The porpoise epiphany

j1b3h0

j1b3h0

Line Up and Wait
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Doug
Just about anyone who has trained in or taught in tailwheel airplanes knows of the dreaded porpoising that can happen, especially when practicing wheel landings with a bit of extra speed. You know the drill, your student (or you) don't quite arrest the sink rate enough before the mains touch down and then, Boing, you bounce back in the air and then, if left unchecked by some back stick, the nose will point down more sharply until you bounce again - this and each time harder, untill either you correct for it, or the airplane does a face-plant on the runway and flips upside down. Many airplanes have been crashed in this fashion. What causes the porpoising phenomena?

I'll give you a hint: When you open the door of a Citabria and unfasten the seatbelt, what happens to the stick?

Took me quite some time to understand why the airplane porpoises well enough to explain it to my students. Enlightenment came to me while attempting to teach wheel landing to a student who (ironically) had taken some of my previous advice to heart: He flew with a very light touch. "Three fingers and a thumb," I would tell him, explaining that it would help him always keep the airplane in trim and be easier for him to tell if he was holding the proper amout of rudder pressure in the climb, because the wing tends to drop, etc. And because I didn't want him to be a hack. After a couple times around the pattern, it seemed to me he was excaserbating the porpoising tendencies of the airplane by actually pushing the stick forward after a bounce. "Don't push the stick forward after you bounce," I would caution him. "I'm NOT!" he told me...which I found hard to believe because I just watched the stick physically move forward about six inches after the last bounce.

Then came the epiphany: The airplane bounces, the nose pitches up (because the mains are forward of the CG), and a split second later, the effect of having hit the ground deflects the elevator downward. It was the G loading of the bounce that made the stick go forward. My student was right, he never pushed the stick forward - the bounce did.

On airplanes with long fuselages - and hence a long distance from the main tires to the elevator, the porpoise happens relatively slowly. Short coupled airplanes, like a Pitts or even a RV can have a viscious porpoise. Stiff landing gear make it worse, but I never witnessed it happen in a Great Lakes with its mushy oleos. Wouldn't happen at all in an airplane with a statically balanced elevator.

I look forward to comments, questions. D
 
FWIW, porpoising is by no means limited to tailwheel airplanes. The gear configuration may exacerbate the problem, but you can definitely get a 172 or PA28 to porpoise.
 
I'm having trouble believing that the inertia of the mass of the elevator will overcome the aerodynamic forces on the elevator even at touchdown speeds. I'd like to see a bit more proof of that.
 
Ron Levy said:
I'm having trouble believing that the inertia of the mass of the elevator will overcome the aerodynamic forces on the elevator even at touchdown speeds. I'd like to see a bit more proof of that.
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It would be interesting to see a slow motion video of it, I agree.
 
This is very nicely described in Plourde's "The Compleat Taildragger Pilot" and called a "jounce". He does a good job describing the difference the CG makes in a tricycle vs convention gear plane with diagrams. Well worth the investment.
 
I was taught that if I bounce a wheel landing you either turn it into a three point or a go around. I don't think you should be teaching a student to chase a wheel landing.
 
In the rear seat I physically guard against the stick coming forward....unless we're doing a wheel landing.
 
Ron Levy said:
I'm having trouble believing that the inertia of the mass of the elevator will overcome the aerodynamic forces on the elevator even at touchdown speeds. I'd like to see a bit more proof of that.
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Agree with that. I've never noticed elevator momentum such as this. Airflow is much stronger than this inertia. The cause of porpoising among new TW pilots is a deliberate or subconcious relaxing of the stick in an attempt to "catch" the bounce. By the time they are pushing forward, the plane is about to start back down anyway, and by the time they've tried to re-"flare", the plane has already bounced again, and they've made it larger with back stick. They are 180 degrees out of phase with the plane. If you hold the attitude of the airplane after it has bounced, and use power to control the descent, you can't stall. Just fly the plane back down. And always remember that there aren't many landing problems in a taildragger that full throttle will not fix.
 
bbchien said:
In the rear seat I physically guard against the stick coming forward....unless we're doing a wheel landing.
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+1

[Disclaimer: I am not tailwheel endorsed] Tailwheel, nosewheel, either way it is better to hold the airplanes attitude than chase the osilations. If you bounce up and then reduce the angle of attack (which means less lift on the wings and less 'lift' holding the tail down because of reduced elevator) than your come down harder than the last time when you were making more lift. This will continue to get worse unless you go-around. Never chase the osilations, hold the stick or yoke and add power or go around if that isn't enough. If anything the pilot should be increasing the AOA (without breaking critcal angle or causing a tail strike) This low to the ground [or maybe the main wheels never left the ground] you will hit the ground before gaining enough airspeed to make putting the nose down worthwhile.

<---<^>--->
 
I suggest reading the FAA-H-8083-3A, pp 8-31, Porpoising. The illustrations assume a nosewheel airplane, but the explanation of the root cause is applicable to conventional gear airplanes.
 
I can't say I have tested my theory in any other airplanes, but I've given almost 1,000 hrs dual in Citabria/Decathlon airplanes and have seen it many times. If can get your hands on a 7ECA, try it yourself...I don't blame you all for being sceptical. It did happen to me a couple of times with a student in a RV-6 with an IO-360/constant speed prop. It could really bite someone in that airplane - it happens so fast. Short-coupled and springy gear legs. I've never experienced it in a Pitts, but I never wheel land a Pitts...you'd have to be doing 90 over the fence.
 
The stick sure bounces when I taxi at a good clip my little LSA - generates quite a bit of force if you try to hold it in one place. It doesn't take a lot of force to push the nose down when flying.
 
j1b3h0 said:
I can't say I have tested my theory in any other airplanes, but I've given almost 1,000 hrs dual in Citabria/Decathlon airplanes and have seen it many times.
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You could test your elevator inertia theory by getting relatively close to touching down during a wheel landing, and then letting go of the stick and seeing what happens when the plane bounces...if there is truly a significant enough automatic stick forward movement to exacerbate the bounce. I feel stick forward movement is pilot-induced, since I had an automatic tendency to push the stick forward as soon as the plane bounced during my first few hours of PPL training. I think this is not uncommon.
 
Ron Levy said:
At taxi speeds, yes, but the aerodynamic forces go up by the square of velocity, and you're probably touching down at about 4 times taxi speed, which creates 16 times the dynamic force on the elevator.
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The force required to push the nose down pretty quickly when at cruise isn't very high.
 
j1b3h0 said:
It did happen to me a couple of times with a student in a RV-6 with an IO-360/constant speed prop. It could really bite someone in that airplane - it happens so fast. Short-coupled and springy gear legs.
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Good! I need a better excuse, than any others I've thought up.
It was fast alright. Bop,bop,flop..........and my prop was dinged.
Was a 6A with a C/S.

L.Adamson
 
Sorry to hear about your experience in the RV. I'll bet you're not the only one who's discovered that quirk. Suprised the heck out of me. I only have 50 hrs or so in it - most of which was giving dual. Anybody who does wheel landings in it should practice the heck out of them. You have to be really quick when an RV porpoises.
 
You'll find that an RV6-7 will land much nicer if you come over the fence at around 65mph. Most RV pilots just try to land way too fast. Also mine handles a lot better with the wood gear dampeners. Don
 
I'll try to remember that, Don. Can't remember what we were using, probably 70ish. Do you wheel land yours much?
 
I find the RV 6-7 lands much better three point. I also use full flaps on almost every landing. Now the 8 doesn't three point as well and I wheel land it most of the time. Something to do with the gear to fuselage junction. I can grease the N3N on in a wheel landing almost every time but a good three point is more satisfying as it is tough to get a perfect one. Same with the SNJ and Stearman. When I did the checkout in the P-51 he had me do some three points and again they were a lot tougher than wheel landing. If I get a bounce on a wheel landing it turns into an automatic three point or go around. I almost lost the SNJ chasing a botched wheel landing on a dead calm day. One other thing is in a cross wind I always three point. I want it all done flying when I touch down. Don
 
yakdriver said:
One other thing is in a cross wind I always three point. I want it all done flying when I touch down. Don
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Interesting point, and contrary to the opinion of some that wheelies are prefereable in X/W conditions. Many pilots (and examiners) who fly the high-wind conditions in TX and OK (I'm one of them) heartily agree with your philosophy. If something happens on the ground, it's going to be much less stressful on the airplane and the pilot if it happens at low speed.
 
wabower said:
Interesting point, and contrary to the opinion of some that wheelies are prefereable in X/W conditions. Many pilots (and examiners) who fly the high-wind conditions in TX and OK (I'm one of them) heartily agree with your philosophy. If something happens on the ground, it's going to be much less stressful on the airplane and the pilot if it happens at low speed.
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What's funny is that I know a few CFI's that 'recommend' wheelies for x-winds, but they will also admit that the worst x-winds they landed in, they 3-pointed.
 
A DPE friend with a Cub says that he knows it will probably ground loop in very strong winds, but that if it's slow enough and three-point it won't tip over. I'm not able to convince myself it would work on the 180.

Fearless Tower said:
What's funny is that I know a few CFI's that 'recommend' wheelies for x-winds, but they will also admit that the worst x-winds they landed in, they 3-pointed.
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I've heard all the arguements, but I'm a three pointer, too.
 
There are airplanes that like to three point, and airplanes that like wheelies.

There are pilots that like to thee point, and pilots that like wheelies.

It's best when the two match.
 
wabower said:
Interesting point, and contrary to the opinion of some that wheelies are prefereable in X/W conditions. Many pilots (and examiners) who fly the high-wind conditions in TX and OK (I'm one of them) heartily agree with your philosophy. If something happens on the ground, it's going to be much less stressful on the airplane and the pilot if it happens at low speed.
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I'm with you, in that regard. Guys I've talked to who wheel land in high winds tell me they hold the tail up and get on the brakes to get the airplane well below flying speed, then let the tail down. I'll have to practice the method some time when it's blowing dogs off chains. One (experienced) guy told me he landed his Stearman in almost 30kts 60 degree crosswind. He's got thousands of hours in the model and his has 4 ailerons.
 
The N3N will handle 20-25 x wind component pretty handily. The stock one I used to fly had skinny 30x5 mains and really crummy Lockheed brakes so you were lucky to have enough to taxi in in high winds so you couldn't use them much on landing or they would overheat.
 
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