I am missing something but I thought that the shorter prop was required to preclude the prop from going supersonic. And, again, maybe I am missing something, a shorter prop isn't as good as a longer prop.

My reply only adressed your statement that you could not get to 3500 RPM due to prop going supersonic.
Why is a longer prop better? (Not trying to confrontational, just trying to get you to question your assumptions.)
I just showed you in one area where the shorter prop was better.
It is all about engineering trade offs to meet your design goal.
In terms of thrust, longer is better, and is largely affected available torque. Need more trust, get more torque with re-drive. Carry that on out to the extreme you have a helicopter. We are not not building a helicopters, but enough thrust to get us off a short strip is a good thing.
Top speed is dependent on airframe, HP available and prop design. The "goodness" of the prop here can only be measured in regard to the airframes performance specs, largely drag profile, and the power output profile of the engine. By changing pitch and airfoil design of the propeller blades you can design a best fit for your goals while being mostly independent of length consideration.

And this brings us back to the original question. Again it comes down to design goals, and the engineering trade offs required to meet those goals, and we have different design goals in play.
For the Sonex, the goal was 2 person, aerovee power and aerobatic, and they met their goal. Though some would say they shaved it a little bit when it comes to the 2 person part.
Both Zenith and Van's decided for 2 person with reasonable x-country appeal, and both are very capable with that goal in mind. That design goal required more power to offset the addition weight and profile drag when compared to the Sonex.
But in all three cases the amount and material used and hence price of the airframe is very similar. But both Zenith and Van's need the larger more expensive engine choice. If you spent the money for a more powerful power plant in the Sonex, you would still not be as suited to the x-country role, but you could literally be flying loops around the other two along the way, or at least until you had to land to refuel.

[Bryan Cotton]

Long props are good for static thrust and climb. The helicopter example is a good one. I was a Sikorsky engineer for 23 years. Smaller props are good for high speed. You need to choose a prop that is a balance between low and high speed performance and you want to avoid supersonic tips. The Sonex climb performance is not due to lots of excess thrust. Is is due to light weight and low drag. A J3 has light weight but high drag, plus low speed, so it does better with a long prop.

[rizzz]

Hi John,

Welcome to the forum.

Looking at this posts here and your posts on the VansAirforce forum I get the feeling you really want to build and (part-)own a 2 seat side-by-side aircraft, however, you are not that keen on the Sonex (correct me if I’m wrong), yet you are drawn to it because of the "alleged" lower cost.
Please take my comments below any way you like, I am certainly not trying to try and steer you in any direction. You should make the choice you are most comfortable with off course.

Firstly, as others have stated, if you’re limited by budget no aircraft will suit 100% of your needs.
Define your “mission” and see what suits your needs/desires the most.
Make a list perhaps, what are absolute “musts”, what are “wants”, what are “nice to haves”. Make sure to include (building AND operating) cost into this and then score the options accordingly.
Then, define your "mission", for an example of a mission just have a look on Jeff Shultz’s website under “Why a Sonex?”: http://www.sonex604.com/

Secondly, as I’ve just touched on, owning and operating an aircraft is not cheap. If you’re limited by budget like most of us you must take this into consideration.

Thirdly, building an aircraft will ALWAYS end up costing you more money than the manufacturer claims (it will also weigh more and take more time to build than whatever they say :-)).
For example: in the numbers they state they assume the absolute minimum in instrumentation, engine, etc.; they don’t add shipping costs (a major factor for some of us); they don’t take into account the parts you WILL stuff up and buy replacements for; they generally don’t include things like electrical wire, terminals, connectors fuses etc., nor do they include fuel/oil system accessories such as hoses, fittings, ….; they don’t include products such a primers, sealers, paint, adhesives, sandpaper, …; they don’t include all the tools you will need to buy; they will usually include the minimum hardware required but not all the extras you’ll end up needing; (the list goes on and on and on and all this adds up to thousands of dollars easily).
Anyway, the total building cost these companies state are very, very, optimistic and they conveniently ‘forget’ to include items you as a potential new customer would not immediately think of either when doing your sums.

Finally, DO NOT DISMISS scratch building as an option. And that goes for the Zenith as well (I don’t believe you can scratch build an RV-12).
Your statement about not many people going down that path is certainly not true in case of a Sonex. Many have been (partly) scratch built. I’m not sure of the percentage of Sonex’s out there flying that would be scratch built to some extent but it would not surprise me the number is over 30%.
I’d really like to talk a bit more about this as I believe you should consider this option if building cost is a great concern to you:
There are degrees of “scratch building” and there is a lot of money to be saved making parts from scratch. I’ve read posts from builders on the old Sonex forum who literally built every component themselves and managed to keep the cost of the entire airframe under $5000! Now those are indeed extreme examples and yes, not many people do this.
HOWEVER,
Most of us who say we’ve scratch built our Sonex, when we do so we’re really only referring to the aluminium parts.
Most “scratch builders” do not fabricate the welded steel components & fibreglass components from scratch, we still buy those from Sonex, and this way we don’t require to have any extra tools/skills/techniques under our belts to complete our project compared to a standard kit builder but we can still save a lot of money, we just need to invest a bit more time into the project.
Remember any kit built aircraft needs to comply to the 51% rule so there is still A LOT of cutting/drilling/filling/sanding to do and many parts still have to be cut from “raw materials” (bare sheet / angle), as a “part scratch builder” we just have to do more of this but there aren’t really any procedures we need to perform or techniques we need to acquire that standard kit builders won’t have to.
I think average built time of a kit builder is about 1000-1200 hours and a “part scratch builder” would be closer to 1500 hours, so yes, significant time added to the project but you will also save anywhere between 5K to 10K if not more (depending on how much you are willing to scratch build).
More on (part)scratch building a Sonex can be found on Eric Witheroon’s website, he scratch built 2.5 Sonex’s (you’ll see what I mean by that when you read his stories/logs):
http://www.spoonworld.com/
(make sure to read his “Low cost strategy article” under “Home Building Hints” -> “Strategy”.

[rizzz]

In all my investigations I assumed that the Sonex was so much cheaper than the other 2 but there is only a $6600 difference between the most expensive (RV-12) and the cheapest (Sonex). Correct me if I'm wrong, but it seems to me that you get a lot more airplane with the RV-12 and CH-650 and they aren't that much more expensive.

Complete Airframe Kits, Kit Minus Tail Packages, and Quick Build Kits will increase in price by $1,000.
Sub-Kits will increase in price by approximately 5 percent each.
Dual Stick upgrade price for the Sonex Complete Airframe Kit, Kit Minus Tail Package and Quick Build Kit will increase to $325.

http://sonexaircraft.com/

Looks like Sonex is closing the gap even further.

[SvingenB]

The OP got a point though. Comparing apples to apples (RV-12 + Rotax 912 and Sonex + Jabiru 3300), same level of instruments, same level of paint, and the Sonex and the RV ends up roughly the same. But he also misses another point, the Sonex can be made for much less, while the RV cannot.

The only point to remember though, is it is guarantied to end up costing much more than you first thought, no matter what aircraft you are building. ;)