Question:

I fly a Tony Clark (Practical Scale) Tiger Moth, 270cm span, 38cc with gear reduction unit, 31.5 inch prop.
Made already hundreds of safe landings with it, like to fly continuous touch and goes with part power, very scale like.

It has a peculiar landing effect which I normally compensate for (increased rate of descent shortly before flaring) but yesterday I was explaining it to a friend and decided I just let it go once, see what happens:

Made a very flat powered approach with normal final speed, a condition from which even without flaring a good landing would result. No wind conditions, so no shear, did not move power.
With wheels about a good foot above the grass, rate of descent increases.
Alas for me the demo went a bit too good, prop broken and axle of gear drive needs re aligning....

I do not recall this effect from other biplanes, nor can I remember if the 1/1 did like this, as I flew it for a few hours, long time ago.

The only explanation I can think of is that the lower wing picks up ground effect, but as it is largely behind the CG, it produces a pitch DOWN effect.

Anyone recognises this effect from other bipes?

The only thing I can think of is that if there is wind, it will blow less closer to the ground.
So when landing in headwind, a rough grass field could possibly create an effect like that...

But if there was no wind, as you say, then I have no idea.

Mr Cox, indeed, I tested w/o wind as said to avoid the effect of windshear (decreasing wind velocities near the surface) Normally models (and all 1/1) aircraft suffer from this phenomenon every landing (unless landing downwind....) and any pilot, model or fullsize will compensate for the increased rate-of-descent automatically without thinking.

Maybe you are flying to close to a black hole! Just kidding,but that does seem strange. Usualy the air close to the ground doesn't move around as freely as it does higher up,and I don't see where a biplane is any different. there has to be a reason though.

If you are landing on a paved runway, especially a blacktop runway, in hot weather, (especially on a sunny day), the surface of the runway will be hotter than the air temperature. Under these conditions, there will often be a layer of hot air just above the surface of the pavement. This layer of hot air will be greater in no wind/low wind conditions. This layer of hot air will tend to be less dense than the cooler air above it. As air density decreases, lift decreases. As your airplane decends into the hotter and less dense air, the wings are suddenly producing less lift and the stall speed increases.
Regards
JC

But doesnt that hot air rise (aka thermal) causing lift?
Sometimes runways will have "sink holes" In the fullscale world, at our airport, runway 26 always has a bit of a sink hole about 1/8 mile back from the threshold of the runway. I think that its from uneven heating in that case.
Orrrrr, maybe that is where the rc gods like to try and strike down aircraft

This is all starting to sound like Stargate to me, where is SG-1 when you need them...
(I should watch less TV...)

Quote: "But doesnt that hot air rise (aka thermal) causing lift?"

Since heat causes air to expand, it becomes less dense as it occupies a larger volume and therefore it tends to rise. However, hot air tends to rise unevenly. Often the hot air rises in circulating columns (thermals). This is great for buzzards and sailplanes. But when you are coming in for a landing and "flair", some other factors come into play. When you "flair", your plane will be in a nosehigh attitude, the wing angle-of-attack is increased, total airdrag is increased, and airspeed decreases. Here is where the layer of hotter, less dense, air can ruin your anticipated (hoped-for) smooth landing. A sudden decrease in air density results in a sudden loss of lift, and the rate-of-decent suddenly increases. At this point, let's hope you have a good strong landing gear and plenty of prop clearance.
Regards
JC

I agree on the hot air thing and over a black tarmac on a sunny day it is a factor, but I thought he landed on a grass field...?

_______________________________________

I dunno.
Here's a rule-of-thumb:
When it's hot.....land hot.
Regards
JC

JC,
Not a bad thought, but it was a grass RWY.....
And it does it always, every landing, no matter what temp. Evening flights are a delight with this model, no thermal/temp influence then.

Btw, correct spelling is landing flare, not flair....

Nothing wrong with landing with flare and a bit of flair...

From having flown a lot of free flight there's no natural flare. They just keep gliding until they kiss "the big green thing".

However in your case if you set up the landing approach so it was coming in with either fixed trim or you are sure you held the sticks with no travel then I suspect there's some freak case of the wing wash reacting with the ground and lifting the tail. I have no idea why but if you're sure that there was no possibility of a "stick thermal" or "stick downdraft" then it has to be some odd effect of this one design.

Mind you, given the results of this demo I'm not sure you're game for more testing....

BM,
I make hundreds of touch and goes lately with my Tiger, a real crowd pleaser and so much fun to do.
But right from the beginning I bought the model, I noticed it, thereby correcting it "automatically".
Due to the number of roller landings, also made by my son who confirms the behaviour we can be 100% sure the effect is there and not caused by fingertrouble.
In the mean time I have consulted a befriended aerodynamics specialist plus some extremely well experienced model flyers.

The influence of the ground effect is a complex one, there are many interactions with terra firma during the last part of the flight. Normally having little effect on pitch, but obviously this plane does.

We considered the lifting of the tail as a factor. Take into account that the Practical Scale Tiger Moth has the tailplane set a 5 degr pitchup. That in itself is quite large, however biplanes have "strange" setups on this matter. But as I said, I notice the effect during touch and goes, aiming for for 2 point landings, the tail sits high and to my guessing outside the ground effect.

One of my experienced friends used to fly a Beech Staggerwing model, he now recalls it was a drag landing this model, as it just did not want to stop flying when landing, the opposite effect thus. As this Beech type, as the name already says, has its lower wing staggered or in front of the top one, it would confirm my thoughts of the lower wing influence.

Coming back on my original post I must say that the higher sinkrate is not only caused by the pitch down effect, but it looks like a general lift loss there, combined with some pitch down. of course the short time span in which it happens makes a precise judgement difficult.

My aerodynamics friend mentiones he had a collegue who spent 10 Years investigating ground effect and still ended up with more questions than answers...




The is not a clear conclusion to be made

Might not be germain to the problem posed but I have noticed exactly the same thing with my Waco. I can only put it down to incipient stall, as with a couple extra notches of throttle trim, and slightly less 'back stick', the problem does not manifest. Could be, with our lack of airspeed instrumentation, the effect of 'apparent airspeed/ground speed' as the airfield comes into vision under the wheels means that we cannot accurately see the true airspeed, and might be just getting a wee bit slow in those final few seconds of flight.
Evan, WB #12.

Since it's not a problem that has come up on any model I've ever flown including a 4 ft span biplane I'm going to fall back on my previous suggestion that tis is a freak occurace for this one particular design. Something about the overall setup and the air off the wings is "packing up" under the stabilizer and forcing the nose down. If the air off the wings is diverted by the ground effect and alters the apparent angle of attack of the stabilizer then yeah, it could push the nose down and resulte in these "arrivals".

But I stress that this is not the case for the vast majority of designs and not something I've ever seen.

BM,

I agree.
Have a long lasting model career and also never experienced this, at least not to the point I noticed.
Flew several bipes, models and 1/1 ones the like.
Along with a professional career as an airline pilot, plus thousands of hours flying GA airplanes I do understand the groundspeed/airspeed effects, which is not the case here.
(btw I drive a motorcycle as well )

I thought the behaviour was interesting enough to spend a topic on the matter....

Ground effect is indeed a mysterious thing.
Years ago I had another strange phenomena with a Graupner Delta, I think it was an X-2000 or something.
Installed a hefty prop engine at the back, plus retracts. Ended up so heavy it failed to t/o when the grass was just a bit too long, or on a windless day.

One time I got so frustrated and let it go till almost the last part of our flying field, riding nose high and cutting grass, not wanting to lift off or accelerating more! At the very last moment I cut the throttle but to my surpride it leapt into the air as a result!

Figuring it out, the speed plus high nose angle did create enough ground effect to get airborne, but the prop was sucking the air pressure away underneath the wing. The moment I took the power away, the pressure recovered and pushed the model in the air! Almost comical to witness. Needless to say I was so surpised I forgot to add power again and it ended up in the next field.
Lost the model soon after that, If I remember correctly due to some interference.

A guy in my club had a Great Planes Cherokee that did the same thing. That plane would stick to the ground like a F1 race car. Must be some thing with the wing incidence.

This rubber plane from 1936 had the same trait. When the motor was fully would and the model hand lunched it would fly as normal. However if it was lunched from the ground it would roll on the ground for 3/4 the motor wind then leap into the air vertical until the motor unwound then stall and crash.

When a tri-geared model tends to stick to the ground, it's usually the landing gear setup. The fuselage has a somewhat nose-down angle when the plane's on the gear. This tends to hold the model down until enough airspeed's built up that the elevator will work. Usually, then, the model will balloon into the air very quickly, and a touch of 'down' will be needed to put the plane into the proper climb attitude.

If the rubber model had a lot of downthrust, it's entirely possible that the relationship between landing gear position, wing incidence, and the amount of power produced early in the run caused the model to not be able to nose up enough to get into the air. As power bled off, the downthrust effect would have lessened. When hand-launched, the model was likely able rotate to the proper flying position, something prevented by the relationship of the landing gear to the rest of the model, when combined with the motor's downthrust.

True.
Try flying a Cessna out of a snow covered airfield.
The nosegear will dig in and pulls the nose downwards increasing the negative angle of attack of the wing, building up drag big time. You never reached liftoff speed.
The only way out was to lift the nose out of the snow, accelerating on the mains only and flying out safely.

But again this has nothing to do with the origin of the thread: groundeffect.

Loved the stories. It's things like this thread worthy 'Moth issue and stuff such as your delta hopping off the ground that keeps us humble.