The aeroplane doesn't know it's windy...

[leccyflyer]

..is a phrase that is often heard.

Well, the aeroplane doesn't know it's Thursday either, but yesterday evening I encountered some rather strange behaviour on several flights, that I'd attribute to the
very windy conditions, but I'm not completely sure how and why.

We had the local Scouts to the club field yesterday evening for a first go
on the buddy lead, using the club trainer. So that they were not all
waiting for the one aeroplane I also remotored our venerable Modelshack
Cricket to share the load. This is a Dave Ridgeway design, with a
passing resemblance to a Uno-Wot, previously powered with a geared Mag
Mayhem motor and 8xRC2400 NiCds. I fitted an Emax 2820/7, 3s1p3000mah
Tipple Lipo and an 11x6" Graupner folding prop for about 300watts.
Plenty of power for a trainer.

We had an 15-20mph crosswind, according to the windsock, gusting considerably higher than that, but didn't want to disappoint the Scouts, having had to postpone a previous attempt due to bad weather. So we just went for it.

Taxiied the Cricket out and made the take-off run towards the corner of the field-
giving a 45 degree crosswind component, after having the wing lifted on
trying a straightforward crosswind approach. A direct into wind take off
was impossible, as it would have been directly towards us.

Immediately on take off the model went vertical, on about half-throttle. I managed
to catch her just the once and force the nose down with a bootful of
down, but it was an un-nerving amount of down elevator to be putting in
that close to the ground. Twice I put her into a loop on the way up, to
maintain control. The Scouts thought I was just showing off, but nothing
could be further from the truth. Once established in straight and level
flight the model was as good as gold, needing a couple of clicks of
down elevator and all but one of the landings, in difficult conditions,
were perfect.

So, what was going on? Even after trimming out for  the conditions she still zoomed horrendously on take off and was as good as gold once above thirty feet. The transition from being in the ground to being in the air was fraught, but the transition from being in the air to being on the ground was smooth and perfectly controllable. Yes,
it was undoubtedly turbulent and very windy, but this was very unusual
behaviour, repeated on each flight.

Any ideas?

Edited By leccyflyer on 15/07/2011 10:08:22

[Bob Cotsford]

the model doesn't know it's windy in a steady wind once above the ground effect.

I got caught out on Tuesday and lost my Gladiator (no great loss really). The model leapt off the ground after a short run, dropped a wing and stabilised once at about 5'. Unfortunately it was now flying cross wind into the lee of a tree, where it suddenly found relatively slow but turbulent air, it couldn't accelerate fast enough to maintain flying speed so down it went. At least that's my analysis of it.

We often forget that the model has mass and inertia, and if the windspeed immediately around the model drops, the model must accelerate relative to the ground in order to keep flying speed. This isn't usually a problem unless it's flying really close to its stall speed to begin with as when taking-off or landing.

 

In your case it sounds as though the model got out of the shear layer at ground level into a headwind so it suddenly had mush more airspeed than expected and zoomed.

Edited By Bob Cotsford on 15/07/2011 10:22:10

[Steve Hargreaves - Moderator]

Well....an aeroplane doesn't know its windy in that the air it is flying in is moving across the ground. It is simply flying in air.

 

However the fact that this air is moving across the ground will result in the air being forced in different directions to go around obstacles etc & this makes the air very turbulent containing updrafts & downdrafts & the model will certainly respond to these. Turbulence generally decreases with altitude so the higher you get the smoother the airflow.

 

Why it would affect a model more on take off than landing I can't answer for certain unless you had some very serious wind shear with the speed of the airflow increasing dramtically between the layers. This would give rise to the model suddenly seeing an increase in relative airspeed as it moved from one layer to the next & this extra airspeed would result in extra lift......this is how an Albatros can fly for so long without flapping its wings

 

Beware this condition when landing because the reverse happens....a model flying quite nicely in one layer of air can decend into slower moving air & fall out of the sky because its relative airspeed is below the stall speed.....

[Bob Cotsford]

Bet you had a higher than usual airspeed on landing to counter the turbulence, so wind shear had less effect.

[leccyflyer]

Airspeed on landing was higher than 20mph, but groundspeed was walking pace. Luvverly The model has always been a real floater and the wind helped landing the model in the width of the strip.

[Ben B]

Once in free air the plane is flying in the wind so no problems, as mentioned the problem occurs on take-off and near the ground because of turbulence. Also, gusting causes problems because the air speed cannot increase as quickly as the wind-speed can. Add in momentum and you've got problems.

[Former Member]

[This posting has been removed]

[leccyflyer]

Posted by Tom Sharp 2 on 15/07/2011 20:25:06:

The aeroplane doesn't know its windy.....

 

Also the engine doesn't know which way up it is...

 

So why do so many people have constant trouble starting an inverted engine?

 

 

 

 

 

 

 

 

I'd have had real trouble installing the brushless outrunner inverted......

[Brian Cooper]

An aeroplane might not know it is flying in windy conditions, but the pilot certainly does.

 

Different techniques are required for differing wind conditions, and anyone who thinks he can fly in the same old way in any conditions will soon be proved wrong.

 

B.C.

[Tim Mackey]

Amen to that - I still don't go along with the whole "it doesnt know its windy" theory - try explaining that when your flying in 80MPH slope conditions.

[Tony Smith 7]

It holds just as true on the slope, I've been there plenty of times both as a modeller and as a pilot.

 

However there are a lot of very powerful illusions to contend with. As a pilot your frame of reference is the aircraft, with the landscape passing underneath, and you have to consciously tune out the changes in ground speed with different courses. It took me quite a while before I could smoothly circle in front of the hill without being freaked out both by the changes in ground speed and by the sudden sight of the hill as you head back towards it.

 

At height and away from the hill its easy to just forget the ground.  I've spent plenty of time circling "hands off" and only the GPS tells me that my ground speed is varying between say 5mph backwards and 55mph forwards.  There's no difference in feel whatsoever.

 

As a modeller its probably even more difficult as you're standing on the ground but trying to feel the airspeed of the model. My experienced pal piled my new glider in on its first flight when he turned downwind over the top of the hill .. he let the airspeed drop too much and suddenly had no control.

 

The big difference on the hill is the change in wind speed sometimes over quite short distances. For example at one cliff take off you could be standing in 50mph barely able to keep the glider under control, but the wind once you're up could be only 15 or so. At that same site you need to land two fields back from the cliff or you'll get trashed by rotor.

Maybe the answer is "the aeroplane doesn't know it's windy but it most certainly knows when the wind changes"

Edited By Tony Smith 7 on 16/07/2011 10:34:16

[Tim Mackey]

I quite like that final sentence Tony - much better

[Martin Harris - Moderator]

Very well explained Tony.

 

Timbo, next time you're flying in 80 mph (or considerably less) fly away from the hill and slow it down well below wind speed pointing into wind. It will flly backwards. Now bank into a turn. What will the glider do? Will it know that because it's flying backwards it had better stall? No, it will circle for as long as the bank is held constant describing a perfect cirle within the air it is flying in. The fact that this air is moving towards your hill is of no consequence to the model but observed from your fixed viewpoint the to the model will follow a very odd ground track. Trying to maintain a constant circle from your viewpoint would require you to slow downwind and accelerate upwind and that's why people suddenly find their models stalling off a "downwind" turn or zooming into an "upwind" one as they leave a little too much elevator on from tightening the "downwind" part of the turn.

 

Let the model do this low down where it's encountering turbulence from curlover from the hill or ground obstructions and it's a different story. The model will rise up and down with the air it's in and the effective angle of attack will change instantaneously as it crosses between these masses travelling in different directions. That's where the extra speed margin that you use when expecting turbulence gives a safety factor to keep the wings flying during these episodes.

Edited By Martin Harris on 16/07/2011 11:02:14

[fly boy3]

here is another answer via our friends over the pond quoting a Brit.! Cheers   Hope it helps, not hinder the thread.

Edited By fly boy3 on 16/07/2011 20:52:42

[Ian Jones]

"The aeroplane doesn't know it's windy" - the aeroplane doesn't know anything about anything. A phrase I put in the bin with "Every landing is controlled crash". (Every crash is an uncontrolled landing).

[Biggles' Elder Brother - Moderator]

Posted by Ian Jones on 16/07/2011 22:56:24:

- the aeroplane doesn't know anything about anything.

 

 

Oh I beg to differ! Mine know when people are watching - that's when they flop nose over in stall turns, screw out of loops, loose height in rolls etc. They don't do it normally - only if someone is watching!

 

BEB

[Ian Jones]

LOL. Ok BEB, I accept defeat

[John Olsen 1]

Well, as per the other answers, the engine may not know it is inverted, but the little puddle of cooked oil that collects in the glow plug surely doesn't help with the starting process.

 

Mind you, they are so much easier to start these days anyway, with one of the electric gizmos...wish I had had one when I was a teenager!

 

We had a gloriously calm day at the field today, about 8km/h breeze according to one source, and even in that breeze you can see the effects on the model from wind sheer and the trees. (At least with the slower models, nobody was flying any jets..)The glider guys were picking up a bit of lift over one line of trees to windward. Note that it is winter here, in the roaring forties, there has been the odd tornado around the country lately, so a day like this has us wondering why it has gone so quiet.

 

regards

John