RV-9 engine power - again

Hi All,

I've read many of the posts regarding using something bigger in a 9. Only a few seem to touch on the use of a bigger engine with a fixed pitch prop.

I have a slightly modified O-320 with Catto 3 blade FP on my 9A. For years I've been mostly flying CS prop airplanes and before the 9A, had never flown a FP airplane with good performance. Here are a few obvious things I've gleaned out of that relatively short experience:

-You can't exceed the recomended TAS of the airplane, as long as you stay under engine redline. This assumes you have the prop pitched for Van's airspeeds.
-A bigger engine won't use more fuel in mid altitude cruise. You will be producing the same HP as with the 320. Otherwise will exceed redline.
-At low altitude cruise, you could burn more fuel, but that can be fixed by pulling out the black knob a little. Same with high altitude (>15k) Just control yourself
-A bigger engine won't use more fuel climbing if you climb at the same airspeed. Drag will be the same, the extra HP will use more fuel, but for a shorter period of time.
-A Catto in front of a 360 may weigh less than a CS in front of a 320.
-The cost trade, like the weight, is probably a wash. Cheaper prop, more expensive engine.

Anyway, as long as you use a FP prop (pitched to Van's airspeeds) and pay attention to the total FF weight, I see no risk associated with disobeying Van's on this point. Here is why I would want to do it:

-Maintenance with the FP will be simpler and less expensive
-Flying behind the FP is simpler. More than half my hours are behind CS (over RG), but I don't miss it a bit. Especially my instrument flying has improved with the reduction in complexity.
-A bigger engine will climb better and have shorter takeoff roll. This can be an advantage when its hot at high elevation airports, when trying to climb over weather, etc.
-You can maintain low 160s TAS to higher altitudes (16k, 17k, etc)
-A bigger engine will be running at a lower % of power in cruise (most of the time), and will likely last longer.
-The bigger motor will be run with greater detonation margin, possibly making LOP at higher airspeeds possible.

Have I missed something or is it perfectly ok to use more HP with properly pitched FP in a 9?

I think you summarized well- I might add one more thought: In mountainous regions flying with non turbo engines, we lose some percentage of rated hp (20-30%?) that were used to rate planes capabilities and flight envelope. Our ground level alt here is ~5000', and we are surrounded on 3 sides by 10-13,000' mountains.

As long as we do not exceed speed limits, it does not hurt to have a few extra horsies available to compensate for altitude-related power losses and help climb rates, or even more important, to have a bit of power available for emergencies.

Check CG and nosewheel weights closely

You may be right about a O-360/FP prop weighing less than an O-320/CS prop but you should check overall performance.

Find out if an O-320/CS prop will perform better at Leadville than an O-360/FP prop.

Find out which will run quieter because of lower cruise RPM.

Find out which will use less fuel for the same reason.

rtry9a said:

I think you summarized well- I might add one more thought: In mountainous regions flying with non turbo engines, we lose some percentage of rated hp (20-30%?) that were used to rate planes capabilities and flight envelope. Our ground level alt here is ~5000', and we are surrounded on 3 sides by 10-13,000' mountains.

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You might consider a new wingtip design. I designed one for Jim Smith's -6 that will give him a 6 to 10 sq ft more area than present, depending on how much area is in the tips he removes, and will increase his AR from 4.8 to 5.8. That will reduce the -6's considerable induced loss at higher altitude for more speed and give better TO at the hotter and higher-altitude airports. "Course, a -9 has higher aspect ratio, but would still profit from the same treatment for those who operate out of the higher and hotter airports. Something to consider!

I think you have one point wrong.

At any given alt I can run my engine at 2700 and put as much hp as the engine will make into the prop.
With your CS, what is your static RPM?

I think most will be about 2300. That is only 85% of the rated power of the engine. Which is 153 hp. A little less then my 160 hp.

If you pitch the prop to get better takeoff performance then you will suffer with your cruse performance.

I would also be very concerned with the weight up front. I don't like all the weight that I am packing, but would want to do a careful check about the claimed weight differences.

I would be quite happy to do a takeoff fly off with the 360/CS, but no I would probably loose in an all out run.

Kent

elippse said:

Course, a -9 has higher aspect ratio, but would still profit from the same treatment for those who operate out of the higher and hotter airports. Something to consider!

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The reason I chose a 9 over a 7. The engine I picked produces 200+ rated hp (presumably at sea level) which should yield around 160 hp at altitude and a bit more for takeoff; also planning a lightweight adjustable prop.

Paul, what was the tip design- extend the wing several feet or some kind of winglet? (removable) tip tanks that also add lift would be a great addition.

Exactly

The difference between the 320 and 360 engine really is very small indeed and if you use that to offset the cost and complexity of the C/S prop then I think you got a winner.

Remember though for max efficiency you want to pitch the prop for wide open throttle at cruise.

As I said when I was engine shopping the 320 was about the same cost as the 360 so the bigger motor with cheaper FP prop would have been an easy choice as long as W&B worked out.

Frank

rtry9a said:

The reason I chose a 9 over a 7. The engine I picked produces 200+ rated hp (presumably at sea level) which should yield around 160 hp at altitude and a bit more for takeoff; also planning a lightweight adjustable prop.

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Just keep in mind that Vne is altitude dependent......I suspect Van had that in mind when he set 160 hp sea-level as the max for his RV-9A design.

Let's be careful.

Hi Steve,

The following was posted some time ago in response to another individual who was considering a 180/360 for his 9a. I think my experience pretty much supports your speculations. One item you suggest which is not correct (or wasn't when I purchased) is that a 360/Catto combo is a wash in price when compared to a 320/CS combo--my Eci0360/Catto was $4500 less than a 320/Hartzell combo.

Aussie 9a,

You asked for input from anyone who has "experience fitting 180 hp to an RV-9(a)". I did just that and have been happily flying that airplane since March of last year. Many of the reasons for the selection of that engine parrallel those of Questair. The following is a summary of the features that we wanted in an aircraft along with the factors that lead to my engine/prop selection;

I wanted a two place airplane that was a stable IFR capable cross country flyer (no interest in aerobatics) so I selected the 9a. I wanted simple operation as my wife intends to finish up her private pilot training in the plane. However, it needed to have excellent take off performance as we will be living in N. Az. where afternoon summertime density altitudes are frequently challenging. I also wanted to keep costs down (who doesn't), wt down, and select an engine/prop combo which provided smooth/quiet operation on long x-c flights.

I boiled my options down to a 9a with an 0320 and a Hartzell cs (Vans recommendation) or a 9a with an 0360 and a Catto 3 blade composite fp.

Cost: The difference in cost of a new experimental 0320 vs 0360 at Vans or Americas Aircraft Engines (ECi) is $500--I suspect Mattituck and Superior are similar differentials.
The cost of a new Hartzell plus governor is around $7K (at the time I was shopping) while a Catto 3 bade with the Sabre spacer is $2K. The cost of the 360/Catto is therefore $4500 less than the 0320/Hartzell.

Weight: The weight difference between the 0320 and the 0360 per Lycomings certificated weight tables averages less than 10 lbs. The difference in wt between the specific 0320 model Vans sells(D1A) and the specific model 0360 they sell (A1A) is less than 6 lbs.
The Hartzell CS with governor totals around 60 lbs. while the Catto plus spacer weighs in at 20 lbs. Using the 10 lbs engine wt differential, the weight of the 360/Catto is therefore around 35 lbs lighter than the 320/Hartzell.

Performance: Although I have never tested this, I suspect that the 0320 with the CS prop will climb better than my fp 0360 but my guess is that the difference is small.
The cruise performance of the 0360/Catto is better than the 0320/Hartzell if you want to use 75% power numbers---if you cruise the 0360 at 65-67% you will see close to the same performance in cruise/ff as the 0320 running at 75% while creating less noise and putting less stress on the engine.

I fly this plane at 66-67% power (seems to have a sweet spot there) per the Lyc chart and see performance #s that exceed Vans published 75% 0320/CS numbers (probably due to the James cowl/plenum). Additionally at these relatively low power settings the engine/3 blade composite prop combo is extremely smooth and with the slower prop tip speeds it is also relatively quiet.

Downside: If you elect to fly at 75% power you will not be able to hit Vne in level flight at altitude (with a stock 180 hp) but you will be closer than a 320 9a and must therefore be careful when transitioning from cruise to descent--although any RV will quickly pick up speed in descent if the power is not managed correctly.

Given the options would I do it again? Lets see: over $4000 less cost, 35 lbs less wt, better cruise capability, similar climb capability, less engine stress, lower noise, less complexity/maintenance, and smooth operation-----yea, faced with the same two options I would most likely do it again!!

I'm going with a similar setup: Superior IO-360 with probably a Sensenich 85" cruise prop. Weight of the 360 is marginally more than the 320 (about 7-10 pounds) which more than compensates for the difference between CS and FP weights. I am a tiny bit concerned about the ultimate load on the nosewheel but can probably shift things around if necessary. I am doing this for similar reasons: more HP if needed on climb but similar fuel consumption at cruise as the 320 (from what I have read it might even be more efficient at certain conditions).

greg

Steve,
I agree with most of your points. I nearly went for the 0-360/Catto combo but in the end decided on the 0-320/Hartzell C/S. The most important factor for me was the better braking effect produced by the C/S prop. My 9A is based on my farm airstrip (1400 ft at 4,300 ft altitude). There is rising ground at each end and I doubt that I could safely and consistently make the desired touch down point without the C/S prop. Once I realised I needed a C/S prop for landing then the 0-360 was no longer an option as the 0-360/C/S prop combo would have been too heavy (IMHO).
I also like being able to alter power and RPM to get just the setting I want. I think the extra knob makes for a more interesting/fun aircraft to fly (again IMHO).

Fin
9A

rtry9a said:

Paul, what was the tip design- extend the wing several feet or some kind of winglet? (removable) tip tanks that also add lift would be a great addition.

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It's a 30" extension, with a straight trailing edge, a tip with a 15" chord, and the leading edge going from the LE of the wing straight back to the LE of the tip, to form a truncated right triangle. Each tip has 7.5 sq ft, for 15 sq ft additional area, and the previous tips were about 5' X 1', so he will net about 5 sq ft with a little increase in profile drag, about 4.5% based on area increase, but a big increase in AR to considerably reduce induced loss at the higher altitudes. I spoke with a crop duster about his biplane which had similar tips, and he told me the handling and carrying capacity was outstanding. I was speaking to the late Gary Hubler about Jason Newburg's biplane, Jamaica Mistaka, with tips like that, and Gary also mentioned about the duster with those tips, and how it had the best weight-carrying ability vs power. Jason told me that the tips changed the whole character of his Pitts, and his first landing with them on resulted in the Pitts floating and floating when carrying the previous approach speed. Tips like this have an Oswald efficiency factor of about 0.82, which, along with AR, is very important in determining induced loss. The higher the value, the lower is CDI, since OWE is in the denominator of the CDI equation. By comparison, a round tip planform, or a rounded square tip, will have an OWE of 0.75+/-, which would give a CDI over 9% higher. It's all in getting the vortex to form as far out as possible, and getting the lift from area to approach zero (zero chord) at the tip to minimize the pressure differential which causes the flow toward the tip and the resulting vortex. I have a similar tip shape on my Lancair, and Peter Garrison has similar tips on Melmoth.

Removable tip tanks & tips

Some input on the wing extending tip tanks and longer tips statements.

When I built my RV-6A I included tip tanks produced by the late Farn Reed. These extend the total span by 18 inches and the stock tips are attached to the outboard ends of the tanks. This system works very well for normal operation. For short races I developed 3 inch wide tips to replace the tip tanks and stock tips. It is 3 kts faster in this configuration at 6,000 ft density altitude. The handling is more like what I expect an F-86 is like - there is no float at all and I can grease it on nose high and fast every time. The roll rate seems faster but the airplane is definitly faster and it has gone from trailing Mike Thompson's RV-6 by several knots to beating him comfortably in the SARL (http://www.sportairrace.org) cross country air races (all races and results are posted at the SARL site). I was going to build a new set of my tips to replace the stock ones outboard of the tip tanks but AirVenture Cup is now allowing a no penalty fuel stop so it wouldn't provide any race advantage.

Bob Axsom