Shop our Airplanes Products Drone Products Sales
Thread Tools
Jan 10, 2019, 10:41 AM
Registered User
Thread OP
Discussion

Speed vs lift/sink


Sorry if this is an elementary question but, is there a "line" that can be crossed with a sailplane that causes loss of lift if it is flying too slowly?

Seems there is a school of thought that focuses toward trying to tweak CG backwards to just above stall speed, so that our gliders just float. But is there a point that is too slow? I know simple aerodynamics requires a certain amount of speed to maintain lift. A bit below that and the plane stalls. But is the sweet spot barely above stall speed or a little bit more glide speed for optimum lift? Or does that depend upon the individual plane/airfoil/etc.?

I'm not really looking to delve into the scientific physics of airfoils and flight dynamics, just a simple question, "Is it better to fly slightly above stall speed or by flying a little faster is more lift produced as a result?" (I guess that is a glide ratio question.)

And perhaps there is no simplistic answer.
Sign up now
to remove ads between posts
Jan 10, 2019, 11:02 AM
Hates Palm Trees
It is definitely possible to fly too slow to achieve the best glide ratio for a given plane.....................The "way back" CG and the flight modes for slow flight are all relative and can vary greatly from plane to plane and pilot to pilot.

It really comes down to what works best for you, your plane and the flying conditions. Nothing can beat investing a lot of time and effort in tuning your plane so that you enjoy flying it.

Good luck!

Steve
Jan 10, 2019, 11:05 AM
Registered User
Crash:

You pose a key question for any serious pilot.

There is indeed a speed, below which your airplane is sinking faster while still going slower. That speed (on the sink/speed curve) is the speed for Minimum Sink. Go faster than that speed, and sink rate increases; go SLOWER than that speed, and sink rate also increases.

There is another, slightly higher speed for Best L/D. But, in the presence of wind, the speed that gets Best Distance for a Given altitude is (1) even faster if traveling into the wind; and (2) a bit slower (but no slower than Minimum Sink speed) going down wind.

I wish we had a white board.

The two biggest factors affecting speed-to-fly are drag-due-to-angle-of-attack (Da) and drag-due-to-speed (Ds). As you fly slower, Da rises, while Ds subsides. However, Da is linear, and Ds is a function of the square of the speed. Combing the two curves produces a sort of parabolic-looking curve with speed as the horizontal axis, sink rate as the vertical axis. The slowest end is (like you suggest) the stall point: the slowest the airplane can fly, before the wing stalls at its highest angle of attack. The fastest end is the speed at which things start to come off.

If you take a horizontal line at the low point of the sink/speed curve, that is minimum sink. If you draw a tangent from the origin to the curve itself, that speed determines the speed for best glide in still air, or Maximum L/D.

If you could read your airspeed, you would fly at (theoretically) minimum sink speed while thermaling. However, practically, you are better flying about 10% faster than minimum sink speed in a thermal, due to the fact that the elevated G in a turn requires flying a bit faster to avoid a stall. We don't have airspeed indications, so just thermal as slow as you can.

Again, if you could read your airspeed, you would fly at (theoretically) best L/D speed between thermals. But, the effect of winds usually requires flying faster than that in order to make the best distance over the ground, especially when trying to penetrate. Again, without airspeed indications, we just do the best we can. But know this: I've flown many hours in full-scale gliders, and the penalty for flying too slow is FAR GREATER than any penalty for flying too fast. In fact, I find it very hard to fly too fast under any conditions, unless in a thermal and trying to climb.

Hope this helps. Feel free to email me.

Yours, Greg
Last edited by glidermang; Jan 10, 2019 at 12:40 PM.
Jan 10, 2019, 11:46 AM
Mark LSF # 3792
+1 to what Greg has said above. When discussing how fast to fly in a thermal I tell people it is better to fly a little too fast as opposed to too slow. What one may loose if the airplane stalls will potentially be worse than one may gain if it does...
Jan 10, 2019, 02:36 PM
LSF303-AMA Fellow
tkallev's Avatar
Quote:
Originally Posted by Soarmark
+1 to what Greg has said above. When discussing how fast to fly in a thermal I tell people it is better to fly a little too fast as opposed to too slow. What one may loose if the airplane stalls will potentially be worse than one may gain if it does...
Not to mention that flying a bit faster makes the aircraft more reactive to control inputs with less movement, reducing parasitic drag from surface deflections ...
Jan 10, 2019, 03:51 PM
Registered User
R.M. Gellart's Avatar
Minimum airspeed does not equate to minimum sink. I cannot tell you how many times I have watched a pilot slowly just sink.

Marc
Last edited by R.M. Gellart; Jan 11, 2019 at 07:56 AM.
Jan 10, 2019, 05:44 PM
B for Bruce
BMatthews's Avatar
This is a bit of a "yes and no" question.

It depends on the airfoil and the wing construction. I've had two gliders that started out sinking more strongly at low speed than when flown slightly faster. I "cured" both with the addition of upper surface turbulators. The fact that this "fixed" them was indicative that the slow speed end of their operation was producing bad separation bubbles on the upper rear sides. The turbulators changed this and kept the air better "glued" to the skin and following the airfoil.

A third glider which liked to fly slightly fast and sunk more strongly when slowed down a lot was also tested with similar turbulators. In that case it didn't fix anything and the solution was to trim it to fly at the speed of lowest sink.

So do you need to fly a touch faster to get to minimum sink? I'd say the answer is "it depends".

I should also note that even with the turbulators greatly enhancing the low speed performance on the tow models I could still trim to a lower speed just on the edge of a repeating very mild stall and the models would both settle faster than if allowed to fly even a couple or three mph faster. So that fits in with what is said above too.
Jan 10, 2019, 08:41 PM
Registered User
I stand by my statement: for most conventional airplanes, minimum flying speed does not correspond to speed for minimum sink. As angle of attack rises to produce lift equal to the weight of the airplane, so does drag. And, down it comes.

Just standing and watching our models will not usually verify this behavior. You've got to devote some time and resources to some sort of objective measurement. In full scale airplanes, that measure is airspeed and rate of climb (which will be showing a descent!).

In powered airplanes, an exercise for student pilots is to slow the airplane down, and maintain a constant altitude. Below the airspeed for best endurance (which is directly comparable to minimum sink speed in a sailplane), power must be increased as airspeed decays. Right up until the wing stalls, and that is the end of the exercise.

Here is a link to the flight test reports of Dick Johnson, who evaluated (in a very objective manner) many of the new sailplanes of the eighties and later. http://web.archive.org/web/200410161...es/Johnson.asp

Yours, Greg
Jan 10, 2019, 10:21 PM
Sagitta Fanboy
Quote:
Originally Posted by R.M. Gellart
Minimum airspeed does not equate to minimum sink.? I cannot tell you how many times I have watched a pilot slowly just sink.

Marc
Greg's right, take a look at the Speed vs sink rate polar for a glider. There is a point where it peaks, and sink rate is minimum, and a steadily increasing rolloff slower than that. That rolloff is from minimum sink towards stall, but the plane is still flying as it approaches the stall.

It would be very rare for an aircraft to have minimum sink at stall speed, which is your minimum airspeed. That would have a polar that has a very distinct cliff at the minimum sink point, rather than the rolloff you see normally.

That said, the difference may not be visible to the naked eye. Put some airspeed telemetry in and you will be able to see it.
Jan 11, 2019, 12:32 AM
who has rabbit ears down
Captain Canardly's Avatar
I remember the first time I found my Best LD with my first BoTime. I was setting up for a landing, coming in under the crosspole on the football goalposts, and i figured I better 'dial in' some down trim. I was standing @ 40 yard line, and as she approached my position, it seemed to accelerate just a bit, but she also flattened the glide slope noticably. I then slowly turned away from me (Right to left path starting) so I wouldn't smash the opposite posts, and spent about 10 seconds going out, and then applied more right turn to come in. I was now 'scratching' the tops of the tall weeds- about 4 ft high, and then pushed for some nose down to land on the field, with a long slide. I then did the marking of positions, recentering trims etc., and fell in love with full flying stabs!
Latest blog entry: HyperFlight Survey results
Jan 11, 2019, 06:13 AM
Registered User
Gratter's Avatar
Quote:
Originally Posted by Captain Canardly
I remember the first time I found my Best LD with my first BoTime. I was setting up for a landing, coming in under the crosspole on the football goalposts, and i figured I better 'dial in' some down trim. I was standing @ 40 yard line, and as she approached my position, it seemed to accelerate just a bit, but she also flattened the glide slope noticably. I then slowly turned away from me (Right to left path starting) so I wouldn't smash the opposite posts, and spent about 10 seconds going out, and then applied more right turn to come in. I was now 'scratching' the tops of the tall weeds- about 4 ft high, and then pushed for some nose down to land on the field, with a long slide. I then did the marking of positions, recentering trims etc., and fell in love with full flying stabs!
The BOT is one of the many planes you hear people talk about being “ one step” when flying. This is usually faster then how people are flying it. Because of this I always assumed slower was not always better. My Orion seems to be like this. To slow and it sinks like a rock. With a little speed it seems to generate lift.
Jan 11, 2019, 09:02 AM
Registered User
One of the impressive things about the F3RES models is how responsive they are to trim changes. I first realized that practicing landings with my first Pures. On a stabilized approach, it is possible to adjust glide slope not with the spoiler, but with very small changes in trim. Landing long? Trim nose up, watch it slow - and then drop more steeply down ( but without stalling).

That is an example of increasing sink rate with decreasing speed.

Greg
Jan 11, 2019, 09:43 AM
Sonoran Laser Art
I just looked at Alegro Lite plan as Dr Drela provides Min sink speed at .87 Ft/Second. The Bubble Dancer shows .80 Ft/Second. These convert to just over 1/2 mph. The speed is listed slightly higher for heavier weights ballasted. Wikipedia says the average walking speed in a crosswalk is 3.1 mph. Makes you wonder doesn't it?
Jan 11, 2019, 09:56 AM
Registered User
Thread OP
Lots to chew on here. I'm REALLY concentrating an wringing out the best performance from my gliders. Being new at this, well, new as far as getting serious about performance, I know these F3-RES planes are way more capable than my skills at this point are capable of achieving.
And so, the little wheels inside my noggin are constantly turning.

I observe the flight with spoilers deployed. The spoilers introduce tremendous drag, so much that lift is effectively, well, spoiled. The plane isn't stalling, it's still going forward, but it is sinking down as if riding and escalator.
So connect that with..

* I observe times when I've trimmed for slowest flight above stall speed. But it seems it really isn't increasing my flight times significantly. In fact, some flights are disappointingly short. This causes me to think of other flights. For instance, my foam Radian I've spent so much time flying this summer. I noticed it does pretty good just above stall speed. But if I add a bit more weight by adding a 2200 battery instead of the 1300, it seems to fly faster but the extra weight doesn't seem to shorten flight times, on the contrary, my best times have been with the added weight. (or is it the added speed with the weight?)
Translate that over to the F3 class, I've observed that it appears at least, that flying as slow as possible, isn't necessarily the ticket to the longest flights. In fact, I've had a difficult time finding lift in the precious few flyable days I've been out. In flight it appears that slow flying is maximizing glide slope but I haven't been able to in any way, observe that no lift days flights are extended by flying as slow as possible. And so I wonder about increasing speed as an experiment to see if there is a speed sweet spot of maximum lift that could be above the slowest glide speed.
And so, I ask the experts! It seems to me the consensus is that indeed, depending on the plane, there very likely is a speed sweet spot that can be above what would be the slowest speed to maintain lift, but not fast enough to introduce "speed drag". I just need to focus in on finding that.

So the next obvious question is, how best to increase glide speed? Simply add down trim? Ballast? Nose weight? Downtrim can introduce drag, ballast may improve penetration in wind but we try to build as light as possible, makes no sense just to add weight unless there is a compelling reason. Nose weight moves the CG forward, but again unnecessary weight unless the CG NEEDS to be moved forward. But move it too much and you get porpoising. Either of these change the flight characteristics, none really positively really unless it's needed for a purpose. So how best to increase speed?

*I realize a lot of other factors have to be removed from this simplistic observations, such as air on a particular day, amount of lift & sink present, wind, humidity, temperature, etc. each of which has a significant impact. Frankly, I've struggled to find what weak lift has been present. Those big boomers of summer are easy. These recent days, no significant lift has been found by me. Add wind into the mix and all bets are off. I've struggled even to get my trim sufficient for wind much less finding lift in that wind. And in the past couple months, I've had precious few flyable days that wasn't a bit windy.
Last edited by Crashbound; Jan 11, 2019 at 10:03 AM.
Jan 11, 2019, 10:03 AM
Registered User
GliderJim's Avatar
To increase flying speed you can increase down trim or add weight. I found this old thread last week and it has a lot of really great information about using ballast to enhance performance and why it works...

https://www.rcgroups.com/forums/show...o-light-Really


Quick Reply
Message:

Thread Tools

Similar Threads
Category Thread Thread Starter Forum Replies Last Post
Discussion measuring sinking speed phil alvirez Modeling Science 27 Sep 05, 2017 08:04 PM
Discussion Does a glider fly any differently in lift than it does in sink? Miami Mike Thermal 67 Dec 19, 2016 06:17 AM
Discussion ESC's timing vs. motor KV vs. switch speed vs. MCU speed juuuut Multirotor Drone Power Systems 5 Sep 13, 2016 03:10 PM
Discussion Propeller confusion, lift vs. speed SlowSL Beginner Training Area (Fuel) 14 Jul 17, 2010 07:47 PM