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Apr 20, 2019, 12:22 PM
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Penetration


Can someone tell me if "penetration" is a real measurable thing like lift or drag or is it kind of a cover all word for how well a plane is performing on a particular day? I've been Googling all morning and have yet to find an actual definition.
Thanks
Don
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Apr 20, 2019, 12:52 PM
Duane, LSF IV
Wazmo's Avatar
Peak lift-to-drag (L/D) is an indication of penetration, but normally that term refers to flying upwind where you usually have to fly faster than peak L/D. So, you'd have to look at the L/D polar to see how steeply it falls off on the high speed side of peak. Rule of thumb for full scale soaring is to fly peak L/D plus half wind speed, so use that to gauge how far to move right on the polar graph. Note that the L/D ratio is equal to no-wind glide slope (height lost per distance traveled).
Apr 20, 2019, 01:28 PM
Registered User
As far as I can tell, "penetration" is a sloppy way to talk about speed, though of course it has to be at a reasonable L/D
Apr 20, 2019, 02:48 PM
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Larry Jolly's Avatar
In full size terms Best Speed to Fly. All flight modes have a theoretical best speed for achieving minimum sink and or best L/D . These are set by the Aircraft Design, weight, and surface and or seal considerations
If the glider has wing camber control the position of the TE will also need to be considered. The next variable is wind. To penetrate or make headway against the wind to cover ground you need to increase speed or add weight. A full size glider pilot seldom flies at minimum sink in a straight line. They are normally nudging slightly forward pressure on the stick to cover more ground. L
Apr 21, 2019, 02:34 AM
Registered User
Penetration is a vague and generalized term and I'm not to surprised you didn't find much in the way of definition. If anyone finds an explanation please post a link. Not an actual measure but its generally accepted a high performance ship will "Penetrate" (Cover more ground) better that a low performance or sport model at any given set of conditions. A commonly held misconception in gliding is that you use a wind additive (Like half the wind speed added to your best glide). This is incorrect but I can see where it comes from. Here is a good way to look at it (As it relates to gliding not power), If the object is to get the best glide in a given piece of atmosphere then flying past the best glide speed doesn't help, It just gets you to the ground faster. Would you slow down in a tailwind (I hope not).
In FS the term "Speed to fly" is based on Macready and Wing Loading. It has nothing to do with wind or camber changing flap settings. Here (Again) is another way to look at it: Ballast does not change the drag polar, It shifts it to the right. So a hypothetical glider with a 48 to one glide at 58KTS at a 7.5 pound wing loading will get the same glide ratio at about 10KTS faster at an 11.4 pound wing loading. Therefore it can be said to "Penetrate" better at a higher wing loading. Models work the same way. Intra thermal glide speeds in a FS racing ship will average 90-110 kts depending on conditions. Wind doesn't play a role in STF computations. A couple of things to consider, And this may relate to model flying, Is that FS gliders are never flown at min sink speed (Except maybe in the traffic pattern) and only rarely at best glide (Usually in the climb). Flap settings are a completely different kettle of fish and probably for another thread. Hope I haven't confused you.
Apr 21, 2019, 07:43 AM
Registered User
Airfoil and construction technology sure has changed over the 30 years I’ve been flying (and crashing...).
I remember when winch survivable wings with monokote covered flat bottom airfoils would fly backwards in the wind.

Back then, “Penetration” meant the plane would get back to the field.
Apr 21, 2019, 08:19 AM
May the Wind Always be Good
That is why the Durex V was made and it was also known as the Minnesota Floater …. But took way over a pound of ballast to make it Penetration
Apr 21, 2019, 08:21 AM
Red Merle ALES VII SJ
Curtis Suter's Avatar
This is a good read....
http://www.5c1.net/Glider%20Performance%20Airspeeds.htm
Apr 21, 2019, 11:35 AM
Registered User
Quote:
Originally Posted by Curtis Suter
Thanks! This link has been posted to RCG before and it points out a big difference between RC and FS soaring. In FS the goal is to cover as much ground as possible in as short a time as possible for a given set of conditions. Even if the goal is max distance one still needs to fly fast because speed is distance when you consider there are only so many hours of lift in a day. OTOH, RC Soaring mainly involves duration (Although there is growing facet of XC style flying and contests). A big problem with the current format of RC contests is that modern RC gliders can boat around at min sink for the duration of a contest round without much skill involved (Except maybe skill in programing your radio to achieve the highest launch). To keep the scoring interesting the concept of Landing points was created where one crashes into the ground at a pre determined point when the time is up for a particular round. Here again contest landings are interesting (And sometimes amusing) to watch but it doesn't do much for a pilots skill set. Modern RC Thermal Duration Gliders are works of art in both form and function (Except maybe the broomstick fuselages) but they have exceeded the tasking system used at contests. I keep hoping that contest rules will be updated to reflect the current generation of RC gliders.
I know this post goes a bit beyond the scope of penetration but the subject of STF in full scale came up and I felt it would be good to point out how it is used differently.
Apr 21, 2019, 12:08 PM
B for Bruce
BMatthews's Avatar
I'm not so sure that there's that big of a difference between full and model sizes. Well, at least not once we hit a windy day.

While that article is geared to full size we still fly like that even if we're not doing XC style flying. We thermal duration folks are faced with the speed to distance curves noted in that article each time we follow a thermal too far downwind and have to get back to the launch/landing point against a firm sort of headwind. And that's where the ability to push the nose down a few degrees and penetrate back upwind comes into play.

Of course if it's a calm day with light fluffy thermals and the wind never gets above a jogging pace there's not much to test the new super gliders. And in such cases a well flown floater can easily beat out the modern "glass slippers". But give us a day where the wind is up and the advantage shifts clearly and strongly to the "glass slippers" and a few woody designs that were built to have a good speed range with minimal use of altitude to fly faster. And it's on those days that talk naturally turns to how well our models "penetrate".

I'd also suggest that while the value for a given model of the best L/D speed is a good indicator of how the model will fly it's not the whole story. A model flown at the best L/D, if not ballasted might well still be losing ground. So we push in a trifle extra nose down trim to speed up past the best L/D speed in order to make distance over the ground against the wind speed. And that's shown in that article as well for both how best to minimize "damage" to our height by how to fly through sinking air and against headwinds to produce distance over the ground to get the models back to the launch point.

So all in all I'd say that the article does have a lot to offer for those of us that regularly fly in blustery conditions.
Apr 21, 2019, 05:45 PM
Registered User
Bruce,
You and I are on the same page but I think we are speaking a different language. It is a great article and explains how soaring performance works (With simple graphics even). Soaring is soaring whether FS, RC, hang gliding or whatever and the same concepts apply. There are two issues to consider with the article. First, on a modern glider they don't use MacCready rings because the STF is computed electronically by a STF Variometer. Second, The concept of adjusting speed for wind has been debunked years ago. When the Glider Flying Handbook replaced The Joy of Soaring as the official FAA reference it was not properly vetted or proofed and many mistakes made it through. I don't know what revision they are on but this was something that made it through the first edition. Do the math (Please) and you will find that this concept does not work. Here is an example: In the real world lets say you are headed to XYZ airport. If you are climbing (In a thermal or ridge) the Glide Computer (Which compensates for wind) will tell you when you can make this airport and it is expressed as "Altitude Required". Keep in mind this is based on current STF which has NO bearing on wind. So if you leave the climb before Altitude required is met you will not make the airport. Also, if you leave the climb after you have required altitude and fly faster than your optimum STF you will glide steeper and you will not make the airport either. Another thing you are doing by attempting to "Minimize Damage" by flying too fast is decreasing your opportunity to find lift. This may seem counter intuitive at first (Just like diving in sink) but it works. If you find yourself low and downwind with your RC glider just forget about a round score and stop and take a climb. At least you will get your ship back in one piece.

Your third paragraph is exactly the point I was trying to make. A current generation FS Open Class glider has the same Max L/D as its predecessors. The difference is that they have vastly improved glide performance at typical real world cruising speeds. I liken this to the current crop of Carbon Slippers we have in RC. They can do what a Woody can do only across a much wider speed and weight range. An unfortunate side effect is that we have to endure watching those silly cartoonish launches.
Apr 21, 2019, 10:26 PM
Registered User
After a few hikes in the woods, I'm sure you'll find yourself putting in a bit of down elevator when returning from downwind. That's just the way it works. Let's say the wind is 12 mph and your floater flies 12mph. You'll make 0 headway. Now you put the nose down a bit and fly at, say, 16 mph. Now you're at least getting closer to the field, at 4 mph. Wind has everything to do with speed to fly if you're trying to get to a specific point on the ground. That's an extreme example, but if there's any wind at all, best speed to fly to make headway upwind will be at least slightly faster than best L/D.
Apr 21, 2019, 11:59 PM
Registered User
Al Austria's Avatar
Quote:
Originally Posted by DougCorrigan
A commonly held misconception in gliding is that you use a wind additive (Like half the wind speed added to your best glide). This is incorrect but I can see where it comes from. Here is a good way to look at it (As it relates to gliding not power), If the object is to get the best glide in a given piece of atmosphere then flying past the best glide speed doesn't help, It just gets you to the ground faster. Would you slow down in a tailwind (I hope not).
If you want to achieve the fastest speed through the airmass, then you would be correct. But STF is not constrained to just the speed through the airmass. If you want to want to fly the greatest distance over the ground, you would absolutely speed up with a headwind and slow down with a tailwind(but never slower than min sink). This is the shifting of the tangent on the drag polar left or right on the X axis by the magnitude of the wind. The context of STF depends on the objective.
Apr 22, 2019, 03:58 PM
Registered User
rrcdoug's Avatar
I agree with Al and BMathews. I fly faster than max LD when trying to cover ground in a headwind. I fly too slow going downwind sometimes... but that is another story. I guess an important difference between RC soaring and FS soaring is that the RC pilot is standing on the ground and MUST keep the glider in visual range and within gliding range of the home field. Lincoln's extreme example probably doesn't sound very extreme to an oldtime RC glider pilot or even a hang glider or paraglider pilot. I'm sure all have experienced going backwards over the ground while flying at max maneuvering speed.
Apr 22, 2019, 11:39 PM
bayagliders.com
In laymans term penetration is used to describe how a given model moves through a headwind. You increase the apparent windspeed and given the mass and drag of the model the model can even move backwards. Increase the mass or decrease drag and the penetration will increase. You can think of it exactly like a car that rolls down hill and reaches its terminal velocity, the only way it could exceed the speed without power is to increase weight or decrease drag.


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