Performance Question

[GrinchF16]

Finally have my 7A almost ready to fly again, this time with its new YIO-360-A1B6. I upgraded from an O360 and also installed a Cato climb prop. Wondering if anyone out there might have SA on what kind of performance I should be expecting? Specifically, what kind of climb performance are you seeing? RPM/Altitude/TAS results etc?

Look forward to hearing from folks...

 

[Mike S]

Prop details, please.

 

[GrinchF16]

Catto Glass/2xCarbon Composite
68” Diameter — 74.5” Pitch

 

[BobTurner]

A climb prop? In Indiana, where there are no hills to hit? And where most people want max speed to go somewhere?

 

[Taltruda]

I’m pretty sure that when I spoke to Catto about climb vs cruise props, they said the plane will always cruise at the same speed, it will just do so at a higher or lower RPM. Power and speed are dependent on Manifold pressure, so even your climb prop will give you 165 knots TAS at wide open throttle.. a cruise prop will hurt your climb a little, but will keep your RPM in cruise a little lower, but still making the same speed..

Back to your question.. I would expect 1000fpm climb initially, at 110 knots indicated, cruise at WOT will be around 158-164 TAS at any altitude, fuel flow around 10.5GPH at 8,500, and 8.5 GPH at 12,500 feet.

You’ll probably climb at 110-130 knots indicated for CHT cooling anyway, and that will make 500 to 750FPM or so at 130 knots.

 

[GrinchF16]

A climb prop? In Indiana, where there are no hills to hit? And where most people want max speed to go somewhere?

Now Bob, all of what you say is true. Our airplane is named The Escape Pod for those very reasons. What you miss though, is that I don’t only fly out of Indiana so climbing is good. We do have a couple of tall trees though…

What you also miss is that it was a used prop and the only one available. That combined with the fact that you do indeed get the same cruise speed, led me to approve the acquisition of the pretty little prop. Life is made up of these little compromises.

But, always remember, Altitude Is Your Friend!

 

[GrinchF16]

I’m pretty sure that when I spoke to Catto about climb vs cruise props, they said the plane will always cruise at the same speed, it will just do so at a higher or lower RPM. Power and speed are dependent on Manifold pressure, so even your climb prop will give you 165 knots TAS at wide open throttle.. a cruise prop will hurt your climb a little, but will keep your RPM in cruise a little lower, but still making the same speed..

Back to your question.. I would expect 1000fpm climb initially, at 110 knots indicated, cruise at WOT will be around 158-164 TAS at any altitude, fuel flow around 10.5GPH at 8,500, and 8.5 GPH at 12,500 feet.

You’ll probably climb at 110-130 knots indicated for CHT cooling anyway, and that will make 500 to 750FPM or so at 130 knots.

Tom, I’m hoping I can beat all those numbers. If not, I may have just spent a lot more money than I should have…. I was beating all of that with my O360 and historic Sensenich prop except only cruising at a touch over 150TAS. Thanks for the words.

Anyone else?

 

[plehrke]

Life is made up of these little compromises.

A very good thought that we all should live by (and reminded of when complaining about the current increasing costs of building/buying an RV)

 

[Freemasm]

I’m pretty sure that when I spoke to Catto about climb vs cruise props, they said the plane will always cruise at the same speed, it will just do so at a higher or lower RPM. Power and speed are dependent on Manifold pressure, so even your climb prop will give you 165 knots TAS at wide open throttle.. a cruise prop will hurt your climb a little, but will keep your RPM in cruise a little lower, but still making the same speed..

Guess I'll step in it. I should know better by now.

HP=RPM*Torque/5252

Below is a typical-ish Lyc HP-Torque versus RPM partial curve (couldn't find a whole one). It's plotted from the peak torque to full RPM.

Torque peaks and goes negative.

The power plant's theoretic torque value is determined by the geometry and kinematics of the engines bore and stroke. The resulting volume sets the physical limit of the combustion charge. an IC engine's performance is determined by how well it can breathe; no more, no less. The only remaining variable to to increase the rate of combustion events -> HP will continue to theoretically increase with RPM; thus the balancing act.

For small bands/relatively flat bands of the HP-rpm curve, you can increase HP by increasing RPM. This is done by reducing counter-torque from the prop-> pitch flatter for the given conditions. Ideally you're doing this around the peak torque RPM. A 1/2 degree on an old aluminum prop could make a noticeable difference. As mentioned, it's a balancing act. If I were in your position and sticking with a FP prop, I probably would have gone with a ground adjustable. A good indicator of proper adjustment would be engine operation achieving rpms for max torque at your desired mission conditions; climb, high cruise, low cruise, whatever. Even then, acceptable, safe performance is needed in the other flight regimes. Clear as mud eh?

 

[Carl Froehlich]

While engine performance and FP prop pitch are always good areas to examine, I find that most people ignore prop efficiency.

In other words, don’t assume any prop will be 100% efficient transforming engine power to thrust. This part touches on the “art” side of propeller design.

I’ve flown one RV with a high end custom composite CS prop. It was a good 5-7 knots slow across the board even though the prop took all the engine power just fine. That prop was replaced by another custom CS prop and the plane was faster at the same power and fuel burn.

I wonder if Catto or other propeller vendors have data on propeller efficiently. I would assume this is very dependent on propeller/engine/airframe combinations.

As with all such things, pick an engine/prop combination that meets the mission you want your plane to do and a budget you can live with.

Carl

 

[71459]

It will be whatever it will be.. you won't know until you fly it. I know everyone says Van's performance figures are completely accurate, but my airplane never quite reached his published numbers. On a climb prop, it might reach the ROC number. On a cruise prop, it might reach the cruise speed number. But my "average" airplane wont quite reach either. That said, enjoy your airplane and try not to worry about the max performance - they are simply a lot of fun to fly.

 

[Steve Watkins]

Climb!!!

That's a pretty flat pitch for 200HP. It should climb extremely well with rpm around 2400-2450 at 100kts. The issue will probably be how far you have to pull the throttle back in cruise to stay under 2700. That is what will limit your top speed. Catto recommends 76" pitch as the climb prop for 200HP RV-7. The 76" is designed to give 2750 wot at 7500' and each inch of pitch will change cruise rpm about 50-60 rpm. Your 74.5" pitch will likely reach 2830 rpm full throttle at 7500'. If you typically cruise in the low to mid teens you will be fine. Otherwise just pull the power back and enjoy low fuel burn down around 150kts TAS.

 

[GrinchF16]

Guess I'll step in it. I should know better by now.

HP=RPM*Torque/5252

Below is a typical-ish Lyc HP-Torque versus RPM partial curve (couldn't find a whole one). It's plotted from the peak torque to full RPM.

Torque peaks and goes negative.

The power plant's theoretic torque value is determined by the geometry and kinematics of the engines bore and stroke. The resulting volume sets the physical limit of the combustion charge. an IC engine's performance is determined by how well it can breathe; no more, no less. The only remaining variable to to increase the rate of combustion events -> HP will continue to theoretically increase with RPM; thus the balancing act.

For small bands/relatively flat bands of the HP-rpm curve, you can increase HP by increasing RPM. This is done by reducing counter-torque from the prop-> pitch flatter for the given conditions. Ideally you're doing this around the peak torque RPM. A 1/2 degree on an old aluminum prop could make a noticeable difference. As mentioned, it's a balancing act. If I were in your position and sticking with a FP prop, I probably would have gone with a ground adjustable. A good indicator of proper adjustment would be engine operation achieving rpms for max torque at your desired mission conditions; climb, high cruise, low cruise, whatever. Even then, acceptable, safe performance is needed in the other flight regimes. Clear as mud eh?

Scott,
Thank you for taking the time to respond. But I have to tell you that was a lot of words and I have no idea what you said. Although I did understand the last part… Clear as mud, eh?

Sincerely though, I do appreciate your response.

 

[Capt]

Every engine/prop combo is a trade off. I bought my vans (owned 2 so far) as they are best bang for buck. Below 10K Ft which is where most fly I look for best cruise speed per dollar, not climb perf. 160HP Sensenich FP is in my option the simplest most easy set up, low cost all round����

 

[maxmirot]

I have a GA prop. I can over speed to 2800rpm if I am not careful at WOT.
I am having to pull it from WOT back to 2700.

Will it go faster I increase the pitch so I max out WOT at 2700?

Max

 

[Steve Watkins]

Pitch adjustment

Max, yes it will go faster if you set the pitch so that you can use full throttle without exceeding redline. You can use a lycoming performance chart to see the power developed at different combos of rpm and manifold pressure. For the O-360 family it is about 8hp per inch of manifold pressure at 2700rpm. I have the sensenich 3 blade GA on my 180hp RV-8 and if I set it for 2400rpm climb @ 100kts it will easily go well beyond 2700 in cruise. This combo results in about 155kts at 2500 and 20". I don't like the sound of 2700rpm in cruise. If I set the prop for 2200 rpm in the climb I can cruise at 2500 and 22" at about 163kts or run wot at 2650 and 168kts. YMMV