Aerodynamic basics- Ground Loop cause/cure

M

Moto-Guzzi

Active member
More cool stuff to learn from the generous experts of the Design Forum!!!

Fellas, is their ONE particular design imbalance that MOST likely promotes the tail swinging around shortly after landing?

The plane in question is a large scale Bristol F2B Biplane, it has a wider wing span to fuse ratio than your typical sport bipe or WW1 fighter. I incresed visual and physics by 50% and put in a gas motor, re-calculated the individual weight of wings, fuse, tail and gear proprotionally up by 25% from the original, and I get a decent handling 12 foot span plane with a more realistic 25 MPH stall speed, so approach and landings at Carl Hansen field look nice and "scale", but....

I get a Left Ground Loop every time just after landing (power on idle before touchdown) OR a 50/50 toss-up left OR right ground loop with the engine killed on final…. Her approach and flair look nice and smooth but she will not roll far once she’s down, just a few plane lengths before she loops around.

I thought the INITIAL left loop on landing may be torque induced because I do need a little right rudder on full-power take-off to track straight… but the 50/50 left/right ground loops with power OFF, plus some difficult taxi habits (zig-zag hard to control) convince me I have a General Airframe Instability Issue….

Yes, I have “G.A.I.I.” and I suspect its not easily treatable with Back Torque or Prop Thrust adjustments...the way my A.D.H.D. is with Ritalin! :eek:

Other than her annoying “ground habits” , I’m happy with her general flight performance from takeoff all the way to touchdown…front to rear CG position seems OK because she flies level with elevator trim centered….(I wouldn’t know what negative symptoms to expect if the CG was too high or too low) and turning looks and feels natural with a little rudder…

So, before I start chasing the Ground Loop Gremlin in the dark, are any of these the primary suspects I should consider first ?

Tail too heavy?
Tail too LIGHT?
CG too High or too low?
Fuse/wing weight ratio: a rough start should generally look like 50/50, 80/20, 90/10?


As always, your replies are greatly appreciated… I am enjoying this learning curve immensely I hope I’m not pestering the board with too many annoying theory questions….
 
That's the problem with just scaling things up.... you don't get accurate results.

Consider what ARF designers do with real RC planes...

Smaller ARF's tend to have flat planar surfaces on the tail. These provide sufficient "lift" to keep the plane somewhat stable.

If they scale things up, they tend to switch to true airfoils instead, or to use "other" airfoils that may generate more lift/authority.

I'll bet that switching to a better ( thicker ) rudder airfoil will help. Also note the placement of the rudder. Is it lodged within the fuselage in the model. You may need to fix this.
 
While Jose is correct, it could also be your inexperience (not knowing your experience level) flying a tail dragger. Usually ground loops are more pronounced with a cross wind, but it could happen landing down wind. Many take offs are riddled with ground loops until the pilot learns to fly front and rear of the aircraft...

Tail draggers - on takeoff hold tail on ground build airspeed comes up, get tail flying before the plane lifts off, at correct airspeed rotate and lift off. Big mistake is to keep holding tail down while plane lifts off... then a stall occurs and crash ensues. Control direction with rudder at all times.

Landing - I find easier, land plane on the mains, roll forward holding tail off until airspeed reduces below flying speed, bring tail down and hold on ground all the while you are controlling direction with Rudder. Taxi at slow speeds with elevator holding tail firmly on the ground. Steer with rudder.

Larger planes are more pronounced ground loopers. Again, I do not know your experience level...
 
Thanks everyone!

My flying experience?
It consists of 40 years worth of….books, movies, museums, and (non-flying!) scale model planes…

And two whole MONTHS of playing with RF7….
I can fly ANYTHING (except Helis!) as long as they are:
A- On a sim (no innocent bystanders to harm!) and…
B- Perfectly balanced (meaning, not tinkered with by me!)

Seriously, my main interest in the sim is tinkering with vintage, large scale planes to handle “realistically”…..takeoff, land, taxi, repeat…Biplanes, War Birds, Golden Age stuff. I keep the physics at max and the wind at zero, just so any changes I make are not watered down or overly influenced by other variables.

But whenever I take off in a tail dragger (there are other kinds?!), I always “drive it on the wheels” for a bit after the tail lifts, even if I don’t HAVE to, even if the plane doesn’t WANT to…. for pretty much no OTHER reason than, well, thats what I saw all the Corsairs and Jennys do in “Baa Baa Black Sheep” and “The Great Waldo Pepper” when I was a kid….

Seems you can fudge takeoffs with reduced throttle to look “real”, landings not so much…

So to that end, I’m up-sizing a few favorites to increase wing loading and stall speeds to get them to settle nicely onto the field more like “real airplane”of that type would.

And I’ll bet I’ll bet that’s exactly what happened with my ground looping issue….the plane had “just enough” tail drag to land straight as it was…Then I added x% size, and x% weight to all the airframe parts, but proportionality isn’t linear…the added tail weight isn’t working, it doesn’t “match” the additional size I added….

So, I’ll bet I have more kinetic energy in motion to slow down “back there’ when landing…. its overwhelming whatever increase in drag the size increase gave me, and she spins when ground contact makes her decelerate faster…like a trailer jack-knifing when you hit the brakes…Thats why I like posting here...the replies make my think throught mechanically what probably happened, and suddenly Aerodynamics its not that "abstract" anymore,

So like Jose says, my next step is to learn the how and why of airfoils….and I’ll most likely need some help making nuts and bolt sense of those intersecting graph lines in the Airfoil diagrams.

And so far everyone on designer forums have been awesome for getting help, making “mechanical sense” I can apply out of aerodynamic theory….
You guys rock
 
As 12oClockHigh posted, bare in mind that the behavior you observe is what many of the real world RC biplanes do, particularly WWI models.

In another sim ( cough * Aerofly 7 * cough ) I can't even land the GeeBee R2 without getting into a ground loop situation.

I've flown various size RC models of the GeeBee and have no such problems, if anything the Realflight physics closely mirror what I experience at the field.

BTW: Your "Back there" analogy is not correct.

The center of mass & C.G. of the aircraft is placed in a location to PREVENT the tail-to-nose reversal you are talking about. Bring the C.G. back far enough and a plane in flight will favor the tail-forward.

Instead think about the angle of attack.

When I see novice flyers having a hard time dealing with takeoff and landing yaw at the field, the first thing I do is I adjust their tail gear to make the plane more level when it is sitting on the ground. They are surprised at the results.

That keeps the tail in the prop's airflow and keeps the wings from having more lift than they should once the wheels touch down due to the increased angle of the tail sitter.

Hint: You'll almost NEVER see a trike gear airplane get into a ground loop situation... why?
 
Motto Guzzi,

It may be a hard pill to swallow, but I suggest that you just fly the stock aircraft and stop your custom modifications for a year or three. If it does not fly like you want it to, move on to another aircraft. The planes are small because that is the way it is in Real Life. You need to learn to keep the aircraft close by. That is the single biggest mistake new RC pilots make. The other is showing up at the field with a P-51 Mustang.

You can certainly ask about aerodynamic aspects and simulator aircraft editing. Push the "learning to fly" way ahead of the designing. You need to fly other designs so you know what to expect. I have and love my WW1 and WW2 aircraft, but they are not for novices. Fly the trainer until you can make spot on landings on the runway center-line every time.

The only guys that were successful designing aircraft without being pilots or learning from other successful designs were from Wilbur and Orville's days. We want the flaws in the WWI aircraft, because that is the way they are supposed to fly.
 
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Gents, all comments are understood and appreciated, no bitter pills for me to swallow, I'm quite thankful you take the time to respond to my endless jabbering.

Dont worry, I'm not frustrated...I'm only changing things to grasp what some of these variables mean.... which ones are the heavy-hitter gross manipulators and which are fine-tuning variables…...flying stock planes is great and I spend a lot of time with my favorites, but for me, making terrible mistakes with design changes on a few test beds is actually just as much fun..

I race vintage class motorcycles, and when adjusting frame or steering geometry I need to picture things like a mental “see-saw"...move THIS fulcrum point, what happens...put a fatter kid on THAT side AND move the fulcrum, what happens...one way restores balance , the other compounds the effect of the fat kid...is that what you WANT to happen…and what OTHER handling aspects will be compounded or negated if you DO….I cant modify (or interpret the consequence of) any frame geometry “on paper” until I see it in my mind as pulleys and levers, loads and resistance….I’m having fun trying to get a bit of intuitive comfort with airframe basics, something I could never do without RF..

And again Jose is right… the rudder is so low from the humpback in the fuse, plus the lower prop spinner riding on the Inline engine crank shaft (instead of centered like a radial) its probably seeing less a lot less propwash than other more “typical” designs…so my hypothesis of added weight doesn’t fly, (pun intended) and I LEARNED something valuable for next time…which is awesome.

And the best thing of all, theirs no road rash here!
 
"Instead think about the angle of attack." and
opjose said:
Hint: You'll almost NEVER see a trike gear airplane get into a ground loop situation... why?
Click to expand...

Hmmmm....I get what youre saying about angle of attack and tail draggers…I’ve noticed the “bouncing ball” effect on bad landings on tail-draggers ….you are in a descending glide (maybe just about stalled, maybe not?) with wings nearly level and you hit too hard... the gear posture allows the tail to drop as the plane “pivots” from impact, and the sudden angle of attack change creates momentary lift …

Lift that (I’m guessing) wouldn’t BE there if the wings stayed level…so you go up, stall, nose down to another impact pivot, and go up and down again.... 4 times in a row like a bucking bronco, wondering “Jeezz, I’m below stall speed, how can this happen?”…

my bet is, you are below stall speed for a certain angle of attack..and you exceed that angle when the tail drops after main wheel impact, and that bouncing cycle begins…

But…as prop and fuse are perpendicular ( perfect 90 degrees like a T-square for arguments sake) the prop wash is always parallel to the fuse as their relative positions don’t change…so it should be(a) constant based on RPM regardless of pitch angle, tail dragger or trike..…..unless the GROUND has something to do with it….

Maybe on Tail Draggers, the ground so close to the tail reflects/disrupts the prop wash over the rear, (causing less straight line stability at low taxi speeds?) and that doesn’t happen with trikes because the tails have added clearance and get more air flow all around them?

Or, does the (greater, yes?) angle of attack (main wing and/or horizontal stab.) due to a tail draggers posture cause some kind of lifting/wobbliness at lower speeds, that a level-wing posture of the trike setup dosnt experience at the same speed….

Or….. a little of both?

(and just for the record, I can bounce-land a trike just as good as any tail dragger…the springy gear is the key! LOL!! The ground loop I’ll be working on shortly!)
 
Tail draggers with high thrust lines and short tails ( aka BiPlanes ) tend to ground loop as their tail comes DOWN.

When this happens there is a momentary increased in lift from the wings, and less authority from the tail.

Inevitably the amount of lift is unequal, making one wheel lighter than the other and starting the plane swinging around ( hint: Toe-in helps as does slightly gunning the engine to stop the loop from happening )... after that things get out of control quickly.

Spread the wheel base out and things improve, lengthen the tail and things improve, lower the thrust line and things improve, level the plane's ground attitude and things improve, decrease prop size and things improve, etc.
 
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opjose said:
( hint:....slightly gunning the engine to stop the loop from happening )

Wow, thats funny....I was just waching "The Great Waldo Pepper" with my son last night, and the Jenny's always goosed the throttle right as tail came down when landing....Couldnt figure out why they did that (arent you trying to slow down? Are you just doing it because the DIrector thought it sounded cool?) and now that explains it....

What is it about a smaller prop that helps?...I thought maybe going a little BIGGER, or a touch more pitch, might make a little more "breese" past the tail and help out, but I didnt want that to become more torque in the airframe as the prop tries to move more air...
Click to expand...
 
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