Lower Cowl Challenges and Features

Update to “This Sucks, But not enough,”: From the last post, there was a lot of discussion on Facebook about my vacuum bagging failure, which is great. My main goal in blogging is to share my learnings with other interested people. There is a treasure trove of information in the Facebook comments, and once I figure out how to connect my Facebook to the blog, we will capture the information. One of the people pointed me towards the unusual way my tacky tape was acting in the vacuum bagging video.

Turns out, my tacky tape had gone bad since it had been sitting on the shelf for quite some time. It had lost its tacky. I haven’t worked with it much so I didn’t immediately realize that. But it turns out that when you fold tack tape back onto itself you should never be able to get it to come apart. In fact it tends to meld into itself. I did a little test with my tacky tape and folded a small section of each roll back onto itself. Within 10 minutes, the first roll had straightened out. The bond didn’t hold. That means my tacky tape was incapable of sealing. It is still possible, even likely that there were other problems. But, we have one really big smoking gun, and new tacky tape on its way.

I haven’t posted in the last 3 days, but rest assured I have still been working hard on Brian’s Defiant. The reason I haven’t posted much is that there hasn’t been much worthy of posting. Or at least, there hasn’t been much worthy of me staying up late to even write a post. A good majority of what I have been doing is noodling (scratching my head) what my options are. But we are due for an update.

Now that we have TCSN2 jigged in place as our shape reference, its time to start working on shaping the lower cowl. This has proven to be a much bigger challenge than the top cowl. The top cowl was pretty simple. Start by R&D-ing some other wide body canards. Then go from firewall shape to spinner shape, and miss the cylinders. The bottom cowl has much more going on.

Note aggressive curvature. This is almost certain to cause flow separation.

The goal for the new shape is for one, to blend the new top cowl. I also want to minimize change to Brian’s design. I want to accomplish these two things while attempting to minimize drag, and keep an aesthetically pleasing shape. With the goals defined, lets look at the challenges.

The Defiant engine is mounted high relative to the firewall, not vertically centered. This means that the lower cowl will necessarily have larger retreating angles than the top to get from point A to point B. In addition to that, the firewall is relatively large and we are trying to fare into the spinner and exit area which is relatively small over only 44″ of distance. Again, causing relatively large retreating angles. Thirdly, we have to integrate the wing intersection into the cowl design. The wing is relatively low on the cowl, which again, makes for large angles. You are hopefully noticing a theme. Many large angles, which are likely to cause flow separation and drag.

So as you can imagine, there are many permutations of how to shape the lower cowl. All of these options have drawbacks and imperfect implications. For me, being a visual person, I had to go to the airplane and hold splines up to the real thing to visualize. While doing that I would realize things like, well lets install an alternator to make sure we don’t have a conflict there, induction tubes, etc. Things like that. Evaluating possibilities and running them by my brain trust has been much of the last three days. With the help of some very smart folks, I think I have a reasonable path forward that I am excited about.

As is the case with all design choices we have to find the best set of compromises to fit our application. In our situation, I think it is to use the “Boat tail,” lower cowl arrangement that some canard pusher airplanes use. Think boat keel type shape. The boat tail will help to minimize retreating angles as you move aft along the cowl and is a well proven low drag shape. A few of the fastest EZs out there use this configuration.

Example of a very fast EZ that has a “boat tail” lower cowl. Klaus Savier has the fastest Long Ez in the world.

To make matters a bit more unnecessarily complicated (which I am usually good at doing, ok I kid) I have decided to try to integrate some underused aerodynamic technology into the aft engine installation. One is an augmenter tube. If you are unfamiliar look at the exhaust setup on a T-34. The exhaust exit terminates inside the cowl exit duct. The high velocity exhaust creates suction in the exit duct sucking the cooling air out with it. This improves engine cooling effectiveness dramatically and allows smaller inlets, further reducing drag. It really is a very elegant concept, and is free performance most designs leave on the table. (You can read more about it in the NACA Report 818, “An Experimental Investigation of Rectangular Exhaust Gas Ejectors Applicable for Engine Cooling.)

The boat tail is the perfect location for a single circular augmenter exit. The other areas on the cowl will be sealed off. That means all cooling air will exit through the augmenter in normal operation. The two cylinder bump out areas on each side of the spinner can be made into cowl flaps if I have a need for extra cooling at high power/low airspeeds.

Since I will have a nice area for a single augmenter exit, that also points to a single exhaust as well. We are going to do a 4 into 1 exhaust for the rear engine. Those of you familiar with my attempt at doing this with Betty may be surprised to see I am going to try again. On Betty, I had heat damage on the propeller from the 4:1 exhaust. Maybe I will do a throwback post to discuss how we did the exhaust on Betty and what the problems were and if I would have continued working the problem some potential solutions. (In my spare time.)

All of the potential solutions to make a 4 into 1 work on Betty started by converting to down draft cooling so we could dilute the exhaust heat. Brian’s Defiant is a down draft cooled engine already, and we are augmenting. This, by definition, mixes cooling air with the exhaust diluting heat. I have been able to get some good data from others who have done augmenter systems so I am confident this will dramatically reduce the temperatures the propeller sees. If augmenting doesn’t reduce the temperature enough, there are other techniques we can use to further cool the exit air. I am confident we can get it to work.

To review: the task is to minimize shape change to the original DDA design, keep it minimum drag, incorporate a boat tail/augmenter/4:1 exhaust. No sweat. In theory. Oh and keep it attractive looking.

My cowl shaping methodology is exactly how I started on the top cowl. This time it is just more complex and taking longer. I started by making a template of the augmenter exit area and location relative to the spinner. From there I started adding control splines. The first one was the lower centerline forward and aft. This was easy to put in as there really was no choice involved. It goes from the augmenter exit to the inlet in front.

From there, I did the sides (3 and 9 o’clock) position of the belly hump between augmenter and inlet. This allowed me to spline in centerline scoop shape.

Then things got a bit less obvious for me as far as where to lay control splines. So I tried many different ones. Connecting different areas on the fuselage to the augmenter exit, the wing root to augmenter, etc. Then I realized I had another control spline that was easy. The cowl seam between top and bottom cowl was pretty simple. So that one went on.

I was quickly back to head scratching. Instead of going through each control spline, I will show you what I ended up with today. I still have more splines to install before I can start sanding. I am not 100% happy with it yet. But think it is a promising start. I can’t quite tell what I am not liking, but hopefully it will be more obvious when the splines are sanded continuous instead of being faceted. The two arrows show areas with aggressive curves and what I think may be what I dislike.

Tomorrow should be an interesting day, either we will sand out a decent shape and fix what’s “wrong,” or we will get the saw out so we can start over.

PS… Sorry there is no video. The go-pros kept over temping in the hangar. I will have to put them in front of the swamp cooler from now on.

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