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How much RPM gain would I get from upgrading to PR wheel pants?

00Dan

Well Known Member
As the title implies, I have a fully faired airplane but with the old one-piece wheel pants and the two-piece aluminum leg fairings. I recently purchased a second hand three blade Catto and found it to be rather cruise pitched for my plane - my static RPM is approximately 2090, and it winds up to 2150 in the takeoff roll. My WOT RPM at 7500’ is 2640.

In speaking with Nicole, it seems I could benefit from a repitch for more static and climb out RPM. In the process of this, I inquired if the additional airspeed from the newer wheel pant design would make up any of the difference; she wasn’t sure.

So that’s my question. Will upgrading my wheel pants let my prop spin up to the magic 2700 RPM as currently pitched? Similarly, while it obviously wouldn’t affect static, would the takeoff run and climb RPM be improved at the same airspeed? If yes, may it prove sufficient or is the static RPM still just a tad too low?

For reference, the prop is a May 2016 production (I’m not sure if that’s second or third gen.) on a 150 HP RV-4.
 
3-5 mph

In an article in the 2000 2nd RVator (200 mph on 160 hp) they saw 3 to 5 mph gain by changing to the newer wheel pants and also said 'as advertised'. So I guess someone back then was saying 3-5 mph and they confirmed it.

Oh, BTW, this was on RV-6A's
 
I bet you'll get most of the 60 RPM increase at 7,500' and a few more MPH.

On takeoff and climb, you'll probably get a very, very modest performance increase, but I think it would take a gen-u-ine test pilot with a kneeboard, a stopwatch, a flightsuit, and maybe a few epaulets to actually measure the improvement.
 
I bet you'll get most of the 60 RPM increase at 7,500' and a few more MPH.

On takeoff and climb, you'll probably get a very, very modest performance increase, but I think it would take a gen-u-ine test pilot with a kneeboard, a stopwatch, a flightsuit, and maybe a few epaulets to actually measure the improvement.

I agree with Kyle. It seems like your prop is almost perfect right now. If you re-pitch the prop for higher static, you’ll probably overspeed if you are trying not to exceed the 2700 Lycoming recommended limit. The wheel pants will give you more RPM at higher speeds because of less drag rise, but not so much when slow. The one piece gear leg fairings won’t make any difference at all, unless the two piece ones you have now are in bad shape or not aligned properly.
 
On the prop, be sure to think through your typical flight habits. My 320 has a moderately under pitched prop and fits my style perfect. I get better climb and at 8000', I could get a bit over 2800, if I ran ROP. However, I almost never cruise ROP. At 8000, I get 2730 RPM at 8 GPH and that is perfect for me. I can also get 2690 at 13000' at peak or slightly ROP. If I had more pitch, I would probably get similar cruise performance (at a lower RPM), but could go faster with more fuel without hitting 2800, however, my climb rates would be lower. Its all about finding the right compromise with FP props.

Remember, lycoming's RPM limits have tach error factored in, as back in the day, the old mechanical tachs often couldn't get any closer than that. Lycoming allows + or - 5% accuracy on the tach, I believe. The means that ultimate limit is really 2835, if you have a trustworthy, modern electronic tach. I do most of my cruising around 2720-30 and engine has done well fo 700 hours. I am not in the camp that thinks the engine will grenade at 2701 RPM, though I do respect the other camp that does. We each have to form our own opinions and limits. I should add that I would not be so cavalier with the RPM if I didn't have a tach that I trusted to be +/- 10 RPM.

Best to think through the options.

Just another opinion to consider.


Larry
 
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RPM

In the era of the Pitt's with fixed pitch prop and parallel valve engine, the engines usually were overhauled at 12-1400 hours because 1200 was the recommended TBO for aerobatics. every aerobatic flight in that era was 3300 to 3500 rpm depending on prop pitch.
The geared Lycomings from the 400 series to the 541 all turned up in the 3500 range. TBO's were in the 1200 hour range.
To have any concern about 2800 or even 2900 rpm is ridiculous.
 
The prop is designed for 2750 RPM on a 160 HP engine. I’m not too worried about the 2700 RPM limit based on what I’ve read, I’m more using it as a baseline for determining over/under pitched.

I ordered the new fairings, so I’ll see how far that gets me before I make a non-easily reversible change like a repitch. Of course, I could also just consider the prop as is future-proofing me when I eventually redo the engine and go 8.5:1.
 
My Catto 3-blade prop is pitched a little bit lower than I thought I'd like at first (WOT at 8000 DA is around 2830 RPM, 2250 static). To Larry's point, although I've considered re-pitching it to hit 2700 RPM at WOT, I find that with my typical flight profile I enjoy the higher climb rate and faster acceleration from my prop. I do fly cross-countries where I wish I could run WOT at a lower RPM, but I honestly don't fly XC that often compared to local fun / acro / breakfast runs.

That said, I feel if I were you, I'd definitely want to have it re-pitched so that I could get more RPM out of it. 2700 at least. Seems like you'll gain top speed and climb performance with that change.
 
My Catto 3-blade prop is pitched a little bit lower than I thought I'd like at first (WOT at 8000 DA is around 2830 RPM, 2250 static). To Larry's point, although I've considered re-pitching it to hit 2700 RPM at WOT, I find that with my typical flight profile I enjoy the higher climb rate and faster acceleration from my prop. I do fly cross-countries where I wish I could run WOT at a lower RPM, but I honestly don't fly XC that often compared to local fun / acro / breakfast runs.

That said, I feel if I were you, I'd definitely want to have it re-pitched so that I could get more RPM out of it. 2700 at least. Seems like you'll gain top speed and climb performance with that change.

Just a heads up on overspeed limits and inspections. If you have a Lycoming engine with a 2700 RPM redline, anything other than a momentary excursion above 2835 requires an overspeed inspection. I know we are exp. but this is for safety. See Lyc SB 369S. You also need to consider any prop RPM limits. In my days of doing engine overhauls, I saw overspeeds result in real damage.
 
Just a heads up on overspeed limits and inspections. If you have a Lycoming engine with a 2700 RPM redline, anything other than a momentary excursion above 2835 requires an overspeed inspection. I know we are exp. but this is for safety. See Lyc SB 369S. You also need to consider any prop RPM limits. In my days of doing engine overhauls, I saw overspeeds result in real damage.

Thanks for the info Dan. I don't run above 2700 other than when I tested the WOT max RPM of the prop / engine combo. I usually run at 2650 or less in a X/C cruise setting. In this case, I do have to pull the throttle way back which is why I debated re-pitching the prop.
 
In the era of the Pitt's with fixed pitch prop and parallel valve engine, the engines usually were overhauled at 12-1400 hours because 1200 was the recommended TBO for aerobatics. every aerobatic flight in that era was 3300 to 3500 rpm depending on prop pitch.
The geared Lycomings from the 400 series to the 541 all turned up in the 3500 range. TBO's were in the 1200 hour range.
To have any concern about 2800 or even 2900 rpm is ridiculous.

My concern is valve float over 2900. My 320 has what I consider to be very light valve springs and just don't have enough data to know at what RPM they will begin to float. I suspect it is well over 3000, but just don't know. I know several of the HIO variants are rated at 3000 and doubt there is anything different except stronger valve springs and possibly tighter tolerances on piston weights and rod balancing.

Larry
 
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200 to 206 mph

I'm doing this right now on my RV4. Preliminary checking with the PR pants on but no gear to pant fairings I went from 200 to 206 mph. I will have the gear to pant fairing finished in a couple of weeks, painted and installed and should be able to give a definitive answer on what I gained in RPM but I'm expecting probably close to 50 rpm at altitude.

Tim
 
I'm doing this right now on my RV4. Preliminary checking with the PR pants on but no gear to pant fairings I went from 200 to 206 mph. I will have the gear to pant fairing finished in a couple of weeks, painted and installed and should be able to give a definitive answer on what I gained in RPM but I'm expecting probably close to 50 rpm at altitude.

Tim

I’m eager to hear your results. I ordered the parts but got an email that shipping is delayed for lack of inventory, so lots of time to figure out how I’m going to go about the install.


On a different note, does my static RPM seem low at ~2090? Nicole says the theoretical static on my engine should be 2150-2200, but at the same time the previous owner who had a 160 HP engine only got 2180 static, so I’m not entirely sure how to view it.
 
I'm doing this right now on my RV4. Preliminary checking with the PR pants on but no gear to pant fairings I went from 200 to 206 mph. I will have the gear to pant fairing finished in a couple of weeks, painted and installed and should be able to give a definitive answer on what I gained in RPM but I'm expecting probably close to 50 rpm at altitude.

Tim

Tim, just confirm, this is from the original Vans wheel pants to the Vans new PR pants?

I am interested in your results as I have James pants on my -7 and am considering fitting Vans PR pants to see if they are faster. The wheel projection of the James seems disproportionate relative to the pant size.
 
PR wheel pant

Hi Bill

Yes, mine are from the Vans original wheel pant in 1998 to the new pressure recovery wheel pant.

I also have the 380*150 tires on there so a little more cutting and trimming to make them fit.

Tim
 
Update

Finally up and flying. Hard to know exactly for sure what I gained as all my data points before were at 8000 ft and 0 c.

Today at 8000' it was a DA of over 11,000' and 204 mph (oat of 76F)

Today at 12,000' it was a DA of close to 15,000 and a 199 mph (oat 60 f)

I gained somewhere around 6 mph but until the temp come down and I can repeat the original test results at similar temps and altitudes I wont know for sure exactly how much i gained.

These were based on a 4 point GPS average.

I was not able to discern if I gained RPM or not due to the extreme differences.

As a side note, this is the first time I've seen my ROC in the mid to high hundreds of feet per min. DA was certainly impacting me here.

Tim
 
Finally got my fairings done and did some initial testing. It was hot today so at 7250’ PA (about 8800’ DA) I trued at 181 MPH (157 kts) at 2600 RPM (I couldn’t spin it any faster). This was at just under 1300 lbs, 21.6” MAP at FT, and 8.2 GPH. This is a bit slower than I was hoping, I’m inclined to attribute it to temperature and humidity but I’ll be retesting - if it’s consistent I may start thinking my engine is talking to me, which should be noticeable with the already over pitched prop.
 
I bet you could convert your 150 to 160 for not much more than shipping and repitch cost. The prop should be perfect with 10 more horses.
 
I bet you could convert your 150 to 160 for not much more than shipping and repitch cost. The prop should be perfect with 10 more horses.

If you search my posts you’ll see I’ve inquired about that exact topic. My hang up is only that my motor has 1900 hours on the bottom and was last overhauled in 1978. I just gotta get to where I can suck up the cost of a full overhaul rather than just pulling cylinders for a piston swap.
 
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