Concepts of engine actuation
It's not news that the use of electronic components in sailplanes is increasing, and that is especially true in complex systems such as motorgliders with retractable propulsion systems. At this time all the manufacturers of high performance sailplanes in Germany offer one or more variations of the retractable systems. For the electronic actuation of the retractable systems, ignition etc. only two systems are being offered:
The "ILEC" - a system in series production and available on the open market, and the "DEI" (Digital Engine Indicator), an in house developed system of DG Flugzeugbau and Schicke-Electronic, which is used only in DG sailplanes.
The ILEC system presents the necessary information to the pilot in a compact package. It is intended to some degree to "control" the pilot, i.e., to prevent some malfunctions. For instance, the engine can only be started when it is fully extended. But it is still the pilot's responsibility to perform the required steps, such as the switching on/off of the fuel, braking of the engine before retraction, retraction of the engine to its proper stop. The high manual participation is intended to prevent possible technical glitches, following the motto: "What isn't there can't fail".
The DEI system frees the pilot almost completely from these manual tasks in the extension or retraction. For the remote possibility of an electronic failure it contains a complete manual system for braking the propeller, and a manual emergency switch which completely overrides the automatic system.
How do these concepts affect the pilot in practical terms?
Jochen Ewald, the editor of Fliegermagazin wrote a test report for the Ventus 2CM which was equipped with an ILEC system. He describes the procedure for retracting the propulsion system:
"To stop the engine, after switching off fuel, closing the main fuel valve and switching off the ignition one pauses briefly at about 85 km/h, until the propeller stops. Then a pull on the propeller lever, turn it 90 degrees and push it all the way in. At about 95 km/h the propeller moves slowly to its stop position. The extended and stopped propeller has less of a detrimental effect on performance than the previous model with an extended engine. A slight shaking in the tail plane reminds the pilot that there is one more chore to do before the Ventus becomes a high performance sailplane. He has to depress the retraction switch to is full stop, until an indicator light signals the end of the retraction procedure in about eight seconds."
The same Jochen Ewald had tested a new DG-808B and described the same procedure:
"For the pilot everything remains as simple as the DG Motor Gliders always were. The entire procedure is executed by the electronic DEI mechanism. Main switch, Ignition and throttle with an integrated starter button - that is all the DG pilot has to be concerned with. The shutting down of the engine and retraction is without problems and as usual: Ignition off, when the propeller stops depress the starter button until the propeller can be seen as nearly vertical in the rearview mirror. The airflow pushes it the last bit until the propeller brake holds it and retraction takes place. There is no noticeably great difference in performance. The performance and flight characteristics even with a stopped propeller are considerably better than previous models with external engines".
In actual use the retraction could be described as follows:
"When the desired altitude is reached the pilot switches off the ignition, and concentrates on the thermal he found".
Everything else is fully automatic - including the vertical positioning of the propeller, and it's braking before retraction.
Only during hot weather conditions - ground temperature of more than 25 degrees C - we recommend a little cooling phase. After retracting the engine press the emergency switch up for one second, fly 5 minutes and press it down again.
To extend the propeller in flight it is only necessary to switch on the ignition and to depress the starter button. When the system is completely extended the engine starts.
To complete this report here is a description of what a DG pilot has to do in the event of an electronic system failure.
After switching off the ignition the propeller is slowed with the manual brake and slowly comes to the vertical position. This can be observed in the rearview mirror. The exact vertical position is also shown by an indicator light on the instrument panel. The extended propeller can then be retracted by the emergency switch. Even the emergency retraction is quite unspectacular. A manual propeller brake is built into every DG-808B, but is only used in emergency retractions.
The DEI system contains a number of functions which are missing in the ILEC system:
- The starter cannot be actuated if the ignition is off.
- The ignition actuates the extension/retraction spindle drive motor and also the fuel and cooling water pumps, normally by battery but in case of battery failure directly from the generator.
- For a simple retraction of the engine the starter is supplied with pulsated electric power. So the propeller slowly turns into a vertical position. In this way you even - if necessary - can retract the propeller on the ground without leaving the cockpit.
- If the starter is switched on via high performance transistors electrical peak charges are basically eliminated and therefore protect electronic equipment.
- Optional there is no choke or manual primer pump. The primer is still being controlled automatically. The pilot does not need to manually control the Primer or the choke flap, especially not with a rubber ball!
- In the event of vapor buildup in the carburetor, the primer can however be applied manually for one second, when you push the starter button while the engine is running. Try that out!! It is great!
- Besides the gauges for fuel, RpM and water temperature there are warning lights for fuel reserve, excess RpM of water temperature and two engine time counters.
- The main fuel valve is closed automatically on retraction, and the DEI system also has an automatic shut off of the pump when fueling the sailplane. No more fuel spills.
- Additional there is an automatic calibration of the fuel indicator.
The DEI system obviously requires considerably less effort by the pilot, and still does not make him wholly dependent of electronics because of the redundant manual system.
Safety Recommendation:
The automatic engine-stowing system must not lead to your forgetting how to use the manual system. To this end, we make the following suggestion:
Every so often at the end of a self-launch, open the red cover of the automatic propeller stowage control and turn off the propeller brake. Then retract the engine using the hand brake and the emergency switch just as in the “good old days” of the DG-400. You will soon notice that this requires three hands.
For the new DG-808B-System there is not red cover any more. Only touch the emergency switch for a second. It will cut the electronics and you can practice the retraction system by hand.
In any case, it's a good exercise!
Can you fully retract the propeller without using the hand brake?
Using the following technique, it is possible:
Let the propeller windmill (or use the starter) to a position just before vertical. It has then passed the motor compartment doors.
Use the emergency switch to lower the motor a little past half-way. That is when you cannot see the propeller tip in the mirror any more.
Wait until the propeller rotates slowly and the LED goes out. That takes a bit longer because at this position, the windmill effect is very weak.
When the LED goes out, lower the propeller all the way down by using the emergency switch.
This method is actually quite simple. Once you master this method, the use of the propeller brake that we have installed becomes unnecessary. You also no longer require a " third hand".
You should practice this technique also from time to time.
Even airline pilots practice emergency procedures even though they seldom happen.
What do you do if the motor won't retract no matter what the reason?
Extend the motor, start it and fly home or to the nearest airport under power!
Land with or without the engine running.
- K.-F. Weber
and W. Dirks -
About the "required" Engine Power:
"It's a crazy situation: We are all soaring pilots, but some people act as if engine power of, for instance, 40 hp was insufficient to start a motor glider safely." This was said by another sailplane manufacturer, and I will admit that he is right. I use the engine as little as possible, and of course one can fly with less power - especially in soaring flight!
On the other hand there are situations where power is really important:
At very high altitudes, when crossing passes or at high altitude and short starting runways.
For me it is also important for a different reason: I can shut off the unloved but essential "noise box" much faster, and enjoy the quiet soaring flight. And if the higher powered engine is available, why not order it?
Permit us a small tip: Two other manufacturers have switched to the motor we have used for the past years.
(There are no longer any airplanes with single ignition being manufactured. The differences in the engine controls remain.)
The exhaust system in a two cycle engine is very important for the performance. According to measurements by the SOLO manufacturer the exhaust system designed by us brings two to three hp more than that of our competitors. This is why we can rightfully say, regardless of the statements in the prospectus:
The DG-808B is still the highest powered sailplane in it's class.
Single or Dual Fuel Delivery:
In the serial production of the DG-400 an electric fuel pump was used, as well as an additional pneumatic pump. In actual use the electric pump never failed, but the pneumatic pump plugged up and failed several times. For this reason the pneumatic pump was eliminated from the first models of the DG-800B.
But this meant that a very important part existed only once and it's failure would mean an engine failure. Even the blowing of a fuse on the DEI or a short circuit would stop the pump.
Result:
Every new DG-808B has two electric fuel pumps, with the second one powered by the generator - isolated from the other electrical system of the sailplane. This means additional security which does not need the pilot's attention, but does increase the production costs. The second fuel pump is not even required by the German Airworthiness Authorities.
But it contributes to increased overall safety, and that was our intent.
Faster Extension of the Propeller:
Several time pilots have asked why the extension of the propeller "takes so long" . Could the measured time of 13 seconds not be shortened? If one is flying on a ridge and the lift suddenly quits one needs the engine right away.
After hearing this question several times I was quite ready to do something about it and talked to our chief designer Wilhelm Dirks, who raised several points:
-
Doubling the speed of the spindle at half power does not work, because at the beginning of the cycle the propeller doors must be opened, which takes full available power.
-
It is required that the propeller can still be reliably extended at a speed of 120 km/h. In real use this means that even at 140 km/h the system must still extend. But at that speed the wind pressure on the large prop is substantial, which does not exist at ground level. The extension mechanism therefore must have quite considerable power reserves.
-
Of course one could make the spindle drive larger and stronger. And of course one could dampen the increased switching power by additional electronic regulators to prevent the cables from overheating.
-
But we have intentionally persued a safety concept which offers an additional manual extension/retraction system. And the battery would be strained at , for instance, 60 amps
-
What are we really talking about? The DG-808C has a sink rate of about 0.6 m/sek with retracted and 2.0 m/sek with extended propeller, with the average extension time perhaps 1.3 m/sek. If in an extreme case one could shorten the time from 13 to 3 seconds one would save 13 m in altitude. Is this worth discussing - who would try to extend the prop at 30 m altitude?
-
The DG-808C has another advantage of much more importance. The pilot cannot make very many mistakes in starting the engine. That is where most of the accidents happened in the past. Fly too low, extend the propulsion system, panic to make a stupid mistake, and then the engine does not start. End of the line! But in the DG-808C, because of it's automatic features, the pilot cannot
-
actuate the starter if the prop is not fully extended,
-
he can't use the starter if the ignition is not on,
-
or use too much choke on a warm engine and flood the engine,
-
or retract the choke too soon so that only one cylinder fires
-
etc. etc.
-
The security of the automatic engine controls surely is worth more than 13 m less altitude loss when the prop is extended.
Let us repeat our safety tips:
Please fly so as not to be dependent on the engine. Every mechanism can fail at some time, and according to "Murphy's Law" will do so at the most inopportune moment. So please start your engine in time!
Automatic Primer
Let me clarify the principle: Every car driver knows that when cold starting a motor the mixture must be enriched and with increasing temperature the mixture should be enriched less.
Normally one uses the choke to reduce the amount of air in the mixture.
This system was used in the DG-400 and worked without any problems if the pilot used the choke properly. The difficulty with this system is simple (particularly with air starts).
The pilot must estimate how much to use the choke. It becomes a difficult problem when the pilot is threatened with an outlanding and when it becomes hectic in the cockpit. In these cases the pilot often uses too much choke and floods the motor. Now he needs patience until the excess fuel is used up in the cylinders. And now is when he probably does not have the extra time.
Instead of a choke one can use a primer. That is simply a tube in the carburetor through which extra fuel can be pumped directly into the mixing chamber. In a rather primitive way, one can make such a pump mechanism with a rubber ball of the same type as used by a barber to moisten your hair. However in an auto, one would not dare offer such a simple solution for that problem.
With our Automatic Primer everything works by itself.
The DEI measures the coolant temperature and delivers the correct amount of additional fuel. The electric primer valve opens the proper amount of time and allows the correct amount of fuel to flow directly into the carburetor during the starting procedure. The fuel is also run through a fine mesh filter to filter out particles that could stop up the primer orifice. The amount of fuel that should be added at different motor temperatures can be set into the DEI if you are not satisfied with the factory settings. This might be necessary for high altitude airports, etc.
Finally, you can get a bad behavior of two cycle engines under control:
They start when they are warm without use of a choke or primer but run for only a few seconds.
If you push the start button briefly with running engine, (yes this is possible with DG-motorgliders!) the Automatic Primer squirts in a little bit of fuel and the motor keeps on running when it otherwise might have quit.
With our Automatic Primer there is no need for either a choke or primer pump and the pilot need not concern himself with a mixture problem. The primer delivers the optimal amount of fuel at any temperature to the engine.
Conclusion:
In developing the propulsion system we tried to build in a maximum of comfort and safety. But all these measures increase the production costs and the product price. Is this the proper concept?
A purist may wish to forego some comfort.
But also the safety component .......?
Customer enquiry regarding retractable engines:
Dear Sirs,
I have a fundamental question regarding retractable engines:
Why can't they be installed such that they can be started in the fuselage before the boom is extended?
This would give the advantage that the pilot could start the motor sooner in a critical situation, before it possibly becomes too late. If it starts, I can delay the cut off point a little. If it doesn't, a safer outlanding should be possible.
Turbos would require a starter, but I'd rather fly around with that than have to extend the motor (too low) and hope that it starts (and if not find a friendly repair shop).
Yours Sincerely
Andreas Junginger.
Short answer:
There are two disadvantages:
1. You would need some kind of de-coupling device, which would cost both money and weight. It would also be susceptible to damage. In effect, you could only really consider a centrifugal clutch for the job.
2.A direct connection between the motor and the propeller is desirable so that you can also start the motor by diving. The most common reasons for a motor not starting in flight are a malfunction of the starter motor or flooding of the carburetor. Both problems can be overcome by a dive start.
Best Regards,
Wilhelm Dirks
Chief of Design
Manfred Koethe from Boston, a former member of Akaflieg, wrote:
Dear Friedel,
When designing the AK-2 at the Akaflieg Karlsruhe, we considered the separate start of the engine. However, we discarded this idea quickly.
The original design of the AK-2 (~1975) had the engine stationary inside the fuselage right behind the cockpit. Only the propeller and propeller tower were extended. The coupling between the cogbelt drive and the engine crankshaft utilized a modified shifter set from a truck transmission. We had hoped the synchronizer would be strong enough to get the propeller moving. However, this was too optimistic.
The whole power concept of the AK-2 was too advanced for its time and engine technology was not ready to realize it. Therefore the AK-2 was never flown as a motor glider.
The power plant concept of DG with the fixed connection between propeller drive and engine, and with the engine rotated inside the fuselage is very elegant and, in particular, much more reliable.
This should not be touched...
Kind regards,
Manfred
Other Articles of Interest: | |
| |
| |
| |