Cowl carbon baffles, CHTs

[rwtucker]

So to climb, don't reduce RPM below 2400, and lean as you get higher to maintain EGTs 100-200 ROP to obtain max hp. By climbing at a "cruise climb" airspeed of 90 knots this keeps CHTs from going much above 400F (maybe 405F) on hot days -- this is not going to hurt the engine when climbing for only 10 minutes or so.

Dave,

I wonder if the differences in typical MSL and atmospherics give us different experiences in managing CHT.

For example, There are many summer days that I cannot keep CHTs below 400F using your model. I may have to reduce climb to 200 FPM, prop to 350, and MP to 22 inches. Even so, I am sometimes chasing 399-405. My CHTs are almost identical so I don't think I have a baffling problem. In the summer where I fly, I'm often taking off at 2,000-3,000MSL and I may be flying into an inversion layer at 4,000-5,000, which is where I have the greatest difficulty maintaining acceptable CHTs and a PRC.

On Max HP for a given MP, I have been working under the assumption that leaning for 1,350 EGT is the best way to approximate Max. Is there a better way? Even if an ROP value is better, I don't like the idea of hunting around for an ROP value in a hot climb since finding that value involves starving my already too hot cylinders for at least a few seconds.

Thoughts?

[David B]

Rick - To answer your question I have no real reason other than the thought that reduced rpm's gave me the false impression I was being easier on my engine.

CFIDave - The thing I love about this forum is the knowledge and advice it provides along with explanations as to why they are being made. I am a better pilot and protector of my expensive asset because this site is available to me. Your explanation of climb performance makes perfect sense to me and as such my next takeoff will endeavor to encapsulate the sage wisdom you have provided...thank you.

[Antoine]

3) lower RPM makes a BIG difference. I find it better to use 2300 RPM and wide open than 2500 and throttled back.

I use fuel as a very last resort (in practice - never) to cool my engine. But then I dont fly in Arizona!

Hi Antoine -

I agree with your first two points, and your third point if you are talking about cruise power. But climbs are where CHT is most challenging for me, and low RPM there can cause big problems.

Everything I have learned from the APS guys (Deakin, Atkinson & Braly) is that at high power settings in a climb you want the prop at high RPM. Otherwise the detonation margins are *greatly* reduced, and peak internal cylinder pressures are very high. The best analogy I have heard is a bicycle: a strong cyclist can climb in low gears, but puts huge strain on the drive train.

OTOH, at cruise power, running with relatively high MP and slow RPM is perfectly safe, since the pressures are greatly reduced. The Lycoming IO-360 has lower power per cylinder than the big bore engines, and can tolerate abuse that would destroy a Continental IO-550. So it could be that "high power" for our engines is closer to cruise power for the big engines. But I wouldn't recommend low RPM climbs as a best practice.

Separately, the rich mixture is not cooler because of the chilling effect of additional fuel. The engine runs cooler simply because the extra-rich mixture burns less efficiently and the engine develops less power (CHT is a good proxy for power). It's just an expensive way to run at reduced power.

Chris

Hi Chris

Firstly just to make sure we are 100% on a positive attitude:
I completely agree with you on the RPM issue - low RPM and high MP is a bad combination, and the engine will actually sound unhappy. I have recently being flying low (headwinds) and had to throttle back to keep it happy.

I suppose my post needs some clarification - sorry for the confusion.

Obviously (to me...) reducing RPM to 2300 versus throttling back only makes sense in cruise. In cruise climb, I "reduce" to 2500 but leave it there and manage temps with airspeed as stated above.
Dropping to 2300 would cost too much power that is badly needed in the climb.
(note: the basic AFM specifies 2400 in cruise climb, fortunately this limitation is removed in an addendum).

Regarding excess fuel flow as a coolant, my understanding for the reason why it cools is partly the incomplete combustion and partly the energy absorbed by the evaporation of unburnt fuel.
It is not only inefficient, it also deposits combustion residues that can cause fouled plugs. I don't like it and try to avoid it to the extent possible.

You are right about our engine being a very different beast than the big turbocharged Contis, and I think this is part of the reason why the guru's advice is only partly applicable here.

It was important to highlight that 2300 RPM is inadequate at high MP climbs and I thank you for doing so.

[Chris B]

FWIW, it took me a long time to figure out how to properly manage CHT. For over a year I routinely let CHT climb into the >420F range. But after much trial and error, CHT now consistently stays below 400F. Here is maximum CHT data stretching back about 2-1/2 years and 250 flights. Many of these flights are with very high OAT and inversions. With few exceptions, all of these data points are from climbs:



The trick is actively managing IAS (FLC mode helps a lot) and target EGT by varying mixture during the climb (or at take-off on high altitude or high DA days). Target EGT = EGT at full throttle, full rich at sea level = ~1300F on my aircraft, but may be different on yours. On really hot days slightly (0.5") less than full MP after pattern altitude can also help, with minimal impact on climb rate.

Chris

[linzhiming]

Just to add some flavour to the thread from flying the DA40 and managing the CHTs in tropical Singapore where on-the-ground OATs are often around 33-34°C and OATs at 9,000 ft are normally around 12°C:

What I have found works well for me to keep CHTs below 400°F is to the following:

(a) Takeoff with full rich mixture setting at Vy (performance is still ok despite the high OAT as the airport elevation is 63 ft MSL)
(b) Retract flaps and accelerate to and climb at cruise climb speed for the relevant weight (either manually or with GFC700 FLC set to cruise climb speed)
(c) Full RPM, full throttle, full rich initially
(d) Passing 3000 ft, still at full RPM and full throttle, start leaning the engine so that there EGT bars are just barely visible at the bottom of the EGT displays (from memory that is about 14.5-15 gals/h fuel flow at 3000 ft)
(e) Repeat step d every 1000-2000 ft, normally this means reducing fuel flow by about 0.5 gals/h per 1000 ft
(f) Level off, reduce RPM to desired value (normally 2400 rpm) and set desired MP (I normally use WOT if above 5000 ft as I normally cruise at 8000-9000 ft) and lean to peak EGT.

Throughout the climb, I monitor CHTs quite religiously and the above procedure normally keeps CHTs around 390°F. If I see CHTs climbing (which normally means I was overly aggressive on leaning), my procedure is to add fuel flow by 0.5-1 gals/h and that is enough to keep the CHTs under control without having to increase IAS (i.e. reduce vertical speed).

From memory, with just me in the cockpit and full ER tanks, I normally manage 700-800 ft/min at 5000 ft with the above procedures (not leaning above 3000 ft causes the vertical speed and engine performance to drop). When at MTOW, I normally manage 500 ft/min at 5000 ft.

I would be very interested to know whether you see any issues with the procedures above, whether there is any way to improve them and whether they work for you.

Regards,

Wolfgang

[Rick]

Wolfgang, I typically operate from airports between 500' and 1500' MSL, and temperatures usually not as warm as yours, but my procedures are almost identical to yours. After initial Vy climb, I retract the flaps and climb at 90 kts IAS (WOT and 2600 RPM), reducing the mixture every so often to keep EGT bars just visible (~1300). Even on the hottest of days (which might occasionally reach 35°C here), my CHTs never exceed 400 in the climb, typically in the 380-390 range. Unless winds dictate otherwise, I always cruise at 8000' or 9000', WOT, 2450 RPM, and just a tad on the lean side of peak EGT, which yields a TAS between 137-140, on 8.2 - 8.5 gph, with CHT's in the 340-360 range. I like consistency!

[David B]

I'd like to thank you gentlemen for this great discussion. Although I've been flying this plane since 2008 I've never really understood the connection between RPM and power. Generally I've flown in cruise at or near 22 squared at 7-7.5 gph with CHT's less than 340 however, this dialogue has enlightened me to maintain at least 2400 RPM's throughout the climb and perhaps throughout the flight. At the above settings I normally get no more than 110 IAS but seeing Rick's note above tells me I've been doing this all wrong.

[Antoine]

This is the AFM data for an IO360 M1A, as installed in the DA42. To the best of my knowledge there is no AFM supplement for the DA40 with the engine settings for more than 2'400 RPM... I don't understand why since there is an AFM supplement to remove the 2400 RPM limit.

Notice how, for the same power setting, fuel flow must be increased when going from 2200 to 2700 RPM. For example, getting 65% power at 2200 RPM burns 9.5 GPH, whereas the same 65% at 2700 RPM costs 10.4 GPH.

But when flying at low alt, I am unable 2200 RPM because manifold pressure must be pulled back too much to avoid audible "unhappiness". The 75% setting of 2200/27.3 is no go with my plane, despite being on the "recommended" group in the chart.

[Thomas]

As I do not have the possibility take a log file with my classic instrument layout ... here the pic from my last cruise over the alps in Austria. Full throttle , 2400, FL140
I keep CHT always below 400 ... It's never an issue in our environment, home airport 1470ft.

[BRS]

When doing the "climb-cooling" certification tests for the SuperCharger we were required to take-off when the OAT on the surface was 100˚F. Then we were to climb at Vx with T/O flaps until temps stabilized or started to descend. We climbed at 30" MAP up to over 8000'. Learned a thing or two about cooling.

First we got rid of the leaks in the baffles (like Antoine said). The biggest offender is where the top rubber meets the top cowl at the rear of the engine (front does it too). It gets all wavy and does not even touch at the low spots. The easiest way to deal with this is to get a few tubes of RTV and completely fill in the low spots with a nice heavy bead, don't wipe it. Then carefully lay wax paper over the baffle and carefully put the top cowl on. Let it cure over-night.

Second, RPM makes a big difference (Like Antoine said). We found that at 2700 RPM no matter what we did we could not pass the test. BTW - cylinders were fine but it was the oil temp that we had the most trouble with. Did I mention we also drained the oil to 4 qts? When we did the test at 2400 RPM (& 30") we passed the first try.