Q: I’ve been in several conversations regarding this question with pilots from all levels of experience and knowledge, but am yet to receive an answer I find to be satisfactory. In my personal experience I have soared (exclusively ridge lift) sand dunes as small as 5 vertical ft and mtns reaching thousands of vertical ft. I’ve observed that it takes a MINIMUM of 15mph to stay aloft on the small sand hills, whereas I’ve ridge soared larger mtns in 8mph wind. Although, this probably doesn’t sound surprising, what accounts for the difference? Put simply, why does it take more wind to soar smaller hills?

A:  That is a GREAT question!  The answer is two-part.  The first part is that we are flying gliders, and as such we are constantly sinking through the air.  If our sink rate at min-sink airspeed is about 200 ft/min, that is a constant.  If you watch a glider flying by and not gaining or losing altitude… then they must be in air that is rising at 200 ft/min.  The air rises at 200 ft/min and the glider is descending at that same rate, netting no visible altitude gain or loss of altitude.  Simple enough, right

We can apply this principle when looking at ridge lift.  We need the vertical component of the airflow to be 200 fpm or better in order to soar.  The vertical component is an important clarification.  200 fpm is less than 2.5 mph… and obviously we need more than 2.5 mph of wind to ridge soar even a vertical cliff face.  The reason is that air is a gas, and gasses can be compressed.  Wind is really air moving from some area of higher pressure to some area of lower pressure, and we feel “wind” as that air moves past us.  This can be air moving large distances, such as cold air from Canada blowing down into the US, or it can occur on a macro level in the form of a “gust” before or after a thermal passes by.  Same cause- differences in pressure.  And the air movement is lateral… from higher pressure at point A, to lower pressure at point B.  Terrain just gets in the way of this lateral airflow… and if that terrain faces the right direction and has the right shape, the air passes over the terrain creating ridge lift.  But above this lift is still horizontal airflow, and that causes compression of the airflow and “squashes” the vertical component of the airflow.  Simply put, a little hill creates a little vertical component, but it’s easier for the air to just bend around the little hill so less air is forced upward.  A big mountain, on the other hand, creates a much bigger change in airflow… and that airflow will have a greater vertical component (even if the sand dune and the mountain are the exact same slope).

The second reason is related to the first, but has to do with Newton’s laws of motion: One being that an object in motion will remain in motion until acted on by another force.  In this case our “object in motion” is the air (ridge lift) and the other force is the descending hang glider (creating lift and “pushing” air downward).  In my earlier example I said a hang glider descending at 200 fpm, but flying in air that is rising at 200 fpm, will remain at the same elevation… and while this is true, in order to get air rising at 200 fpm past the glider it actually needed to be rising faster than that before it encountered the glider, because the descending glider exerts a force on the rising air and slows it down.  I think this is the bigger reason for the phenomena you mentioned, where small dunes need more wind than big mountains.  The vertical “lifting” component of that wind is clearly one aspect, but a small cliff is still a cliff, and a big sloped mountain is still sloped.  What is different is the VOLUME of rising air.  The force the descending glider exerts on the air it flies through remains constant in either scenario, but since momentum is the product of mass and velocity (momentum = mass x velocity), a larger volume of rising air will have more mass.  Yes, air has mass!  The tricky thing to wrap our minds around is that the larger volume of air, with it’s more momentum at a given speed, exerts a greater force on our wing even though it contains the same exact vertical component of rising air.  Real life example is that getting hit by a ping pong ball going 10 mph doesn’t hurt all that much, but getting hit by a car going the same speed might leave a mark.

Air density can also be a factor, where denser air (lower altitude, colder temp, less humidity) will have more mass than the same volume of less dense air… which I suspect is why we can ridge soar in lighter velocity winds in the winter (the colder, drier air has more mass and can have the needed momentum with less velocity).

In your question you said that you have yet to receive an answer you find satisfactory… I’d welcome your input here as well!  I am not above learning too, ya know?

Cheers!

1/16/14 EDIT: “Unfortunately the physics behind this isn’t quite correct. It is ultimately the density (mass per volume) of air that that effects pressure (force over an area). Pressure on a hillside increases based partially on Bernoullis Law, and thus since the Force=Pressure*Area, the increase in pressure increases the force. The completely watered down bottom line is that on large hills, a much larger volume of air is funneled into a small area as opposed to a small hill where a smaller volume of air is funneled into a similarly small area. Again, comes back to Bernoullis Law.”

Thanks to Raffael Housler for that!  I am admittedly not a physics major, and clearly there is a LOT going on when it comes to wind hitting a hill!  Learning is fun!

After re-reading my post- after reading Raffael’s addition- I see that I was not so clear about why the air having more momentum makes it easier to soar.  The airflow will follow the path of least resistance.  It goes over a hill or mountain only as much as it has to and no more.  Adding a glider that is making lift by deflecting air downward will slow the air rising over the hill, and/or it can change where the “path of least resistance”.  If the rising air has less momentum the impact the glider has on it will be greater (takes very little to stop a ping pong ball, more to stop a baseball, more still to stop a bowling ball).  I have noticed flying crowded ridge sites like Point of the Mountain, that adding more gliders seems to make the lift weaker.  I always just thought that was because the increased traffic meant sharing or yielding the best lift… but thinking about it now, if you put 20+ wings deflecting air downward along a ridge that the wind has to flow over, that could be a pretty significant impact on the airflow!  Either the air trying to flow over the hill will be slowed (squashing the vertical component?) or perhaps then the path of least resistance for some of that airflow is up and over all the gliders flying along the ridge as well?!  Fascinating stuff…