Monday, April 5, 2010

Using Teknodur polyurethane paint like topcoat, two layers of paint to finished surface without any pinhole problems

I have noticed (well, might be that it is a usual way to use it but I just haven't heard of it) that Teknodur polyurethane paint that can be used to paint composite structures like those on experimental aircraft, can be applied with brush and then perfected with sanding like on applying topcoat (/gelcoat) on a sailplane.

This just works for me, please do not follow if you are not willing to take the responsibility of potentially ruining your paint:
0. Do not use base paint or raw epoxy method, you don't need to fill pinholes, just forget about pinholes with this method! In other words, you can directly apply like this on top of smooth sanded dry micro or automotive polyester filler!
1. Apply thick layer of Teknodur 2 component polyurethane paint (e.g. white) on top of the composite structure. Any other similar polyurethane paint works too (I have also tested with Hempel 2-component boat polyurethane paint). Base paint is not necessary, the Teknodur takes on a bare epoxy surface which is sanded to dull (be sure it is sanded to dull, if it is not, then it will not take, but peels off). Do not use solvent to make the paint thinner, the thick property is desirable. The thick paint blocks the pinholes on the surface below.
2. Let it cure and then inspect. Look, 1 layer of paint and no pinholes! There may be runs, but you can get rid of the runs easily!
3. Wet sand the surface smooth. Use quite coarse grit at this point.
4. Add second layer of Teknodur paint. You can use a bit solvent now, and you will get no pinholes. Try to avoid runs more carefully at this time.
5. Wet sand to completely smooth finish.
Use all available wet sand paper grits up to 2000 if you can find 2000 grit. 1200 grit is fine though.
6. Use polishing compounds to finish the surface to high gloss.
7. Add vax and polish.

A little bit tedious with all the wet sanding, but on the other hand: full control over pinholes, no base needed, and most sanding goes to the paint without harming the critical glass/carbon fabric under it.

I am just in middle of painting a little composite part this way and I have noticed that it works. Before you ruin any large parts by using a method where the paint is misused and done differently than all painters will teach you, please try it to some scrap part first. I have finished two scrap parts like this and they have been in the snow and ice the whole winter without any harm done to the paint surface, so I would guess that this sanding method does not ruin the paint.

I am not sure, but it could be that:
- You would be even better off if you first apply a very thin layer of paint that enters the pinholes. Sand dull. Then don't care about the pinholes, just add the thick layer of paint on top of the thin layer.

On the base and on the first layer, the sanding result does not need to be smoother than 240 grit. Anything more than that is waste of time because the thick paint rounds the minor irregularities.

- Polyurethane paint is easy to sand, very very very very easy compared to sanding epoxy
- Runs on polyurethane paint is no big deal, just sand them off in a minute and you are done!
- Quick to finish
- The thick paint is very weather resistant and is as smooth as you sand it

- The layer of paint becomes pretty thick and it is heavy, and in some cases might be undesirable.

Thursday, April 1, 2010

Idea: Series hybrid in airplane using auto engine and avoiding the pitfalls of auto conversions

I have been thinking this back and forth now quite some time. This idea is quite simple, the purpose is to fix the most critical problem with auto conversions, achieve better aerodynamics, propeller placement and mass and inertia distribution.

Auto conversions most often fail, no surprise, because of the reduction gear or belt. The core engine is not the root cause in the problems and many problems with the reduction belt or gear system can not be seen beforehand because the dynamics of the vibrations of the engine, propeller and their inertia forces affecting each other is a bit more complicated than one could think at first - it is not that simple to make these parts to last for hundreds or thousands of hours.

So we came up (with Kate, we usually talk with Kate about these things and we kind of invent these things together, I usually happen to be the one who writes them down - and it is usually so that Kate is the opponent into which I test my idea's feasibility before I write it here) with the idea of having a auto engine, possibly a diesel engine, running at constant power, most likely exactly at the optimum point of the engine, always. Then all the power variation would come from the electric motors which would drive the propellers. The idea is that the diesel engine only runs a generator.

The downside of this idea is the additional weight from the generator, batteries, motor controllers, electric motors and the props (depending how many electric motors are used, it is also possible to use just one if that is preferred). However, there are two several things possibly good about this:

- First the diesel engine burns less fuel, resulting smaller fuel tanks.
- Secondly the gearbox system is saved. The gearbox system can be very heavy duty in a high power aircraft engine and they still have tendency to fail. Possibly something like 40-50 kg is saved straight away.
- Thirdly the aerodynamic advantage - optimal aerodynamic shape without using long extension shafts and couplings to deal with the dynamics of the rotating shaft connected to a non-optimally rotating propeller and the power pulses of the diesel engine. Now there is the chance to put the engine anywhere in the airframe where it best fits and propeller drive don't need to be considered at all.

Then there is the redundancy thing. Brushless DC electric motors usually never fail, but the prop can still fail in bad circumstances. Therefore having two independent props for the one diesel engine could be advantageous. Same thing with the batteries - if the diesel engine fails, the batteries could be sized such that the aircraft can fly without the diesel engine for example for 30 minutes in level flight. That might be enough in most cases to get safely on the ground, except on middle of an ocean. The most likely place for the engine to fail is the takeoff. This takeoff stress would never happen with this engine configuration - the engine would be run always at optimum and safe power, never on takeoff power. The extra power for the takeoff can be easily taken from the batteries if they have proper capacity and the electric motors are powerful enough. On takeoff the batteries at full power are not discharging that quickly, because the diesel engine is recharging the batteries at the same time. The takeoff power can be rarely used for longer than 5 minutes on an aircraft equipped with Lycoming engine either, so having a limited period of time for the full power is not that big problem.

Generator and electric motor can have very high efficiency, and the gap to a efficiency of a reduction belt system is not that great. Best electric motors (though heavy ones) are around 98% efficient.

On descent the diesel engine could be shut down providing there was enough battery capacity. The motors could actually regenerate also batteries when the pilot wants to decelerate the plane.

Maintenance cost would be like a single engine aircraft, but the reliability geared towards a twin. Of course there is the one little fine print: the battery pack is expensive and it has an expiration time and date, unfortunately. But nothing is perfect and without compromises.

Any comments about this idea? This surely would not be a racer as the power to weight ratio would be rather poor, but anyhow I am thinking, providing it would be efficient enough to climb adequately, this would be a quite economical thing to fly and also easy conversion-wise, almost stock auto engine would be okay, no reduction gear and prop installation and an assembly that takes the push or pulling loads, would be needed. Also waiting on the airport would not waste any energy, since props can be completely stopped when the plane does not need to move. For example Lycoming IO-360 consumes about the same amount of gasoline per hour when waiting on IFR clearance on the ground than our Toyota Prius car on highway. Consuming zero amount of fuel when still on the ground, but still being ready, would save some liters.

And answer to the question, why diesel and not gasoline when gasoline engines can be run very lean and quite great specific fuel consumption values can be achieved in optimal conditions - it is quite simple: availability of the 100LL/Avgas seems to be becoming poor. There has been three 100LL operators in Finland, but two of them decided to discontinue this year. There is only one left. When that only one decides that it is not profitable enough, there is no 100LL available for anybody and the whole country's fleet of Lycoming and Continental based planes are grounded. The Jet-A1 is not going anywhere, so engine that can burn the jet fuel would be a safe bet. Jet engine, turboprop, or turbofan are out of the question because those are not available in meaningful sizes and power classes - there is not a small turbofan that would have high pressure ratio and bypass ratio available, nobody manufactures such a thing. And it is unlikely anybody will in the future because this personal flying all is a very niche market unfortunately until it changes for better (if it ever does).

The implementation possibilities have challenges; namely no such electric motor available (would require custom motors possibly), etc. And the weight also causes penalty for the efficiency and speed of the plane. But the power to weight ratio will be with this arrangement a lot better than on a pure electric aircraft. And pure electric aircraft is feasible, why an electric aircraft with a generator and a fueltank added would not be.

And by the way, even if it is first of April at the time of writing this, this blog post is not an April fool.