Ground effect on a Bipe
 

Can the ground effect lift on the lower wing override the aerodynamic lift of the upper wing? I have a bipe that is a ***** to take-off and land. The plane will taxi nice and straight and will roll out straight during take-off until it reaches a certain speed then it turns hard left (ground loop) just before reaching full flying speed. If I time the rudder input just right I can get it in the air with a very ugly take-off. On landing the approach is straight down the runway and just after touchdown, as the plane slows it will again turn hard left. It seems that the take-off and landing hard left turn happen at about the same speed.

The upper wing is swept back with no ailerons and the lower wing is straight with ailerons. There is a very slight warp in the upper wing that should bank the plane right so the ailerons are trimmed to the left for straight flight. With the ailerons trimmed for left roll the right aileron is down a bit more than the left aileron and as such counteracting the warped upper wing. This is all well and good while the plane is flying, but on the ground it may be a different story.

I do not know the lift factor of ground effect so my question is, is it strong enough to cause the lower right wing to lift more than the upper left wing and turn the plane left when it is on the ground??

Has anyone else had this experience?

Oldbob

RE: Ground effect on a Bipe
 

I,m not sure about the lift on your wings would affect it but I had a similar problem that had to do with my wheels one of them was just slighty tighter than the other maybe something to check

RE: Ground effect on a Bipe
 

Not sure if this is the problem in our case, but I know from experience with several bipes that they all have what I call a "no man's land" right about the time that the tail comes up during a take-off roll (and, consequently, at the same speed during landing).

What happens, best I can tell, is that as the tail comes up, we lighten the load on the tailwheel...thus reducing its effectiveness in steering the airplane. However, due to the rather disrupted airflow at what is, essentially, a high AOA, the rudder has not yet become terribly effective as a steering mechanism. What's left is a plane that's almost entirely at the mercy of torque...and it tries to snap left. I've seen this happen in all 4 of the bipes I've owned, and, honestly, it's just something you have to get used to.

RE: Ground effect on a Bipe
 

What you have is not caused by ground lift on the lower wing. It sounds like it's far too consistent for that. It may be that you've got a loose wheel bearing that resonates at a certain speed. You've probably seen this from time to time. Try putting a bunch of different wheels on a wire axle and give 'em a spin. At least a few with the "right" size wire axle will sort of flutter at some rotational speeds. If you left wheel is doing that then it'll add a LOT of drag on that side of the model. Or it just may be that your landing gear struts are just flexing for some reason at around that speed? Without seeing what it's like it's hard to say

Another possible cause could be that aileron trim. For a lot of reasons related to stall charactaristics you should strighten up that upper wing and remove the aileron trim on the lower. Nothing good can come from such a fix. It may just be that this is part of your problem. As the tail settles the down aileron on the right side could be generating extra lift thanks to the pinching of the air compared to the left wing where the up aileron leaves a more open gap. That could be enough to drive the left wheel down and produce more drag on a grass strip.

So check the wheels first. See if you can get the left one to flutter. If you can then try a bit of oil on the axle and try again. But if it's noticably wobbly where you can TILT the tread more than about 1/16 to 1/8 side to side then you should assume that it could flutter on you and fix it with a better fitting bushing from some brass tubing or just get new, better fitted wheels. If you got it to flutter and fixed it try it again. If it still does it anyway then it could well be due to your trim scheme and it's time to get serious about that warp.

RE: Ground effect on a Bipe
 

The wheels roll good. I even changed them to a different size, and this did not help. I have checked and changed toe-in and toe-out, etc. I have eliminated any possible problem with the wheels. The hard left turn is the same no matter what I do to the wheels.

Sometimes it’s good to share your problems. I think I can verify if it is ground effect on the bottom wing. The next I fly this beast, during take-off, I will hold in just enough right aileron to cancel the left trim setting. If the rollout is straight all the way to liftoff this will verify the ground effect theory, if not???

Oldbob

RE: Ground effect on a Bipe
 

For ground effect to be a cause the lower wing has to be les then 1/2 chord width above the ground, and providing enough lift to fly the model. I doubt `Ground Effect' is in any way implicated here. It sounds more a combination of normal taildragger stuff, wheels too far foward of the CG, rudder effectivness low at low speeds, possibly blanked by the fuselage at high aoa's and torque effects as the tail rises. Try these `fixes' one at a time. See if you can move the wheels back a bit, with some ally u/c's you can turn the whole thing around. At flying attitude the wheel axle should be about 15 deg. foward of the cg, i.e. on a line drawn at 15 deg. through the cg. As the takeoff is commenced try using foward elevator stick to raise the tail as soon as possible, this will help the rudder bite in the prop slipstream and provide positive yaw control on the ground. Try not using full power, 75% should be enough to take off, and there will be less torque effect to have to control. Try not using ailerons until the model is ready to turn on the crosswind leg after takeoff, biplanes are RUDDER airplanes, not aileron airplanes, make sure you have at least 45deg. rudder movement each way. Then go out and practise lots of touch and goes, using RUDDER lots, and only rudder when the wheels are on the ground. Good luck!
Evan.

RE: Ground effect on a Bipe
 

Not a great builder nor expert on aerodynamics but have built and flown several bipes without too much trouble. Your situation reminds me of past experiences and might suggest considering the amount of right thrust to engine. 2 or so degrees of right thrust to plans that called for "0" made a big difference. FWIW

RE: Ground effect on a Bipe
 

Ground effect reduces the induced drag and starts to make a significant impact when the wing is less than 1/2 of the WING SPAN above the surface. Even a Cessna can fly in ground effect.

RE: Ground effect on a Bipe
 

Norm, the original design of the plane is 0-0 engine thrust. Because of this problem I have changed it to 2.5 degrees right and 1 degree down and this change made no difference in the sudden left turn during take-off.

Evan, I must use 50% to 60% power or else I can never get the plane into the air. If I apply power to quickly the plane will do a 180 ground loop and come back at me.

The weather here had been rainy the past few days so no chance to try out my theory.

Oldbob

RE: Ground effect on a Bipe
 

As a last resort I would suggest that you only use the grass strip for takeoffs and landings, and always `into wind'.
Evan.

RE: Ground effect on a Bipe
 

Run into a couple that had bad attitude on takeoffs. All of them required their own throttle on takeoff technique. Some could take 50% throttle applied quickly, some wouldn't take any quickness on the throttle no matter what percentage was given.

I've always thought that if they'll taxi with an degree of success, they'll take off if you just get the acceleration right.

We're all spoiled with the short takeoffs most models will do. Some of these suckers however just take some runway for them to behave.

I've got a Decathlon that's a beast if you push the throttle forward too fast. And it does great with gradual throttle advance. Watching a gradual throttle advance takeoff you'd never suspect that sucker was such a witch.

RE: Ground effect on a Bipe
 

Someone has mentioned wheels. There is another facet to "wheels". When the engine torques into the airframe and before the wing is carrying the weight, that torque unloads one wheel and loads the other. Any misalignment of the "heavy" wheel is magnified.

Also, if the airplane has wheel pants, look to see if the tire can scrub in the pant. With the weight of the airplane and the axial roll on takeoff loading that wheel, it just might be distorting and grabbing in the pant. ..... or not........

BTW, my Decathlon is still a witch on landings unless...... I flip high rate on rudder for the landing. As it slows down, that rudder that was SO EFFECTIVE in the air seems to go worthless. It's the first thing that quits flying as the airplane slows down. When I first flew the sucker I wondered why they recommended so much throw on a rudder that'll almost break the airplane in half when jammed in flight. Then it dawned on me that the rudder needs all the deflection possible on the ground when it's "not flying yet". Your bipe might be the same.

RE: Ground effect on a Bipe
 

Ground effect is noticeable within one half wingspan of the ground (as mentioned earlier), not merely 1/2 the chord.

I corroborate some of what has already been written. See if this makes any sense to you:

Consider the following, it's a common cause of sudden left turns with taildraggers: As the tail is coming up on takeoff, gyroscopic precession causes the nose to yaw noticeably left. This is not a theory: it's a physical fact. The only way to properly correct for this is to anticipate it and apply a bit more right rudder as the tail is coming up, then decrease it once the tail is up and it's rolling along the ground on only the mains. Otherwise, if you wait to see the yaw you will yaw harder left than you mean to and/or over-correct every time. This agrees with your comment that if you get the rudder in just right, you can take off instead of ground looping. It also agrees with your statement that you have to use reduced power for takeoff. Precession is a strong force and shows up well on short-coupled biplanes with small rudders. This assumes you are allowing the tail to come up on the ground and are not taking off in a three-point attitude. You can reduce the precession-induced yaw moment by using a lighter weight propeller or reducing RPM (by going to higher pitch) or reducing propeller diameter without increasing propeller mass or RPM.

As for the landings, it's likely that as the airplane slows, the rudder loses it's ability to steer the airplane. That's why it's important to have a tailwheel rather than a skid, and gradually feed in up elevator as the airplane slows, to make the tailwheel steering make up for the loss of yaw moment from the rudder.

It seems unlikely to me that ground effect has anything to do with it - primarily all that does is reduce induced drag. Prop slipstream and tailwheel steering are life when it comes to directional control on the ground. Also remember, that almost all full-scale airplanes have differential braking to help with ground steering at low speeds when the rudder is ineffective. Models have to make up for this lack of differential braking somehow, usually by using a larger than scale rudder and extra-careful use of elevator to maintain tailwheel steering at slow airspeeds.

RE: Ground effect on a Bipe
 

This is a commonly encountered problem with all taildraggers, not just bipes. The cause is insufficient yaw resistance. To fix it, the following will help:

-Add washout to the wings.

-Extend the tailwheel, to allow the tailwheel to give steering control until the rudder and fin have sufficiently fast airflow over them to take over.

-Move the landing gear back as far as possible without getting into nose-over trouble.

All of these will help, but the surest fix is to increase the rudder and fin area. A simple way to do this is to simply tape a temporary cardboard extension onto the trailing edge of the rudder, then permanently enlarge the rudder when you are satisfied with the results.

Bipes are common offenders in this department because the airflow over the rudder and fin are messed up by two wings, as well as the cabane struts. The Ultimate is a good example of a bipe with sufficient vertical tail area.

A local low-time modeler had the same problem with a Sig Smith Miniplane. I roughly doubled the rudder area with a cardboard extension, which cured the problem. After about 20 good flights, he removed the extension. Guess what happened? Right! He got a bit slow on final approach, and spun in on the next flight.

RE: Ground effect on a Bipe
 

Rotaryphile, did your rudder extension extend above the vertical fin or just aft of the rudder? At present the movable rudder surface is about equal to the stationary vertical fin. Do you think it needs to be bigger than that?

Mease, what you say makes sense but might not be the answer to my problem. The gyroscopic precession may be a slight contributing factor but the force is not that great and besides 2.5 degrees of right thrust should compensate for most of it. GP is a law of physics and should be the same for all planes of the same size with engines of the same size and props of the same size. I have five tail dragger planes, three of about the same size, with engines of the same size, turning props of the same size, and only the bipe has this crazy left turn on take-off.

Since gyroscopic precession cannot be eliminated only an aerodynamic solution is left for me to try. The weather is still messy here but as soon as it improves I will try to hold right aileron during take-off. If that does not work I’ll try a bigger rudder.

Oldbob

RE: Ground effect on a Bipe
 

My temporary rudder extensions normally only add chord to the rudder, maintaining the original height. I find that subfins and subrudders are particularly effective on bipes, since they are in clean air while the airplane is flying at a high angle of attack, unlike the rest of the vertical tail. In my book, getting really solid controllability extending into stall territory requires a vertical tail area of about 15% of the wing area, for a normal tail moment/wingspan proportion, and that the larger the vertical tail area, the better the controllability.

RE: Ground effect on a Bipe
 

I know that what I wrote is not the only solution to your problem, but it may be part of it. Things happen so fast with small models that unless you know what to look for, it can be difficult to determine the cause(s) of a given departure from course. Gyroscopic precession can be very noticeable, especially at low airspeed and high pitch rates. Right thrust does nothing significant to correct for gyroscopic precession, since GP only occurs while the airplane is rotating about the pitch and/or yaw axes (even that statement is oversimplified but we'll call it close enough). Right thrust has the greatest effect against spiral slipstream and P-Factor. The ONLY way to correct for GP is with with timely rudder application, or some form of yaw damper (like a gyro in the yaw axis). Or, as is the practice with pattern, make the yaw stability so great that GP is small in comparison, making it relatively unnoticeable. You probably already know this, but many of those full-scale WWI bipes were HARD to takeoff and land, and many, many of them met their end in a ground loop on takeoff or landing. They required very careful and specific techniques to safely and consistently take off and land. Any departure from those techniques was likely to cause a ground loop or runway excursion.

The rest of what you wrote makes sense. I suspect a larger rudder and/or fin will be the best solution, but not being able to inspect the airplane puts us would-be problem solvers at a disadvantage.

Please let us know how it goes, and good luck.

RE: Ground effect on a Bipe
 

Well the problem is solved. I made seven consecutive takeoffs and landings that were normal and straight down the runway. The first few takeoffs I held in just enough right aileron to take out the left trim I had in the bottom wing. After that I cut-up a pop can and made a trim tab that I taped to the top wing. A few more flights and the old bipe was flying straight and level and now has ground handling characteristics like my other tail draggers. Boy, am I happy! :D

I think what was happening is that the lower right wing was producing a strong ground effect lift and the plane wanted to roll left. But since the wheels were still on the ground it could not roll and the plane would turn hard left instead.

It seems logical that ground effect lift increases as wings get closer to the ground. On by bipe the lower wing sits 4 inches off the ground and the upper wing sits 12 inches off the ground. The wingspan is 4 feet so the height of ground effect field on my particular plane is 2 feet. With the plane sitting on the ground the upper wing is in the middle (50%) of the ground effect field and the lower wing is in the 84% ground effect field. This assumes 100% ground effect at zero (0) inches and 0% ground effect at 24 inches.

It will be interesting if anyone else out there can verify this by getting his or her ground handling monster bipe to behave on the ground. Obviously those that have bipes with perfectly straight top and bottom wing are wondering what all this bumbo-jumbo is about. But if your bipe is nutty on the ground you more than likely have one or both wings with a warp. To improve ground handling set the bottom wing ailerons as equal as possible so that the left and right wings produce about the same lift during takeoff. Then trim the upper wing for in flight straight and level.

I learnt a lot trying to solve this problem. And thanks for the good input from all of you who took the time to jump in.

Oldbob

RE: Ground effect on a Bipe
 

Sounds like it was just out of trim. One wing was "taking off" before the other.

RE: Ground effect on a Bipe
 

Here is a chart on ground effect.

RE: Ground effect on a Bipe
 

Hi Oldbob ,
Looks like you have an answer. I have had two biplanes and just loved them. They are a handful to get off the ground and land. I always tended to forget this. Everytime I stopped flying it for a while the first takeoffs would be horrible. As I recall, during takeoff I used various combinations of slow throttle input, some up elevator, lots of rudder and finally full throttle. Staight down the line takeoff were the exception rather than the rule. It has been at least two years since I flew one last so I cannot seem to recall some of the details. Good luck with yours.

RE: Ground effect on a Bipe
 

Ville, if you still have one of your bipes that is hard to handle on the ground maybe you can apply my theory to your plane?? I believe the theory can be used as a general rule but since I have only one bipe to experiment with I am reluctant to go that far at this point.
One of my flying buddies has a bipe that does a lot of left-right left-right during takeoff. Maybe setting both sides of the lower wing for equal ground effect lift and than trim the upper wind for straight and level flight can eliminate this. This will take several flights, doing upper wing trim adjustments between flights, to get the upper wing trim set correctly. It took me five flights. Now it’s a real joy to watch the o’l bipe track straight down the runway, have the tail come up, roll another 15-20 feet and fly into the air.

If you OR ANYONE ELSE try this please let the rest of us know how it turns out.

RE: Ground effect on a Bipe
 

First, let me say that I am glad you found a solution to your problem. However, I do have a question that perhaps someone could shed some light upon. This takes the possibilities of any trim, setup or dissimilar wing incidence issues out of the equation and relates specifically to the assessment of a ground effect induced problem.

If I understand correctly, the contention is that ground effect was causing the left. Was this left in one axis or more than one? It seems as though you were discussing a yaw AND roll movement (which was corrected by your trim tab retarding the roll axis movement). I guess it was perceived as a roll more than yaw. If roll was part of the movement, is the roll the cause or affect? Even considering ground effect, what is producing the asymmetrical lift? Is the aircraft tracking with right side leading edge in front of the left (aircraft crabbing instead of straight)? I just don't understand how if everything was straight and true, the right wing (bottom wing at that) produce more lift than the left wing. I have reviewed P-factor, torque, and gyroscopic procession, along with tail-wheel -vs. - rudder effectiveness and can understand the movement based upon all of these things, but I cannot get my head around an asymmetrical lift produced specifically based upon ground effect. If all else is equal, shouldn’t ground effect be produced symmetrically?

Please let me know where I am going wrong and not seeing it through.

Thx,

RE: Ground effect on a Bipe
 

I'm glad got your bipe to track and fly straight now and please forgive me because I dont mean to sound antagonistic, but why dont you just straighten the wing? Boil it down and that is the problem here. The rest of it is 'laws' of physics and the natural tendencies of sport bipes - neither of which you are going to 'fix'. The trim tab is a patch but not a good solution for a warped wing.

RE: Ground effect on a Bipe
 

Oldbob
Sorry, but I do not agree with the ground effect explanation.