Preheat - Test results - Cowl Cover or Cowl Plugs

Please see the attached document for charts of engine preheat ( SR 22 with Reiff TL6 and oil cooler option ) with either a cowl cover (see photo) or just cowl inlet plugs.

The bottom line for me is that unless you are starting in temperatures below 0 degF, the cowl plugs are quite effective and a lot less bulky to handle.

Effect of Blanket.docx (990 KB)

Interesting…am I reading this correctly to say that at 10 degrees F the cylinders would increase ~10 degrees per hour with cowl plugs? So if I want to get them to 100 degrees it would take roughly 9 hours?

IF the temperature rise was continuous, you would be correct. However, the temperature increase is not a linear function with time since there are all sorts of heat loss paths that reduce the rate of increase as the temperature increases above the ambient value until those losses match the power input. Then the temperature approaches a maximum. This happens asymptotically, that is, the rate of increase continuously diminishes to zero at infinity!

Most preheaters can be left on indefinitely and use of a cover will give you a higher temperature than using Cowl Plugs. My interest was to find out how little lead time would be necessary.

Many articles suggest that the engine should be at least up to 40 degF to avoid unnecessary wear. This is often used by flight schools as the point to start using engine preheaters. Also, here in the Minneapolis area, most GA flying grinds to a near halt at 0 degF so I was not interested in temperature increases of much more than 40 degF. In either case (with a Cover or a Plug) this is reached in 3-3.5 hours. Therefore I did not present any data beyond 4.5 hours.

My planning is based on getting the engine to 45 degF. In either case, the nominal rate of rise for this range is about 15 degF per Hour. So, if the ambient temperature is 0 degF, I ask (or otherwise program) that the plane be plugged in at least 3 hours before flight time; this would be 2 hours ahead of time for OAT of 15 degF and 1 hour for 30 degF.

As mentioned above, you can leave the heater on a much longer time, I rely on the heater suppliers to not burn up the plane. I was most interested to find out, HOW SOON CAN I FLY? For a short time application (e.g. 3 hours), there is no significant difference between using a Cover or Plugs. I find the Cowl Plugs easier to manage.

As the difference in temperature increases, the time to heat one more degree will increase, ie it will be much harder to go from 90 to 100 than from 20 to 30. The reason is intuitively obvious, but as the delta T increases, there is faster heat transfer from the engine compartment or engine parts to the surrounding cold air.

If 10 degree air is flowing through the engine compartment, then it’s going to suck heat out of the engine really badly, so blocking air flow is at the top of the list. Next presumably is reducing conductive heat loss through the cowl fiberglass itself, which the blanket does. And the graph probably shows this heat loss becoming incrementally more important as the delta T increases. With cowl plugs only, the rate of increase drops off more as the engine gets hotter. The blanket stops heat from going through the fiberglass as fast.

But the overall point is pretty powerful: stopping air infiltration dominates insulating the conductive surface. Kind of like in a house.

I have always wondered about air flowing through the cooling air exits at the bottom/back of the cowl, where the exhaust pipes also exit. Seems like it would be useful to plug this area somehow. Doing so would be hard because of the pipes. Just curious if people in cold areas using heaters try to block the exit as well as the cowl air entrance?


Thanks for sharing this as I have both the cowling plugs and blanket. I noticed a big difference with temps with cowling plug only while the plane was on the ramp in Grand Marais, MN a few weeks ago. Outside temp was 10F. No doubt the blanket is bulky and more work.

Maybe see you around the Minnesota neighborhood.

Fly safe!


Curious, did you consider doing both the plugs and the blanket?

And would you happen to know someone with an IR gun? So we can see where the greatest heat loss is?


In my opinion, the blanket restricts air flow through the cowl inlets.

I really would not expect a significant change by adding cowl plugs to the cowl cover configuration.

In my opinion, the blanket sufficiently restricts air flow through the cowl inlets.

I really would not expect a significant change by adding cowl plugs to the cowl cover configuration.

Sorry, I don’t know anyone with an IR gun.

Since hot air rises, I am skeptical about the effectiveness of closing off the passage around the exhaust pipes at the bottom of the cowl in addition to the cowl plugs. Blocking the air flow on one end of the path (the inlet openings) effectively reduces the flow thru the cowling due to both thermal convection and wind.

I do not have the heater connection in the inlet so the oil filler cover was not totally closed; if that was closed, the results would be better, how much is not known. I was just happy to learn that the cover was not as important as I had thought.

Another point got lost in the presentation: I have the impression that cowl blankets are much more effective on aluminum cowlings. Maybe that is another plus for Cirrus’s construction?

I used to use plugs for my exhaust pipes as well as standard cowl plugs. I used them in summer for limiting Bird entry and in winter for isolation with my heating system.

Of note: I used to keep my Reif heater all the time in winter. I did use a c owl blanket and would put it fairly fair up the windshield of the plane. I found that it helped in keeping the avionics warm.


I love experimental data. And that was nicely done.

John, interesting experimental results. From your studies can you deduce any rules of thumb for the heat loss time of the engine. Example would be engine at 40-50 deg F (in heated hanger) brought outdoors into ambient temperature of say 20 deg F. How much time before it cools to 30 deg F?

John, thanks for putting this together. Any thoughts on the best way to keep the battery warm? My engine heater and plugs do a good job on my engine/oil temp, but if it’s been cold, the battery doesn’t seem to do as good of a job turning the prop, making for rough (very) cold starts. My shop says the battery is fine, and suggested plugging in my gpu for 15 minutes before starting in the cold (without the gpu plugged in), thinking that would warm the battery up. Thanks.


Your shop is correct. The battery in that mode
Is acting as a big capacitor. More important, you need to have a load engaged while doing the 15 min battery warm up trick or it won’t come up to temp and it’ll remain cold killed.

As Alexander has stated, you need a load on the battery (pitot heat, landing light, avionics) for the battery to “heat itself” and be ready for your start. I think it is possible that, with the GPU connected, the current would not come from the battery and thus defeat the purpose.

Here is some cool down data taken after a flight.

In your example, of 50 F in the hanger and 20 F outside (30 F above ambient) it looks like you have about 1.5 hours for the engine to cool to 40 F (20 F above ambient).