What's new
Van's Air Force

Don't miss anything! Register now for full access to the definitive RV support community.

Engine operating techniques, Racing

sailvi767

Well Known Member
I am curious how those who race operate their engines under race conditions set the mixture. If CHT's are not a factor do you normally lean to best power or do most feel detonation could be a problem at full throttle leaned to max power at low altitudes? I know most operate at 2700 RPM and some exceed that number with the prop control but I am wondering how mixture control ties into this. Leaning to best power would certainly be the fastest mixture setting but I am concerned about long term effects of leaning under full power operation at low altitude.

George
 
Balls out

:D depends on the race and the conditions also different racers have different limits.

For me I stay within the manufacturers suggested limitations. so that means 2600rpm prop limit from Whirlwind. Everything full forward, then lean for best power. I watch CHT's and EGT's. I have also ran at 2500rpm and lower depending on turbulence.
 
Thanks, It sounds like leaning to best power is acceptable even at low altitude as long as CHT's are reasonable. I have a whirlwind RV200 prop however the limit on it is listed as 2700 rpm.

George
 
I Like to see that Brian

I run the highest RPM I can get - usually 2720 rpm with a Hartzell blended airfoil prop using F7496 blades 72" diameter. I lean for what I hope is best power but it is strictly a gut thing, I have no instrument that indicates a direct power difference as I vary the mixture control in the normal operating range. I have come to find that an EGT of 1300 on cylinder #4 works very well so that is what I target. CHTs are never a problem on our airplane - the highest I ever see is in the 370s and typically in the 350s. I determine my altitude for each leg of the race based on plane old climb, descend, and cruise performance calculations and the head or tail wind component. I haven't found a need to go into cross wind component effects on speed yet.

Bob Axsom
 
Last edited:
Detonation

Just as I expected, more rpm's less detonation. Remember your racing, so 50 to 100 degrees rich of peak is max power. Lean of peak is for economy, not speed. Max power = max speed.
 
Forward

Forward

Forward

Concentrate on the corners

Don't look at the wings when you turn

If it starts to smell - I guess you are probably winning........ :cool:
 
Schneider crew twice

City Livery Cup crew once

Piper Lance

Oh, boy - you really don't want to look at the wings in the turn......

Or the yachts, or the sheep ...... ;)
 
Mooney 201 racing

All 3 knobs full forward, CHT 330-340. ~1280 EGT, or about 250 ROP. 18.8 GPH. 165-170 KIAS.

The last two races we experimented with leaning about 75-100 degrees from full rich, so that puts it about 150-175 ROP. 16 GPH. Picked up a knot or so. The CHT in that case rises to 360-380. That's as far as I was to go. This is summer time as well. You can detonate an angle-valve IO-360 with normal CHTs in very cold weather at sea level with not much leaning at all. Cylinder pressure is already pushing the maximum at full rich.

I dont think you could get to 80 ROP (remember, peak is going to be higher than cruise) without exceeding 420 or more on the CHT. I am not going there.
 
Just as I expected, more rpm's less detonation./QUOTE]

Enlighten me, what is the logic to this exactly?

The graphs in Dan's link to the old post were very informative - somehow hadnt seen those before. You have to blow the graphs up and its still a little tough to match up the cylinder-specific CHTs that correspond with the cylinders that are detonating, but I note that detonation is occurring well north of 450 degrees, and approaching 500 degrees for the more severe cases.

thanks guys

erich
 
George, when did you get your prop? the paperwork I got with mine bought in 2009 says 2600 max continuous.

I took the number from the company website. They only listed the one number for max RPM. I purchased the aircraft used. Your number may be correct if you have all the paperwork that came with the prop. Its also possible they raised the number since they have raised the overhaul interval to 650 hours.

George
 
George, you afraid I'll out run you?

BTW, I was pulled back on Sunday and we were still smoking along.
 
Detonation

I should have said max power (50 to 100 degrees rop) at 75% power. North of that you need more fuel. I believe Lycomings CHT redline (never exceed is 500 on the parallel valve) and don't operate continuously above 435 should give plenty of leeway to lean to max power when racing. Always use max rpm, your not worried about noise when your racing. If your worried about making TBO by the book you won't be flying at max power. Racing can be expensive!!
 
In the Big Muddy Race, three weeks ago, I recorded the following engine data:
RPMs 2700
MP 28.9
Fuel flow 22.1 to 24gph
oil temp 199 to 205
Max CHT 400

I start full rich, 24 gph, then reduce to 23gph after I am on course and have a few seconds to play with settings. Depending on engine temperatures I will vary slightly from 23gph. The engine is cooled by air, oil and fuel. I am well on the rich side of peak but how does one know exactly where peak is at this power setting unless you go there, a place you should not go at full power!
I use my peak EGT numbers from lower power settings as a rough guide but really let the engine temperatures detiremine mixture settings. In really hot conditions I might be full rich and sacrifice one or two knots to keep the engine at my personal temperature limits of 200 OT and 400CHT. For me oil temps can vary from that but I am pretty firm about my 400CHT limit. Most of the race will see CHT temperatures in the 375 range with #6 being my warm cylinder and it will gradually climb to 400 towards the end of the race, increasing fuel flow will hold it at 400.
It is nice to see so many interested in race power settings, hopefully this means more competitors for the SARL races!
 
Where do you get all those charts Dan? I would like to see some specific to the parallel valve 540 mass cooling air flow, detonation, all the cool stuff. Interesting.

Google....they were in detonation study comparing to ethenol. I too would love to see the same charts for a parallel valve.
 
I took the number from the company website. They only listed the one number for max RPM...
George


Here is the pertinent document.
http://www.whirlwindaviation.com/pdf/WWA_200RVManual_Rev2012-1.pdf

And the callout from the manual:
5.4 RPM Recommendation
It is recommended to avoid continuous operation of the 200RV
Series propeller in the RPM range between 2050 to 2300 RPM and
2600 to 2700 RPM.

The question then is if you're O.K. to fly AT the upper or lower RPM in the "avoid" range. Just to let you know a bit of the history, I purchased mine when there was NO rpm restriction. Hartzel then came out with their restrictions on rpm ranges for certain props with either high compression pistons or electronic ignition. Seemingly immediately after that, WhirlWind put THE SAME "avoid" rpms on their props, even though the materials are completely different and would presumably have different resonance characteristics. I called Whirlwind and asked point blank if they had done the vibration and resonance testing that Hartzel had done and the Whirlwind tech guy said "no". When I asked what constituted "contiunous" the tech guy hemmed and hawed and then said "just don't cruise there". I got a very nebulous feeling from the whole conversation.

The identical restrictions between the different props makes no sense to me from a resonance point of view. Different materials, length, weight, mass distribution, stiffness etc. Rap your knuckle on a Hartzel aluminum blade and do the same on what sounds like a hollow carbon fiber Whirlwind 200RV blade. VERY DIFFERENT sound. You're hearing its resonance characteristics. It may be that there should be some avoid rpms on the WW, but I can't imagine they would be identical to the Hartzel.

All that said, I operate my prop honoring the published avoid ranges.

In some very rough hacks at optimum speed runs, I have found that on my plane with my prop, 2700rpm does not equal max speed. I have also wondered how you tell what the best power mixture is without first leaning yourself into the detonation range at high power (high rpm, high MP, peak egt) and then enrichening. It is something I'm a bit timid about trying. I just feed it too much gas and presume I'm not getting everything out of it. If someone has a safe technique that applies to most altitudes and atmospherics I would love to hear it too.

YMMV

All Best

Jeremy Constant
 
I wish I knew that I was getting maximum racing speed

Your comment about 2700 RPM not equalling maximum speed has been seen by me randomly but when I plan a test for it I can't repeat using my standard test method.

My most dramatic observation was cruising along in a straight line to somewhere and I do not remember the exact sequence but I was at one RPM and I changed to another and the speed changed in the opposite of the expected direction. As I recall right now I was at 2610 RPM and went to 2720 RPM and the speed went down and when I went back to ~2610 the speed increased (GPS ground speed).

A couple of times since that incident I have run a triangular speed test at 2720 then flew another one at ~2610 and the speed was a knot or so slower each time at the lower RPM. Obviously atmospheric conditions were not exactly the same and the mixture was not exactly the same and the governor is controlling the prop pitch so I have no idea what the actual prop pitch was under the different actual operations. Hartzell has told me there may be some efficiency loss at the higher RPM that would be reflected in a small speed loss. Whatever the reason I have seen it in clear enough terms to know that there is some optimum combination for speed hiding there but it is so elusive that I can't risk race speed searching for it. Manifold pressure is so insensitive under these conditions that the gauge might as well not be in the plane. The only things that move in response to control inputs are EGT and speed and the speed change is so subtle that by the time you see which way it is going, if it is down, you have already lost the race. After several races

Race Date Class Aircraft Type Pilot Speed Kts Speed MPH
AirVenture Cup 2008 7/27/08 RV Blue RV-6A Axsom, Bob & Jeanine 194.56 223.89
Rocket 100, 2010 11/20/10 RV Blue RV-6A Axsom, Bob 192.80 221.87
Texoma 100, 2008 4/20/08 RV Blue RV-6A Axsom, Bob & Jeanine 188.36 216.80
GNAR III, Ephrata, WA 6/16/12 RV Blue RV-6A Axsom, Bob 187.74 216.05
Tennessee Valley Air Race 2010 10/30/10 RV Blue RV-6A Axsom, Bob 187.52 215.83
AirCap 200 8/24/08 RV Blue RV-6A Axsom, Bob 185.16 213.08
Hill Country 150, 2012 4/21/12 RV Blue RV-6A Axsom, Bob 184.97 212.86
BCAF Time Trial 4/28/12 RV Blue RV-6A Axsom, Bob 184.38 212.18
Rocket 100 2008 11/23/08 RV Blue RV-6A Axsom, Bob & Jeanine 183.83 211.55
Taylor 150, 2011 4/2/11 RV Blue RV-6A Axsom, Bob & Jeanine 183.41 211.07
Big Muddy, Carbondale, IL 6/9/12 RV Blue RV-6A Axsom, Bob 183.34 210.98
Taylor 100 2009 3/16/09 RV Blue RV-6A Axsom, Bob & Jeanine 182.35 209.85
Taylor 150, 2012 4/14/12 RV Blue RV-6A Axsom, Bob 181.47 208.84
BCAF 150, 2008 5/4/08 RV Blue RV-6A Axsom, Bob & Jeanine 181.45 208.81
Grace Flight 2010 10/2/10 RV Blue RV-6A Axsom, Bob 181.23 208.54
Texoma 100, 2012 3/31/12 RV Blue RV-6A Axsom, Bob 180.87 208.14
Memphis 100 2008 10/19/08 RV Blue RV-6A Axsom, Bob & Jeanine 180.26 207.44
AirVenture Cup 2007 7/23/07 RV Blue RV-6A Axsom, Bob & Jeanine 179.99 207.13
AirVenture Cup 2010 7/26/10 RV Blue RV-6A Axsom, Bob 179.83 206.94
Memorial 150, Terrell, TX 5/26/12 RV Blue RV-6A Axsom, Bob 178.44 206.28
Tennessee Valley Air Race 2009 6/14/09 RV Blue RV-6A Axsom, Bob & Jeanine 178.98 205.97
Colorado 150, 2008 6/29/08 RV Blue RV-6A Axsom, Bob & Jeanine 178.63 205.56
West Texas 100 6/6/10 RV Blue RV-6A Axsom, Bob 178.04 204.89
Memphis 100, 2007 10/28/07 RV Blue RV-6A Axsom, Bob & Jeanine 176.86 203.52
Rocket 100, 2007 11/18/07 RV Blue RV-6A Axsom, Bob & Jeanine 176.33 202.92
Wichita 300 7/17/07 RV Blue RV-6A Axsom, Bob & Jeanine 169.91 195.53
Taylor 100 2007 5/20/07 RV Blue RV-6A Axsom, Bob & Jeanine 168.72 194.16

it seems I am stuck with max RPM and 1300 EGT on Cylinder #4. I'm still hoping to find the magic but I'm not going to pull back the RPM in a race in search of it.

Bob Axsom
 
When you decrease the rpm without changing the throttle position there is a short term transient that may come into play. When you pull the prop control back, the prop has to increase the pitch to get the rpm to decrease. This will lead to a transient increase in thrust that may lead to a temporary increase in speed.

Another way to look at this is there is a bunch of rotational energy stored in the engine and prop. If you pull the prop control aft to command a lower rpm, the rotational energy must decrease. That rotational energy has to go somewhere, and it gets converted to thrust as the prop blade pitch increases. This is only a trasnsient effect though, and it is over once the rpm has stabilized at the new value.

Bottom line - eventually things will reach a steady state condition, but you need to ignore the transient effects that occur immediately after you move the prop control.
 
My method is very much like Tom's...in fact the Rocket boys in SARL and I have had a number of discussions on this, as we all want to find best power without first finding the red zone...kinda one of our holy grails, if you will.

I have 10:1 pistons, and looked intently at those charts Dan posted...then read his caveat about hi compression, etc. I approach best power gingerly at race power...as Tom said. I look for the same 400 CHT max (prefer 380-390) and 200 OT (I'm usually below 195 at full power, unless its a second or subsequent hop on a warm day...heat soaking is a real game-changer in this pursuit).

I did some high(er) altitude testing in preps for the 2010 Airventure Cup, and leaned to and through peak at altitudes ranging from 9,500 to 17,500 (WOT and 2740 RPM). I was happy with my engine's performance when LOP, but the drill was to find best power at altitude, so I could get there more quickly during the race.

What I found was that for each 1,000' of altitude decrease (from 17,500 down), best power mixture (100 ROP is what I used in the tests), my engine's fuel flow increased 0.5 gph...or a 1 gph delta for each 2,000 feet of altitude change. It was fairly linear up high, but I'm not sure it is linear from sea level to 17,500'. The 13.5 gph I saw at 17,500 on test day would extrapolate to 22 gph at SL, and thus far I would not feel comfortable leaning that far at SL at max power. Perhaps best power is there, and the temps Tom and I use as our comfort limit are just not acheivable in an IO-540 at best power at SL. It may be out of detonation, and still be hotter than our limits, but how do ya know? Better safe than sorry in our books!

Interestingly, at Reno, Mark, Greg and I were using 21-23 gph in the races, and watching our temps in the same manner. The linear extrapolation from my tests would put best power at 19.5 gph at Reno's 5,000', but I would not lean that far there either. I was running 23gph at the recent PRS, and had good temps.

How this relates to 4-banger RVs would be an interesting comparison as well. Would it be a 2/3 relationship? Seems logical, and the faster of the Lancair 360's at PRS that we were with was running about 17gph at max power. Maybe Slick Cone, who ran his O-360 RV-8 at PRS has some numbers on that front.

Different ambient temps, different density altitudes, etc, will likely have an impact on those fuel flows and where best power is, so it still is an elusive target to bracket towards. And the above numbers are from one series of tests over a few days...certainly not enough to draw a final conclusion. The consequences of missing the target and getting into detonation are too severe to chance going there, so I choose to move the red knob very gingerly in SL races. Wish there was a cylinder pressure/deto-meter...another of the holy grails, eh!

On the speed versus RPM issue, in the Rocket/540 world, seems the MT 3-blade lost speed over about 2650, while our tests with the Hartzell 80" BA seem to show that that prop keeps giving back when you give it more RPM...pretty phenomenal propeller in that regard. Bob, your 76" BA may be the same kind of animal...that one test may have been an outlier...the other results you discussed (higher RPM is faster) seem more in line with our BA results. I think Slick may be trying out various RPMs with his WW200, so perhaps he can share his results as he tests and preps for Reno.

If anyone figures out the magic best power-finding method...please let us know (well let me know...don't tell those pesky Rocket guys!) :p

Cheers,
Bob
 
Last edited:
in a few hours it will read incorrectly and fail with the lead in the fuel. O2 sensors are not tolerant of leaded fuel. A short burst of leaded race gas never hurt my turbo AWD Eagle Talon TSi's, wideband O2 however.
 
I feel better about my decisions now

When you decrease the rpm without changing the throttle position there is a short term transient that may come into play. When you pull the prop control back, the prop has to increase the pitch to get the rpm to decrease. This will lead to a transient increase in thrust that may lead to a temporary increase in speed.

Another way to look at this is there is a bunch of rotational energy stored in the engine and prop. If you pull the prop control aft to command a lower rpm, the rotational energy must decrease. That rotational energy has to go somewhere, and it gets converted to thrust as the prop blade pitch increases. This is only a trasnsient effect though, and it is over once the rpm has stabilized at the new value.

Bottom line - eventually things will reach a steady state condition, but you need to ignore the transient effects that occur immediately after you move the prop control.

Thanks Kevin, your description seems right and certainly is consistent with what I experienced. I will remember that information.

Bob Axsom
 
Back
Top