Well that is your problem
86WC said:
I'm new at this, so if the replies are not done correctly, forgive me, I'm trying!
I replaced a Sensinich 78" pitched prop for this new 3 blade Catto prop. I have the earlier style of wheel pants. I do have some slight gaps and edges and the elevator is not perfectly aligned, but could this make that much of a difference?
Don't worry about it. First the piece of the puzzle about the prop change is a big one. It more likely is the prop does not match you plane (right now), but that does not mean it will not work. You will need to find more HP or less drag to get through the air faster to turn the RPM's up.
You went from two metal blade prop to a 3-BLADE wood/glass prop. You have more blade area. You should expect to go SLOWER going from a Sensinich to a Catto for two reasons. 1) The Catto is wood fiberglass and has thicker blades than a metal Sensinich (you loose efficiency with thicker blades); 2) Three blades are slower than 2-blades because of loss of efficiency with the extra blade. Simply the blades are running in the wash of the others. More blades, more wash and interaction-interference between blades. Now we are talking about small loss, like say 1% loss, and if you have lots of HP you need more blade area and have to add more blades sometimes.
When you a 350hp TSIO540 or a Garrett 1200HP turbine, you are not going to care about 1% in loss of thrust. You have plenty of HP to spare.
Three blades can give better climb, better tip ground clearance and lower noise levels from a ground observer. The metal blade is thinner and Sensenich is MADE specifically for the RV airframe! That is a big one. Catto makes props for all kinds of planes, not necessarily a RV specific prop. Therefore you may not have the exact match for your plane. Even sometimes people get the wrong Sensinich for their plane. However with the Sensenich you can add or subtract pitch by tweaking them (bending them with a big bar).
New wheel pants will help and the intersection fairings that open up in flight of have blunt edges have been known to be a big speed killer. Those two mods will free up several MPH and will increase RPM by a know amount. There is math I could do, but not off the top of my head, but lets say you pick up 4 mph 180-184 mph, you RPM will go from 2580 to say 2650 rpm? Find a little more rigging and baffle seals for another 2 mph you are getting close to 2700 RPM. So your prop may work.
HP wise it seems you need to do some more flying at fixed benchmark altitudes and get a TRUE base line. At solo weight WIDE OPEN, 75% power (about +8,000 ft std day) you should true out at 188 mph.
You can also do sea level wide open top speed runs, but that is a little more risky because you have to fly fast low to the ground. Also getting a place to do it, unless you live in the low flat lands is hard to do. That is why we use 8000 ft DA / 75% WOT for a standard for all to use. You never get it perfect but close is good. If you can fly low wide open you should see Van's spec top speed for a 160hp RV-6a of 200 mph. That means you should also see +2700 rpm. MANY RV's WITH FIXED PITCH PROPS are under pitch and allow higher RPM's such as 2,800 rpm. This does pick up speed since every 100 RPM is 3-6 hp. 6 more hp is an extra 2.4 mph. Of course this cost gas and is above lycs recommended red line.
86WC said:
I get no advantage from leaning the engine, it runs lean. EGT's are 1400-1500.
You may need more jet in the carb. RV's are famous for running lean. With the free flowing exhaust, better airbox and higher speeds forcing quite a bit more air into / through the engine, lean conditions are common with Carb Jets for a 120 mph C172 is common.
86WC said:
On the flight home my indicated airspeed was 180mph and the GPS showed 140nts TGS, but had a 15 knt headwind. I was burning 10.5 gph, manifold pressure is unknown. The flight home was 1.5 hours, and the outside air temp was in the 20's. After 30 minutes of flight my oil pressure jumped to 99lbs. and the oil temp dropped to 149.
You are doing fine with the info, just when you go up take notes not only of of the airspeed, GPS speed and RPM/FF but
add altitude, baro, OAT temp.
Doing it at the same throttle setting and D-altitude helps compare from run to run.
The "STANDARD" is wide open throttle, leaned to best power (100F-150F) ROP, 75% power at approx 8,000 feet density altitude.
That allows other RV'ers to compare to your numbers as well.
If you are going to be able to make adjustments and compare you will need to make dedicate flights for testing. The standard technique or the best one is using three constant GPS course runs and average them with a spread sheet. You can search on this forum for the excel spread sheet to crunch the numbers. A simple average is not really accurate depending on winds. You can use the 4 runs on
headings 90 degrees apart and do a simple average of all 4-runs. It is OK but not as good as the 3 constant
course method with spread sheet calculations.
Search for Kevin Horton's post or web site.
http://www.kilohotel.com/rv8/
You just have to fly a steady course (GPS not compass) that are different but not necessarily 120 deg. Than plug the numbers into a spread sheet and get your true Airspeed. You do it the same way each time (DA altitude), you can make judgment calls of what helps and what does not. (web site was not responding, if you need kevins excel spread sheet I'll email it to you.)
I assume 180 mph is true at 75% power at 8,000 ft DA? That is pretty good, about 8 or 9 mph under vans spec. 10.5 FF is plenty of fuel flow and more than 75% power. Because you don't have MAP gauge you really don't know you true percent power but from 10.5 gph, I guess you where making at least 87% to 91% power and where at a lower altitude than 8,000 feet.
86WC said:
I throttled back and seen little change, as I flew further the outside temp climbed to 32. My oil temp climbed to 160 and the pressure dropped to 93. I have never had these problems before.
The compression is good. The exhaust system is basically straight pipes. As for the intake, we removed the air filter, flew it, so no change.
I still think that I'm missing something with the engine. I will try and get some pictures posted of the airframe hopefully within a few days. Thanks
160F oil temp is low but expected in winter. OAT can vary as you fly. Pressure of 93 psi I assume is very high? That may be a O320H2AD thing I guess, but that is way high. 75-85 psi is normal for other 320's.
The Air filter is BETTER than no filter if running I assume the Van's FAB air box. Just make sure you do the K&N filter cleaner. Again if there is any chance the fairings can float or get sucked into the wind while flying they can cost several MPH.
Also you will GO slower with three blade Catto vs The Sensinich. I know the Catto "looks fast" and is pretty but that is not how aerodynamics always works. The Sensinich is much thinner because it is metal and as I say two are better than three for low HP engines. At some point as HP gets higher, like 300-400 HP you NEED more blades (blade area) to absorb the HP. You can only make the diameter or blade width (cord) greater than you need another blade. However 160 HP is no where near ++300 HP. The rule is use as few blades as possible to get the job done with reasonable diameter and blade shapes. However you should be enjoying a smoother ride with less buzz or vibration?
Sorry of not help. Do you have any data for the Sensenich prop to compare with? RPM, True airspeed? You could be down on HP still as you suspect, but if you had the Sensenich prop performance that was good, RPM, True Airspeed you would have a data point to go on. Any chance of putting it back on?
Cheers George
