MT reversing props for piston engines

[PilotDAR]

German Propeller manufacturer MT Propeller has offered reversing versions of several of their hydraulic constant speed propellers for many years. They are notably useful for floatplane operations, where small amounts of reverse thrust can be useful for slowing and stopping. They've been STC'd on Lake Amphibians and Renegades for many years. More recently several US vendors of MT props have began to supply these props STC'd to a number of other aircraft. A few points are worth discussion:

The operation of the prop in forward thrust is completely normal, and intuitive, as the governor operates the prop with the same function in forward as usual. Reverse, within the permitted operating range, is selected electrically/hydraulically by a guarded toggle switch. The permitted operating range for reverse will be engine [propeller] RPM less than 1350 or so, and the airspeed must be slower than about 35 knots. This is to prevent the operation of reverse in flight.

This system is not a "Beta" system (although some MT data incorrectly uses this term). Beta propeller systems are common to turbine engine installations and use the power lever to control both power and blade angle - forward or reverse in certain ranges. You know you're operating a Beta system when the power level has a gate, and moving the power lever backward form that gate selects prop blade angle less than fine pitch forward, and begins to increase engine power (now in reverse). The MT reversing propeller systems on piston engines are not that, they're just reverse. When you want to increase engine power in reverse, you still have to advance the throttle control forward. When you select reverse blade angle, the blades move to a preset angle only, you're not "controlling" a varying blade angle in reverse, as you would in a turbine Beta system.

The whole system is operated electrically, without aircraft power, and the closing of switches, reverse cannot be achieved. The very well thought out interlocks which prevent entry into the reverse blade angle are a pitot system electric switch (which opens above a certain speed) and a simple and very effective centrifugal locking system within the prop hub. If the RPM is too high, the locking rings "throw out" and the blade angle will not move out of fine pitch. This locking ring system is designed to allow a very slight RPM increase, so if you have selected reverse below the specified airspeed, though above the limiting RPM, the engine speed will increase slightly as a warning that you have selected reverse, but it will not enter that condition. Therefore, with these two interlocks, errant entry into reverse in flight is impossible.

The most effective use of the MT reversing system is on the water. Very precise maneuvering is possible, including backing the aircraft up. The system works while taxiing on wheels, and is handy for saving brakes, but backing airplanes up on the ground has risks, so must only be done with lots of caution. The use of reverse on a Lake Amphibian in an attempt to back the aircraft up is ill advised, as with the high thrust line, you're much more likely to lift the nosewheel off, and bang the tail. For landing runout reduction, it will in the latter stages, but it is not set up to provide massive reverse thrust, so by the time you're slow enough to operate reverse, most of your slowing has already occurred naturally. It's not like some turbine aircraft where you touch down, and command a huge cloud of dust, within which the plane stops. The use of reverse in a floatplane while still on the step is also ill advised, as the reverse will greatly reduce the desired affect of airflow over the tail. The aircraft can begin pitch and yaw excursions which are scary.

While taxiing, you can select in and out of reverse very quickly. Taxi speed can be modulated with a rapid in and out repeated selection, which gives pleasing results. As the operation of the blades is engine oil pressure hydraulic, if you reduce to idle power, there may be inadequate oil pressure to change the blade angle. Thus it is better to keep some power on all the time, and control the blade angle, rather than to reduce to idle, then try to restore forward thrust. This is a practiced technique. It is very easy to hear the differences in blade angle, so you get used to what you're doing by ear. There is a reverse light, but you really should be looking outside - not down at a light! Another nice feature is that if you shut down the engine in reverse, when you start next time, the blades will be stopped very near to flat pitch, and it will be a zero thrust start. You can warm up, and untie at the dock, then pull away when you're ready.

It is noteworthy, that like some boats, steering is very poor while in reverse. So it takes some practice to maneuver close in, and backing around a corner should not be attempted in a confined area. Similarly, the reverse is a system, which like the engine, may not always operate perfectly. So, racing straight on at the dock (or worse, the plane parked there ahead of you) and expecting to stop on a dime is foolish. I train that you still approach the dock at an angle, slowing as you would in a regular floatplane, and able to steer away (or into the dock) if suddenly required. You also have to watch the use of reverse with a cabin door open, it'll tend to pull them forward against the stops. I also assure (particularly for floatplane amphibians) that the lines on the bows of the floats are short, such that they cold not be pulled into the prop.

The system is super, and now available for a number of floatplanes by FAA STC. Installers should be aware though, that as there are no Canadian STC's for floatplanes yet, TC could take an interest, and ask to "familiarize" themselves with the "foreign" STC going on to a Canadian aircraft.

[Chris M]

The permitted operating range for reverse will be engine [propeller] RPM less than 1350 or so, and the airspeed must be slower than about 35 knots. This is to prevent the operation of reverse in flight.

Thanks for the post, I'd been wondering about these things.

With regards to the RPM engagement limit, is the limit only to engage reverse or is it an operational limit as well? For example, if you engage reverse at idle and then push the throttle up, will the prop exceed the 1350 RPM in reverse or will it disengage and go into FWD pitch?

A while back I came across an add for a Velocity kit plane with one of these props on the back. Obviously the builder intended to use it to reduce the landing roll, but if the airspeed limit is only 35 kts you've already burned off most of your kinetic energy so I don't know how helpful the prop would be at that point. Better than nothing, but if you need help getting stopped at 35 kts you've made some pretty bad mistakes.

[PilotDAR]

The interlock system prevents entry into reverse above 1350+- RPM. Once you're selected into reverse, you can select more power, up to full power, thought there is nowhere near the thrust in reverse, as there would be in forward, as only the outer part of the blade is in reverse blade angle. If, however, you've got lots of power in in reverse, and you select forward without going to idle, it's going to jump into forward at that power, and it'll be quite a jolt.

There should be only very small [slow] expectations of reduced ground roll with the use of reverse. The effect is there, but it's modest. The installation of the MT reversing prop on a piston powered landplane is probably of limited benefit proportional to the cost.

The other thing to bear in mind is that for a standard tricycle aircraft, the prop is fairly close to the ground. Using reverse on a non paved runway creates quite a risk of leading edge damage. Though the leading edges can be replaced, they cannot be dressed in the normal aluminum propeller sense. So it works out better with a prop which is farther away from the surface by the arrangement of the aircraft. The leading edges are available at the time of order in stainless steel or nickel. they are both superior to aluminum in terms of erosion, with the nickel being far superior. After 300+ hours in service on the 182 amphib I occasionally fly, with lots of water work, there is not a mark in a blade leading edge.

[Broken Slinky]

Didn't Hartzel already have a "beta" prop for piston Cessnas? If I remember correctly, they touted it as a way to get slowed down quickly on final.
Be a great feature if some boob parks in front of you at the pumps. Just slam it into reverse and...

[Cat Driver]

Reverse pitch sure helps when docking on the water.

The first time I learned to use one in a light single engine airplane was in 1954.

https://www.youtube.com/watch?v=ZL7siF_XAGI

[cgzro]

Slightly off topic but have you seen the reversible pitch propellors used in aerobatic model airplanes.
They literally can fly towards the ground, go full power full reverse and fly away from the ground backwards.
Insane.

[peterdillon]

Have one on a super cub on floats for 10 years or so. Works great for docking in the wind. Nose into a beach and when you go to leave just jump in and warm her up in forward pitch and when your ready to go back out and away you go. If your healed in you can start in reverse and flip her into forward and go. Makes things quite simple if you have to hand prop.

[JasonE]

If I had a flying boat or amphib this would be the way to go. It would make docking/undocking (or beaching) much nicer.

I've been more interested in flying boats recently (especially after reading about the Seawind), and the reverse prop sounds ideal to me. There's a Seawind 3000 for sale in QC if anyone's interested! It's likely out of my price range.