Questions about gear speed and flying fixed gear

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pantdino

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As some of you are aware, my Titan T-51 had a gear collapse on initial touchdown -> pilot aborted landing to try to resolve it -> which led to a loss of power because of a bad electrical circuit design and subsequent landing with gear collapse.
In other words, the root cause of the incicent was the failure of the main gear to lock down. It seems to me the main gear in general on this airplane are of borderline design and strength, so I am interested in what it would take to just fly fixed gear. No, it doesn't look as cool, there's more drag, and you won't be able to go as fast, but the trade-off would be worth it to me.

My question revolves around the "gear speed" on a retractable aircraft. On this plane its about 100mph, and the plane lands at about 70. So that's too narrow a margin to fly around with. But fixed gear airplanes fly a lot faster than 100mph, so it must be possible to make the gear strong enough to withstand the forces exerted on them during flight.

Is the "gear speed" limited by the vulnerability to damage during the transition more than their locked down position?

I assume the major force on an extended gear leg would be a horizontal one pointed rearward, from the airflow on the wheel and tire. Correct? Unless you're going to be doing aerobatics I would think the other forces would be minimal in comparison.

What changes would one have to make to make the gear strong enough to withstand, say, 160mph, which is the cruising speed of this aircraft?

Thank you
Jim
 

wsimpso1

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Retractable landing gear are only intended to be used at the lower end of an airplane’s speed range. To design it for higher speed would add weight and cost that adds no utility. Several things can put the speed limit on a retract:
  • Ability of the actuation system to unlock and/or move the gear;
  • Strength of gear doors and linkage in resisting wind load;
  • Aerodynamic disturbance from the draggy gear, doors, and opening;
  • Ability of the system to hold the gear down;
There may be others. But whichever is lowest airspeed for gear in transit set the gear in transit limit, and whichever is lowest airspeed for gear locked set the locked limit.

To make it work, you would have to come up with some new parts to make it be down and welded, be sturdy that way, remove the hydraulic pump, circuits, cycle valves, locks, plumbing, etc, fix the doors closed over the openings, and to approach the intended performance, fair the landing gear.

This is sort of like operating with a fixed pitch prop. While it can be done, that really is counter to why one has a high performance airplane like a replica Mustang.

The initial report on the gear issue seemed to indicate a non-standard way of running the gear that makes it vulnerable to unintended collapse. A switch that shuts off the pump by position will leave the gear unlocked if the switch triggers even a tiny bit early.

Normal design practice for gear extension systems is to turn the pump off when pressure goes high in the down circuit, and turn the pump back on any time that pressure fades, holding the gear in locked position. Pressure goes high in the circuit when the actuators run out of travel at gear lock. This is simply fixed by removing the down switch from the pump circuit. Regular checks of gear systems (annually during Condition Inspections) usually results in their being reliable.

If you want high performance two seat airplane with fixed gear, they exist, and may be way less fuss to maintain and operate.

Billski
 

BBerson

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The gear doors should be the weak link and high drag. Remove them and install simple fairings and seal the wing holes, that should improve takeoff climb and remain at about the same cruise speed because doors don’t seal that well.
Insurance might be more available and affordable.
 

Tiger Tim

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When the T-51 was first conceived, wasn’t it as a fixed-gear LSA? Perhaps Titan has notes on how to do it.

A slightly different tack would be to rebuild it with retracts as intended, correcting the shortcomings in the systems design. Maybe team up with TXflyguy here to come up with a well thought-out solution that can be offered for sale to other T-51 owners. People are making money on Stewart S-51 parts and there are like six of those so if they can do it then someone ought to do okay with the broader audience of Titan owners and builders.

Or fix it as designed, test fly it, and sell the thing to buy a Navion or something.
 

pantdino

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Billski- removing the down switch is an interesting idea. It would leave the pilot with one more thing to do during landing, though, wouldn't it? He'd have to watch the pressure gauge in addition to the other things he has to do during that busy time.
But I'm thinking once the gear locking arm was overcenter the gear would be locked anyway, wouldn't they?
 

Vigilant1

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As billski mentioned, the normal way of doing things is to have the pump cycle off when the pressure reaches a certain (high) pressure in the "down" circuit. IIRC, this pressure switch already exists in the stock pump setup you've got, but it isn't used in your installation (per Titan plans?). If you use this pressure switch, there's no added pilot workload or anything to do after the gear handle is down. And if pressure (for whatever reason) decreases while the gear is down, the pump cycles back on. That's a good feature it seems to me.
 
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TXFlyGuy

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The Titan aircraft hydraulic pumps normally are pressure regulated. Example...when the gear is retracted, and the system senses a drop in pressure, the pump will run briefly to restore pressure. This sometimes happens if a gear sags a bit, but the pump goes to work and picks it right back up. This operates as a fail safe, so the gear does not accidentally extend while inflight.

My plane is different as there are inner gear doors installed. So the pump will run until all contacts are made with the closure of the inner doors.

I also have an amber HYD PUMP light located near my gear lights which indicates if the pump is running, or if it is off.
 

wsimpso1

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Billski- removing the down switch is an interesting idea. It would leave the pilot with one more thing to do during landing, though, wouldn't it? He'd have to watch the pressure gauge in addition to the other things he has to do during that busy time.
But I'm thinking once the gear locking arm was overcenter the gear would be locked anyway, wouldn't they?

From Elliot Seguin's video, he described a limit switch in the circuit that powers the hydraulic pump. When the gear swings to the down position, it cuts off power to the pump in the down direction.

The common scheme in hydraulically driven landing gear systems is that the system powers the gear all the way to down and locked, and then keeps it there. When all the cylinders hit the end of the travel, the pressure climbs high enough for the high pressure switch on the pump to open and shut off the pump. There is also a low pressure switch on the pump that energizes the pump to keep pressure above some minimum amount too, and this system is set up to do this anytime the gear switch is in the down position. No added effort for the pilot. Select gear down, the electric motor spins in hydraulic pump to drive the gear down, sequencer valves make sure the doors are open before the gear actuator is pressurized, then the gear is driven down and overcenter to lock down. Most of us have three gear legs to get down, so the system should make pressure in the down direction until all three go over center and lock. Once everything is in the down and locked position, then the switches/valves keep it all pressurized in the down and locked direction. No big deal, and pretty robust. Not as robust as down and welded, but pretty darned good. Many thousands of airplanes running some variant of this, and malfunctions are rare. There might be up locks, down locks, gear doors closed again after the gear swings, etc, but these do not change the system that swings the gear and then holds pressure in the down and locked direction.


After take-off, move gear switch to up position, the pump runs and sends oil in the opposite direction (either by valving or pump running in the opposite direction, the down locks (if applied) are released, doors opened (if they were closed), the sequencer valves apply pressure to the up side of the various cylinders, and the gear swings up into the wells. Once there the sequencer valve closes the doors, everything hits its stops, the pressure comes up, and the high pressure switch cuts power to the pump until pressure comes down enough to make the low pressure switch, pump comes on again... Some systems throw up locks, others just hold it shut with pressure.

Last step is to move the switch to the down position with the pump not able to run (for whatever reason). The switch is connected to not only the electro-hydraulic pump, but releases all system pressure and pulls any uplocks. Then some sort of springs and/or gravity pulls the gear down and hopefully to locked position. There are airplanes that do not swing down by gravity, but would have to be manually pumped down, or even blown down by stored pressurized gas.

Your airplane appears to have had a limit switch set up to cut power to the pump as the gear hits the down and locked position. It is nice to know the gear is locked - those three little green lights are there for a reason - but if the switch is made just a skosh early, the pump shuts off and will not be powered again in the down direction. If the gear leg linkage went over center, it might stay put until you have it back in the barn. But if it is not quite over-center and vibration or landing loads try to fold the gear, there is nothing maintaining pressure to stop it. Most systems have the down and locked switch, but it only turns on the green light for that wheel, not disconnecting the pump. Then there is the whole issue of the switch is one more thing that can lead to the gear not working, etc. It sounds like someone thought they were the smartest person in the room, but ultimately made a choice that left Elliot with no way to get the gear down and locked...

The pilot never switches pumps on and off, only moves the switch, the hydraulics move the gear, and the high and low pressure switches on the pump keep the system pressurized no matter which way the little wheel shaped switch is moved.

Please be very skeptical of every system in this airplane. You have already seen an electrical system that shut off your engine for no good reason and a gear system that leaves your gear unlocked. I would be reviewing every system and its pieces from a couple perspectives:
  • What is the usual way this is done on other reliable airplanes? When you find stuff done differently, get other folks involved;
  • Look at everything and say "what failure modes can happen to this part and what effect will that have on my safety?"
Both paths should lead from you to some other smart folks who know how these things are normally done and can give advice on how to do it right on your bird. I would not go changing your system based upon some free commentary given on the internet by a retired guy who used to engineer automatic transmissions. Nope go back to some engineer who is temporarily working for you and see how they think it should be set up. Then go through everything. Personally, I recommend the AeroElectric Connection and then review that electrical system. Once you are used to asking the what if this fails this way questions and figuring out if your electrics make sense or not, you can apply that thinking to the landing gear, but I suppose you could do it the other way around too.

I do believe that a system using best practices running the structures you already have may be your single best path. Lots of folks are safely running electrically dependent engines and retractable landing gear. You should be making sure yours is set up so it runs safely too.

Billski
 

Dan Thomas

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As Billski says, fix the system. Making it a fixed-gear P-51 replica is like making it a trike. An abomination, and for no good reason.

Billski outline what the engineers of certified airplanes do. They do it that way because they have found, over many decades, what works safely. They have huge liability reasons for getting it right, and they don't put in switches that shut the pump off just because the gear is down. The downlock switches are there to fire the gear-down lights on the panel, not for the pump. They also put in squat switches to try to prevent the pilot from retracting the gear on the ground.

Besides pressure switches, they'll have a relief valve to relieve excess pressure if the pressure switch fails closed. They'll have a thermal relief (not sure why they call it that) that opens if the system has a hand-pump to manually extend the gear. It prevents overpressuring the system with the hand pump. Lots of stuff to keep the pilot's workload down and make it safe. I wish I knew why some of the designers of homebuilts don't study those certified systems, and think about why they're like that.
 

TFF

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The real question is why can’t it be fixed correctly? Gun shy while hard to reason away, is not fixing the problem.
 

pantdino

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Sounds like just using the switches for the green lights and changing to a pressure-controlled system will work.

Thanks for all the advice, everyone.

Jim
 

pantdino

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I'm looking on Aircraft Spruce and not seeing an adjustable valve that would turn a pump off at a set pressure.
Or do you have to buy a pump that is pre-set?
My plane has a Parker "Hydraulic Power Unit" and on their website it seems you can order these with whatever pressure limit you want.
 

wsimpso1

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I'm looking on Aircraft Spruce and not seeing an adjustable valve that would turn a pump off at a set pressure.
Or do you have to buy a pump that is pre-set?
My plane has a Parker "Hydraulic Power Unit" and on their website it seems you can order these with whatever pressure limit you want.

Elliot pointed out at about 48'20" in , the high and low pressure switches are normally part of the gear power pack. Elliot's video showed somebody's information on the Parker power pack with high and low limit switches. Part numbers for the switches are probably available directly from Parker. The switches are probably available directly from Parker or from the normal supply houses for GA. I am amazed that anyone would add the gear limit switch to the gear down circuit killing the pump, which may be part of the source of the gear not staying locked down... I defer to your paid engineering help on fixing this.

Elliot's summary on the cited video indicates that the main bus was brought down by a tripped breaker. That is also a serious WOW for me. Neither old school airplane wiring schemes nor schemes being promoted over the last 30 years for failure tolerant designs put a breaker on that kills any bus. This video should make its way to Marc Zeitlin so Marc may confirm the electrical diagnosis and drive the process of rewire to a much more failure tolerant electrical design.

We can talk on other threads about the bigger picture on wiring design.

Billski
 

wsimpso1

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Sounds like just using the switches for the green lights

If This Was My Airplane, these would be the limit switches that close when the over center links are locked on each of the gear legs.

and changing to a pressure-controlled system will work.

ITWMA, these would be high and low pressure switches usually on the power pack.

Billski
 
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