I have stated here previously that the boundary layer suction maybe requires jet engine for having enough power to be wasted for the suction. However, a knowledgeable friend just sent me couple of (more) links as he has used to do now for quite some time. (Thanks by the way). Interestingly enough on this ppt: www.aoe.vt.edu/~mason/Mason_f/LaminarFlowS04.ppt on page 17 it has been stated that the example case of Piper Super Cub only required 2.0 hp for suction. Another example was Cessna L-19 with 17 hp used for suction.
This is very interesting since taking 2-17 hp out of e.g. 200 total hp (=2 x Rotax 914) is quite doable. With smaller engine power as the previously discussed 2 x HKS700E, the available excess horse power for suction would be obviously smaller and taking 7 hp out of the available thrust would be unwelcome whereas taking only 2 hp out of it would be clearly still within limits of potentially feasible and that benefit outweights the loss.
The achieved Clmax increase with boundary layer suction is significant. If on the Cessna example the Clmax increased from 2.5 to 5.0, that makes a whole lot of difference in wing sizing and in turn this affects drag and efficiency significantly.
The downside is that if the wing sizing is done with the expectation of Clmax of 5.0, and then because of mechanical failure, the suction is not available, the stall speed in such emergency would be high. Also potential failure modes are that the suction disappears on final approach or shortly after takeoff.
How to mitigate this potential problem? The suction mechanism would need to be very reliable and most likely it should be doubled. In other words, in twin engine aircraft, either engine should alone be able to supply enough suction so that in case of engine failure of one suction pump failure, the aircraft would not crash but could still safely land on the airport (with the remaining engine and remaining suction pump).
Another way to mitigate the problem could be to not count on the achieved Clmax but only take the benefit of the drag reduction caused by the suction. There comes the question then of the justification of the added complexity. One of the unknown issue to me is that how water ingesting through the perforated skin would be dealt with - it would be pretty severe condition to have whole suction slot full of water. In addition to the suction not functioning properly, the wing would weight significantly more.
What the complexity adds to the manufacturing cost? For small commercial general aviation aircraft (which is targeted to masses and which does not try to achieve anything special but be good all-arounder) it could add more than is justified for the benefits gained from market - simplicity and low cost manufacturing drive these rather than the last decimals in the efficiency. However, for experimental prototype aircraft which is built on basis that price of a work hour is not counted, at least then this might be a feasible idea to incorporate. This would require more investigation, and it could depend quite much on the aircraft configuration, how much gains this could add and what kind of tradeoffs there are to be expected in turn.
7 comments:
Light (GA) aircraft might be the ones to have the most to gain from boundary layer control.
Savings in drag would offset complexity/cost with use of a smaller engine and less fuel consumption.
Publications from TU Delft mentions power requirements in the 1hp range for a high performance glider. A light plane with 2x airspeed and half the wingspan should have a similar requirement.
Also, the engine needs the air, or it might be used for pressurization.
Thanks for your comment. After all, it may be true that this product segment would get the most benefits.
However, I currently have no idea how to solve the potential issues with water ingesting in the suction slots and getting frozen inside the channels. So would it be somehow solvable also for IFR conditions?
I have thought a bit about that too. I think the problem should be split into several parts:
1. Avoid water ingestion by using the system mainly at cruise altitude.
2. Water that enters the plenum is given a gravity path to a sump where it can be pumped out, even while the suction system operates.
Ice residue may sublimate away if cruise altitude is high enough?
As you already have pointed out, there is a substantial risk involved in usig the a mechanical suction system at critical IAS in landing and take-off. The main part of the fuel consuption and thus savings can be expected to occur in cruise.
So the concept would be like this:
- engine power would be decided by the needed power for takeoff
- on cruise with suction on, the engine could be throttled back very much
- it would be beneficial if the engine was having a good specific fuel consumption figure at the reduced power setting - e.g. Rotax tells the specific fuel consumption for the max continuous power setting which is 95 hp for the 100 hp model and 100 hp for the 914. I am not sure what the figure is with reduced power setting, there are some fuel consumption curves in the Operators' manual, I would need to convert the liters/hour values to SFC values to get the comparison right (assuming the liters/hour values represented in the Rotax Operator's manual are correct and that they are just like that on a constant speed prop).
- I have not seen HKS700E specific fuel consumption curves anywhere.
- would be very interesting to see also a SFC curve of couple of auto engines - they are designed to be run at low power setting, maybe they would have a good SFC at low power. There is some data available about the Eggenfellner H6, but it is using auto ECU which is really bad for aircraft use, it runs in the open loop mode in high power settings which makes the fuel consumption same or potentially even worse than on old Lycosaurs at the given power setting.
I think it would make sense to cruise on only one engine. It would run at a higher percentage of max power, and you would save 50% money on engine overhaul costs.
Airframe and wing might be assymetric to balance thrust at cruise and take-off/climb.
Using one engine in cruise was used in the Burt Rutan's Voyager. It might be difficult to implement that with asymmetric trust since the caused side-slip causes increased drag. At least the Voyager style twin engine configuration would be easily implementable - the forward engine could be shut down for cruise. It could be implemented like on Stemme S10VT, so that there would be a huge spinner and the propeller would get inside when the engine is shut down, the aerodynamics of the forward fuselage could be with only small sacrifices then. Ideally the engine air intakes would be after laminar-turbulent transition. Might be very hard to achieve laminar flow after the discontinuity because of the propeller mechanism between the fuselage and spinner though. The rear engine placement in terms of tail can be easily accomplished by using a twin boom tail. The twin boom tail could also take the benefit of the inverse V-tail which reduces adverse yaw (which is high on high aspect ratio plane otherwise).
Yes, any sideslip must be avoided, at least at cruise; at climb and decent a small slip can be trimmed out. Rutans Boomerang has an asymmetric configuration meant to cancel out unequal thrust from the engines. Feathering one propeller will probably not alter propeller drag much from running both engines at 50%.
(The nose of the Stemme S10 is not a spinner, it is an extended part of the cowling).
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