stunthanger.com
Design => Engineering board => Topic started by: Chris Wilson on October 27, 2011, 03:40:27 PM
-
Not sure that this is the right place to ask this quandary but lets see if the mods will let it settle here.
(I expect this to evolve into some kind of formula as a solution.)
Why does the models forward speed wash off when landing upwind?
I 'was' thinking that the airflow that is supposed to be across the wing gets diverted along the wing and so lift is destroyed, but that is also the case when landing downwind and no ill effects are felt there.
Hmm.
-
The ground speed is lower than the airspeed. i.e. A plane gliding at 40mph into a 20mph wind will have a ground speed of 20 mph. This can cause multiple take offs and landings if you aren't careful.
-
Hi Phil,
I see that I am not have been perfectly clear in my original post.
I mean 'dead' upwind as in the lines are pointing into the wind. A gliding model seems to stall very easily even when whipped through the uphill part of coming into the wind.
Perhaps its just a the residual effect of losing airspeed before the upwind point is reached but I don't think so
Upwind just seems to be a dead zone when on the glide.
-
Rank beginner here but I have always assumed (Dangerous).. It is because the combination of the slower airspeed (Having spent the previous half lap gliding into wind) and reduced control authority due to less line tension than when downwind means the model is just not as responsive to control inputs so it's probably not stalling at less airspeed in so much as it's just not responding to the pilot trying to "flare" the landing quickly enough. But yes I have noticed this, and will always drag the model past the upwind part of the circle before attempting to land.
TTFN
John.
-
When the engine quits the model bleeds stored energy. Heading into the wind the drag coefficient of the model and the lines cause the model to bleed energy at a faster rate. When the model gets within one wing span distance it comes in to the ground effect region. This produces additional lift. So you combine the reduced air speed, higher energy bleed, and ground effect. These conspire in the loss of line tension, slower forward speed and higher lift. This results in those hard to control "Balloon" and bouncing landings unless you actively and consciously fly the model towards the ground. Whipping helps add energy back into the model which helps maintain line tension and some forward air speed.
Models that are less draggy whip better than those with higher drag, funny that heavier models also are better at this because they have more stored energy to shed than lighter models due to the need for more power to get them to move in the first place.
When the model is dead up wind and the breeze is blowing parallel (cross wind) to the wing the air speed that the wings see is cut, effectively the airspeed of the forward motion of the model minus the speed of the wind. As the model has not yet reached the down wind leg, where the wind helps add some energy back to the model (push it) and it is past the upwind leg where the model is bleeding more energy yet the effective airspeed the wings see is the forward speed of the model plus the wind speed, results in a null zone where things can get really tricky.
Its a balancing act, impart enough energy by whipping and help from the wind during the down wind leg to get have enough energy to bleed off on the upwind leg so you have enough line tension to get past that null zone.
-
A plane that is not ideally balanced (i.e., tailheavy) will slow quickly and won't want to glide a full lap...like most combat models, OBTW. It's either that, or lack of piloting skills? :-\ Steve
-
A plane that is not ideally balanced (i.e., tailheavy) will slow quickly and won't want to glide a full lap...like most combat models, OBTW. It's either that, or lack of piloting skills? :-\ Steve
Hi Steve,
at the moment I will go with option B "lack of piloting skills."
I admit that and got caught out on the glide, its just that I found it curious that dead upwind is dead zone for lift and wondered why.
Oh, and the model was a new Thunder Streak powered by a new Merco 29 and trimmed fairly well - so its all my own doing here!
-
Increased drag from oncoming airflow.
-
Increased drag from oncoming airflow.
Going into the wind, yeah I agree, but being dead upwind is the position I am talking about so there is no 'oncoming airflow' - the oncoming wind is all span wise.
Perhaps its due to the speed wash off finally manifesting itself upwind and the damage is done getting there in the first place.
-
OK : Dead upwind.. so Infront of the Judges you mean ?
Thats the same Phenomena, your forward airspeed compared to the airflow moving spanwise from the crossflow of air isnt enough to maintain adequate lift over the wing.
Wings need airflow moving over the wing from L/E to T/E, With a substantial cross wind, you have airflow moving from Outboard to Inboard - There is very little airflow to maintain lift.
-
For a full size airplane, spanwise flow has a deleterious effect due to a thickening of the boundary layer. This is the case on swept wings. Whether it influences a model or not, I can't say.