Sunday, 14 July 2019

Twister 4 ship formation

We had a memorable flight of four Twisters, at the home of the Twister, Paderborn Haxterburg, on Sunday 7th July.

Flight lead, Matthias Strieker D-MTMH

Number 2, Ingo Zoyke D-MTMN

Number 3, Johan Teerlink OO-162

Number 4 and cameraship, Andy McKee G-FUUN

Can we make it 8 Twisters next year? Let's hope so.

Click on this link for a short video:

Monday, 3 June 2019

Energy management and controlling the monkey brain

So there I was, part way into my second sequence of an aerobatic competition, when something happened.

I was looking to the left as I was just about to pull up to the vertical for the next figure which was a stall turn, so my eyes were on the sight gauge and horizon to the left.

I was not looking straight ahead at the time of the incident.

My first thought was 'Bird Strike' as I'd seen lots of gull's circling in thermals during the climb and during the first sequence earlier in the day. Fenland is quite close to The Wash so the coast is nearby and many sea birds are sharing the air.

There was a change in sound - a notable buzzing noise coming from the front of the aircraft. I immediately reduced the throttle to just above idle. 

A quick glance at the engine instruments confirmed nothing untoward with the engine so the sound reinforced my first thoughts of a bird strike to the prop.

Although it could also be a loose piece of bodywork or fairing which was 'buzzing' in the airstream? I didn't know for sure but suspected the prop despite there being nothing different vibration-wise from normal operation.

A glide approach was then started back to runway 26 - which was the wind favouring runway of choice below. I was very fortunate as to where and when this incident happened BTW, high and fast (a high energy state) and right over an airfield with 2 runways.

A call was then made on the box frequency to the Chief Judge to let him know I had a problem and I was ending the sequence. He acknowledged and I immediately went to the airfield frequency which was blocked due to an incoming aircraft 5 minutes out making a long drawn out call. I took the time to eyeball my situation some more and visualise the approach to 26. Once the other traffic and the tower had stopped talking I made a Pan-Pan call, stating I had a problem and that my intention was an immediate forced landing on 26. The tower continued to talk back to me and also talked to me when on finals giving me wind information but I was mentally blocking out most of what he said as it was irrelevant at this stage and my brain needed to fully focus on the aviating from now on.

During the approach I verbalised to myself out loud "No go-arounds!". This acts as a sort of 'order' so your monkey brain does not take over and do something stupid. I often used this technique when I was Gliding - which if you have ever done in a competitive environment is just a series of decisions, the winning pilot usually having made the best decisions as flying skills don't count for nearly as much as good decisions in the results.

Coming onto the final approach and now lined up with runway 26 I again verbalised my situation and gave myself an order to correct. "Too high, too fast!" - "Side Slip!". So a side slip was engaged to lose the excess energy I had.

Once flap limiting speed was reached and the runway was certain to be 'made' I went for full flap. A final little side slip to lose that last bit of excess energy and then a reasonable landing was made with no further damage done.

Now comes the question - if you have an excess of energy then should you go for best glide, that is; fly slowly and extend your thinking time, or should you just aim to get back on the ground whilst things are in your favour - in this case an empty runway. More time would not have helped me in this case - if anything I have learned from this experience that taking excess energy down low is a good thing and a safety net - typically in an emergency you can always get rid of energy but not put it back. So if I were to do it again I would do the same thing.

Once I had reduced the throttle to just above idle I treated the rest of the flight as a glide approach - something I do often as practise anyway so I'm quite familiar with the 'picture', performance and sensations that go along with it.

Having said that - if for some reason I had to add some energy there was still the option to do that with the engine running but that would have been with some risk - still preferable to crashing into trees or water if that had been the other options though. ie: the risk was worth it if the other outcome was worse. As it was I had an excess of energy so did not need to ask anything of the powertrain after the incident.

I was asked "Why didn't you turn the engine off" by one pilot (the only one out of the hundred or so that have talked to me about the incident to ask such a question BTW). The answer to that is quite simple and 4 fold. 

Firstly there was nothing wrong with the engine so shutting it down would not have 'saved' it from further damage (anyway engines can be fixed, people not so easily. As a pilot you should always be concerned with doing the least dangerous action to yourself rather than the least damaging action to 'some-thing'). 

Secondly the prop had been through the most intense energy state at the time of the incident, which was - high speed, high rpm. Once the throttle was reduced to just above idle and the airspeed reduced somewhat then the chances of the prop failing anymore was reduced to the point of 'highly unlikely'. 

Finally and this is the main point, if I had shut down the engine then I would have had a completely unknown performance state to deal with that could not be altered and one that I had never practised (a dead stick) so this would have been very unfamiliar to me, which would have increased the workload and therefore the risk.

So the safest thing to do was to keep the engine running - albeit at just above idle as that is it's smoothest setting and provides the airframe with the least amount of drag - much less drag than a stopped prop would do.

It was only when I had taxied in and shutdown that the cause was confirmed - the prop stopped with the blade in front of me that suffered the most damage and was split. So it was confirmed as prop damage after all. At this point I suspected the spinner causing it and a friend coming up to shake my hand confirmed that with "the spinner is gone" (you cannot see the spinner from the cockpit of a Twister).

Luckily there was no other damage to the airframe as the spinner was disintegrated into thousands of tiny pieces which made there way either over or under the wings and tail. There is a very small scratch on the outside of the cowl but other than that I can find no other damage at all.

I had cleaned the spinner and prop before this flight too and did not notice any screws missing or anything else untoward so it is still somewhat of a mystery as to why the spinner failed in the first place. I would only add to this that the spinner was made by Hercules Propellers and was not a factory supplied item. It was made to a different schedule and was said to me (by a third party) to be   'not fit for purpose' (not their actual words, I am being nice here). I will do a separate post later on about the new and improved spinner I am working on as a replacement.

As usual I was filming the sequence from the tail looking forward so there is video of the whole event. Even though the damage itself happens in part of a frame - it happens that quick - from nothing untoward to prop damage in an instant, the video hopefully shows something someone else can learn from.

I edited the video to show what happened and how I dealt with it. I hope only that it informs others in some way. Check your spinners! And also practise your forced landings. If you found the 'Verbalising orders' thing helpful then think about employing it - one day it may help you make the right choices in a high pressure situation and not let your monkey brain take over.

To that final point there are many examples over the years of seemingly very experienced and capable pilots doing the 'wrong thing' in an emergency due to letting their monkey brain take over. The gentle pulling back on the stick 'I don't want to crash feeling after engine failure' (when it should be pushed forward) which results in a spin into terra firma.

Video on this link:

The leading edge urethane material stayed together - despite hitting the spinner at approx 600 mph and then the prop split along the next weakest point back which is where the urethane is attached to the wood.

There is a small notch in the urethane which shows how much force is going on as you can hit this material with an axe and it does nothing normally.
No other damage to the airframe, so no harm done except to my wallet and total score in the competition. Second place was on the cards again until this happened.

Wednesday, 8 May 2019

Twister hybrid - More details

Here are some more details regarding the Turbine GenSets currently on offer.

If anyone knows of any others please let me know in the comments section below.

First up the MiTRE from Delta Motorsports.

Their latest model has an output of 35kW with a weight of approx 50kg.

It is quite a compact unit as you will see from the below pics.

I would need double this output and if anything have a slight reduction in weight. Bear in mind that some of that 50kg is for emissions reduction for the car industry which is not necessary in experimental aviation, so with that equipment removed the weight should go down. Also more exotic materials can be used to reduce weight further, although that will drive the cost up.

The battery pack is a 'moveable ballast' so if needs be It could be relocated to another area of the airframe - perhaps under the safety cell or even split in two with part of it in the baggage area (as Pipistrel do with their electric model to retain the correct C of G).

The other turbine Generator on offer is the TurboTech from France.

Unlike the Delta MiTRE this unit is specifically designed for aviation.

They are quoting an output of 55kW with a weight of 50kg.

With 40kg of fuel (54 litres) on board this unit has the equivalent energy of 1000kg of Lithium-Ion batteries!

The size of the Turbotech unit means it will be a squeeze to fit it under the Twister cowling and to include the electric propulsion motor. So the battery pack and inverter would almost certainly have to be moved elsewhere in the airframe to make room for it.

From their Linkedin picture it seems like Turbotech are using a very similar idea (but in reverse) to the pre-cooler on the Sabre (single stage to orbit space engine). I believe the Sabre pre-cooler is made from micron-thin-walled Inconel.

More about the Sabre here:

More about TurboTech here:

And finally a link to the Delta MiTRE which is a few years old so talks about 17kW rather than their latest 35kW version:

Delta Motorsports MiTRE turbine genset.

Installed in the bottom of a car.

Compact size of the Delta MiTRE is clear from this video screenshot.

Turbotechs offering. Larger in size than the MiTRE but getting close to the figures I need for the Twister. They are quoting 15 litres per hour in the cruise - same as my current fuel consumption.

Turbotech's recuperator design seems very similar to the Sabre Hybrid rocket engine.

Monday, 6 May 2019

Twister hybrid - revised version

It's been 7 years since my first design of a hybrid Twister. (See my previous March 2012 post).

A lot has changed in that time and I've also learned a lot more about the possibilities of electric hybrid powertrains.

For the immediate future a hybrid solution is the only one that works if you actually want to tour in your aircraft (which I do).

Fully electric is only good for aerobatics and local flying of no more than 1 hour.

So with my latest design the bias is with much less batteries and a normal fuel load.

Delta Motorsports here in the UK are developing a gas turbine generator as a sustainer/range extender for Ariel's new HiperCar and for Morgans new electric model, both coming out in 2020.

Normally a gas turbine is very inefficient - typically 15%. However through the use of a recuperator and some other secret tech Delta have managed to get the efficiency of their turbine up to 30%. This makes it a game changer.

Now the generator is as efficient as a conventional internal combustion engine.

The electric propulsion motor is 96% efficient so there are hardly any losses after the GenSet.

In terms of Aerobatic performance this new hybrid Twister would have the potential to compete at the Advanced level (although the roll rate would also have to be increased, the easiest way to do this would be to dispose of the flaps and go for full span ailerons although this would lengthen the landing distance by quite some margin).

Comparing to the Extra 200 and 300, which are both quite a bit heavier, the power to weight ratio is very much in favour of the Twister. With the Extra 200 being 540kg empty, plus 120 for fuel and pilot gives a power to weight of 0.22 kW per kg. The Extra 300 fares only a little better being 682kg empty, plus 120 for fuel and pilot the power to weight ratio is 0.28 kW per kg. The Hybrid Twister with 150kW and an estimated empty weight of 330kg (+ Pilot and fuel 450kg) would have a power to weight ratio of 0.33 kW per kg. This would give it a phenomenal climb rate of at least 3,500 fpm.

With that kind of climb rate then the lower VNE becomes less of a problem as the display/sequence can be flown in a smaller space and at a slower speed as there is less need to 'wind up' to a high speed to gain energy before starting a vertical figure.

Watching the new GenPro recently on YouTube it has shown me that with a superior power to weight ratio you do not need to fly so fast and take up so much room in the box. 

This suits the Twister as it has a lower VNE and G limits than the Extras. (160kts vs 217kts and +6-4 vs +-10G)

As you can see below the turbine would be housed in it's own heat proofed section of the cowl. The exhaust can be made parallel to the airstream for a little extra thrust. The turbine would be fed either by a dedicated NACA duct on the side (shown) or taking part of the air from the LoPresti inlets at the front - if there is some spare capacity left over from the liquid cooling requirements.

Hopefully the existing cowling and oil cooler scoop can be used.

With the small battery pack you would have 2 mins of max power - 201hp. Then in the cruise the power output of the GenSet would slightly exceed the propulsion motor requirements and therefore it could be used to charge up the batteries.

There is also some nice redundancy here. If the turbine GenSet fails after take off then you still have approx 4 mins of battery power to get around an abbreviated circuit and back on the ground.

Cost is a big unknown at the moment as the turbine is not ready for market. This design would also need a new variable pitch prop to deal with the greater power.

The existing fuel system can be used and with 30% efficiency from the turbine it should mean that the range is not compromised. Jet A-1 is less than half the price of AvGas in the UK too (and if you can buy at the corporate rate then it is even better as it is approx 20% of the price of AvGas). 

See the below illustration for details and layout.

Monday, 29 April 2019

Aerobatics comp result

Yesterday was the first aeros comp of the season and it went better than I expected.

From 12 competing in the Sports class I managed to get 2nd place and a silver medal.

I managed to beat all the pilots in Extra 200's and all but one in a Pitts S2. Both those aircraft have double the horsepower of the Twister. Plus 4 times and 2 times, respectively, the roll rate of the Twister. Plus all those other pilots are professionally coached while I am self taught.

Video of the sequence as it was flown is on this link:

Happy days!
Some of the aircraft competing. There were 3 classes. Sports, Intermediate and Advanced.

Wednesday, 20 March 2019


I thought I would share my checklist for G-FUUN - which is type specific but generally applies to any fixed gear Twister with a UL engine.

Feel free to download and print out - it is set up to be A5, a good size for knee boards. PM me if you want the original PDF - I cannot upload it here as Blogger only allows images not files.

Strike out the cowl flap references if you do not have one.

I personally don't use this checklist anymore having committed all of this to memory but if you are starting out it would be wise to have a Checklist.

Sunday, 3 March 2019

Twister Pilot Notes

The following is meant to help anyone who has yet to fly their Twister and should not be taken as any kind of instruction - it is merely a guide.

As I've only ever flown G-FUUN this is type specific - your results may vary :)

(This is for a fixed gear, integrated tailwheel Twister with a UL260iSA engine.)

How I fly a Twister.


I start my pre-flight inspection at the prop, checking it for any damage and general secureness, I look underneath the cowl for any leaks and at all 3 tyres to see they are inflated correctly. 
Then the oil level is checked, then all cowling fasteners.
I then visually check the contents of each fuel tank. 
After that I walk around the wings running my hand along the leading edge to check for any damage before stopping at the pitot to check it is clear and the ailerons to check they are operating in the correct sense and are free. I also check the aileron linkage at this point. 
Proceeding back along the fuselage I check the static vents are clear and then that the fail safe screw for the tailplane attachments is in place. The elevators are checked next for integrity, full and free movement and no play.
Next the rudder is checked, that both pins are in and both cable attachment bolts and nylocks are secure, also checking for full and free movement.
The walkaround continues to the other side of the aircraft with the same checks as above where appropriate.  


After getting strapped in and applying the parking brake the canopy is closed and locked (you should never even start a Twister, let alone run one up with the canopy not locked down). Throttle is set to just above idle and all switches are set to ON from left to right. A brief pause to check you have correct fuel pressure and then looking outside for any people who may have strayed close to the prop before shouting "clear prop". Starter is then engaged. Oil pressure is monitored closely and must come up in 5 seconds or less or shut down. Battery voltage should also begin to rise.


Whilst warming the engine I perform all pre-take off checks (bar one). Controls, full free and in the correct sense (the most important check in any aircraft). Straps, secure and any loose ends tucked in. Instruments, set altimeters to local QNH, turn on radio and transponder. Flaps, set to first position, 10 degrees down. Trim, set to slightly forward of centre. Engine, check battery voltage is now 14.5, then check each ignition coil in turn by turning them off one at a time there should be a small rpm drop of only about 20-50 rpm each time. Oil temps coming up, I taxi when it is 35 degrees using only low revs, no more than 1,500rpm.


The view for me over the nose is possible but a shorter pilot will not be able to see over the nose so you should use the Pitts method of taxiing in a series of left and right curves. The turning circle on the integrated tailwheel Twister is quite poor so you need to take that into account before taxiing into a confined area, this takes up quite a bit of your attention when at an unknown airfield. Braking won't help tighten up a turn either as the standard brakes act on both main wheels simultaneously. If the surface is bumpy then it's best to hold the stick hard back to ensure the nose does not tip over.

Take off:

Final check before take off is a full power static rpm test after first checking oil temp is at minimum 50 degrees C. Depending on the outside air temp and wind direction I get between 2,500 and 2,600 rpm. 
Once the take off roll has begun do not be tempted to raise the tail too soon, wait until you have 30 knots or so and have good rudder authority, the integrated tailwheel will help you greatly to keep straight but not when it is in the air! Once the tail is raised your view ahead improves greatly and the usual footwork will be required to keep straight, at this point I glance down to check that my ASI is live, when you get to 50 knots things will become light and shortly after it will want to fly off. Let it accelerate in ground effect until you have 60 knots before easing it up into a climb attitude.


On the initial climb out I leave the flaps down at 10 degrees and climb until I have about 300 ft or more before raising them. Once raised the Twister will naturally assume a lower angle of attack and the speed will come up.
At this point I let it accelerate to 90 knots before turning onto the crosswind leg. I continue to climb at 90 knots, this gives 1,300 fpm in the Winter (and as little as 500 fpm in the Summer when hot, heavy and high). There are many good reasons to 'cruise climb' at 90 knots rather than try to climb out at a much slower speed. Firstly you have a much greater margin over the stall giving you much more time to react to any emergency, Secondly you have a better view over the nose for possible conflicting traffic, Thirdly the climb rate is no worse at 90 than it is at 70 because with a fixed pitch prop the engine is not able to rev out (and therefore produce more power) when you fly it slower, Fourthly when you are actually trying to go somewhere then a faster climb speed will make your journey time less, and finally, Fifthly, the cooling is better for the engine when at a higher airspeed and a lower angle of attack.
Caveat to this; my Twister is probably the heaviest one out there and I may also be the heaviest Twister pilot(!) so your combination may climb better at a slower speed, but I have found this is the best speed for me.


I climb above my target altitude by 100 feet or so then shallow dive down to the target altitude to accelerate quickly to cruise speed and get on the 'step'.
I typically cruise at 2,700 rpm giving a TAS of 125 knots with a fuel burn of just under 15 litres per hour. 
If I'm just flying around at low level for an evening 'bimble' then I'll reduce the revs to 2,600 or even less.
I change fuel tanks every half hour to keep the wings balanced.


The Twister is a slippery beast so it is best to plan well ahead with your descents. I tend to reduce power a little and then head down at max rough air speed - which is 120 knots indicated.


The more familiar I am with an airfield the tighter I will fly the circuit. I tend to fly the downwind leg quite fast - at 120 knots indicated and then go below circuit height on the last bit of the downwind leg while setting the throttle to idle. Then as the speed comes slowly back I will raise the nose by quite a lot to slow it down further until the flap limiting speed. If you don't raise the nose it would take forever to slow down to approach speeds. Flaps come down in stages. Only use 20 degrees of flap if you have a strong crosswind and plenty of runway length available, otherwise I always use full, 30 degrees, flap.
If you're starting out then fly a big wide circuit and only go at 100 knots until you get used to everything.


A steady approach at 60 knots is what you are after, nail that 60 knots and make sure you carry that speed all the way until the roundout. The Twister side slips quite well, so if you are too high then throw in a side slip to lose height.


A minimum runway length of 500m is a good idea for a Twister - certainly in the early days of flying one. (In nil wind I personally would not try to land anywhere that was not at least 500m)
The Twister floats quite a bit on landing but don't try and slow down your approach below 60 knots to land shorter you will only end up running out of energy to round out and hold off if you do. Stick to 60 knots and you'll be fine, just accept the long hold off period until touchdown.
I virtually always 3 point for a landing. What you are trying to do is come down very slowly in the last foot or so above the runway so that you touchdown without much vertical speed at all. If you have a sudden drop to the runway it will bounce, the undercarriage does not have any kind of damping effect and is more like a spring so you end up going back up to where you came from. If this happens then hardly anything needs to be done, just be patient and let it come down again gently with a very small back movement on the stick at the last bit (do not pole forward!). Unlike most other aircraft you should not stall a Twister onto the ground as you are landing, the reason for this is that it has a sudden wing drop at the stall (for me it's the right wing) and if this were to happen with any height above the runway you would come down very quickly on one side. It has happened to me once and it was not nice.
Due to the Integrated tailwheel the Twister is a pussycat of a taildragger. As soon as that tailwheel is on the ground you have good directional control (unlike most other taildraggers). So that's why 3 pointing is a good idea, even in a crosswind. I have done some wheeler landings (where you touch down on both mains in a level attitude and then lower the tailwheel later on) but to be honest they are no better than a 3 pointer if you have the integrated tailwheel.


So you're ready to have some fun then? Good stuff. Here are a few things I've learned while aerobating the Twister.

Safety first: Do it high so you don't die. If you're trying anything for the first time then do it high - minimum 3,000 ft or preferably 5,000 ft.

The main thing to watch out for in a Twister when doing aerobatics is speed control when you are heading ‘downhill’. The Twister accelerates much faster than more draggy airframes so if you are not careful it can get away from you a bit. If in doubt always roll out of any manoeuvre rather than trying to pull out (if you are upside down) if this is happening.

The Elevator has a nice feel to it, the Ailerons are heavy (spades are needed really) and the Rudder is much too light and so quite sensitive.

The roll rate is pedestrian - 90 degrees per second - which is very slow for an aerobatic aircraft. Also it has a low power to weight ratio which makes it lose energy going 'uphill' quite quickly. Conversely as it is so slippery it accelerates and gains energy going 'downhill' very quickly (as cautioned about above). 

120 Knots is a good all round entry speed for most manoeuvres.

Loop, Stall turn, Half Cubans etc.

A big input of right rudder over the top of any Looping manoeuvre is needed to stay on heading.

The Immelmann requires a higher entry speed, 140 or even 150 if you can manage it as the Twister loses so much energy and has a relatively slow roll rate at the top.

Any other figure which requires a long up line - first half of a Goldfish or reverse half Cuban for instance - also require a higher entry speed - 140 is good.

Snap (Flick) rolls should be entered at a much lower speed, well under Va. 80kts is fine for those.

Normal aileron rolls and hesitation rolls can also be done at a lower speed. Around 100 to 110kts or so gives a clean look to those.

Stall turns should be to the Left. Once the nose has passed the horizon on the first half of the turn then I go to idle with the engine. This helps in two ways. It lets you make a long down line without gaining too much speed, it also gives you much more right rudder authority to 'stop' the turn on a vertical downline with the wing leading edges parallel to the horizon, it also reduces the torque effect of the engine.

With spinning I prefer to spin to the left as it gives more control and enables you to stop it on a heading better (more right rudder authority at idle). Spinning to the right it happens quicker so is harder to stop on an exact heading, normally you should start recovery from a spin 1/2 of a revolution before you want to stop to get it to come out on the right heading.

Enjoy your Twister and be progressive in exploring it's limits (and yours).

Eye Candy Part 2

Some more pics that others have taken.

At the Sywell Rally in 2018

At Breighton for the Club class aeros comp.

Parked up in Reykjavik Airport, Iceland on my way across the Atlantic.

Parked up next to Johan's Twister at Sanicole for the EFLEVA weekend.

I made FlightRadar's Year in Review with the Spitfire outline drawn over Southern England as my tribute to the RAF100 anniversary.

A more detailed view of the path I flew. The coverage is not perfect hence the jagged lines here and there - the actual SkyDemon path flown was much smoother than this.

Landing at Lee-On-Solent in October 2018.

At Oshkosh in the flypast.

At Oshkosh, and featured on Kitplanes 'Guess what' section of their website.

Over Lake Tahoe as part of the Kitplanes early morning photoshoot.

A 'break' from the photo ship.
The best shot from the Kitplanes photoshoot. I like the reflections off the prop disc.

Close up from the Kitplanes photoshoot.
Landing at Conington during the Aeros comp.

Thursday, 28 February 2019

Fuel overflow

When upside down the fuel overflow outlets are in an area of low pressure and so vent out a lot of fuel.

This has been a problem since day one and I've only just got around to a long term workable solution.

My idea was to restrict the size of the outlet hole which as standard is quite excessively large at about 6mm.

For a while I was just using a piece of duct tape over the outlet with a small hole pierced in it but that did not last very long as the fuel soon got to the sticky part of the tape and made it fall off. I needed a more permanent solution to the problem.

So I made up two small discs out of thin ali sheet and drilled them in the middle with the smallest drill bit I had - 1mm.

These were then glued to the outlets using 5 min epoxy.

A recent flight test proved that these to work well - there is still a tiny bit of fuel leaking out during and after inverted maneuvers but it is hardly anything compared to before.

I never fill my tanks till they are overflowing so there is plenty of space in the top for expansion should it sit in the sun - the fuel can still flow out of the outlets at a reasonable rate anyway with the 1mm hole so I don't think expansion is going to be an issue.

Fuel venting out whilst upside down. This is before the fix.
Ali plate restrictor with 1mm hole drilled in it.

Thursday, 21 February 2019

Wheel pant mod

With the fixed gear Twister it is impossible to pump up the main gear tyres while the wheel spats are in place. You even have to remove the wheel pants just to check the tyre pressures, which is quite impractical.

So I decided to mod the pants by making a small panel which is easily removable.

This panel allows enough access room to both check pressures and pump up the tyre if necessary.

I used 3 layers of carbon for the flange with small patches to make 4 layers where the riv-nuts were to be located.

The panel is flush and the two small M4 dome head Allen screws don't make much drag.

Size and location of the panel. Tape was left on till I painted the flock (which was used to fill the cut gap).
View of the flange and riv-nuts.

Once finished it is hardly noticeable and adds very little drag.

Monday, 18 February 2019

Cowling outlet experiments

I long had the suspicion that my outlets on the cowling were too big. 

I erred on the side of caution when I was making my cowling and made both the inlets and outlet quite a bit bigger than the calculations in case things did not work as I first thought, it being easier to make them smaller than make them bigger afterwards being my thinking.

Until recently I had not bothered to restrict the outlet to see if it made much difference to temps but finally got around to doing that this Winter.

For the first trial I blocked off approx 50% of the outlet (the lower half) using a piece of foam pipe insulation. This didn't seem to make any difference to the temps at all.

I then tried blocking off approx 80% (some of the side outlet being blocked off this time). This did make a difference - I saw a 10 degree increase in the CHT's with a slight rise in oil temps (as the cylinder heads are oil cooled - when they run hotter the oil does too).

On the second test the hottest CHT in the cruise was 99 degrees or some 60 degrees below Max continuous. It was 115 degrees in the climb. All CHT temps stay within 3 degrees of one another (thanks to my 7 odd iterations of baffles on the front cylinders).

These tests were conducted at 5,000 feet with an OAT of approx 2 degrees.

It goes to show I went a bit overkill on the inlet and outlet sizes when building the cowling and it also shows that the sealed plenums work very well.

It also shows that the lower outlet area doesn't do much - it being in a high pressure region does not encourage flow out of the cowl. However the side outlet area works very well as they are in a low pressure region which means the low pressure air coming out of the cowl is more likely to flow to this region.

The beauty of the foam pipe insulation is that it is very easy to remove when the weather gets warmer.

I've also conducted some True Airspeed Tests to see what the exact cruise is at certain RPM's. I used to guesstimate at 125 knots True when light and 120 knots True when full of fuel and baggage (when touring).

I flew 4 way compass headings then added up these speeds (from the GPS groundspeed) and then divided by 4 to get the average. At 2,800 rpm (max continuous) at 5,500ft this resulted in a speed of 129 knots True.

I then flew the same 4 way test at 2,700 rpm (my usual cruise setting) at 5,500ft and this resulted in an average of 125 knots True.

Flying at 2,800 burns around 17 litres per hour but at 2,700 I burn just 15 litres per hour so that is a better rpm to cruise at for economy (and the engine seems to like it better too).

In hotter weather it will not cruise quite as fast due to reduced power from the higher inlet air temp for the engine and the extra drag from the open cowl flap - which I have found adds about 2 knots to the cruise from fully open to fully closed.

Approx 80% of the outlet blocked off with foam pipe insulation. The first test was without the small additions of foam at the top on the sides.

View of the left hand side. The approx 20% remaining outlets are more than enough in Winter to adequately cool the engine.

Monday, 4 February 2019