Cruise Performance: Peak vs Lean of Peak

[andrewbutler]

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.

[LarryT]

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

[jetdriven]

Originally Posted by LarryT

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

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.

[jchang10]

Originally Posted by RV10inOz

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.

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...9-7c2a96f34a1a

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

[RV10inOz]

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.

[RV10inOz]

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.



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?

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.

[Bob Redman]

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