Rotax 912 Airbox mod or replacement

[petercoota]

I'm considering buying an aircraft with 100HP, 912 UL engine. The Rotax airbox has been removed & replaced with two separate air cleaner type fittings.

 

I've read that the Rotax needs its specifically designed airbox, without it, the engine won't develop full HP.

 

Can anyone either confirm or rubbish this theory? Thanks

 

 

[Downunder]

Plenty come with just the 2 airfilters on the carbs. 

 

This means they are using hot internal air and won't make as much power as a cold air fed engine. Whether it is a realistically quantifiable difference is debatable. 

 

Often just having the filters means you don't need a de ice carb heat system. 

 

 

[petercoota]

Thanks, that's an interesting point about the carb heat you made, hadn't occurred to me.

 

 

[skippydiesel]

I have been flying my 912ULS motivated (Czech designed) aircraft for about 10 years and some 500 + hrs, or so.

 

Cabi filters are bog standard Rotax supplied oil /medium cone on each carbi.  Aircraft manufacturer does offer an air box/carbi heat facility but as far as I know non of the aircraft (of my type) have gone down this route.

 

"Touch wood" have never had an icing issue - due in part to air being slightly preheated by engine..

 

Engine delivers sufficient power for a single pilot 1500 ft/min climb and 120 knot max cruise17 l/h ( more usual econo cruise 13 L/h at 100-105 knots) - cruise performance slightly down due to ground adjustable prop being set for advantage climb.

 

In short - I have no doubt that cooler air would slightly improve power (basic science) but would also necessitate heavier more complex system incorporating a carbi heat facility - cant see the cost benefit - love the simplicity of my aircraft as is.

 

Hope this helps in  your decision making.

 

 

[petercoota]

Thanks, I'd never encountered a Rotax without an airbox, so I was a bit taken aback. Your comments, along with those of Downunder, are reassuring. Again I understand & appreciate the point about carby icing.

 

I see you're in a Zephyr, I'm only interested in a high wing, but I've admired the Zephyr. Thanks for the comment

 

 

[skippydiesel]

Thanks, I'd never encountered a Rotax without an airbox, so I was a bit taken aback. Your comments, along with those of Downunder, are reassuring. Again I understand & appreciate the point about carby icing.

 

I see you're in a Zephyr, I'm only interested in a high wing, but I've admired the Zephyr. Thanks for the comment

 

At my age I will take all the admiration you can dish out without any discrimination (high or low wing).

 

High or low wing will make no diference to the 912 motivation itself. Fuel delivery  & tank(s) may differ.

 

Out of interest  - why so definitely desirous of a high wing??

 

 

[petercoota]

At my age I will take all the admiration you can dish out without any discrimination (high or low wing).

 

High or low wing will make no diference to the 912 motivation itself. Fuel delivery  & tank(s) may differ.

 

Out of interest  - why so definitely desirous of a high wing??

 

High wing is good for old age & bad back, avoiding stumps & fence posts, reducing sunburn.

 

Really lousy for refuelling.

 

Fuel delivery & valving arrangements vary widely. specially in the kit built world.

 

 

[facthunter]

High wing is also good for going through GATES and high grass and crosswinds and you can stay under the wing when it's raining. Also fuel runs downhill which helps keep your motor happy. Nev

 

 

[JG3]

Plenty come with just the 2 airfilters on the carbs. 

 

This means they are using hot internal air and won't make as much power as a cold air fed engine. Whether it is a realistically quantifiable difference is debatable. 

 

Often just having the filters means you don't need a de ice carb heat system. 

 

I have a Savannah with an airbox. I did lots of comparison testing of power with outside air vs inside cowl air. Temp difference usually 3-4 degrees. Couldn't tell any difference at all.... So long ago removed the scat hose from outside and just run on inside cowl air. I reckon that the slightly warmer air will avoid any carb icing - if Bing carbs are actually prone to icing..... 1600 hrs now never a problem. The airbox now just makes a good support to tie the carbs together and prevent any vibration, and gets in the way when I need access to the back of the engine......

 

 

[skippydiesel]

High wing is also good for going through GATES and high grass and crosswinds and you can stay under the wing when it's raining. Also fuel runs downhill which helps keep your motor happy. Nev

 

Agreed! except the X wind statement - how so? please explain.

 

 

[facthunter]

 You can drop the wing further without tangling it in the grass when you are holding off in a x-wind. (A REAL one).Nev

 

 

[Thruster88]

I have a Savannah with an airbox. I did lots of comparison testing of power with outside air vs inside cowl air. Temp difference usually 3-4 degrees. Couldn't tell any difference at all.... So long ago removed the scat hose from outside and just run on inside cowl air. I reckon that the slightly warmer air will avoid any carb icing - if Bing carbs are actually prone to icing..... 1600 hrs now never a problem. The airbox now just makes a good support to tie the carbs together and prevent any vibration, and gets in the way when I need access to the back of the engine......

 

Each 10 degrees Celsius increase in inlet temperature will cause about 3% loss of power.

 

 

[facthunter]

You can calculate that on the universal gas equation, or the density altitude adjustment rules of thumb which you are probably more familiar with than I am at the moment. Sounds about right. Nev

 

 

[skippydiesel]

 You can drop the wing further without tangling it in the grass when you are holding off in a x-wind. (A REAL one).Nev

 

Thanks - I never thought of it that way

 

 

[Thruster88]

 You can drop the wing further without tangling it in the grass when you are holding off in a x-wind. (A REAL one).Nev

 

If the grass and cross wind are that extreme it may be prudent to land elsewhere.? 

 

 

[skippydiesel]

Each 10 degrees Celsius increase in inlet temperature will cause about 3% loss of power.

 

That's okay then - I have about a 10 degree rise over ambient, within my cowling during climb. As expected this difference is reduced down below 5 degrees in cruise. I can accept a 3% loss of power - to overcome this would require a considerable increase in cost, complexity and some added weight - could easily nullify the benefit.