www.davidcookaviator.com

DAVID COOK AVIATOR

Greg Pedersen

Streak Shadow ZU-BFO

E-mail converstaions with other Shadow pilots about modification.

2nd Batch of E-mails

3 August 2002
Dear Ajab,
Here is the next round of photos and hopefully some useful information that will be of benefit to you. I hope these don't flood your computer. There are seven photos in this batch.
As far as 1 know, 1 don't think anyone else worldwide has done this amount of work and experimenting on the top of a Streak wing - stall fences, vortex generators and enclosing the gap between aileron and wing. All this work was only done after approximately 150 hours of flying with the standard unaltered wing so that I could experience any changes.
My Findings
The first experiment with which I started was the stall fences on the tips of the wings based on a 'thumb suck' by reading magazines and articles on work done in the USA on single-engine light aircraft, etc. I then flew the Streak for a period to make sure the flying characteristics had not changed. Satisfied with that, I then moved to the centre hard point and did the same, curving them slightly around the leading edge of the wings. Again, no noticeable change in the flying characteristics of the plane, but a noticeable change in the stalling characteristics - a bit slower.
I then thought 'What the hell, let's put on a third set" and these were placed on the hard point where the wing is joined by the wing attachment pins but set back to avoid messing too much with the air flow over the leading edge, but keeping the air flow straight over the widest part of the trailing edge. This was a concern of David's.
Reasons for all the Fences (my version)
A stall starting from the root, especially where I have placed the fences, may be delayed more because of the containment of air, between the two inner fences with the flaps down, crossing over the wing faster thereby further delaying a stall because of increased air flow. And of course, with our thick wings, air passing over the wing would tend to move across the wing at an angle so all those fences straighten that out, getting the air over faster, producing more lift and less drag (my theory, anyway) and it seems to have worked as there is definitely more speed and the fuel consumption figures are very good at cruise speed. I could never get 110 m.p.h. straight and level with a passenger in the back seat at 4000 ft. before I did all the work I have listed.
Material used was 1mm stiff aluminium sheeting glued and pop riveted to the hard points on the wings. Ajab, the height of the stall fences varies between 40 - 60 cm at the rear and only a true test in a wind tunnel would determine if they were really performing correctly.
Gap Covers
The second experiment was gap covers placed over the upper front over the ailerons (see photo) to reduce drag and air turbulence when in the normal flying attitude - again 1 mm thick stiff sheet aluminium with a 1 cm gap so that no chafing occurs.
The result - a change in handling performance of turns - air spilling through from lower wing high pressure to the low pressure upper surface, almost acting like a fowler flap in reverse, thereby seemingly equalising the pressure, so that no flutter of any sort was noticed up to speeds of 130 mph on the ailerons to date. So, again, in my humble opinion, another improvement to stop drag and turbulence at low and high speeds in this area, which is quite considerable.
Vortex Generators
Third experiment was vortex generators glued on the upper leading wing surfaces making 'little mischiefs' to slow down a 'bigger mischief in respect of air separation off the rear of the wing. It has worked because the dust pattern goes all the way from each vortex generator to the rear of the wing, unbroken.
What has the end result been, all round, so far?
1. The original normal flying characteristics of the Streak haven't noticeably changed.
2. Definite better cruise and penetration
3. Better fuel consumption
4. No real change in the take-off roll but I think that is due to the lazy four-bladed
propeller - no 'massive kick' on take-off
5. Landing and stall speed has been lowered. At 4000 ft. with full 30 degree flaps and full up elevator I can mush her down gently at 20 mph indicated air speed starting at 100 ft per minute drop, increasing to about 300 ft per minute drop with full control being maintained and no buffeting noticed - and neither wing wanting to drop away, even in a tight downward flat spiral manouvre.
Into the wind is more fun. It's almost like a helicopter descent, happily landing at 35 mph at our altitude of 2600 ft. This is of course in calm weather at 20 - 25 degrees C, full fuel load but with no passenger.
6. Now very difficult to push her over 1000 ft. per minute descent. She wants to keep on nosing up and flying with a long, shallow glide and will overshoot the runway if the approach is too high. And of course with this sort of glide angle there is more time to pick a good landing spot if the need ever arose.
Ajab, bear in mind that my wing surface is bigger than yours so comparable figures would probably be quite different.
Overall Opinion -
I am happy with what I have done thus far. My improvements may not have been earth-shattering because the Streak wing is efficient to start with, but there have certainly been no negative aspects that I have noticed. Taking into account all the modifications that have been done, the overall weight of the aircraft has increased up to about 240 kg at the last 'weigh-in', although the aircraft definitely feels and flies better and she very reluctantly wants to drop out of the sky under normal conditions, even with the additional weight.
I am still toying with the idea of using vortex generators right around the back canopy to get the air to stick to the body-work more efficiently to reduce turbulence just in front of the prop, but, with that in mind, I will use bits of wool first and then get one of my friends to videotape the results in flight so we can see if it would be worth doing or not. Should be interesting.
My philosophy is make an aircraft as slippery as possible by rounding off corners, closing gaps and holes and there has to be an improvement - even if it is slight - to the all round performance including getting the air over the top of the wing as fast as possible with the minimum of disturbance.
The rest of the photos are of different angles of the plane and 1 would appreciate your valued comments. I still need to have a spinner made for the new four-blade propeller and also replace my front spat which I broke, with one of a better design. The last one dug into the ground and 'popped off in a number of pieces.
The black fin on the underneath belly is the air scoop for the oil cooler which is built in under the engine sump. I had to build this out a bit further because the belly was not allowing enough air into the oil cooler.
Lastly, the last picture was taken at the Himeville airstrip near Underberg, in Southern Natal - altitude 5050 ft. - with the Drakensberg mountain range in the background (10 - 11 000 ft). A nice picture re the positioning of the stall fences. The aircraft in the background is a friend's Bushbaby.
Oh yes, headsets used are American Flightcom Eclipse to match the Flightcom intercom system and, like your aircraft, I also have an electric trim built into the elevator (standard). (Left out of specifications in last email.)
Ajab, I look forward to hearing your comments on my conversions thus far and maybe other ideas with which you and your fellow flyers have experimented to get better performance.
Pictures sent so far - 4 with last email plus these 7. Have you received them?
Take care,
Greg

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20 August 2002
Dear Ajab,
Sorry about the delay in getting back to you. I only received the email from Lindsay on Sunday afternoon, 11 August when I went to visit. You will only receive this after the 13th because I needed to speak to two of my friends to collect more information and ask them if they would be able to correspond with you as well.
In response to your last email on 10 May, I am happy that you received all the photos and hope that the detail is clear enough.
Firstly, let's talk propellers -
I have spoken to my friend, George Killey, the gentleman who made my propeller and showed him pictures of your Star Streak and your emails. George is happy for me to give you his details and for you to contact him to discuss propellers and propeller types, etc. George has been making wooden propellers for about twenty five years and has been in the aviation game just about all his life.
As far as I know, he is one of only a handful of people in the country that still makes and designs propellers, mainly for microlights only, and his work is outstanding.
The downside is that he is a one-man operation, working from his double garage at home, with one semi-skilled helper. He shapes and makes all the propellers himself. I am extremely fortunate to live only a few kilometres away from George, so we were able to experiment extensively with different shapes and designs over a period of about eighteen months.
Ajab, looking at pictures of your propeller, our conclusions were that you are probably going through some of the problems that I experienced in the early days with my Arplast propeller. (I think I sent you a picture of this propeller.)
Think about it - the ends have been chopped off to allow for the smaller diameter, but what has probably been lost is the efficiency of the blade. Those blades were probably designed to work from about the middle to the original tip and not designed to be cut, so it makes you wonder how much thrust has been lost by butchering the ends.
This is why we decided to eventually go for a four-blader (the one that you see on my Streak is the second four blader). The first one split a blade at the end so we decided to retire it. Anyway, the second is more efficient.
George designed this blade for the engine so that it would be as efficient as possible, ie. it starts working right from the roots to the tip of the blades, and with all the knowledge we had with the previous blades we were able to determine blade width and maximum pitch to the power output of the engine, so George said.
The blade is probably working more efficiently than all the other three-bladers I had on previously.
I think that has become evident in the performance figures to date, compared to all the other propellers.
With your horse power and gearbox, a four blader propeller turning slower would probably be an awesome combination. One of our Streaks on the field has a four-blader propeller driven by a 62 hp Rotex engine and it delivers a staggering performance on take-off.
Ajab, another reason for the smaller blade was to move the blade tips away from under the wing to reduce noise and the problem of tip vortices slamming into the underside of the wing which set up vibrations throughout the airframe in the early days. Not clever!
Also, a four-blader needs to be slightly smaller to soak up the power output.
Unwittingly, by doing this, I think in the end we hit on a good all-round compromise - for my application anyway.
Going back to adjustable composite propellers, George does make them, but mainly for small standard-powered paraglider engines. He makes all his own designs and moulds and does export them to other countries from time to time. So, Ajab, the possibility is there.
However, he was hesitant when I mentioned your dilemma. The problem lies with the huge amount of work to make and design the moulds for the blades and then make the blades themselves. One would need to research propeller types for your application.
Machining a four-bladed hub would be easy in a CNC machine, however, it is the design, the application thereto and the moulds that would probably have to be a one-off for your specific application. Powered paraglider propellers require far less strength and workmanship.
George has produced numerous standard composite 72 inch, ground-adjustable blades for the Rotex engined weight-shift trikes which, from reports that I have heard, have proved to be quite exceptional and apparently outclass most other similar propellers.
We did experiment previously with three-bladed ground-adjustable wooden propellers with metal hubs but we found that the hub was too heavy and cumbersome and the wooden blades at the root were very thick and not efficient enough because of the blade and size restriction. George was not producing composite blades at this time so that was not an option then.
We did experiment with wooden blades and a thinner neck section, strengthened with metal bolts and expoxy, but we encountered serious problems with heat transfer from the crankshaft directly through the metal hub onto the semi-exposed blades, causing serious cracking and burning of the wood. Hence, the solid four-blader that eliminated that problem eventually.
However, the possibility also exists now that George has become better at composite work, and we now know of the related heat problems (you would not have this problem because of your gearbox configuration), he could probably design a hub raised away from the propeller flange to eliminate heat transfer. Custom blades could be made I suppose. I must admit that for now I have been spoilt with the wooden propeller because it is so much more forgiving as it soaks up vibration from the engine and twists far more freely than the rigid composite propellers, especially my original Arplast.
So, like with everything, there is a trade-off.
I have tried in the past to twist George's arm to make a ground-adjustable composite four-blader propeller for me, although I would not part with my wooden four-blader. I am still trying to work on it, but George seems reluctant at this stage. Maybe one day I will get my wish. I will really be in trouble if I broke my present four-blader.
If you do wish to correspond with George, his email address is gsMQ@w$bm<uLco.za. Maybe you will have better luck.
For interest, George makes light composite blades out of a special honeycomb glass fibre material used in the manufacture of surfboards for our local surfers, which has proved to be very strong. He also uses carbon fibre in the blades.
Ajab, in my opinion, I think that for your application you definitely need to find someone to make a custom-built propeller to suit the engine/airframe configuration for maximum efficiency, whether it is my George or someone closer to home.
I hope my findings so far will be of some use to you, or thought-provoking at the least.
I have also contacted another friend of mine, Flynn Elliott, who also owns a Streak Shadow like mine who has also been tearing his hair out because of propeller problems. I have asked Flynn to email you with his findings. He is looking forward to contacting you.
Very briefly, Flynn lives 100 miles south of Durban on our Transkei wild coast, in a little place called Port Edward. Like me, Flynn changed to a Jabiru engine and also used the three-bladed Arplast propeller in the beginning.
He then, like me, bought the American Warp Drive three-blader propeller and found it a bit better.
He then obtained a four-bladed hub from Warp Drive and has been experimenting with that.
Lindsay has sent him pictures of your super aircraft so that he can appreciate looking at another super-looking, well-finished Streak. I am sure Flynn will give you more information on his findings comparing the different performances between the two propellers and maybe information that I am not aware of.
Ajab, also bear in mind that these propellers have also been reduced in size from their original shape. Flynn's email address is flynno@,yebo.co.za
Let's talk wings -
Since doing the wing modifications, to the best of my knowledge, I have gained about 8-10 mph at the top end. As mentioned in previous correspondence, I could never get 110 mph straight and level with a passenger at 4000 ft. altitude but barely managed 100 mph by myself. In early races with my friend, Jas, in the other Streak, he used to beat me, but now I can pull away from him. His Streak has never been modified and remains stock standard.
In the cruise at 80 mph there has been a change in fuel consumption which has dropped from about 13 litres/hour to about 11 litres/hour with a passenger. By myself, the fuel drop is to about 10 litres per hour with an altitude of an average of 4 - 6 000 ft.
Looking at figures of when Jas and I flew to the Cape at an average height of 2000 ft. above sea level, fuel consumption dropped to 8,5 litres/hour at a speed of 80 mph at 2 500 rpm. For interest, Flynn apparently gets about 9 litres/hour along the coast in his Streak.
Stall Speed -
My Streak no longer stalls but simply very gently mushes out of the sky. I gently take off power to idle and pull 30 degrees of flap and, with full elevator up, she slows to approximately 20 mph indicated air speed and starts to float down. The stall, or 'mushing' was never that low or slow and I think it was in the region of about 30-35 mph, if my memory serves me correctly. I have fiddled so much in the last three years, I have lost track, just a little.
One more little trick I did (and don't you dare tell David) is that I increased the flaps down another half a notch which creates a real "barn door" effect - this had the effect of popping me back into the air - but I only do this manouvre at very low speeds of about 35 mph and below. Otherwise it is impossible to pull the flaps down. Hence the 'helicopter effect' I mentioned in my previous correspondence.
Once again, this has not changed the flying characteristics of the Streak. I would love to try this at sea level one day to see how low and slow I can land her. It feels very safe and she has never wanted to stop flying, side-slip or drop a wing.
Lastly, Ajab, I am honoured that you feel that my ideas and these modifications would be of benefit to you and of course you can use the lot with my blessings including anything else I can dig up to help you to improve efficiency. I hope that these modifications will work with your wing.
It may be a good idea to discuss these ideas with your fellow colleagues in the 'know' to maybe improve on what I have already done.
Please keep records of your speeds and efficiency as it will be interesting to see the results you get.
Let me know how you are going with your back cowlings so far. It looked very interesting on the one picture I saw. Try and keep everything as light as you can.
I managed to find a picture of Lindsay and myself taken about a year ago, so at last you can see the two hooligans on this side of the world with whom you are corresponding.
Very lastly, I am going to try and send you a story (in a few days' time) which I was asked to write for our club newsletter some time back, all about my adventures with the propellers I went through and it is very appropriate in the light of propeller problems that we all seem to be experiencing. It's a lighthearted dig at myself which had a lot of club members in stitches. But, Ajab, read between the lines. There were many times I felt like putting a match to the entire project and my flying career because of all the frustration. Enjoy.
Please take care. What we are seeing on CNN with all the fighting around you must be giving you quite a headache and lots of unnecessary worries.
Regards,
Greg and Lindsay

E-mails from Greg Pedersen S.A.