Options for controlled descent

[Extremeone]

Hi,
I have not posted to this group before but I have learned a lot from it. I have a lot of experience with glow and gas but not much electric time, expecially sailplanes. I have an Aspire with the stock 600 motor and an MPI 3.8-1 gearbox with a 14/7 folder. It climbs really well now and my main problem is how to get it back down when I catch that boomer thermal. I know the wings flutter on this design which leaves high speed descents out. I have thought about parachutes and dethermalizers but would rather go with spoilers and flaps because they leave you in control and give you some bail-out options during the approach to landing (like going around).

I especially like spoilers because they give a very positive control of the rate of descent and, if you retract them, you are instantly flying again. Flaps are not like this at all and you get into situations where you have to milk them up to avoid stalling. On a difficult approach I would use spoilers first and then flaps only if really needed to get a steep rate of descent. The main problem with these two options is retro-fitting them on an ARF wing which has questionable structurable integrity to begin with.

One thing I had thought of which, if it works would be great for ARFs is installing a Hi-Amp servo controlled switch that would reverse the polarity of the motor. This would allow the application of what they call Beta Pitch in turbo-prop aircraft. The turboshaft engine runs a constant speed and the pilot just varies the pitch of the prop. It is really interesting to land a Turbo-Porter and then back it into the parking spot.

Anyway, with reverse polarity, any application of throttle would start the prop spinning backwards. This should result in a variable speed reverse thrust which would be very useful on steep approaches. It could also be used to get the plane down from high altitude. Just point the nose down and use reverse thrust throttle to manage the speed. I would like to know if anyone has ever tried this and, if they have, could someone point me in the direction of the results.

Thanks in advance for any advice.

Regards, John

[JimNM]

I can only think of a few things to point out.

#1- Your motor will not like to run in reverse, most likley. The brushes are broken in for one-way travel, and reversing it will cause A LOT of arcing (read potential radio problems)

#2- Your gear box will not like ANY sudden starts or stops. The teeth on the pinon will rape the inside of the gear box - do it slow, and you will be OK.

The best way I have found to get down is to pull into a verticle climb, and just before stalling - give full rudder and about half up- elevator. My 72" ARF will safely spin down to a managable height in a matter of seconds. I leave the power on, and the spins are about twice the diameter of the wing span.

Good luck - and go slow with the learning process. My flying buddy has a graveyard of plane parts from finding the critical stress load the hard way.

Fly'em high
JimNM

[John Gallagher]

Spinning the plane down is a low stress way of bringing the plane down. When the glider is in a spin it is flying at it's stall speed. As long as you keep up elevator and full rudder it will not pick up speed but will descend quickly. Most planes will come out of the spin simply by centering the controls but some will require reverse rudder to stop the spin and then easing off the elevator.

[WimH]

Originally posted by Extremeone
I would like to know if anyone has ever tried this

This was tried , and very quickly abandoned, as a landing aid in the beginning of electric flight. Approach was a bit (too) steep and the plane difficult to control. You might as well open up the throttle just enough to make the prop spin and apply down elevator if losing altitude quickly is your main goal.

[uscra112]

I do the spin-down method to get my big electric homebuilt down. Useful that you see the top and bottom of the wing alternately - since the top of mine is dayglo orange I can keep the thing in sight. I use the trick of applying a little power to make the prop unfold and windmill only on finals (instead of a spoiler, which mine doesn't have). It doesn't do nearly enough to pull the thing off a boomer, but it does make it settle a bit faster without speeding the plane up.

[TStokes]

Interesting idea.

Reverse would not work with the folding props most of these guys use.

Tom

[WimH]

Originally posted by TStokes

Reverse would not work with the folding props
Tom...

Oh but it does!( Why would it not???) It's just that there are better (read safer) ways to get a plane down.

[uscra112]

Actually they extend quite well. Centrifugal force. Try it.

If the plane is equipped with a brushless, there would not be much difference to the motor if it was reversed. Except the current might be kinda high. And you'd obviously want to stop it before reversing.

But I'm trying to imagine what the flight path would look like. Might be a candidate for "World's Funniest Videos". Good Lord, I can see it now - the newest 3D maneuver - nose-down hover!

Anybody ever tried dive-brakes of the kind they had on the F-86? i.e. panels that swing out from the sides of the fuselage? Could be easy to actuate with a toggle linkage and a servo.

[tic]

I wouldn't want to pay the bill on the turbo porter for a hot section inspection after it's been repeatedly backed up on the ground.

[uscra112]

It occurs to me that the visibility of the plane coming down is key, after all, why bring it down if you can still see it?

[Extremeone]

Because there is little point in trying to bring it down if you can't see it. John

[Extremeone]

Hi Guys,
Thanks for all the input. I know about the spin technique but that does not help on landing. If I were building the aircraft I would install flaps and spoilers, or spoilerons, during construction. It is difficult to retro-fit those things in a wing that has marginal structural integrity to begin with. I know this is because of the high aspect ratio with poly-hedral and the built-up construction. It was not meant to dive.

I did not think about the reverse effect on the brushes and potential radio problems. I am using a JR PCM 10 and one of their 950 receivers which is bulletproof. There are a lot of things in flying that can result in unusual attitudes but they can be handled if approached in the right way. I don't know the skill level of the person(s) who tried it before but I do know mine.

I have every intention of stopping the motor before switching polarity. It seems natural that transitioning in and out of beta pitch would be a gradual process anyway. Assume you are descending in a steep nose-down attitude with just enough revs in beta pitch to keep the speed within limits. As you reach a lower altitude it seems natural to decrease the revs as you ease th nose up to level flight. I would think the motor would be stopped before achieving level flight and the polarity could be switched back to the normal position. Landing would be similar but the sequence would take less time. Keep in mind, when you stop the motor you have a clean sailplane which will fly the same regardless of the polarity.

I think I am going to get a brushless setup to try it with. Perhaps an Axi which could also eliminate potential problems with the gearbox. I really don't think there would be any though unless I were foolish enough to switch polarity while the motor is running. As for the hot section of the Porter, not a factor. The turbine always spins the same direction and only the propellor changes pitch. The hot section does not know the difference. I have also seen four-engined C-130 turboprops back up the length of the runway when there was no room to rurn around. A fairly routine operation in some parts of the world.

I am going to do some experimenting. I will keep you posted on the results (or lack thereof}. Beta pitch is a very useful capability once you learn how to handle it. I am just looking at an unusual way of achieving it. I am sure that once a person got comfortable with it they could get some spectacular maneuvers. (Hmmm, 3D with a 2M sailplane}.

Regards, John

[uscra112]

Yes - try it. Who knows? Every once in a while some seemingly wild idea works. If not, then we will have proof that it doesn't. That's better than guessing and speculating, any day.

[tic]

It has nothing to do with which direction the turbine wheels are spinning.. It has EVERYTHING to do with what gets picked up and injested while operating in reverse pitch moving backwards on the ground.. Any mechanic would shudder to see this.. Most POH's have minimum FORWARD speeds allowed while using reverse pitch on the ground.. As far as the C-130's are concerned, who cares about the cost of blades when it's tax payer money?

[Extremeone]

Yes I know all that, I am an A&P myself and I do shudder when I see it. However, you have to see the fields some of these folks operate out of. I would rather risk the engine and the prop than risk the trees or the rocks.

I am sure you would not find this kind of operation at LAX.

By the way, my wife and I are both from PA originally. I grew up in Warren and she in Kane.
John