Leanings effects on CHT

[beetlebug]

I think I've read almost every thread on leaning in this forum but I'm still having some issues with it.

If I run ROP I cannot keep the CHT below 380 unless I'm burning around 12-12.5GPH. At that point the mixture is practically full forward. If I run LOP I can easily keep the CHT below 350 and only run about 8-8.5GPH.

Also, generally I'm just not seeing the TAS numbers other folks see. I have a 2010 DA40 with A/C so I expect my TAS will be slightly lower than most due to the scoop but the best I can get is about 130 KTAS at 8/8.5GPH LOP at about 23in/2500rpm. I'm honestly happy with that since a few extra kts doesn't really help much for short flights so it's more of a curiosity than anything.

What am I doing wrong here?

[LTek1]

For what it's worth, my 2003 is also a 130kt machine. No powerflow, though, which I suspect is most of the difference.

[smoss]

I have never heard how much power the AC robs, but clearly it uses some of it. Is there much difference with it off?

[Tom Davis]

I have a 2005 model. I usually fly between 7000 feet and 8500 feet. I lean to 8.2 to 8.5 gph and use the manifold pressures and rpms set out in the handbook. That usually gives me between 128 ktas and 135 ktas. When I go higher, I get better tas, but not by a lot.

[Lou]

That's strange. I am no engine genius but I get 145TAS at 10.5 gph no problem ROP, and 130TAS at peak EGT with fuel flows around 7.5 gph. CHT's rarely above 380. How many hours on the engine, and are the cooling baffles intact?

[Chris B]

Hi Jeremy -

are the cooling baffles intact?

^^^ Check this first. Even seemingly insignificant gaps dramatically reduce cooling efficiency. RTV is really cheap, too.

Brock's 2014 post in the supercharger thread (see page 20) was very helpful.

Second there are a lot of air leaks in most planes. Start by removing your top cowl and looking carefully at the marks left from the baffle material. Where it is black it is sealing. Where it is not black or worse you see kind of a faint spray pattern then you have a leak. You can also look at the baffle material. It will be worn where it touches the cowl - this is good. Where the seal is new looking is where it is not sealing. It takes very little leakage to make a significant difference so be a little OCD about it. Most front and aft baffles have a wavy scalloped shape and don't touch on the low points. We took RTV and filled in the low spots, covered it all with plastic wrap and put the cowl back on while it dried. The sides baffles are made up of two overlapping pieces (on my plane). I found that in the middle above the cylinders one was a bit taller than the other keeping the lower one from sealing. Have your A&P carefully trim them so they both make good contact with the top cowl. Next there are many leaks where the fiberglass touches the engine especially near the case halves fore and aft as well as around and in grommets etc. Seal all that stuff up.

In my 2008 the gaps between the composite shrouds and engine case were huge, particularly near the prop. This is easy to miss unless you look carefully.

Here (link) is a more recent discussion about baffling with a happy outcome.

Other considerations:

• AC probably accounts for 3-5 kts. In addition to the scoop, the cowling has a noticeable flat spot directly behind the prop.
• 2-blade props apparently cruise a little faster (but climb slower).
• CG near the aft limit is fastest.
• IME hot weather (/inversion) reduces top-end speed ~3 kts. Power & cooling are both worse.
• GAMIs help run fast LOP. After three iterations, generally all of the cylinders are now pulling strong, though I am planning one more tweak after the results of our recent heat wave.
• IMO, fine wire plugs are essential if you run LOP.
• EIS (currently only Electroair) helps ~3 kts, IME even without the timing advance. Whether this is "worth it" is totally subjective. FWIW, I mostly went this route for smoothness.

Chris

[beetlebug]

Thanks. This is all helpful.

@Steve I don't notice a difference with the A/C off. I think the primary loss of kts is from airflow because of the bump on the cowling and the scoop on the belly of the plane.

@Tom I think I need to print out that page of the manual and make absolutely sure I'm flying with those settings.

@Lou 754 hrs on the engine right now. I'll check the baffles.

@Chris Thanks. Sounds like I need to look at the baffles. I know I have fine wire plugs but I don't recall seeing anything about GAMIs in the logbooks so I don't think I have them but I'll double check.

I was thinking more about my last few flights where I've really tried to hone in on leaning and speed. Really trying to learn this plane better (only owned it since Feb, flown ~70 hrs in it now). My last few flights have been lower altitude due to clouds (I'm VFR only at the moment). Somewhere between 4500-6500ft so it's possible based on Tom's comment that I'm not flying the right numbers for these lower altitudes.

[Joey]

At 7 to 8k I am generally cruising 137-139, 2400 rpm and 9.3 gob. Lower altitude at 4500 I can see 142 at 2500 and 9.7. But pulling back to 2400 and 9.3 only takes a few knots away.

I have a 2008 model xls w power flow and a 2 bladed aluminum prop.

[Antoine]

I have done a lot of speed tests with the DA40 and explored ROP, peak and LOP.Here a few hints:

When flying low, the engine has a lot of available power and the dense air causes a lot of drag.
So you will need considerable fuel flow to generate the power to fly fast. But you can. You want to run VERY rich if ROP

The IO-360 really does not mind 2500 or even 2600 RPM continuous, contrary to what the base AFM and some mechanics would lead us to believe.
The AFM does state a 2400 RPM limitation. This was aimed at reducing the noise signature, but there is an AFM supplement that removes the limitation. So If flying low, use 2500 at least and you will be able to exploit the available high MP to maximize power. My 2008 XLS without AC will easily exceed 150 knots true on 11.5 to 13 GPH down low. Remember to stay deep into the ROP domain, no peak EGT

The DA40 strives at medium altitudes (6-9 K). Using wide open throttle and around 2450 to 2500 RPM, with a fuel flow of 10,2 to 10.5 GPH (near peak) , I typically see a solid 150 KTAS. No excuses, no BS. 150-151 KTAS every day, all day.

At higher altitudes, speed drops off but so does fuel flow. Use higher RPM (2600) to get more power from the engine if needed.

In order to maximize speed, use the aft half of the CG envelope.
If your post 2007 DA40 cannot go north of 145 KTAS, you are probably one of many victims of the corked nose wheel problem. Do a search on this forum. You will find it hard to believe, but your aircraft was delivered from the factory with the nose wheel pointing sideways and the trim tab bent to correct for this.
How to find out?
1. Make a test flight and note speed and settings,
2. Land, remove the NLG fairing and fly again.
If you're SLOWER, you NLG is fine and you can reinstall the fairing. End of the test,
If you're slightly FASTER (by one knot), you've just proven that your nose gear fairing is slowing down the plane: you have a "corked" plane. Go check the thread for more explanation and fixes

[beetlebug]

Thanks Antoine.

I'll try the nose wheel test when I get a chance. I wouldn't be surprised if I have that issue. I've also noticed in some cases I have the issue of the nose wheel locking drastically to one side causing the plane to be uncoordinated without lots of rudder input until I touch down and then you can feel the nose snap back to straight when it touches the ground.

I glanced at the baffles but haven't given them a good look yet. Honestly, not entirely sure what to look for yet but I'll figure it out.

I was able to get about 137ktas yesterday at 5500 (KJYO-KFME). According to the recorded data (I always leave an SD card in the MFD to record flights) I was at 2550RPM, 23.5in, 12.5GPH, peak CHT was just shy of 380.

Then another leg (KFME-KJGG) of the flight was 117ktas, 2500RPM, 23.5in, 8GPH.

Here is a link to the analysis from KJGG-KJYO and is more typical of what I see: https://www.savvyanalysis.com/flight/19 ... fe3abdc22e