Airspeed readings in the 9A
G.P.
Here are a couple of numbers that might help you with your decision. These are not to sway you one way or the other, just to give you an idea of what to expect and will probably not see if they are not shared with you at this point of your building. I'm using numbers that you can confirm on Van's website under the RV-9A's performance. Also I'll give you markings from the 9A's airspeed indicator as it comes from Van's. These are numbers that you can clearly see when flying in a 9A, but in the building stages they are hard to put together.
Airspeed indicator:
flap extension 90mph or 78kts
manuevering speed 119mph or 103kts
yellow 180mph or 156kts
red 210mph or 182kts
75% power @ 8,000' 161mph or 140kts (indicated)
187mph or 162kts (true)
These are actual numbers that you could expect to see if you install a 160hp engine on your 9A and very similar to what I see on mine. I'll try to explain what they mean, or better yet how they translate to the question that you are asking without insulting you hopefully.
To obtain true airspeed the indicated is increased by approx. 2% per thousand feet. This is ballpark, but close enough for this. With a 140 knot indicated airspeed at 8,000 feet you would take 2% of 140 knots which is 2.8. Then you would multiply it times 8 representing the 8,000 feet which gives you 22.4. Adding 22.4 to the indicated speed of 140 knots results in just over 162 knots true. This is exactly what Van's claims for performance on the 9A for these conditions.
So, how does this answer your question? Simply put, at 8,000 feet your indicated airspeed is 19mph under the yellow! As far as I can tell there is room for the 180 hp in the 9A as long as you respect it when the bumps come. This also keeps you under the 210mph speed if flutter is your concern during normal cruise. (It was recently explained by Van's that Flutter is caused by velocity not pressure. This means under extremes you could get flutter before the indicated speed shows into the red)
Now that I have a decent amount of time in my airplane and can see that there is room for 180hp there are still different questions that would need to be considered before going with one engine over the other.
1. Fuel tank size, 36 gallons is 3.5 hours running an 0-360, and 4.5 hours running an 0-320. Of course the 180 could be powered back, but...
2. Weight, the RV's are desired for their feel. A light plane is a treat to fly.
3. Prop pulses- these are a debate and will bring responses from both sides of the fence. Early RV-4's and 6's had thin skins on the tail feathers and developed cracks when mated to a 180hp airplane. Van's responded with thicker skins on these designs. The 9 has thinner skins, but also different trailing edges that the other designs. Time will tell on this.
4. Insurance- Not being a recommended engine by Van's it could possably become an issue some day if you run an 0-360. This will probably get me flamed by the XP-360 folks, but who knows how it will all play out in time. As one of the earlier builders (kit #8) I checked on this early on with the insurance companies. I was told back then that as long as the engine fit the mount that Van's sold me it would be okay. Since they would sell the mount that would fit either installation it wasn't going to be a problem then. Newer builders, what are they saying now? Hopefully the insurance will not ever become an issue for any of us.
5. Most importantly is the designers intent for the airplane. Van's still strongly recommends the 160hp as the max. power for this airplane.
I hope this offers you more information to think over. There is nothing like displacement, but it might not be the best answer on the 9. Frankly, the Sam James cowl with an 0-320 will net you more than the bigger engine in cruise and will be much more efficient. However the 180 will climb like crazy.
Best of luck to you,
