B

Bob Redman

Active Member
Sponsor
Good morning, afternoon or evening,

I have been reading many of the posts re lean of peak operations, and considering the various arguments for & against while studying Figure 3-1 in our Lycoming Operators manual - 'Representative Effect of Fuel/Air ratio on .... and Specific Fuel Consumption at Constant RPM & Manifold Pressure'.

Our copy of Fig 3-1 illustrates that:

a. at Lycoming's 'Max Power Range' between EGTs 100 & 200 deg F Rich of Peak (ROP), percent of best power is relatively flat and at about 100% of 'Best Power'.

b. at Lycoming's 'Best Economy Range', leaned to peak EGT, percent of best power has dropped aprox 5% from 'Best Power'.

c. leaned to aprox 50 deg F Lean of Peak (LOP), percent of best power has dropped aprox 8% from 'Best Power'.

d. leaned to aprox 90 deg F LOP, percent of best power has dropped aprox 16% from 'Best power'.

e. if you lean further, then the drop in power becomes much steeper.

f. the specific fuel consumption (SFC) curve is minimum and flat from peak to 90 deg F LOP EGT, and SFC increases either side of Lycoming's 'Best Economy Range'.

Generally when I was recording cruise data for our RV-7, I chose peak EGT as a setting to maximise repeatability of the data. However, I have been experimenting with LOP cruise (below 75% max power, and no leaner than 30 deg C/ 50 deg F LOP).

I have now plotted fuel flow against observed airspeed (call it KIAS), differentiating as to whether I was at peak or LOP. The two curves are the same shape, but the LOP curve is showing:

a. about 3 litres per hour (aprox 0.8 USG/Hr) less fuel flow than the peak EGT curve at about Carson's Speed (aprox 105KIAS at 816kg/1800lb AUW), and

b. about 2 litres per hour (0.53 USG) less fuel flow at about 150 KIAS.

If valid, this data shows a clear benefit in operating LOP, eg, at 105KIAS, fuel flow is about:

a. 19 Litres per hour (5 USG/Hr) at Peak EGT, and

b. 16 Litres per hour (4.3 USG/Hr) LOP.

For comparison, at about 150 KIAS, fuel flow is about:

a. 38 Litres per hour (10 USG/Hr) at Peak EGT, and

b. 36 Litres per hour (9.5 USG/Hr) LOP.

Given the curves illustrated in Lycoming Fig 3-1, I was surprised at the result. I expected that LOP would have resulted in a lower KIAS for the same fuel flow because of the power reduction resulting from leaning below 'Max Range Power'.

The caveats on the data and result include the following:

a. the curve for peak EGT is faired to data with much less scatter than the curve for LOP data (but all LOP data is below the curve for peak EGT).

b. all data as observed on the Dynon D100 EFIS and D120 EMS - no calibration - and no correction for pressure, temp, weight, etc.

c. the data were recorded on many different days and conditions over a 18 month period, but on those days, data was either peak or LOP.

d. the pilot is now referred to as an 'older gentleman' on buses and trains, & often offered a seat by people who appear to me old enough to be my parents, but younger than my grandfather's image I see in the mirror when I am shaving, so residual competence in data collection & analysis could be an issue.

Aircraft is a standard RV-7, tip up, Aerosport IO-360M1, standard compression, ECI components, E/PMag combo, A curve advance (jumper fitted), NGK BR8EIX auto plugs, Hartzell BA constant speed, basic weight 492kg/1085lb, empty CofG 78.24 inches aft of datum, painted and complete.

I would be interested to hear if other people have noticed a similar result, and even more interested in a rational explanation for the benefit of operating LOP given my expectations based on Lycoming figure 3-1. If you use senility to explain the anomaly, I will feel aggrieved, but the offer of a fine ale would compensate my bruised ego.

Best regards
 
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I have found that the best SFC is between 15-25 LOP on an IO-360-A3B6 engine. There is no need to lean further than that unless you have CHT problems or just want to fly slower. As you noticed the speed falls off precipitously after 50 LOP. In fact at 100 LOP the speed had fallen off a whole 15 knots from peak EGT. Absolute maximum NMPG was found at 70-80 LOP, but the speed was 10 knots slower than peak EGT, and I guess the lower drag from lower airspeed biases this figure to favor leaner mixtures and slower airspeeds. Attempting to factor this out shows that 50 LOP should be the leanest mixture setting to use for cruise, with a .2 to .4 NMPG penalty for flying at 15-25 LOP in return for 3-4 knots more airspeed. In short, the most ecomonical cost to fly a trip is found at 15-25 LOP for any power setting between 55% and 75%. Below that we just run peak EGT, the window for fuel savings before major speed loss is too small to accurately target.

On our plane, 80 ROP is 5 knots faster (155 TAS) but 1.9 GPH more fuel than 25 LOP (150 TAS). Peak EGT was .3 GPH more 1-2 knots faster than 25 LOP.

Byron
IO-360-A3B6 1977 Mooney M20J
 
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Thanks

Thanks Byron,

You are seeing what I would expect from the Lycoming Fig 3. Thanks for the data. At the next opportunity I will look at setting MP/RPM, varying the mixture and watching KIAS, fuel flow, % power, & nm/litre.

Regards
 
Constant SFC when LOP

It is correct that speed should drop as you go more lean. The SFC is constant whenever LOP. So the speed loss is exactly linked to the reduction in fuel flow, since the available power is exactly linked to fuel flow.

Because power required is proportional to the velocity^3, the speed reduction should go as the cube-root of the fuel flow reduction....(this assumes constant drag coefficient, which is not really right; if you are at a speed above the best L/D speed, then as the speed drops, drag coefficient decreases) so the speed will not drop as fast as the cube-root.

The reason the SFC is constant is that when LOP, all the chemical energy is being recovered from the fuel, with no fuel wasted. There is residual oxygen, not residual fuel. Only when you lean so far that you start to misfire will you deviate from the constant SFC curve -- because the misfire wastes fuel.
There is a weak second-order effect that the available chemical energy in the fuel is related to combustion temperature, which is affected by mixture. But certainly to O(1), the SFC is constant when LOP.
 
Logical but not supported by evidence?

scsmith said:
...

The reason the SFC is constant is that when LOP, all the chemical energy is being recovered from the fuel, with no fuel wasted. There is residual oxygen, not residual fuel. Only when you lean so far that you start to misfire will you deviate from the constant SFC curve -- because the misfire wastes fuel.
There is a weak second-order effect that the available chemical energy in the fuel is related to combustion temperature, which is affected by mixture. But certainly to O(1), the SFC is constant when LOP.
Click to expand...

This makes sense but does not agree with the data from the Advanced Pilot guys who publish actual test data from a well instrumented setup. Their data show an SFC curve against EGT very clearly.

I have not examined the data from the thread-starter, but I would expect it to show logical differences in TAS vs. fuel flow with varying mixtures. As Steve points out, the cube rule is not really accurate. At around 200 mph, the RV7 still has 10% or more of its drag from induced and at the speeds for this subject, that will be even higher.

On a related note, the SFC as reported by an EMS or EFIS - at least in my case (GRT) is highly suspect because the BHP is being calculated from MAP and RPM, etc. I can get my instrument to show me SFC's that I think are simply not realistic. That is because it is assuming my power table is correct. That internal table cannot be correct at various mixtures, only one or some that happen to agree. I admit that I don't know how other EMS/EFIS systems compute SFC, but unless they have data on torque and RPM together, their HP calculations must be based on some assumption regarding mixutre. No?
 
But the trend with EGT is a much flatter curve than the rapid change in SFC on the ROP side, and at the extreme LOP point where misfire occurs.
That is what I meant by the secondary effect of combustion temperature.

The HP table must also include altitude, but you are right -- without knowing the mixture, it is only approximate.

We were discussing the altitude effect on a thread a while back -- it comes from the reduced back pressure on the engine at the exhaust.
 
scsmith said:
It is correct that speed should drop as you go more lean. The SFC is constant whenever LOP. So the speed loss is exactly linked to the reduction in fuel flow, since the available power is exactly linked to fuel flow.

Because power required is proportional to the velocity^3, the speed reduction should go as the cube-root of the fuel flow reduction....(this assumes constant drag coefficient, which is not really right; if you are at a speed above the best L/D speed, then as the speed drops, drag coefficient decreases) so the speed will not drop as fast as the cube-root.

The reason the SFC is constant is that when LOP, all the chemical energy is being recovered from the fuel, with no fuel wasted. There is residual oxygen, not residual fuel. Only when you lean so far that you start to misfire will you deviate from the constant SFC curve -- because the misfire wastes fuel.
There is a weak second-order effect that the available chemical energy in the fuel is related to combustion temperature, which is affected by mixture. But certainly to O(1), the SFC is constant when LOP.
Click to expand...

I can't explain it, but our airplane's speed drops proportionally to fuel flow when LOP, but takes a massive loss after ~75 LOP although the fuel flow only decreased a small amount. Its pretty linear above that. BTW the engine runs smoothly to 100 LOP sometimes a bit more.
 
MORE DATA

Yesterday I tried a different approach to determine the validity of the Lycoming figure 3-1 in the original post given that, for the same KIAS, my data illustrated LOP cruise was at a lower fuel flow than cruise at peak EGT (see the original post for the caveats on the validity or otherwise of my data). The weather was not ideal for data collection - summer, MSL OAT 32 C (90F), fair weather cumulus base 3500, tops 6 to 7000, & light to occasional moderate turbulence. Military airspace above, & controlled airspace adjacent, prevented preferred higher & longer runs. I tried to hold constant manifold pressure & RPM, Dynon auto pilot engaged (alt & heading) while recording various parameters.

All data as observed on the Dynon D100 & D120: 6500 alt at 11C (52F), DA 7400, AUW at takeoff 1720lb, power set at 23/2390RPM (not full throttle). Results (in the sequence of data collection):

a. Lean of Peak (< 30C/55F), 64% power, 29 Litres/hr (7.7 USG/hr), 135KIAS.
b. Peak EGT, 76%, 35 Litres/hr (9.2 USG/hr), 142KIAS.
c. ROP (80C/144F), 41.5 Litres/hr (10.7 USG/hr), 142 KIAS.
d. Peak EGT, 75%, 35 litres/hr (9.2 USG/hr), 142 KIAS.
e. LOP (< 30C/55F), 64%, 29 Litres/hr (7.7 USG/hr), 138 KIAS.

I will try again under more favourable conditions, probably during late autumn/early winter (May/June here). Although I tried to wait for conditions to stabilise, there was a little too much turbulence for me to gather good data. Never-the-less the peak & LOP data sat within the scatter of the previous data. But LOP cruise was clearly slower than cruise at peak EGT which agrees with the Lycoming Figure 3-1, and is consistent with the comments of the other posters and my expectations. Now I suspect that the anomaly is due to plotting all (raw) data as fuel flow vs KIAS, rather than looking at still air specific fuel consumption.

I was hoping a generous flight test engineer would gently indicate the erroneous assumptions in the original post. I may be forced to review theory after a lapse of many years.

Regards,
 
Bob,

Why are you doing tests other than WOT. This is the most efficient the engine will be, and at 55dF LOP this is past the point of best BSFC.

Have a closer look here.
Landmarksgraph_zpsbfb07cbb.gif


And after the online course you will gain much as far as the best place to operate. At the moment you are dancing from the Lycoming book point to a point equally past the best BSFC.

Great to have your phone call tonight, but if this post is not making sense, call me again. You are missing the sweet spot by a small amount.:)

cheers!
 
David,

Any chance you might have accurate speed performance data to superimpose
on that chart?
For the Rv-10 possibly?
 
Not really, but I reckon you would have ;)

I am a lazy pilot, down low, like 1500' its WOT/24-2500 RPM/70-80LOP. I get good results.

Up higher I find about 164-6 knots at around 42LPH. In the FL's slight bit less TAS but a lot less fuel, maybe 37-8LPH.

I think I have lots of drag in antennae for storm scopes and the mutli radio gear.

Slack I know :rolleyes:
 
David, I'm confused. How can you run 70-80 LOP at WOT at 1500'? Does this power setting not go against the advice of running less than 68-70% power?

Please explain, because I've read all of your advice and that of others, in -the-know, about running LOP.

Thanks,
 
Speed Data

I'll get on it then.
Although hard proof must be provided to back up a claim
I have enough data to know that the speed curve from best power mixture ROP, is almost flat over to about 10 to 20 degrees LOP.
This is close to what the OP found.
It drops off dramatically after that and I don't fly there very often.
What is obvious is the approximate 1 GPH per knot of speed loss in that range
until you get to the 10 to 20 Degree LOP side.
I'll find some proof and post it on that graph.
 
Ernst, indeed it is, there is only a slight loss of speed, and hence the comparison point of a nice conservative ROP vs appropriate LOP (ICP and CHT for similar powers) means a hige fuel saving and not much speed losses.

Pierre, you can run LOP at 100% power ;) which is what your diesel cars and pickups do too! But not so easy in your RV10 though.

The higher the power the further LOP you need to be.


________________________________________
Red Box = No Fly Zone
? At and below about 60% power, there is no red box. Put the mixture wherever you want it.
? At about 65% power or so, 100?F ROP to Peak.
? At about 70%, 125?F ROP to 25?F LOP.
? At about 75%, 180?F ROP to 40?F LOP.
? At about 80%, 200?F ROP to 60?F LOP.
________________________________________


________________________________________
Outside the Box
? At 65% power, use richer than 100 ROP, or leaner than peak EGT.
? At 70%, use richer than 125?F ROP, or leaner than 25?F LOP.
? At 75%, use richer than 180?F ROP, or leaner than 40?F LOP.
? At 80%, use richer than 200?F ROP, or leaner than 60?F LOP.
________________________________________

Courtesy of Advanced Pilot Seminars.

On that topic, as a matter of full disclosure, myself and Andrew Denyer of Riverina Airmotive have formed Advanced Pilot Seminars Australia. And we work closely under licence of Deakin, Atkinson and Braly.

And just for the nit pickers watching, no when I suggest you do the APS course in Ada OK (like March 22-24) I am not making a cent out of it, so I have no vested interest. In fact I do not expect to be making money out of the Oz ones either, or if there is loose change left over that will be it as funding for the next course.

As for the course in Ada in March, I am going to be taking part of the class, and doing so at my expense I might add. Why not fly over from GA, you will be the first RV to fly into one of the courses I think.

We teach far more than LOP ops, in fact that is about 15% of the course. 15% is on how to run ROP properly, which most folk don't get either from flying school.

Off for my IR renewal.:)

Cheers
 
pierre smith said:
David, I'm confused. How can you run 70-80 LOP at WOT at 1500'? Does this power setting not go against the advice of running less than 68-70% power?

Please explain, because I've read all of your advice and that of others, in -the-know, about running LOP.

Thanks,
Click to expand...

Pierre,

As David already states, the key is avoiding the "red box" which is the operational danger zone of high internal cylinder pressure where detonation becomes a concern. The only thing "magical" about 70% power is that, above this, the red box is big and below it, it's small.

Another thing to note is that at the RPM and altitude that David notes running 70-80 LOP, he's only making around 75% power (I assume his EGTs are in *F). Using the mixture and prop knobs to reduce power at low altitude is more efficient than using the throttle because you avoid the pumping losses of an engine working to draw intake air through a partially closed throttle plate.

Skylor
 
dtw_rv6 said:
If I lean to 8gph under WOT, does this always yield 65% power, regardless of altitude? This is a 180hp, normally aspirated engine with fixed pitch prop.
Click to expand...

When running LOP, % power is determined by fuel flow. In normally aspirated 4 cyl. Lyc's you multiply the fuel flow by 14.9 and divide that number by rated HP. In your case, 8 gal/hr X 14.9 = 119.2/180= 66.2% power.......as long as you are LOP, it doesn't matter what altitude you are at.

Jon D.
 
skylor said:
Pierre,

As David already states, the key is avoiding the "red box" which is the operational danger zone of high internal cylinder pressure where detonation becomes a concern. The only thing "magical" about 70% power is that, above this, the red box is big and below it, it's small.

Another thing to note is that at the RPM and altitude that David notes running 70-80 LOP, he's only making around 75% power (I assume his EGTs are in *F). Using the mixture and prop knobs to reduce power at low altitude is more efficient than using the throttle because you avoid the pumping losses of an engine working to draw intake air through a partially closed throttle plate.

Skylor
Click to expand...

Almost slight clarification, once you are LOP the HP is not RPM and MP dependent it is fuel flow dependant, and I think closer to 80%, give or take depending on QNH and temp. At 75% I would be 40-50LOP.

Here is photo at 1000' and around 79%
photo2-3.jpg
 
dtw_rv6 said:
I've been running LOP at higher altitudes since first breaking in my Superior, and I"ve been curious about running WOT down close to the ground. Since I have a fixed prop, my only way to regulate power at WOT would be to pull the mixture far enough to get below the red box. Above 4000, I lean to 8gph, and then cross check the EGT's by slowly enriching that I"m truly on the LOP side of the curve, then pull back to 8gpm again.

If I lean to 8gph under WOT, does this always yield 65% power, regardless of altitude? This is a 180hp, normally aspirated engine with fixed pitch prop.

Don
Click to expand...

Yep thats fine. Just remember when LOP with a fixed pitch once you go past about 75dF ROP towards peak and beyond the power is rolling off, so the RPM starts dropping, and so will the EGT, so you don't have to go far.

Remember that these numbers are fairly flexible, 1% who cares :)
 
David, any reason why the lower D-180 doesn't show LOP? When I select "Lean" mode on my D-120, a little box appears with -20 or similar, after the EGT peaks..the big numbers (1425) disappear.

Best,
 
Jon. That makes it real easy based on the formula. Just to make sure I have this correct: if I have a 193 HP 4 cyl 360, as long as my FF is less than 9.7 GPH (or use 9.5 to be on the safe side), I will never be above 75% HP at any altitude? 9.7 x 14.9/193 = 74.9%

Therefore, you should never be able to hurt the engine by leaning too much below this FF (again, no matter what alt)?
 
Safe Leaning

db8 said:
Jon. That makes it real easy based on the formula. Just to make sure I have this correct: if I have a 193 HP 4 cyl 360, as long as my FF is less than 9.7 GPH (or use 9.5 to be on the safe side), I will never be above 75% HP at any altitude? 9.7 x 14.9/193 = 74.9%

Therefore, you should never be able to hurt the engine by leaning too much below this FF (again, no matter what alt)?
Click to expand...

That multiplier is only correct when LOP. However, the correct, if unknown multiplier for richer mixtures is lower. For that reason, you should be safe.

HOWEVER, the Advanced Pilot folks also have a recommendation for determining the "target EGT" so that you can begin leaning at lower altitudes and during a climb. I have mine labeled right next to my EFIS. It is a number you determine experimentally for your individual airplane. That seems to me to be a good safety concept for the engine. I don't want to try to explain their stuff, but it was on their website the last time I looked.
 
RESPONSE TO POST #10

Thanks for the graph David, it seems broadly consistent with the Lycoming graph. The subtle differences are in the detail, as usual.

Thanks to all the posters, particularly for the reminder of the fuel flow multiplier - if I stay below 34 Litres/Hr (9.1 USG/Hr) I should be below 75% regardless of the Dynon display. Our indicated fuel flow seems accurate when compared to fuel dip/added.

My response to David's comment:

a. When operating my preference is to cruise at full throttle, set my preferred RPM, and lean, usually at 8.5 to 9.5K alt. I have been leaning to leanest cylinder not beyond 30C/54F LOP. After seeing your graph, I will use leanest cylinder not beyond 25C/45F LOP. Note that I am limited to a maximum of 22" manifold air pressure (MAP) when below 2350RPM because of electronic ignition.

b. For the results listed in post #9, I could not collect data above 6.5K alt. Also, I was not sure whether or not I would have to adjust throttle to maintain a constant MAP. So I decided to reduce throttle for the 5 data points, but did not need to adjust either throttle or RPM.

c. In any case, from time to time I have needed to, & will need to again, cruise at less than wide open throttle (WOT), so the data is useful.

For interest, here is more (winter) cruise data (all observed):

a. At 8.5K alt (DA 9K), WOT, 22.5"/2360RPM, 66%, peak EGT, 30 Litres/hr (7.9 USG/Hr), 138KIAS (141KCAS) 158KTAS, 86C/187F oil temp, CHTs 150/162/170/164C (302/324/338/327F).

b. At 9.5K alt (DA 10K), WOT, 21.6"/2200RPM, 60%, peak EGT, 27 Litres/Hr (7.1 USG/Hr), 129KIAS (132KCAS) 150KTAS, 86C/187F oil temp, CHTs 138/149/160/152C (280/300/320/306F).
 
Pierre,

I assume you mean the %age power and the ROP LOP indicator? If so your D120 needs to be connected to an AOT source, via a D100 and DSAB, and altitude etc will be as well.

Then go to the engine setup page in the D120, and input the HP rating etc.

Download a D120 manual and it will be explained.

One other critical thing to note here for all viewers. The latency in probes and subtle variations often mean if you lean past peak, from the rich side you get a different peak number every time.

If you want to spend some time poking around, do a BMP (big Mixture Pull) and sneak up to peak from the Lean side. Results are sometimes more consistent and its less effort finding the what was last to peak (now first) and then go from there. It is also less stressful when at high power on the engine to find peal from the lean side.
 
Hi Bob,
it seems broadly consistent with the Lycoming graph.
Click to expand...

The APS graph was carefully constructed from real Dyno values, to get the relationships very accurate. I know your penchant for manufacturers data, but depending on which graph you look at the Lyc data can be rough or accurate. So I would say that the Lyc stuff is bradly consistent ;)

a. When operating my preference is to cruise at full throttle, set my preferred RPM, and lean, usually at 8.5 to 9.5K alt. I have been leaning to leanest cylinder not beyond 30C/54F LOP. After seeing your graph, I will use leanest cylinder not beyond 25C/45F LOP. Note that I am limited to a maximum of 22" manifold air pressure (MAP) when below 2350RPM because of electronic ignition.
Click to expand...
I would say, try this, at 8-10K' WOT, 2400-2500 RPM, and the last cylinder to peak from the rich side, or first to peak from the Lean side, set at 10dF LOP. Anything else is a compromise.

c. In any case, from time to time I have needed to, & will need to again, cruise at less than wide open throttle (WOT), so the data is useful.
Click to expand...
Not sure when that might be, but lets assume you are in a WOT cruise, and you want to slow down quite a bit (ATC maybe...or just the view, or flying with a Cessna) once you have the mixture set just pull MP back, the Bendix fuel system is pretty good at keep the F/A ratio consistent.

This is why on descent from even in the FL's I rarely touch the mixture until the hangar door, apart from maybe a tweak. Refer the Airsport article back in JUN or there abouts.;)

Have you considered bribing your daughter and shifting the wedding plans a month or so. :D;)
 
db8 said:
Jon. That makes it real easy based on the formula. Just to make sure I have this correct: if I have a 193 HP 4 cyl 360, as long as my FF is less than 9.7 GPH (or use 9.5 to be on the safe side), I will never be above 75% HP at any altitude? 9.7 x 14.9/193 = 74.9%

Therefore, you should never be able to hurt the engine by leaning too much below this FF (again, no matter what alt)?
Click to expand...

Dave, your math is correct provided you are LOP at that FF regardless of altitude. Remember, the only time you do damage to your engine is when you lean too far while still ROP and get into the red box, or you don't lean far enough once you are LOP and are still in the red box. Once you are LOP, you can't do any damage by leaning too much. The engine will eventually just stop running from an excessively lean mixture, but you won't do any damage to it.

Jon D.
 
Mixture Variation Testing

To add some real data to the discussion, below (I hope as I have never inserted external links) is some data from recent testing varying only mixture settings.

VH-XRP%2026In%202600RPM%20Mixture%20Variation%20Testing.pdf


VH-XRP%2026In%20Mixture%20Variation%20Testing.pdf
 
RV10inOz said:
Hi Bob,


The APS graph was carefully constructed from real Dyno values, to get the relationships very accurate. I know your penchant for manufacturers data, but depending on which graph you look at the Lyc data can be rough or accurate. So I would say that the Lyc stuff is bradly consistent ;)

I would say, try this, at 8-10K' WOT, 2400-2500 RPM, and the last cylinder to peak from the rich side, or first to peak from the Lean side, set at 10dF LOP. Anything else is a compromise.

Not sure when that might be, but lets assume you are in a WOT cruise, and you want to slow down quite a bit (ATC maybe...or just the view, or flying with a Cessna) once you have the mixture set just pull MP back, the Bendix fuel system is pretty good at keep the F/A ratio consistent.

This is why on descent from even in the FL's I rarely touch the mixture until the hangar door, apart from maybe a tweak. Refer the Airsport article back in JUN or there abouts.;)

Have you considered bribing your daughter and shifting the wedding plans a month or so. :D;)
Click to expand...

I have noticed my new Precision RSA-5 makes quite a drop in fuel flow from full throttle to about 90% throttle. You have to reset the mixture then, it gets too lean. Below that, say from 40-90% throttle, it stays pretty consistent. There must be an additional WOT fuel circuit, like an economizer in a carburetor. I dont think our old Bendix RSA-5 unit did that as much.
 
Procedure for applying WOT at low altitude

Query for David Brown on WOT at low altitude.

I have about 100hrs on my IO360 CS prop engine in my RV7, EI-EEO. I have and AFS3500 with EMS.

I have been an advocate of LOP operation from the start (having read John Deakin over and over and over until I "got" it) and regularly operate at in the 25 to 28 LPH range. Out of the box, my CHTs are all practically equal when operating LOP. Engine stays perfectly smooth all the way down so much so that my air speed is a good indicator of how lean I am.

Normal process after take of is to pull the prop slightly to 2500RPM (to be neighbourly), then when settled, pull the throttle to about 21" or 22", then pull back the prop, then lean.

What is the process when wanting to operate at WOT after take off or in the cruise?

Why is this not a problem (high MAP, low RPM) for the engine, when all the standard training says reduce the MAP before reducing the RPM and vice versa?

Thanks for your help,

Andrew Butler.
RV7 EI-EEO
EITT Ireland.
 
Reply to post #28

That is good information.

It seems like diminishing returns leaning to less than 9 gph, unless there is a purpose for going more slowly.

Based on your data, the 8 gph @ 2200 and 2000 RPM yielded 4.3% speed reduction and 7.7% mpg (actual trip cost) improvement and 8.3% speed reduction for 5.5% mpg improvements than 9 gph, respectively.

2600/24/9 yielded a fuel economy of 10% over WOT numbers with only a 2 kt. speed reduction.

Thanks for posting your data.

Larry Tompkins
N544WB -6A
W52 Battle Ground, WA
 
LarryT said:
That is good information.

It seems like diminishing returns leaning to less than 9 gph, unless there is a purpose for going more slowly.

Based on your data, the 8 gph @ 2200 and 2000 RPM yielded 4.3% speed reduction and 7.7% mpg (actual trip cost) improvement and 8.3% speed reduction for 5.5% mpg improvements than 9 gph, respectively.

2600/24/9 yielded a fuel economy of 10% over WOT numbers with only a 2 kt. speed reduction.

Thanks for posting your data.

Larry Tompkins
N544WB -6A
W52 Battle Ground, WA
Click to expand...

I just lean to peak below 55% power, the speed loss is massive more than 25 LOP and the economy improvement is likely only a couple %. 75% power is where the savings definately are the greatest.
 
RV10inOz said:
This is why on descent from even in the FL's I rarely touch the mixture until the hangar door, apart from maybe a tweak. Refer the Airsport article back in JUN or there abouts.;)
Click to expand...

When I do this, i notice my EGTs will spike up to over 1500F or higher when MP is pulled back to 15 to 13" or so on the downwind.

https://www.savvyanalysis.com/flight/221385/e02d9708-f396-4359-a569-7c2a96f34a1a

You can see that spike up in EGTs at 1:05 on the graph above. In fact, you can see the gradual rise in EGT during the slow descent.

I asked Mike Busch about this in email, and he said i should be adding Mixture during the descent, which i sometimes forget to do like above.

Jae
 
Jae, you can add some extra fuel on descent, just no need to do the full rich by landing. That is a waste.

Couple of comments if you like though.

Your EGT went to 1500dF......So what!!! Who cares. That is fine. It is probably because the flame front is slow enough to do that.

Mike Busch does a lot of TCM engines and mixture is driven from the mechanical fuel pump so RPM/pressure related. If you have the typical Lycoming Precision RSA type it is a MASS AIRFLOW device so as the air mass gets greater it responds accordingly which is why I say touch nothing, or only a fraction just to stop it grumbling at you. I can do a partial power go around with out pushing the red knob in so it is not about to quit.

So only richen up if it grumbles at you, and only just enough. :)
 
Andrew,

If you go back and read some of JD's articles you will find that this is what he has said.

From Takeoff leave the throttle WOT, only reduce RPM a bit for noise when actually necessary and then once above 1500 and away from the field put it back in again. As you pass say 700-1000' take note of an EGT, say EGT1. Use this as a target EGT every couple of thousand feet in the climb.

We have the data, and I did some recent tests with even more data capture and GPS tracks which show that the WOT target EGT method at the same IAS produced a quicker climb, on less fuel and lower temperatures. All the old wives tales were shown to be that. :) The old 25/2500 after takeoff is actually the worst way to climb.

So unless you want to slow down to C152 speeds, use WOT all the time. You would not run around with a clogged air cleaner, so why do the same thing?

At low level, and WOT the power you can produce is going to be higher, so you need to be further from peak EGT to keep the peak pressure away from TDC and the CHT's going up.

here is another photo from a couple of weeks ago. Try to formate with a Lancair IV....which must have been hard for him.

80LOP_zps6e22f7f3.jpg


This would have been around 70-80LOP, thus peak EGT was over 1500 by a fair bit. Look at the CHT's on a hot day (26dC OAT).

I must stress it is important to learn and understand. I can only throw you tid bits of info in a forum post. I see you are in Ireland so getting along to an APS course is not easy unless you are in Oklahoma in March or down under in May. So the next best solution is do the online course. Take a peak.

Why is this not a problem (high MAP, low RPM) for the engine, when all the standard training says reduce the MAP before reducing the RPM and vice versa?
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When you have done the course this may make more sense, but the reduction in RPM moves the peak pressure up closer to TDC, this increases the pressure magnitude and the CHT's. Not so much a problem when the MAP is 29" and RPM is 23-2400 RPM but getting down to 2000 is starting to have negative effects. The old flight instructors usually have no idea why they are teaching that to you anyway. But it cam from the days of big multi row, compound turbo supercharged radials where MAP was 40 or way more inches and RPM quite low. This is where the OWT of over square came from. So in your Lycoming or TCM, the MP/RPM and mixture can be moved in any sequence you like, its no big deal at all.

Happy to take a PM or two if you like. :)
 
REPLY TO POST 26

Thanks David,

So, pull the mixture until it squeals, sneak it rich towards peak, and lean to 5C/10F below peak EGT, first cylinder to peak.

I do leave the mixture lean through descent until the base turn - roll, throttle, prop - and mixture rich, just-in-case the earth sucks.

I will fall off my bar stool laughing if at 10F LOP the Dynon says I am still at peak because of data scatter in our setup.

Regards
 
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