DA40 CS Power Settings

BravoZulu

Well-Known Member
Hello all, I am relatively new teaching in the DA40, looking to see what other folks out there are teaching for power settings for climb.
We have the 2 bladed prop authorized for continuous operation up to 2700RPM, but would prefer to teach 25 squared as that is pretty much the industry standard.
the manual suggests max throttle and 2400RPM (against convention, and for 3 blade prop as standard)...not exactly conclusive.

Thanks for any advice.
 
would prefer to teach 25 squared as that is pretty much the industry standard. the manual suggests max throttle and 2400RPM (against convention, and for 3 blade prop as standard)...not exactly conclusive.

I don't fly this airplane, but I'm almost horrified you're prefer to teach an industry practice that borders on superstition rather than what the manual instructs. That isn't what teachers are for, IMO.
 
I flew it at max power and 2400 rpm per the book. No problems at all.

I've even flown planes at 35" and 2400 RPM before. Scary.

-mini
 
25 squared for *many* airplanes is simply a good rule of thumb that typically provides adequate climb performance without overworking/overheating the engine...that is all I'm referring to.

The whole reason I started this thread is because I don't want to teach based on any sort of assumption, but the guidance from Diamond is not conclusive and I'm hitting a dead end with them.

Flame away...
 
I've got about 30 hours in a DA40 in the last month. Both the 2 blade and 3 blade models. I've played with the whole 25 squared vs full forward RPM and MP, and can say that it really doesn't make a big difference in Rate of Climb, IMO. So, teach however you want I'd say. But don't do it just because you do it that way in one airplane.

I've also found the cruise performance charts to be very inadequate. They just don't have enough data there to get a good setup to run the plane for a good X/C. Sure, you can guess, but more data would be great
 
25 squared for *many* airplanes is simply a good rule of thumb that typically provides adequate climb performance without overworking/overheating the engine...that is all I'm referring to.
Reducing the throttle to 25" closes the throttle valve restricting air (which cools) from entering the cylinders. I'd say that could lead to overheating the engine.

The whole reason I started this thread is because I don't want to teach based on any sort of assumption, but the guidance from Diamond is not conclusive and I'm hitting a dead end with them.
What's not conclusive? They list the numbers for the climb (IIRC) and then they have a cruise performance section. FT/24 in the climb and then whatever the cruise numbers are.

Flame away...
Don't get so defensive. I know I'm an ass, but I'm really just trying to understand where you're coming from or why you wouldn't want to follow what Diamond says to follow.

-mini
 
I've taught using the 25 squared method but you will find when you are teaching brand new pilots, first time in the airplane, it might be a little to much for them so I will teach them 23 squared, that is all pre-x/c training, once they get to the x/c portion they have a good feeling of the airplane and then teach them to bump it up to 25 squared to really get the feel for the plane. Not sure what you think about that but it seems to work out just fine for the student pilots.
 
"I flew it at max power and 2400 rpm per the book"

the problem is that the supplement for the book per our 2 blade prop says
Max rpm 2700 for climb out which makes the A&P side of me cringe as the temps bump the yellow and red on climbout...and i'm sure our nearby town is not a fan either.
 
What is the fuel flow on a "standard day" at takeoff power on your bird? If the temps are that high, your fuel flow may be a little on the low side at that power setting.

-mini
 
Do what the book says.

There is nothing magical about operating at anything squared.
 
the problem is that the supplement for the book per our 2 blade prop says
Max rpm 2700 for climb out which makes the A&P side of me cringe as the temps bump the yellow and red on climbout...and i'm sure our nearby town is not a fan either.
4-point CHT? If so, try pulling the prop back and running full throttle and see what happens.

The Diamond test pilot working on certification of the DA-42L360 (essentially using the same engine as the DA40 CS) said in the DA42 running full throttle and 2400 RPM gave nice performance and kept all the temps pretty healthy.
 
It will be hard to see but if you look on the left side toward the middle a little low there is a comment that says "limiting manifold pressure for continuous operation" which I was told is used for judging the limit difference between MP and RPM. Can someone verify?

If we compare numbers that would mean at 25 MP the limit is 1800 rpm minimum, 26 mp limit is 2000 minimum, 27 is 2150, and 28 is 2300. This is for an IO-360-A, -C, -D, -J; AIO-360 series engine and is figure 3-21 which is the sea level and altitude performance.

At the college we were taught this and shown that the maximum difference in engine RPM vs. MP will be 4 so that we have a safety margin from this data. We were also taught that there is absolutely no reason, other than emergencies and overheating issues, in cruise to not be at this limit. We were told: it is quiet for passengers, provides better fuel consumption, less ware and tear on the engine, less prop drag (I will explain in a second), and since tach time bases off engine RPM then longer hobs times between overhaul.

Quickly to explain the less drag at 25/25 we have the same power as 26 MP 24 rpm but we have slightly less prop drag which means the equivalent power is giving slightly better cruise speed. It only amounts to a couple knots but the fuel burn and other aspects show a far better improvement.

That being said I try to teach them to think about this and teach it the same way even though it might be a little over doing it. It doesn't make sense to me to teach how to fly a constant speed aircraft, which was designed specifically to be more efficient, and then not teach the student how to use its efficiency.


Reducing the throttle to 25" closes the throttle valve restricting air (which cools) from entering the cylinders. I'd say that could lead to overheating the engine.

Just thought I would mention that the 200 rpm reduction is probably more the cause for the cooling. Also since he is referring to a climb at a high power setting with slower airspeed generally we are trying to keep the aircraft cool which is likely the reason for the procedure in some aircraft like the RG (which at full rpm/power does have has overheating issues).

I presume someone just read it in another manual and decided to apply that to every aircraft which simply isn't the right way to operate the aircraft since the manual isn't for that aircraft. Thats my 2 cents on where this 25" squared and 20" squared for the pattern stuff might have come from.
 

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"I flew it at max power and 2400 rpm per the book"

the problem is that the supplement for the book per our 2 blade prop says
Max rpm 2700 for climb out which makes the A&P side of me cringe as the temps bump the yellow and red on climbout...and I'm sure our nearby town is not a fan either.

There is a difference between max rpm for climb out which is a limitation and recommended procedure for climb out which is not. You can use any procedure you want as long as you don't exceed limits and you certainly don't have to follow that recommendation.

That being said who am I to think I know more about what procedure is best for this an aircraft climb out over the engineers and test pilots that designed and flew it? So I follow the book and certainly still run my own tests with slight changes to see if their procedures yield the best results run down 30 yr old aircraft, but typically they are fairly accurate in my experience.

The way I do it is max rpm till safe altitude and then back to the procedure which knocking down 300 rpms would certainly control those temp issues. I typically ignore cruise procedures for the favored approach described in my previous post.
 
25 squared for *many* airplanes is simply a good rule of thumb that typically provides adequate climb performance without overworking/overheating the engine...that is all I'm referring to.

Remember the "economizer valve" aka "power enrichment valve" from your A&P class.

At full throttle the mixture is SUPER RICH which cools the engine and prevents detonation. If you pull the MP to some arbitrary number you lean the mixture out considerably.

By "babying" your engine, you are in fact subjecting it to higher temps and shorting it's life. I ALWAYS climb at full throttle.
 
Do what ever the POH says can be done. You have a lot of options. Do whatever you want, except for passing anything on to students about "MP never being more than RPM" or "must be squared".

I liked running that airplane down around 2100 RPM (if i remember right) in the summer because with the vents open, it get loud.
 
Fantastic Gentleman, I spent a fair bit of time flying last evening, and the airplane performs well and cool at max throttle and 2400RPM recommended in the POH, and I will definitely take SHDW's advice and be more aggressive in taking advantage of the passenger comfort afforded by really bringing the rpm down during cruise phases.

Great to debunk the squared myth/rule of thumb...just goes to show there is always more to learn on a topic...looks like I've got a lot more reading to do!

Thanks all.
 
be more aggressive in taking advantage of the passenger comfort afforded by really bringing the rpm down during cruise phases.

Just be careful if you go beyond the suggested differences in the POH for your cruise. If you go beyond them make sure you check the DA40 engine manual for the figure on sea level and altitude performance and see if they have a limiting distance between MP and RPM.

You don't want to go outside the engine limitation, the ones I gave only work for an IO-360 which is only in one of the 3 DA40 models. One is a diesel which is certainly going to be different and the other is an O-360, would anyone know if that would make a difference for these limitation? I would assume it would not, but I would risk an engine on my assumption.
 
25 squared is a bastardization. I have flown that plane many times before with instructors who do that to keep it simple or whatever. Thankfully most all the DA40s (fixed pitch or not) have excellent engine analyzing equipment which allowed me to prove we were close to burning up the engine to those instructors. Fly it like the book says. 25 or 24 squared would produce some pretty toasty CHTs and not to anything for your climb performance in that plane, good or bad.
 
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