Cowling melted from leaning during taxi

Just so people are aware of potential issue.

I have an RV9, that I didn't build. I bought it with 183 hours, and have put another 150 on since purchase.

The builder did not put any heat shielding on the lower cowl.

I flew it with just the smallest dark spot on the left lower cowl for most of the 150
Hours. The cowling was fine at annual 2 months ago.

I had an issue with fouling spark plugs during taxi, delayed take offs (busy airport).

After reading a bit, there seemed to be consensus that you can lean quite a bit at idle without problem. I did this for the next several flights.

I recently installed all of my egt and cht gauges, and when I went to put the cowl back on I saw these two huge bubbles where the inner fiberglass layers swelled up like balloons from the honeycomb structure underneath.

After some help from my hangar mate, I removed the old fiberglass and repaired the cowling as best we could. Needless to say heat shield was added.

Looking at it, there seem to be two small areas where the exhaust actually contacts the cowling. Huh!

Just posting this so people are aware what can happen with leaning (apparently too much) at idle.

leaning

I would just create a min 3/4" to 1" gap between the cowling and exhaust pipes.

Leaning on ground is a good idea - keep doing it.

Just fix the cowling.

I have one exhaust pipe on the left side that is 1/2" or less to the edge of the exit chute. It was so close a standard exhaust pipe heat shield would touch the cowl. I removed the shield, and added 1/8" of fiberfrax covered by Vans aluminum to the cowl. It has remained cool, no discoloration - -I checked and replaced.

I lean at idle all the time, so you could be confident that using the Vans aluminum shielding, a little 1/16" or 1/8" fiberfrax under it will not cook your cowling.

The shielding is necessary on the sandwich areas of the cowl that are within 2-3 inches of the exhaust pipes.

Exhaust near cowl

Keeping the exhaust away from the cowl was a harder part of my 9A build. I still need to tweak it some more to get it away more.

I don't think leaning would have anything to do with the cowl getting to hot. Perhaps there is less airflow around the exhaust in lower cowl while taxiing but EGT's are several hundred degrees higher during cruise and cool off as power is reduced. Several hundred degrees cooler while taxiing too. Don't be afraid to lean on the ground.

Mdragon said:

Just posting this so people are aware what can happen with leaning (apparently too much) at idle.

Click to expand...

There is no such thing as "leaning too much at idle." On the ground you should be leaned to the point where advancing the power past normal taxiing RPM makes the engine stumble. On my airplane, that means pulling the red knob all the way back then advancing it about 1/4" in one fluid movement before the engine cuts.

It looks like you don't have a problem with leaning for taxi, you have a problem with the exhaust making physical contact with the cowl. That shouldn't happen. The cowl is not supposed to be a heatsink for your exhaust system.

Remember that the engine vibrates, and if components fixed to the engine are in contact with the cowl that means your cowl will vibrate too, and you'll end up with gelcoat cracks, delamination, abrasion and eventual failure of the piano hinges which hold the cowl together, and (in this case) excessive heat transfer.

RVs are tightly cowled, but they shouldn't be so tight that anything downstream of the engine mount isolation blocks touches the cowl during normal operation.

- mark

BillL said:

I have one exhaust pipe on the left side that is 1/2" or less to the edge of the exit chute. It was so close a standard exhaust pipe heat shield would touch the cowl. I removed the shield, and added 1/8" of fiberfrax covered by Vans aluminum to the cowl. It has remained cool, no discoloration - -I checked and replaced.

I lean at idle all the time, so you could be confident that using the Vans aluminum shielding, a little 1/16" or 1/8" fiberfrax under it will not cook your cowling.

The shielding is necessary on the sandwich areas of the cowl that are within 2-3 inches of the exhaust pipes.

Click to expand...

That's what I'm having. The standard exhaust pipe heat shield is touching the cowling. Of note the side with plenty of space is the one that was damaged the most.

What Mark said. This has absolutely nothing to do with leaning and everything to do with exhaust to cowl geometry.

sjhurlbut said:

I would just create a min 3/4" to 1" gap between the cowling and exhaust pipes.

Leaning on ground is a good idea - keep doing it.

Just fix the cowling.

Click to expand...

Yep, Yep, Yep.

newt said:

There is no such thing as "leaning too much at idle." On the ground you should be leaned to the point where advancing the power past normal taxiing RPM makes the engine stumble. On my airplane, that means pulling the red knob all the way back then advancing it about 1/4" in one fluid movement before the engine cuts.

Click to expand...

This thread exposed a hole in my knowledge for sure. I never had a CFI adequately explain to me proper ground leaning procedure (or for that matter, leaning procedure for takeoff at high density altitudes either). They all just kind of glossed over it: “Oh just lean it a few turns when you’re taxiing” and that’s it. I don’t recall reading a POH that mentions leaning on the ground either. It seems to be lore that you just pick up either the easy way from other pilots or the hard way after fouled plugs and valves.

Draker said:

I don’t recall reading a POH that mentions leaning on the ground either. It seems to be lore that you just pick up either the easy way from other pilots or the hard way after fouled plugs and valves.

Click to expand...

In the service of the propagation of knowledge:

We want to lean for taxi because excess fuel carries excess lead which, at low power levels, doesn't get hot enough to exit the combustion chamber as vapour, and instead condenses on (among other things) spark plugs and valve stems. That's why you get lead fouling from taxi but don't get it in flight.

To reduce excess lead, we can reduce excess fuel, i.e., lean the mixture.

At takeoff power levels, a rich mixture provides a detonation margin, and running lean erodes that margin and can hurt things.

Thus: It is important to not attempt to takeoff with the mixture leaned.

Conversely, at taxi power levels, nothing you do with the red knob can hurt anything. Infinite detonation margin, zero problem.

If you set the mixture extremely lean at idle power, so that it's only just capable of sustaining combustion, you'll find that any extra air introduced into the system by advancing the throttle more than a small amount will make the mixture too lean to light off, and the engine will stumble. If you open the throttle all the way the engine will probably stop altogether.

If you taxi with it leaned like that, you'll never be at risk of takeoff power damaging anything if you forget to enrich as you cross the hold short line.

My usual cockpit flow for commencing taxi is to rapidly pull the mixture knob all the way out and then advance it by about 1/4". Any advancement of power past about 1200rpm, or any further leaning, will make the engine stumble. Minimum fuel, minimum lead, and no chance of damaging anything.

Your setup is probably different, maybe it isn't 1/4". If you set your engine to 1000rpm on the ground and wind the mixture back until it rises to about 1020-1030rpm, you'll be in the right spot. Notice the way the engine roughens if you ask for more than a couple of hundred rpm more power; That's where you want it to be. You can always wind it in half a turn if it's too lean to break resistance when you start taxiing.

Hope that helps,

- mark

Draker said:

I don’t recall reading a POH that mentions leaning on the ground either. It seems to be lore that you just pick up either the easy way from other pilots or the hard way after fouled plugs and valves.

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It's in there.

Both of the things you mentioned are in section 4 of the POH . I would have to go hunting for it, but I'd be willing to bet that it's also in the documentation that comes from Lycoming with the engine.

Here's a screen shot from a C172 section 4. I grabbed a screen shot from a 172 because it was handy, but also because it a Lycoming IO360.

Auburntsts said:

What Mark said. This has absolutely nothing to do with leaning and everything to do with exhaust to cowl geometry.

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I agree

Fix the root causes, then lean as much as you desire.

Leaning while taxiing is straightforward. Lean until the engine nearly quits, then enrichen just enough so it doesn’t quit. The idea is that set this lean, if you advance the throttle for takeoff, the engine will hesitate badly-reminding you to set the mixture full rich. It is very important to not attempt full throttle (near sea level) takeoffs while leaned, due to the risk of detonation.
High density altitude take offs are a bit trickier. Some POH’s are so concerned about detonation, they say ‘lean above density altitudes of 5,000’, only enough to get smooth engine operation’ (this is from a 182 POH). This approach is safe from a detonation stand point but means low power take offs. Above 7000-8000’ density altitude power will be limited to 75% sea level power or less, and detonation should not happen. Set the mixture to best power (run up at full throttle, lean for max egt, note value, enrich 150 F) If you have a fixed pitch prop just lean for max rpm. Now the real problem: density altitudes 3000’ to 7000’. You would like to lean more towards best power, but avoid detonation. Use some judgement. If you’re running high compression pistons and it’s hot, be conservative about leaning. And watch the CHTs - you may need to run richer just to keep cht temps within limits.

Don't want to throw you all in a tizzy here, but at least for fuel injected engines with an Airflow Performance system, leaning on the ground is unnecessary when the idle mixture is properly set. Dont agree? Don Rivera at AFP says its so and I tend to think he knows what he is talking about. Check it out, making sure to read all the posts from Don in the thread.

https://vansairforce.net/community/showthread.php?t=94002

Leaning on the apron

Eric, good day sir
Regarding your remark for no need to lean, running an Airflow performance fuel controller. Question please.

At 1000 RPM, as for taxi, what is our fuel flow when leaned ("short of stumble") versus full rich?

GOFT said:

.
At 1000 RPM, as for taxi, what is our fuel flow when leaned ("short of stumble") versus full rich?

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I don’t know, I don’t lean on the ground

On a fuel injected engine at 1000 rpm, you can expect a 50 rpm rise just before cutoff as you slowly lean the mixture.

That means it's running rich. You get the 50 rpm rise because it passes through "best power" on the way to shutdown.

- mark

Increased CO risk from not leaning on ground

Leaning also reduces CO emissions. With a half life of 3-4 hours (depending on your reference) when breathing fresh air, you are not going to get it out of your blood for that flight you are getting ready to take.

Like others have said, proximity and lack of shielding need to be addressed.

Exhaust Leak

Check your ball joints in the exhaust and make sure they have not started to leak.