RCU Forums - View Single Post - CLmax noob question
View Single Post
Old 12-24-2020, 04:22 PM
BMatthews's Avatar
Join Date: Oct 2002
Location: Chilliwack, BC, CANADA
Posts: 12,393
Likes: 0
Received 17 Likes on 15 Posts

Airfoiltools has a Rn calculator you can use to get a feel for the range that a model of a set speed range and size might encounter.

Reynolds number calculator (airfoiltools.com)

And if you pick and look up an airfoil and go to the Airfoiltools link that comes typically near the top of a search you get the Xfoil predictions. For example let's look up a Selig 3021. <--click the blue link

First is a picture to admire. Next is a box full of similar airfoils. Then comes the list of Reynolds numbers and assumed conditions used for each curve in the charts that follow below that list.

You'll see in the Cl vs Cd chart that each curve reaches a different max Cl value and then rolls off. Some roll off slowly and continue to climb others hit a hard peak and fall off like the S3021. But that's not the whole story. If you look at the Cl vs alpha (angle of attack) you'll see that the Cl maxes out at around 10 and higher. But if you look at the Cd vs alpha you'll see that the drag turns around a bend and starts to rise more strongly at around 7 to 8.

What this means is that unless you have a lot of power you're going to find that as the alpha rises so does drag. And at some point on a glider or low power setting the drag will overpower the forward push and that makes the model slow down and start to descend. Or we see that happening and pull back slightly. Either option results in the alpha increasing and pushing deeper into the fast rising drag with alpha. And really fast all this builds up and the model stalls and comes to almost a halt for a moment before the nose drops and down it comes.

Or we have more than enough power on hand and the model flies on in "harrier" mode hanging half on the prop thrust and half on the wing. Because wings still make lift even when stalled. They just make a lot of drag along with that lift.

Play with some examples in the Reynolds number calculator and you'll get a feel for things soon enough. A 2M Gentle Lady glider for example might have a minimum speed of around 10mph. And it has a roughly 9 inch chord. In the metric values (use the Windows calculator in the units conversion mode to make this easier) I get 4.47 m/s and 0.228 m for the wing chord. And hit "Calculate" and I get back 71.7K for a value.

So clearly more focus could be put onto the 100K curve than on the 50K curve.

By the same token a Gentle Lady would be maxed out at what any of us would consider still gliding when it's nosed down and perking along at around 20 to 25mph. And I'm thinking even 25 is pretty generous and that more likely it would be in a shallow dive by that time. So let's say 20mph. The chord is the same so we're looking at a doubling of the Rn to 144.4K.

So to some extent we want to look at the 50K curves as a hint of possible low RN hiccups. We'd pay close attention to the 100K and 200K curves and ignore the higher ones. The actual performance in real world flying would occur as a fuzzy cloud between these curves based on what we do during any moment in the flight for flying speed and angle of attack. For example when in a tight turn the wing's angle of attack is higher and we might be pushing up towards CL=1. For casual flying along at a speed which moves around somewhat to look for lift the Cl might be down around 0.1 to 0.2. When flying near the stall very slowly the Cl might be up around 0.5 or 0.6. You'd want to run the numbers on that

So put some time into playing with these features so you can build up a feel for what the actual models out there are doing. You have to know that before you know what to look for in Airfoiltools.

I found another calculator for Cl values. I'm not sure how good it is because it does not allow for span and chord. Just area. But when I punch in the slow fly numbers from above and use a density for air at 1.204 for air at sea level and 20C I get back a Cl value of 1.65. Obvious that isn't happening. So my guess of 10mph and a 910gm(32oz) model is way off. Dropping the weight to 800gms (28oz) and raising the speed to 5.5 m/sec (12.3 mph) gives me back a far more realistic Cl for slow flight of 0.96. And looking back at the CL vs alpha that appears to be a 6 angle of attack. And far more what I expect in real world operation.

So it looks like 12 to 14 mph would be our slow range flying speed for a slightly under 30 oz Gentle Lady glider. And that means that the top cruise but not quite shallow dive speed would likely be up around 25 mph. Of course all this slightly increases the Rn values so suddenly the 100K and 200K plots have a lot more meaning as our likely upper and lower boundaries for normal flying as we move around the sky flying straight and through moderate angle turns.

Hope that helps out.

Last edited by BMatthews; 12-24-2020 at 04:24 PM.