Cowling design - Part 3

Well I have done absolutely nothing on the plane now for 10 days. Been struck down with a bad case of 'man flu'.


Seems to be taking ages to get rid of it so in the meantime I thought I would do a further update on the cowling design.


An interesting read was a piece about calculating total drag on a Messerschmitt 109E by Hoerner (considered by many as the God of aerodynamics).


In this article is transpired that exhaust thrust was quite a significant factor. Much more than I was expecting it to be.


The propellor produced 1,000lbs of thrust and the exhaust stacks produced 140lbs of thrust. So the exhaust was adding approx 14% of the total thrust.


Of course the 109 did not have mufflers to slow the exhaust gases down so this figure of 14% would be high compared to the UL Power set up which has a muffler.


Still, why not take advantage of that thrust rather than ejecting it at a steep downward angle and wasting its energy?

Exhaust detail. BF 109E

Another book which is well worth getting hold of is Economy with Speed by Kent Paser. No longer in print - I was able to get a copy from the States but it was not cheap (£30 I think).


Kent Paser was a NASA engineer who also built his own plane and then over the course of 20 odd years modified it. All the while recording the results of each modification so he knew absolutely what worked and what did not.

Without increasing engine size or burning more fuel he was able to increase the top speed of his plane by 64mph.


A good bit of this speed came from cleaning up the draggy parts of the airframe. 


What's relevant to us is a whole section on engine cooling. Which improved speed by 7mph. Part of it is about exhaust augmentation but mostly it is about improving the efficiency of the system.


His main points were:



· Add full pressure plenum to the top of the engine for efficient and equal pressurization of cooling air to all cylinders. The plenum also removes the problem of the cowl rising off of the traditional baffles at higher speeds.


· The smooth outflow of air at the cowl exit is just as important as a smooth inflow of air at the cowl inlets.


· Exhaust “jet-pump” helps pull air out of cowl.  Resulted in significantly cooler cylinder head and oil temperatures.  If the cooling is more than is needed then the cowl inlet size can be reduced, resulting in significant Aero-dynamic improvements.


· Reduced cowl inlet size 4 square inches at a time until CHT’s hit the bottom of the temperature range.  Initial size was 60 square inches.  Final size was 30 square inches!!  This experiment cannot be optimized until a CHT/EGT is placed on each cylinder and the under cylinder baffles must be optimized to get equal CHT’s on all cylinders.  At this point reducing the inlet size until temperatures begin to go up means that all of the air going into the intakes is being used and none is spilling out as excess, thus reducing drag.


All good. And another thing that came out of this book was possibly a better location for the oil cooler.


He copied the Mooney system of putting the oil cooler on the back of the plenum as shown below.

Mooney style oil cooler placement (pic is not of a Mooney BTW)

There may just be room to try this on the back of the UL - right hand side as viewed from the cockpit.


If this works it would certainly reduce the drag compared to the existing set up - a massive hole at the front of the cowling.


This current set up is not at all efficient - Pete has the largest available oil cooler on there and no ducting of any kind to slow the air or smooth the flow at either side of the cooler.


It does work - he has good temperatures but it most certainly is not the most efficient set up.


Radiator theory states that the opening can be 1/6th of the size of the cooler itself if set up correctly. Pete's set up is 1 to 1 - ie: the opening is the same size as the radiator face.


The problem is one of space - and trying to do this under the cowl at the front is a bit impossible - time will tell if things can be improved with this Mooney style set up.


Also if my memory serves me correctly Pete does not have a sealed plenum on top of his engine - he relies on the top part of the cowl to seal - and everything I have read says this is quite inefficient as far as cooling goes with big loses in the gaps that open up due to this system.


Last pic to close out this post is of the round inlet composite air box plenums for the UL Power engine. (which do seal completely).