Friday, 25 November 2011

Cowling design - Part 2

From my previous post about the cowling you may have got the impression that I was only concerned with the inlets, as that was all I mentioned.

However the inlet to outlet ratio is crucial to get right and there is much that can be done with the outlet to improve cooling performance and efficiency.

Very few aircraft make use of the exhaust gases. If anything they tend to exit at a fairly steep angle to the airstream and make no use of the small amount of thrust that they could provide but more importantly the exhaust gases can be used to accelerate the outlet air speed. 

Cooling drag typically accounts for up to 37% of total drag so it is well worthwhile to try to reduce this while still maintaining good temperatures on the engine.

Most outlet cooling drag is caused by the outlet air having to accelerate back up to the free airstream speed and this costs energy and adds drag. By utilising the exhaust gases to accelerate this air we can make them less wasteful and improve overall performance. This is called exhaust augmentation.

Two aircraft that have done this very successfully are the Arnold AR6 (Multiple winner at Reno) and Dave Anders much modified RV-4.

Pictures of both are below.

Dave Anders makes the most of smoothing the air before it gets to the outlet with a simple duct. Piper also did this with the Comanche.

If you want to see a really smooth cooling air outlet - take a look at the radiator exit air duct on a P-51 Mustang - that is very smooth and narrows down at the exit to accelerate the airflow. Of course the P-51 was made famous as the first aircraft to achieve Zero cooling drag. (a function of the thrust that was made with the warm compressed air leaving the exit duct.) It still impresses me.

The AR6 averages more than 220kts (while turning the course at Reno) on 100hp. Look at the bump under the fuselage - that is the cowling air exit and exhaust exit all in one. It worked so well when first made that the owner had to reduce the size of the already tiny inlets to get the temps up!

It is amazing that most aircraft designers pay so much attention to smooth airflow on the outside of the aircraft but almost never pay attention to smooth airflow under the cowl.

On the Twister it will be a challenge to include these design features as space is very limited under the cowl - it was originally designed around a compact Rotary engine - and has changed over the years as various powerplants have been installed. The biggest of these is the UL260. However I believe I can still employ the Lo Presti style inlets and UL power offer a carbon plenum with round inlets as an option which will make the job somewhat easier to implement. I would still like to explore different exit ducts to improve that area as best I can in the space.

Dave Anders RV-4 exit with exhaust augmentation

Note aluminum sheet exit duct to smooth airflow

No muffler though so it must be noisy...

Arnold AR6 - bump underneath is cowl and exhaust exit

The small but perfectly formed Arnold AR6

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