An article from Barry Wrenford (wrenford@snowy.net.au ) about a propeller and CS regulator for the Europa and a Rotax 914.
THE EUROPA AND CONSTANT SPEED PROPELLERS
Description and Flight tests
GENERAL
The Europa with the Rotax 914 Turbo has such a wide speed range, from 43 to 165 knots, that a fixed pitch propeller cannot hope to give the full performance of the aircraft
Cockpit adjustable propellers do fulfil the need, but experience has shown that you can go nuts in the slippery Europa, trying to keep revs within limits for all except constant speed cruising. I counted over 20 manual adjustments being required for a brief circuit of the airfield.
The problem arises not only with each power and attitude adjustment, but with the changing airspeeds involved in this. ie You increase the power and the revs increase. You compensate, and then the revs increase more as the aircraft accelerates. You have to continually adjust the revs during this period. Every change to the aircraft attitude requires adjustments, until it has stabilised. In addition when you climb, the engine revs increase by some 200rpm for each 1000ft. This is because the propeller loses bite in the less dense air, while the turbo maintains engine power. This of course requires rev adjustment both on climb and descent. Taken all together, the constant adjustments become rather tiring.
Now there is a little inexpensive black box that does all this for you, called the Propcon.......
THE PROPELLER SYSTEMS
Fitted to my Europa now is a Rospeller Hub with a rather mean set of near feathering Scimitar carbon fibre blades and a Propcon constant speed controller, all supplied by Rospeller-Aero in Germany. Rospeller-Aero is a small private company owned by an ex Luftwaffe engineer. They have designed and tested a propeller system, which has been verified by the Stuttgart Aeronautical Institute, and which has a design safety factor of 3x for the blades and assembly. Strength tests resulted in the machine collapsing, not the propeller.
The pitch control mechanism for the propeller is a motor driven pushrod, operating through the hollow drive shaft in the Rotax gearbox to the propeller hub. The Propcon speed controller is linked to the engine RPM sensor, and feeds inputs into the electric motor operating the pushrod.
With the engine running you simply dial in the revs that you want for the engine, and they are automatically controlled at all power settings within a 100rpm, as far as the pitch stops allow. The Propcon also allows for manual pitch control, which you may wish to use under some conditions.
The Propcon is a tiny box that fits into a 2 1/4” panel cutout with a switch on top which allow you to set auto or manual for the pitch control, a bottom on-off-on momentary switch which controls the pitch directly in manual mode and which sets the revs you want if in auto. The upper readout shows the actual engine revs and the lower readout the revs that you want in the auto mode. These only activate when the engine is running. The default rev setting is 5024 rpm when turned on. See pic.
Rospeller supply an assortment of blades and shapes apart from the scimitar blades tested, which include special 2 bladed props for the 912 and 912S engines, with both glass sheathed foam cored, and solid carbon fibre blades.
For those unfamiliar with the Rotax 914 turbo, you set the desired power using the manifold pressure gauge. ie 100HP as max continuous power setting is set with 35” of boost on the gauge. This power setting has a recommended rev setting of 5500rpm. Each lower power setting has its own reduced recommended revs for the engine. If the power is to be increased, the Propcon “auto” setting for the recommended revs should be set first, followed by the power, and vice versa.
The following is both in flight comments, and some comparisons of prop performance against the cockpit adjustable NSI hub withWarp Drive blades.
The Rospeller scimitar blades and the Warp Drive parallel blades are opposites. The scimitar blades are very large in area while the Warp Drive are really too narrow for the the 914, especially in high density altitude conditions. Here the blade area appears too small to efficiently absorb the power of the 914, and would be better suited for the 912S. The fine pitch stops for the Warp Drive has to be set at 20° at the tips, which is quite unusually coarse, and results in a slippage/stalling at my 1000M altitude, where the engine overspeeds at 40 knots during the takeoff run and then slows back until 80 knots. This means that I could never get full revs for max power for the takeoff/climb, as I had to set the prop even coarser to stop rpm overspeed. The Rospeller blades bite and give far more thrust during these initial stages of takeoff.
TAKEOFF
Part of the cockpit check is to preset the revs to a little less than 5800 rpm (5700) to allow for control tolerances. The power should be fed in slowly to minimise the use of right rudder to offset the engine torque. If you don’t, the engine power for such a light, short aircraft can cause quite a powerful tendency to swing to the left, and all the more so if you should raise the tail quickly. Here you add gyroscopic forces which require considerable force to the rudder to conteract. If the power is fed in progressively as the speed increases and the tail left to fly itself up, the takeoff is quite smooth and without drama. When full power of 40” boost is added, there is a slight delay before the turbo cuts in, when a strong surge of extra acceleration boots you in the back and you are airborne almost immediately.
CLIMB
You climb at 65 knots at about 800 - 1000 fpm depending on load and density altitude, until you raise the flaps and accelerate to 75 knots which will give you 1200 to 1600 fpm again depending on load, temperature, and humidity.
When solo the transition from flapped climb to unflapped is a real eyeopener when first experienced. The nose attitude with flaps is conventional, and when they are raised the aircraft sort of hesitates while it builds up speed, then you raise the nose to hold the speed to 75 knots but the speed still builds up, so you raise the nose more.... and more... until the angle looks impossible and you still have 80 knots. Then you retrim and get the nose higher and the rate of climb just goes from 1500 fpm up to as much as 1800 fpm and you just seem to be laying on the back of the seat with nothing but sky in front.
The aircraft just seems to hang under the Rospeller prop which efficiently uses all of the 115HP of the Rotax engine at these lower speeds to achieve maximum climb.
You don’t have to climb at this rate, and the power may be backed off to 35” boost, and 5500 rpm set with the Propcon, depending upon your urgency to get up there. For cruise climb you can just accelerate to 100 knots with the prop holding 5500 rpm, which still gives up to 1000 fpm and keeps the engine cooler as a bonus.
CRUISE
If you are really in a hurry to get there, you can just level out at this same power setting without touching the engine controls, and the aircraft will accelerate to 150 - 160+ knots at 5000ft depending on ambient conditions. The NSI system surprisingly gives a similar turn of speed, where the small blade area is working more efficiently.
I have found a wide variation in cruise speeds at various settings with the turbo under different outside conditions. The turbo at high power can heat up the intake air up to 40C, so when conditions are near freezing the aircraft behaves like the scalded cat. When the air temperature is high, or in conditions of high humidity when the air is less dense the cruise speeds are often some 10 knots less. Of course wing lift and prop efficiency are less too, so it is not all loss of engine performance.
In summer I generally achieve 140 knots with 31” boost/5000rpm (75HP @ 18 litres/hr) and which is a good compromise for range when wanting to go places quickly. In winter this lifts to 150 knots. Otherwise a summer dawdle cruise is around 125 -130 knots with 28” - 29” boost/4300 - 4600 rpm using about 14 litres/hr. It is very quiet at these revs. Out of interest I found that when I had to purchase Avgas, the fuel consumption at 31” boost rises to 20 litres/hr. Enquiries confirmed this as apparently the combustion energy from Premium unleaded is higher than for Avgas.
During flight you will find that because the drag is so low, very small changes to the nose attitude can make marked changes to the airspeed, so it has to be trimmed and flown carefully compared to more conventional aircraft.
When cruising in stronger turbulence with convection, speed and engine revs vary quite a bit with the lift and sink encountered. The Propcon does seem to work a bit here to hold the revs, and I wonder that in the interests of longevity, the Propcon could be switched to manual in these conditions.
DESCENT
Descent in the aircraft requires some care. My first powered descent nearly put me through the VNE, as when the power is reduced the aircraft accelerates. This is because the nose drops upon reduction of power, and while it doesn’t look enough to worry about it sure has an effect.
I usually set the Propcon for 4300 rpm when letting down and reduce the power to 28” or less depending upon required descent rate. It is surprising just how much the power has to be reduced to get a reasonable descent angle. At around 27” - 28”you can achieve around 300 ft/min (3 knots) at 150 knots and have a glide angle to target of 50:1 which is equal to an ultra high performing sailplane. From 6500ft you then need 50 nm to get down to ground level! So with a bit of planning you can pick up quite a bit of speed on the descent AND save fuel at the same time.
CIRCUITS AND LANDINGS
Slowing down in the circuit for the first time with the NSI prop was a revelation. My first time I did 2 circuits around the strip before I could get the speed down enough to lower the flaps. The reason was the slight nose drop on power reduction, coupled with a slow descent, low prop area, and not enough reduction of power. A little nose down attitude doesn’t give much rate of descent, but by golly it gives a boost to the speed, specially at lower speeds where the drag is minimal.
My first landing/test flight with the NSI prop was in light variable winds where the ASI had a 20 knot error, and I had to use the GPS as a guide. For safety I came in a bit fast, maybe 65 - 70 knots and floated for nearly 800 metres before touchdown. Later analysis showed that the aircraft is too clean with the fowler flaps and really needs a lower flap position. In addition the small area of the NSI blades at idle does little in the way of providing drag.
With the flaps down, on the approach in turbulent conditions, it is very slippery and the speed varies considerably, and requires more than the usual attention to the ASI.
The Rospeller with its large area can be used as a very efficient airbrake if the power is reduced sufficiently. In addition it acts to significantly steepen the glide path at idle on the approach, and shortens the landing run as well. Later tests gave about 400ft/min flapless descent rate on the idle with NSI, and 600ft/min for the Rospeller.
So, in the circuit one can now go fast on the downwind leg,
cut the power way back for rapid braking, raise the nose to
maintain height and as the ASI passes below 100 knots, reset the
Propcon to 5700 rpm in case of a go around. From here on you just
use the throttle to adjust the descent rate and let the revs take
care of themselves. With low power settings and low speeds the
revs can go way back below 4000rpm and will only come back up
with the application of power.
Others have commented upon how quiet the propeller is, however I have always been in the aircraft and cannot comment upon this.
PROPELLER VIBRATIONS
The NSI system gave unacceptable vibrations in flight, occurring in harmonics at various rev settings. NSI generously replaced the first set of Warp Drive blades, but which only made the problem worse, and also gave 1” of fore-aft flutter of the blades at higher temperatures. Both sets of blades were dynamically balanced, requiring substantial (28grm) weights fitted at the spinner plate perimeter. I was advised by a LAME not to continue flying with this prop system and it has been returned for a promised refund, which after 5 months is still outstanding. The Rospeller required no weights at all, showing a reading of 0.1 on the dynamic balance, which is virtually perfect.
Another local builder with the NSI system has not had this vibration problem at all, yet after 2 sets of blades and even another gearbox trial on the Rotax 914 failed to cure it.
While one cannot be certain as to the reason for the vibrations/flutter, it is first of all apparent that the higher the temperature of the blades, the more the vibration and flutter occurs. As the Warp Drive blades are carbon fibre and I guess epoxy resin, they should become less rigid and wave around more as the temperature rises. In this case I suspect that the blades are not uniform along their length either in rigidity or in density. In the latter case the CG of each blade may differ. It seems also that Rotax gearboxes by design also contribute to harshness in propellers, and that in some minor cases the addition of the slipping clutch has helped.
Enquiries also indicate that the use of wooden cores in props may help to damp these vibrations and are not affected by temperature. Also that the use of foam cored composite fibreglass props also damps vibrations. Rospeller comments were as follows - The general advantage of composite blades is their dampening effect compared to the much harder carbon material blades. They run smoother - but - are more critical when not properly adjusted one to the other.
INFORMATION
I asked Rospeller to provide information on their props for other builders, and their answer is as follows:-
We have carbon fibre blades in a standard shape or the Scimitar type, AND composite blades with a foam core.
These glass resin blades are used on the Fascination UL in Germany in almost 150 pcs.
Due to the more flexible character of the glass-resin material it could be probably that what you are looking for. These blades have an excellent finish - they appear polished - and are available in different colors, but towards the root they are much broader than the carbon blades. They also are much twisted and have a maximum width of 18cm at 1/3 from blade root for engine cooling reasons - and are very good for low revolutions too - due to their significant rounded profile shape. They can be shortened for a small additional sum.
I recommend them as well as the carbon blades, in a two bladed version for the 912S-engine. The two bladed prop is quite a bit lower in price as well.
Please see our webside Rospeller-aero.com for the photos of these blades.
The cost to order direct from the factory are as follows:-
2 bladed VPP with carbon or composite blades, complete EUR 3054,-
3 bladed VPP with carbon or composite blades, complete EUR 3654,-
PropCon constant speed regulation EUR 400,-
Composite spinner with aluminium bulkhead EUR 150,-
Shipping costs per prop about EUR 125,-
You can see that you have a wide choice of blades for your own aircraft. For the Europa with its smaller 64” diameter propellers, the scimitar blades work fine with the 914, giving great bite and efficiency in the takeoff and climb with no losses in the cruise. These would be too much for a 912S where I would suggest the 2 bladed composite setup. Rotax do have a recommended maximum for the moment of inertia of fitted propellers and hubs, and my impression is that lighter is better, and less critical in the balancing. Hence the 2 bladed prop. For the 914 you do need a larger area to absorb the power of the engine at an efficient angle of attack on the blades, and you really need the 3 blades to achieve this.
Barry Wrenford - Australia - Builder 308