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