The fastest RV's all have.............
Bryan Wood said:
There have been posts pertaining to RV's that have flown with both an 0-320 and then later with an 0-360 with information on performance gains. There are also the folks who have installed a constant speed after flying behind a fixed pitch and have shared information.
Right that data is almost exactly right off the Vans web site and specs.
Going from 160 to 180 HP is only 7 or 8 mph! HP translates more into better climb performance, not top speed. HP never hurts but drag reduction is key to going fast. BTW Constant speed v Fixed and top speed - Constant speed advantage is on takeoff, climb, landing and better overall efficiency but not necessarily top speed.
Are there any of you who have flown your RV with the cowl and baffles that come with the kit and later installed the Sam James Cowl and plenum? If so what kind of differences did you see between the performance of the two?
Unfortunately most do this mod from the start or don't take good before/after data. Some time builders make many mods at the same time but James aircraft claims:
"Average speed increase with the cowl and cooling plenum alone is 7-10 m.p.h."
I have no reason to doubt this from my research. Take a look at James aircraft yahoo group and read back messages. You may want to join and post there.
I'd point out that even if its only 7 mph, which is really significant, it is equal adding 16 HP or more! Look above 20 HP is 8 mph! So its at least like adding 16 mph or even 20 hp, depending how you cut it. However its not a miracle. If the plane is cockeyed and draggy to start with, your milage might vary as they say.
The difference with drag reduction verses HP is it's the gift that keeps on giving and does NOT BURN MORE FUEL!
Another data point.
Every RV that holds a record or top race finish almost with out exception has a "holy cowl" and solid plenum (no soft seals) of some kind. Whether it's a custom homemade or Barnard/James aircraft cowl/plenum, the fastest have some sealed set-up. The engineering or science behind this is without question and well documented.
Now why would you not do it? Well I suggest you do, but there are some draw backs I suppose. Some minor like upper spark plug maintenance may be more difficult. The decision to make this mod is up to you. Going stock and following the plan is usually always a good suggestion when it comes to time to build, cost and weight. In this case the gain I think is worth any extra build time, cost or weight.
Of any one of the major airframe mods, the cowl/plenum is top on my list. It is easier and cheaper to do the first time on a new build. Fitting soft seals on the stock baffle is not much fun either.
Most find fitting James Aircraft cowl takes longer to install than stock. I do prefer Van's stock epoxy/nomex core cowl construction over the James aircraft solid laminate cowl. The quality of glass is very good but it is a little heavier than stock. He has plans (I think) for an equivalent epoxy/nomex core cowl. James aircraft has offered a short cowl version for some time. The big issue, at least for me, was SJ cowls only came in a "long" version, which required a longer prop hub or extension. Not the case any more. "Long" cowls need constant speed props with a longer hub and they are more expensive and are not approved for aerobatics. With fixed pitch its a non issue, most go for the longer cowl. It is only an inch & half or two inch longer. Fixed prop extensions are relatively cheap.
You can modify the stock cowl and make your plenum as the above poster did and I am. It is more work but I would not say its super hard; you can always learn. However this may be the choice for those with existing fitted cowls. The savings in cost and time to keep your old cowl which is already fitted, may off set the involved fiberglass work you need to glass rings in and make a plenum. Now you can just glass over the existing inlets and plunk in rings, it will not look as pleasing or be as aerodynamic. The inlets are built forward of the stock inlets just behind the prop. Aluminum rings are available to purchase, or you can glass in the inlet shape directly, if you are good.
Frankly on the aesthetics front, I like the round cowl look but some do not. Some go half way with a sealed plenum, while keeping the stock cowl inlet. There was a thread on this recently. Some chimed in it would not make a difference in speed, but I would submit they don't know. The issue we are discussing is good before/after data again. You will not see 7 mph from a sealed plenum only, but you may pick up some speed. Part of the whole package, the full meal deal, is going from about 44 sq-inch inlet area to about 25 sq-inch while maintaining the same cooling. That is done by eliminating the internal leaks, reverse flow and making better transitions, ie less turbulent.
