View Full Version : Tanis Preheater install Rotax 912ULS

Dave S
05-29-2015, 12:52 PM
OK....I realize the season for thinking about preheating has passed...but, I completed this project this spring so here goes while the idea is still fresh.

Last winter we acquired a Tanis preheater kit for our Kitfox equipped with a 912ULS. I spent a lot of time studying this deal and made some changes to the recommended installation so did some addittional analysis to better understand the whole thing. One conclusion I came to is the Tanis heats the engine from the inside out while the method I had been using heats the engine from the outside in. Rotax has a video on the Tanis on a 912 on their web site.

Here is a couple photos (2846 & 2849) of what the kit looks like First one is the heaters and the second is the harness - total installed weight was 20 ounces. One pad heater for the oil pan, four heater bolts for the cylinders and one for the crankcase - total designed wattage is 230 watts while the install checked out at 227 watts. The harness has one extra plug if a person wants to add a battery heater.

The instructions call for screwing the cylinder heaters into one of three places - Upper coolant flange, intake runner flange or the baffle ears. Tanis leans towards the coolant flange because of its location to the center of the cylinder & coolant chamber. I discoverered an issue with the intake location which I shared with the company and I believe thay have modified their instructions to exclude this location. I chose the baffle ears because that location is not used for any other purpose except the front port side location which holds a cushion clamp. The baffle ear location places the heater a couple inches further from the center of the cylinder but later performance testing indicates the location works fine - performance testing data at the end of this post.

Next photo (2890) shows the heaters installed in the (Tanis instructions indicate 6 hours normal run time) baffle ears. I fabricated 1/2" long aluminum spacers to go under the heads of the heater bolts to meet the Tanis spec of not running the threads out the far end of the hole more than a couple threads. Tanis does supply short spacer which are normally used on the coolant flange location. The best length for the spacer would be 7/16" and I will probably do that later.

The Tanis instructions call for removing one of the bolts securing the crankcase halves on the bottom of the case towards the front. I elected to use one of the 6 mm holes on the back of the gearbox in order to place the heater in a hole that was not used for anything else. The back of the gearbox is integral with the case. See photo 2895. I used 3 of the thin Tanis aluminum spacers plus one washer on this location as this heater is longer than the cylinder heaters.

Oil tank location was per Tanis instructions see photo 2903. Note that the location is below the oil level. I elected to include a modification to my operating instructions to burp the engine before plugging in the heater.

Now - a couple issue that arose. Tanis sells a socket with a slot in the side so a person can use their torque wrench. Photo 2847. Probem is the outside diameter interferes with some locations. I set the socket aside and went back to my calibrated fish scale, combination wrench (open end on the heater, box end to hook the fish scale) my metal ruler and and a little math - which works without any interference. If you use a fish scale, do calibrate it, they all read high to make fishermen and women feel good about their fishing prowess.

Next issue - Due to manufacturing variation of the Rotax intake manifold, you cannot be assure this location will work....maybe it will, maybe it won't. I shared this fact with Tanis and I believe thay are changing their instructions due to the situation with the manifold. See photo 2862.

Now the fun part - how did this work out performance wise with my modifications? Set the plane up for a 20 hour overnight run with nine channels recording the temp every minute - approximately 10,800 data points. Photo 3011 shows the hook up. Note the sensor on the prop hub. Engines with an aluminum prop shed a lot of heat out the prop hub and get rid of it on that great big cooling fin. Other sensors were located at the top of the crankcase, bottom of the crankcase, inside the oil tank, one for each cylinder and ambient. The attached PDF shows (click on it to open) what the temperature rise was, how long it took to stabilize. The green line at the bottom is the ambient - you can follow that dropping a bit overnight, the next recording up is the prop hub - some heat but not a lot, the orange line is the bottom of the case - you can tell when the sensor fell of the case and came to rest in the bottom of the cowl - next lines include the case top, 4 cylinder locations and the oil. A note on the oil - there is such a thing as too much heat here as a person does not want to end up with spot coking inside the tank - temp was low enough to avoid that. FYI - the little squiggle about an hour from the end of the chart is when I opened the hangar door in the morning.

Basically the temperature rise is comparable to installations utilizing the other locations for the heater bolts - 50 to 60 degrees above ambient. Stability achieved at four hours. This is in a drafty hangar with no cowl blanket or cowl plugs so there was connvection cooling going on through the cowl (in the bottom, out the front).


Dave S
KF 7 Trigear
912ULS Warp Drive

05-29-2015, 06:41 PM
I have been using a 60watt light bulb inserted in the bottom of my Rans S-7 cowl once the temp lowers to 40 deg here in pa. I have used this method for the past 4 years and the engine starts easily all the time. I put a thick quilted blanket over the engine compartment. I put a 40 watt bulb in the cockpit under the panel on all winter and spring to help keep any moisture from penetrating the instruments.

05-29-2015, 09:16 PM
Thanks for the write up. Looks like a good direction to go for our -25 days. Any colder than that and I probably won't fly. I'm guessing it works for a 914 as well?

Dave S
05-31-2015, 05:18 AM

The preheater kit I got from Tanis is described in the instruction manual as being for both the 912 and the 914.


Dave S
KF 7 Trigear
912ULS Warp drive
St Paul, MN