KenyaCub
Registered User
Kenya, Africa
It’s been a few years since I’ve written an update on the DoubleEnder Project and a lot has happened since. The project had started in 2007, and it first flew in 2010. It demonstrated early on that it had good potential to meet its design goals, so the idea was then to tackle each area that could be improved upon.
Engine cooling has been tricky, but we’ve finally gotten there. It’s actually quite amazing the amount of effort that went into designing the cooling for the engines. We’ve made many changes to scoop sizes, cooler sizes, locations and shapes. The changes not only allowed the engines to run cool under any condition but also reduced drag and enhanced aerodynamics.
Finding the right balance of wing modifications has also been a fun challenge. I was interested to see where we could get to with today’s modern technology and design tools. After all, a cub wing was designed in the early 20[SUP]th[/SUP] century and was meant for a different mission than what some of us use it for.
At the end of the 2010 trip to Alaska, I had in mind to tackle the flaps. I decided to design a double slotted flap system for the cub airfoil. Doug Keller took care of engineering the flaps to be able to fit them onto a cub wing. At that point I did not know what to expect, weather or not these flaps would work on the cub airfoil. The gap, overlap, size and airfoil could all have an impact on the performance, so in order to minimize the risk and get the best chances of success from the beginning I decided to approach an aerodynamic company known for its work in the STOL domain.
Many months were spent using Computational Fluid Dynamics software analyzing a stock supercub wing, and the modifications it had at that time (such as Mackey’s slats, extended chord flaps etc). We even hooked up a data acquisition module, with an alpha/beta boom and had a professional test pilot gather accurate data. We needed a baseline in order to compare all future modifications from.
Then they analyzed the double slotted flaps in CFD as we had made them for the cub wing. It showed a decent improvement in CL max. The engineers doing the aerodynamic studies led us to believe that a lot more performance could be attainable, and so they proposed a different airfoil, flap, aileron, spoiler and slat which would have a CL max considerably higher than the cub wing.
The CFD results on the CL max of this new wing were quite promising: almost twice the amount of lift that the modified cub wing generated.
It has 70% span fixed vane double slotted flaps, with a huge fowler motion. The ailerons are 30% span and very deep chord. The airfoil is custom, and is derived from the LS416. The spoilers are located at the trailing edge and span 20% of the wing. The slats are fully retractable and automatically operated by airflow.
We had big hopes for this wing. To my knowledge it is the most complex and aerodynamically advanced wing that has ever been tested on such a light aircraft. I decided to pursue this new wing instead of the double slotted flaps. Doug decided to complete them, and made them available as a retrofit to stock cub wings via his company Performance STOL (www.performancestol.com). I’m glad he finished the project as they work very well, and they are now in use on the current DoubleEnder wing.
We spent an entire two years analyzing and building the new wing. A lot of time, work, money, hopes and dreams went into this wing. The complexity was way more than what I would of liked, but the idea was to reach the limits of where modern aerodynamics could take us, and we would work on simplifying the mechanism and design later, if it worked out as hoped. The wing finally got completed in 2013, and the first test flight took place in March. It is a work of art. Gorgeous! A credit to Eric Lewis and Pete Anderson who built it.
As with many things in life, it’s a compromise and some things worked and others didn’t.
- The airfoil worked well. It was designed to have no more drag than the USA35B, while having a much thicker aspect ratio lending itself better to our application. It achieved its design goals.
- The flaps did not work as expected, and this was probably the biggest disappointment. While the increase in lift could be felt, there was a lack of drag. You’d expect that with such a large flap the drag would be considerable, but there was actually less drag than what a cub’s plain flap generates. The landing and takeoff speeds were about matched with our previous wing. When flying with the flaps down at about 50 mph it was like hitting a wall of air, with tons of drag, but once slowed up to below 40mph the drag would disappear and would make it difficult to slow down even further. My personal conclusion is that these flaps are more adapted for faster airplanes. We will eventually redesign a new flap for this airfoil.
- The ailerons worked beautifully. For such a short span it had good roll control, even at slower speeds. It still lacked a bit of authority if it were to be on its own, but combined with the right spoiler this aileron is very effective for such a small span.
- The spoiler did not meet the design criteria. Although it was effective with the flaps deployed, it didn’t have much effectiveness when the flaps were retracted. We have been more successful with other types of spoilers we have designed over the years, and will eventually modify this wing to accommodate those.
- While the slats did enable the airplane to fly at high angles of attack when open, and considerably reduced the drag when closed, they did not consistently operate how we wanted them to. We’ve already designed and tested a new slat mechanism, which works well and is considerably lighter, and will eventually adapt it to this wing.
Concurrently to developing this new wing, I have also been trying different modifications to the cub airfoil.
The wing that is on it now, is the USA35B airfoil with an automatic slat, the double slotted flaps (75% wingspan), short span and deep chord Frieze ailerons (25% wingspan), and roll control spoilers (25% wingspan). The roll is almost entirely done by spoilers as the ailerons are still quite ineffective. So far, out of all the wings we have tried, this has proved to be the best performer. But we’re not done with its design either. We are currently designing a new aileron for it, and refining our slat mechanism. The new slat system, compared to the original one, allows the slat to fully close in the cruise position. This substantially reduces the drag, enables you to cruise faster and burn less fuel.
In the meantime we have also been testing several different horizontal tail configurations, as well as different vertical stabilizers and rudders. We’ve tried about 5 different horizontal tail configurations. They all have their strong and weaker points, the best tail we have tried is super effective and light on the controls but it weighs a lot, the lightweight tails we have tried are heavier on the controls and less effective. The key is to find the right balance between it all. To tell you the truth, all of the configurations we have tried would fly just fine if you didn’t know any different, but why settle for something that works, when you can achieve something that works better? That’s basically been the philosophy behind the entire DoubleEnder project.
We have managed to considerably increase single engine performance and cruise speed, while also lowering the fuel burn. This was primarily done by reducing drag in several areas of the plane. We modelled the entire airplane in CFD, and analyzed the airframe as a whole in order to quantify the amount of drag that each element creates. This helped us understand where the drag is coming from and narrow down the areas that could be improved upon. At the same power setting we used to cruise at 95 mph, we are now cruising at 108 mph.
Single engine performance, and safety in general, was always a crucial point in the design of this airplane. I did not want to end up with a twin engine that had marginal performance when one engine was lost. That defeats the purpose of having the redundancy of both engines. If it could not keep going under most circumstances on one engine then a single engine design is safer, so this was a crucial design goal.
As of today the DoubleEnder can climb at around 300 ft/min at sea level, and can maintain altitude at 10,000 feet on one engine. These results are at a weight of 2300 lbs (800 lb of load), at lighter weights it performs considerably better.
In order to enhance the single engine performance even further, we have designed a fuel dump system. There is now a fuel dump mechanism on the belly pod that enables the pilot to pull a lever in the cabin, which dumps over 300 lbs of fuel in just a few seconds. This was done so that gross weight is set by structural limits and not by single engine performance (as so many certified twins are). Being able to dump the fuel via the belly pod in the event of an engine failure, allowed me to set the gross weight at 2,500 lbs, and know that even if the engine failed at 10,000 feet, I would still be able to maintain altitude. The heaviest condition you could ever find yourself once the fuel is dumped is under 2,200 lbs. It is rare to fly at gross weight (1000 lb of load), so in most scenarios, the fuel dump will allow the pilot to substantially reduce the weight of the airplane and get better single engine performance, should he require it. Care would have to be taken not to dump over a populated area, but then again it might be better to have fuel come down, rather than an entire airplane. This was really designed as a last resort solution to increase single engine performance, but in most cases it would not be necessary as it already does pretty well even with all the fuel in it.
The belly pod, which incorporates the fuel dump mechanism, is also unique. It is entirely constructed in carbon fibre, and envelops the cabane vee and part of the suspension system. It was designed to reduce the drag caused by this area. It can hold 55 gallons of fuel, and has a small cargo section in the front.
We still have a few things we are testing to increase the single engine performance even further than what it already is. You can never have too much safety. Between the wing slats that make this aircraft stall at a much higher angle of attack than a normal airplane would, and the performance still available when one engine quits, the DoubleEnder is able to tackle two of the most common safety issues in a cub. That has always been one of the important design goals of this project.
We used Oratex fabric on our last two sets of wings and might use it again on the next wings. This fabric has promise. It seems durable, but I have not put it through the test of time yet, and seeing how often we uncover and rebuild wings, I might not get to test the longevity of it for quite a while. The finish is not as shiny as what I am used to, but the weight savings are certainly there. We weighed both fabric processes and concluded that you could save about half the weight if you were to cover an entire Cub or DoubleEnder with Oratex. Basically saving about 20 to 25 lbs on the entire airplane.
In July 2013, I took the DoubleEnder back to Alaska for another round of real world testing. The airplane as it was then weighed about 1500 lbs. and did not have all of our latest wing mods. It was still using the Mackey slats, and a standard aileron profile so mostly relying on the spoilers for roll. It was flying side by side and landing in the same places as the lightweight cubs. It’s performance is no longer comparable to what a cub would do at the same weight, it is a different animal all together. It is rewarding for me to experience that after so many years of development. It gives me the motivation to keep evolving the project even further.
Here is a link to the new DoubleEnder website:
http://www.bushplanedesign.com
It’s been a relentless few years, with failures and successes, but in the meantime we have gained a lot of experience and are better equipped for our future plans. And above all, we’ve been having a hell of a lot of fun.
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