Foxbats & Constant Speed Propellers

Posted on May 10, 2014 by Scott

Here’s another frequent question customers ask about Foxbats – is there any benefit in fitting a constant speed (CS) or in-flight adjustable (IFA) propeller?

Unfortunately, there is no simple answer. However, here are some thoughts to consider when weighing up the pro’s and con’s. Please remember – these comments apply to the Foxbat. Other aircraft can and will have different considerations.

1. The maximum permitted continuous rpm for the Rotax 912 series engine is 5,500, whatever propeller is fitted. We therefore pitch the standard, on-ground adjustable propellers on the Foxbat so they will just reach 5,500 rpm at full power, straight and level at about 1,000 feet amsl. This gives a maximum true airspeed between 95 and 105 knots, depending on the propeller type, the size of the wheels and whether spats are fitted. At this pitch, the prop typically gives around 5,250 rpm at full power on take off and best climb speed of around 60 knots.

2. The maximum power & torque of the standard Rotax 912 carburettor engine is achieved around 5,800 rpm, which you can use for a maximum of 5 minutes at a time. Ideally, you’d use this full power rpm for take-off, and for the first couple of thousand feet of climb if you really need it. Rotax specify a full power take-off rpm of no less than 5,200.

3. Therefore, with the standard prop pitch setting on the Foxbat, although you are within Rotax limits, you are not getting full power from the engine at take-off. Even so, take-offs at maximum weight can still be achieved in 50-100 metres (at sea level), and this setting gives you the best cruise speed.

4. To give a higher cruise speed on the Foxbat, a CS/IFA propeller cannot be set to run any faster than the on-ground adjustable prop. But it could be set to run up to 5,800 rpm on take-off. This will shorten the take-off run in the Foxbat and increase the climb rate. A bit.

So, in summary, a CS/IFA prop on the Foxbat will improve the already impressive take-off performance but will not increase the cruise speed.

BUT…there’s more: there are couple of uses for a CS/IFA prop – other than better climb and/or cruise – which are not often considered.

The first is to use the prop as an air-brake, when you need to descend quickly, by setting it fully fine pitch and engine at idle. The Foxbat is already fairly draggy, so doesn’t pick up speed quickly in a dive. With a fully fine pitched prop, it’s like having a great big parachute holding back the aircraft.

The second use is to allow you to keep the engine warm by using higher rpm at slower airspeeds. For example, a fully fine pitched prop on the Foxbat could allow you to run at 5,500 with an airspeed of only about 65 knots. This may be useful in poor weather when you’re looking for a landing site, or for mustering pilots who need to fly slowly most of the time.

A third use relates more to an emergency. If the worst happens and the engine loses power or stops, by setting the prop fully coarse, you can substantially extend your glide range, as there is less wind resistance on the windmilling or stationary prop.

AND…yet more: there are a couple of major disadvantages to CS/IFA props.

First is the cost. A good CS/IFA prop for a Light Sport Aircraft (LSA) like the Foxbat is going to cost at least A$5,000 more than the standard on-ground adjustable prop and probably more. You won’t get this back in fuel savings for a very long time, if at all.

Additionally, most if not all CS/IFA props weigh considerably more – as much as 10 kgs more, in some cases – than the standard props. All this weight goes right on the nose (affecting weight and balance) and comes straight out of your usable load. This isn’t so much of a problem with the Foxbat’s excellent load carrying capacity but could be on some of the heavier aircraft in the LSA category.

And don’t forget, you need a CS/IFA endorsement on your RA-Aus Pilot Certificate or PPL to legally operate an aircraft with such a prop fitted.

My conclusion – while a CS/IFA prop may have big advantages on a more slippery, faster cruising aircraft, the benefits for the Foxbat are more debatable. If you want to be able to run on the limit for the complete flight envelope (and your pocket can handle it), maybe a CS/IFA prop is worth it. Me? I like the simplicity of the Foxbat and the take-off performance is more than enough for all but the most extreme conditions. So I’m happy with one of the standard props.

What about you?

Foxbat flaperon hinges

Posted on May 6, 2014 by Scott

Flaperon hinge no safety pin – correct

Flaperon hinge with safety pin – incorrect

Sharp eyed Foxbat owners have noticed that the flaperon hinges have small holes drilled in the spindles which run through the centres of the hinge bearings. One or two owners have even added washers and inserted safety pins in these holes, assuming that this will add to the security of the flaperon hinge.

The factory has confirmed it is absolutely not necessary to fit safety pins – the geometry of the flaperon controls and hinges means that the flaperon just cannot come off its hinges. In fact adding safety pins may actually be detrimental to the wear of the hinges. As the wing and airframe flexes under flight loads, it is important that the flaperon is free to move on the spindles, not just up and down but also laterally.

It is also important to ensure the hinge bearings are kept lubricated, even between services if needed, to ensure they are free to slide. If you have installed washers and safety pins – please remove them now.

True Air Speed (TAS)

Posted on May 3, 2014 by Scott

I regularly hear and take part in discussions about True Air Speed (TAS) – I sell aeroplanes and a common question is: “What’s it TAS at then?”

The resulting discussion indicates strongly that there is not much real understanding of how the speed indicated on the Air Speed Indicator (ASI) can vary widely from the true or actual airspeed. And since the calculation of TAS is quite complex, based on indicated air speed (IAS), temperature, barometric pressure and altitude, many pilots can’t be bothered to work it out and assume IAS and TAS are much the same. Or at most a couple of knots different.

In general terms, the higher and warmer you are, the higher the TAS compared with the IAS. While this may help aeroplane sales people who can say that the ‘real’ air speed is greater than indicated on the ASI, there is in fact a very real use for being aware of TAS.

For example – say your aeroplane has a manoeuvring speed limit of 90 knots, ie the maximum speed for full control deflections or cruise in rough air. At 5,000 feet on a warm 25º Celsius day at 1013 Mb QNH your ASI indicates 85 knots – are you under the speed limit? Or are you running the risk of damaging the airframe through over-speeding?

When you work it out, your TAS is actually 95 knots and if you hit a big thermal, you run the risk of structural damage to your aircraft. That’s why calculating your TAS is important.

Many of today’s digital instrument panels calculate TAS for you, provided you set the barometric pressure (often called QNH) correctly.

For those of you without the benefit of one of these panels, here’s a link to a website that will calculate TAS for you: TAS Calculator

Boeshield lubricant

Posted on April 26, 2014 by Scott

A long-time and multiple aircraft owner has told me about an excellent lubricant made by Boeing, suitable for use on light aircraft – it’s called Boeshield T-9.

Boeshield was developed – surprise, surprise – by The Boeing Company for lubrication and protection of aircraft components. It is a combination of solvents, lubricants, and waxes designed for penetration, moisture displacement, lubrication and protection. T-9 dries to a thin waxy film that clings to metal for months. It will loosen rusty and corroded parts and is safe on paints, plastics, and vinyls. To me this sounds just like the description used on other products like Lanox and WD40. However, T-9 seems to work more effectively and helps to remove old grease and lubricant film as well as add its own protective layer. Having used it myself now for almost 2 years, I can confirm it does the job well! Boeshield T-9 is available through many aviation stockists or directly on Ebay – it isn’t cheap, typically around $25+ for an aerosol can – but one can will last a very long time on a Foxbat!

10 things to know about Light Sport Aircraft (LSAs)

Posted on April 15, 2014 by Scott

1. LSAs were originally devised in the early 2000’s in the USA where they were intended to bridge the gap between unlicensed ultralights and fully certified GA aircraft. The objective was to make non-ultralight flying less expensive, through cheaper aircraft and reduced pilot license requirements. Instead of FAA certifying aircraft, the responsibility was shifted to the the manufacturer to confirm their aircraft were compliant with a number of quite rigorous ASTM standards (formerly known as the American Society for Testing and Materials). These standards cover everything from original design through to manufacture and flying characteristics. FAA continues to police the manufacturers through full-blown inspections of their factories and processes to ensure ASTM standards are being met.

2. As of 15 April 2014 there are 134 different approved LSA aircraft available in USA. The number approved in Australia is unknown as neither CASA nor RA-Aus publishes this information.

3. The very first officially approved LSA aircraft in both USA and Australia was the Evektor SportStar Plus. Thus with some pride, Evektor claims to be the ‘Number One LSA’ company. In USA sales terms, they rate at No. 5.

4. The ASTM LSA standards were over-ridden by CASA in Australia in a number of areas. The reasons for this are unclear but rumour has it that some local manufacturers felt some of the standards could not be easily met by their products at the time. The main differences are:
– the USA straight & level, full power, maximum speed limit is 120 knots. There is no maximum speed in Australia
– the USA stall speed at maximum take-off weight (MTOW) must be under 45 knots ‘clean’ – ie no flaps. In Australia it is 45 knots in landing configuration – ie with as much flap as you need.
– the USA allows both glider and banner towing by LSAs. Australia only allows glider towing.

5. LSAs may be factory manufactured – in which case they are known as ‘Special’ or S-LSAs – or built from approved kits – in which case they are known as ‘Experimental’ or E-LSAs. In Australia, E-LSA aircraft registration numbers on RA-Aus aircraft (but not CASA VH- aircraft) are preceded with the letter ‘E’ – for example: E24-8460. Under E-LSA regulations, there is no ‘51%’ rule, so an aircraft can be almost complete, with only a few items for the builder/owner to finish.

6. An LSA aircraft may only be modified from its delivered configuration with the manufacturer’s written approval. This includes adding to or changing instrument types on the panel (including changing the radio type), changing any of the installed equipment, even installing bigger (or smaller) tyres. Contrary to popular belief, a CASR Part 21 engineer (previously known as a CAR 35 engineer) cannot legally approve modifications to an LSA.

7. In Australia, LSAs can be either be VH-registered with CASA or 24-registered with RA-Aus – the aircraft are identical, only the paperwork and pilot license requirements are different.

8. CASA-registered LSAs (but not RA-Aus registered LSAs) can be flown in Night VFR conditions, provided they are fitted with the required Night VFR equipment and the pilot has a night rating or higher.

9. Retractable (‘re-positionable’) landing gear is only permitted for amphibious LSAs . Landplanes must have fixed landing gear.

10. The Aeroprakt A22LS Foxbat is an approved LSA aircraft both in USA and Australia. Customer aircraft are registered both with CASA and RA-Aus. Among them in Australia, there are both amphibious and Night VFR rated aircraft.

Auto fuel and Rotax engines

Posted on April 8, 2014 by Scott

Just recently, I have heard stories of rough running Rotax engines. This has been traced on several occasions to problems with the carburettors. It appears that the plastic floats in the carburettors are slowly dissolving in the fuel, causing them to become porous and cease to float! Equally dangerous, small parts of the floats are breaking off and blocking the carburettor jets. I looked at a couple of these floats and sure enough, they were soft and spongy to the touch and there were tiny pieces of black material in the bottoms of the float bowls.

Rotax recommends the use of unleaded automotive fuel – ‘mogas’ – in their 912 series engines. The fuel must have a minimum 95 octane rating for use in the 100hp 912ULS and 912iS models,.

You can also use 100LL aviation fuel – ‘avgas’ – in your 912 if you have to, but you’ll need to change the oil more frequently to help mitigate the effects of lead build up on the valves and other parts of the engine.

All fuel companies use petroleum blends to increase the octane rating of their fuels. Originally the main octane raising additive was lead but this has been phased out for automotive applications, although lead remains in use for aviation fuel. Significant octane-increasing additives in unleaded fuels include ethanol and toluene, both of which work well but they do have their limitations in some applications.

Generally, the more ethanol or toluene, the more the octane rating is increased. However, this is not a golden rule and some high octane ‘low aromatic’ fuels – like BP Ultimate 98 – do not contain ethanol and have a low levels of aromatics such as toluene.

Both ethanol and toluene can affect rubber and plastic components in the fuel system. Toluene also has a high carbon content that may lead to sooty spark plugs – so don’t assume that this indicates an overly-rich mixture.

So it seems some unleaded fuels which contain substantial ethanol and/or toluene levels may be having a negative impact on plastic and rubber components in your engine!

Rotax recommends fuel with no alcohol additive for their engines but has approved the use of up to 10% ethanol in fuel. They do not mention toluene anywhere in their technical information. Your airframe manufacturer may or may not have approved the use of ethanol in any proportion. This is important to note, as the fuel tanks, fuel lines and auxiliary pumps they install could be affected.

My feeling? Stick with fuel that has no alcohol and meets at least the minimum required 95 octane for the 912ULS and 912iS engine. The higher the octane, the more likely it is to include higher levels of additives. Certainly, steer clear of any premium unleaded quoted at 100 octane or at least check if it has ethanol in the product. And maybe it’s a good idea to stick with the well known big brand name fuel distributors rather than fuels from an unknown source, that is, until we get more of a handle on the exact nature of the problem. They may be a cent or two more expensive but in my opinion, that’s a small price to pay for some peace of mind!

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