Ya think? <g>

The cooling drag increase can be in the 15-20% range.  Now, calculate the increased HP required to overcome that increase in drag.  It's not insignificant.  Were this simple to address the diesel aircraft would be faster and more efficient than they are.

Walter

Sorry Walter,

 

I didn't intend to talk over anyones head. Here's how you calculate the power change. We'll use Scott's estimate of 20% for cooling drag as a fraction of total drag and your upper range estimate of 20% increase in cooling drag for a diesel compared to a gas engine. The increase in total drag due to cooling for the diesel installation is then 20% * 20%, which yields a total drag increase of 4%.

 

Power is a function of drag * velocity. Assuming velocity is held constant, then the power requirement will vary directly with the change in drag. Thus the required power increase for the additional drag will be 4% as well.

 

If you need help with any other concepts, just ask. I'm sure someone on the list here will be happy to help you.

 

Rob