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Discussion in 'Aircraft Design / Aerodynamics / New Technology' started by Vigilant1, Nov 13, 2018.
Folding props have rarely worked on IC engines (from the torque pulses).
And the prices I saw for the powered sailplane models were about $5k each. But I'm sure they are very nice.
I wonder if instead of the complexity of a folding or feathering prop, you could just have a really skinny one like a Warp Drive? Those blades are narrow enough that when stopped they have to be like 90% of the way to a fully feathered prop.
Stopping a spinning propeller is not optional it is mandatory on a multiengine aircraft. On a small aircraft like the Micromaster with fixed pitch propellers a simple device like a brake for example is most likely an adequate solution. Driving the aircraft for extended distance after loss of one engine even if possible against a spinning propeller is inadvisable do to the possibility of insufficient lubrication to the dead engines crankshaft and the possibility of sudden engine stoppage and inertial forces separating the propeller from the crankshaft.
I have watched the oil pressure drop and the oil temperature rise on many occasions. At some point it is mandatory to stop the propeller from spinning or the crankshaft will seize and the inertia of the propeller may be catastrophic (also with a oil controlled feathering prop if the oil gets to hot it will be to thin for the feather pump to produce adequate pressure to force the propeller into the feather position) and again a spinning prop producing no power is akin to putting a piece of plywood in front of the engine of the same dia.as the prop.
There is a procedure in the event that a propeller cannot be feathered it can be placed into "flat pitch", not as good as feathered but much better than spinning (it will not remove itself from the aircraft in flat pitch in addition to the drag reduction)
With little excess hp available on a twin like the Micromaster the additional drag of a spinning propeller is likely to void much of the benefit of having a second engine, as someone who has experienced that drag phenonium let me tell you that you immediately have no doubt which engine is the trouble maker and initiating proper procedure to feather or stop rotation of the problem child to reduce drag will be decisive.
Relying on the idea that a engine "may" stop spinning on it's own even a small engine like a 810cc is in my opinion "folly". If you know it will stop spinning on it's own each and every time that is another matter entirely, but otherwise a method of securing a dead engines propeller from spinning is mandatory for any hope of success in flying the aircraft on the remaining engine from critical situation like one engine failure shortly after rotation in my opinion. And there is always that the spinning propeller on a dead engine not receiving proper lubrication and having the crankshaft seize could separate from the aircraft going in god only knows which direction possibly causing additional damage while also causing a change in weight and balance which depending on design factors may be more or less of a problem as well.
"Yes" to this and all that followed. I think we can get useful info from the folks now flying similar setups (direct drive V-Twins with similar props). Also, in the process of getting a good baffling system worked out (for both the front and rear engine) I would imagine at some point there will be a test rig towed behind a trailer, and if the breeze is 70 MPH back there (measured at the prop) we'll get an easy opportunity to see if the prop will spin. Finally, with a twin that can be restarted in flight, doing a "no kidding" flight check near a nice runway at various speeds should be low risk.
The main thing is stopping the prop. But, if we must have a brake of some kind, I wonder if it is worth trying to get the prop stopped at a particular spot. Getting it stopped at 3/9 o'clock would reduce chances of breaking it on landing. If we end up with a high wing Micromaster, stopping the rear prop at 12/6 o'clock would get it out of the high-speed downwash from the wing and maybe reduce drag a little. >If< such indexing can be done without any loss of of reliability or increased pilot workload or weight, it would be worth thinking about.
Usually the prop stops only when slowed to near stall speed. I have done it hundreds of times in the Grob motorglider. Once with my Aeronca Chief.
Often after feathering an engine it would still rotate very slowly, fortunately usually it was slightly "overfeathered" and gently and carefully pulling on the feather button would move the blades slightly and they would stop completely. Attitudes varied about this of course some wanted the prop stopped, others felt the slow rotation wasn't as bad as messing with the feather pump.
Anyway I believe that an engine that hasn't suffered catastrophic failure and already stopped itself will rotate if the brake is eased off a bit and you would be able to clock it in any position you wanted, once you were in a comfortable position/situation to "play" with it.
Sometimes when an engine fails you are busy, flying the airplane, terrain, traffic, communication etc and were the blades are clocked is going to be down the list of priorities for a while. Other times an engine will fail at altitude in cruise and you'll be more comfortable playing with such things as blade position.
Keeping the Microlight as simple as possible is a key element in its ability to be successful and although a engine/propeller brake of some kind is an additional complication it is a necessary one however it should kept be as simple as possible.
Another thing unrelated to propellers that may fly in the face of simplicity to some is my continued support of EFI but I believe it is, considering 2019 electronics simpler than carburation in regards to functionality and reliability. Additionally and especially in a multi engine environment it reduces pilot workload regarding engine management providing additional time for other tasks.
Minimal required pilot attention should be a goal. That's why I think a brake arm under spring pressure but that is held back by a pin attached to a cable to a handle in the cockpit makes sense. When the pilot pulls the cable/pin, the brake is applied. Done, no more attention required. If the spring tension is right, it will stop the prop. If we're worried about the possibility of pulling the wrong handle, we can make the thing resettable (adds weight) or just choose a brake pad material that will wear down to nothing in a reasonable period if the engine is turning under power. If we want to stop the prop at a particular spot, we add a "catch" on our brake arm (e.g. maybe a ball under spring pressure that clicks into a shallow hole in the flywheel every time it goes around, or a paddle that clicks over a peg on the flywheel, like a Chuck-a-luck wheel), when the prop gets slow enough it will stop against the catch. Again, no pilot action required after the decision is made to stop the prop.
It can. But, once the plane's electrical system becomes critical to flight, then we need 2 independent ones to maintain our goal of 2 engines with no possibility of a single systems-related total loss of power. "Front Main Bus", "Rear Main Bus," two batteries, bulletproof isolation if we want flexiblity to power systems from either bus, etc. It's just more complex. And as far as simplicity of maintenance, looking at the B&S EFI guide doesn't fill me with hope. What are the values the system puts in when a sensor fails? But preventing carb ice is worth special attention--it could put both engines out at once.
I plan to keep the stock electrical system to each engine including an appropriate sized battery so that each engine remains independent electrically. Additionally each engine electrical system will be breaker protected and supplied with a switch to the aircraft electrical common buss. No engine systems may be on the buss if all switches are off. The buss will have it's own battery with breaker protection. The buss battery will have capacity to run all systems on the buss for at least the legal requirement of time in an emergency. Any engine system may be switched on to the buss to charge the buss battery and if a problem develops in that charging system for any reason, switched off (additionally the line is breaker protected). The buss battery will continue to keep buss systems on, and another engine system can be switched on to the buss or not at the pilots discretion. If a complete buss system meltdown occurs the engine systems are breaker protected from the buss and further no more than one engine charging system needs to be on the buss at any one time therefor ensuring that at least those engines not connected to the buss should continue running unaffected and the system charging the buss is likely to be unaffected as it has protection. Individual electrical engine instrumentation would be powered and grounded to its respective engine and would not share buss power or grounding. Considering this is a VFR machine buss system meltdown likely means I am navigating by pilotage (oh my!) and if landing at a towered field will have to recall my light gun signals (I always had them on a little card affixed were I could see them even though I never used them). Batteries today and especially for a system of this size need not be of excessive weight.
My problem with factory EFI is that most of these engines come set up very lean to meet EPA limits. If you hang a prop it and try to coax more hp out of it you are in uncharted territory.
I wish N8053H would jump in here and on the other thread as he is a certified B&S Tech IIRC.
I plan to not coax more hp out of it. 810cc approx. = 49 cu. in., 28hp/49ci = .57hp per ci. That is a significant ratio. While peak hp is at 3600RPM peak torque is at 2600RPM
and is about 21hp, Considering runway lengths and etc., it would be rare to need full 3600RPM/28hp from each of multiple engines. Reduced power take-offs would likely be safe as 3100RPM/25hp approx. for two engines would be 50hp. Considering the plane in question is being designed to fly with an engine out, reduced power for normal operation would be considerably more than sufficient and reduce stress on the engines. HP costs and it ain't just money, it's reliability as well. Reduced power for take-off will likely add many hours to the life of the engines for a very small price in performance.
That sounds good, and it's clear you've thought this through. Some thoughts:
1) Contagion. I'd think a voltage spike or other "unclean" power (voltage spikes, noise, etc) could take out any electronics (to include EFI boards) exposed to it, and it won't be stopped by a breaker. So, the two EFI systems should never be connected to a common bus or each other.
2) Maybe during critical phases of flight you'd isolate the three systems? If so, your main bus (i.e. not the engine specific busses) would need to run on battery for a few minutes on TO and landing. Not a problem, just another thing.
3) Simple? Well, it's way simpler than the diagrams and interdependencies of many certified aircraft. Still, if I'm trying to make the case to a DAR that this plane is " a lot like a single, only safer, no need for a multiengine endorsement" . . .then I'd probably use a hanky to cover that bank of breakers and if-then switches and hope the subject turns to baseball.
There are a couple of ways to "trick" these systems into run richer than originally designed. Its often as simple as putting a couple of resistors in the right wire.
These use the old school .1V-1V O2 sensors. A voltage divider can offset the output to make the engine run richer.
A similar strategy can be used to offset the thermistors that measure intake and head temperature.
There are some other tweaks that might be useful when converting to aircraft use. I think we are going to find that these EFI units are more reliable than the carburetors. That doesn't mean that they are suitable for aircraft.
Edit: Reading a bit further in the B+S manual it says that if the crank position sensor fails the engine quits. This is unacceptable, IMHO, for aircraft. It doesn't say if this failure only shuts off the ignition or if it also shuts off the injectors. Adding a limp mode if the crank sensor fails could get involved.
You could make it simpler … kind of. Two buss batteries. If you could run the buss on one or both buss batteries, then taking a battery off the buss and putting it back on is not a problem. It is then just a matter of taking a battery off-line charging it, putting it back on-line, taking the second battery off-line yada yada. Not very sophisticated but in it's own way simple enough.
I agree there is no evidence to support these systems in flight. I would argue they are probably going to be OK up to 5 or 6 thousand feet because people run lawn mowers in Denver. Further I volunteer Vigiant1 to test at higher altitudes.
I would put a resistor in between the batteries and the circuitry so the circuitry doesn't get voltage spiked when putting a freshly charged battery online.
In the famous words of the platoon sergeant: " I need a volunteer for a routine recce mission. Somebody who doesn't owe me any money."
Another simple way to do electronics is buy one of those little tractor alternators for about $50 that POPS pointed out for the VW's and mount it the mower blade side of one of the engines with a pulley and belt, then it has it's own battery for the instrument panel, and your done. Start engine on its own electrical system, engine powers separate electrical system for radio's and lights and transponder and miscellaneous, it has backup battery power to support the electric components for a legal amount of time in leu of alternator failure. And the original engine electrical systems remain virgin.
Worst is you are pilotage and light gun signals. And reality is how likely is it to happen that you lose alternator and completely run down the battery before you land?
Put the alternator and extra batt. on whichever side of the W&B teeter totter you think you need the most help with.
The alternator grounding wire and power would have to be run shielded to the battery isolated from the engine compartment.
Could you use twin pusher engines with folding props, this is so simple; engine stops, blades fold; engine starts blades unfold.
Each blade is hinged, that's the total mechanics of the system.
The company's statement:
"The propeller is a three-bladed design which folds by aerodynamic drag when is engine stop in flight, extending by centrifugal effect on engine startup."
That type of folding prop can also he used in tractor installations. Food for thought....
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