Dick, daRock, TallPaul, BMatthews (no particular order)

Penetration

I feel this is misunderstood (at least by me) as most seem to immediately cite mass (really inertia)
Undoubtedly mass and inertia have a bearing but I don’t think that is even the major factor.
Reason: Sailplanes have very light weight yet excellent penetration.

Enlighten me please.

What is drag, the lack of it, Alex?

And ballast up when they wish for better penetration.

For their cross country events, they usually fill their ballast tanks with water. It's fun to watch them scream through the last gate and pull the plug. They're actually not very dainty either. You can't have that high an aspect ratio wing and it be what you'd call light.

Many years of excellent fun were spent slope soaring R/C gliders. Every one had a way to ballast. I built a series of 2Ms that were excellent thermal soarers and did very, very well on the slopes. They had a ballast box to fit a lead weight I'd cast for the purpose that was about 2x2x4 and was almost what the glider weighed.

Go straight to models for examples.

Foamys are a good example of the lack of penetration from lack of mass. When our Tuesday gathering has high winds predicted, the guys who fly small electrics often don't show up. They fly mostly scratch built. Some are balsa and some are foam. All are lightweights.

There is a reason kites have no mass compared to their "wing area".

This term is a little pet peeve of mine, like "ballooning". People talk about these all the time without being precise about what they mean, and it leads to confusion. So good for you for asking what it means. I can't tell you, but I can tell you what makes it confusing.

It is often spoken of in conjunction with wind, as in "wind penetration". But wind is just moving air, and the ability to move forward against it relative to the ground has another term that IS precise. It's called "speed". People will say that a heavier plane has better "penetration".

I think the confusion is this: in level flight, a heavier plane, all else being equal, is NOT faster than a lighter plane. It's slightly slower since it must fly at a slightly higher angle of attack and create more "induced drag" (drag created by producing lift).

But if your airplanes get to coast downhill, which is what gliders do, then it is a different story, and that's where Rock's comments hit the mark: more weight means more speed because they are always going downhill relative to the air. They're like a soapbox derby racer ... the heavier, the faster.

For powered planes, I think what people mean is that if your plane is downwind, and you need to get it back, the heavier plane has an advantage as long as there is some height to spare, because giving it a little down elevator will bring a heavier plane back faster. But for level flight, the extra weight does NOT improve "penetration".

I think it's a term that should not be used, and I don't think you will see it in aerodynamic texts.

Jim

Apples and oranges.

Kurt

Not really Kurt. Buzzard's points are all dead on. I'm not sure what you're referring to with your "apples and oranges" comment as it pertains to the portion you quoted.

Onewasp, not all sailplanes are good at punching out to get back upwind. Fly a Gentle Lady on a blustery day and you'll learn the meaning of fear And while it is a delight to fly in calmer conditions it has met it's match once the wind comes up. And due to the design choices in a model such as the Gentle Lady it will not respond well to ballasting other than maybe just a little to gain just a little. The airfoil and overall design just has too much drag to do well in a high speed mode. To see the gains from ballasting the airframe has to be clean enough and the airfoil one that works with low drag at very low lift coefficients or you don't gain at all or can even lose a lot like with the Gentle Lady.

With power planes such as flat bottom wing trainers this shows up as well. When you pick up speed you actually end up trying to fly the airfoil with a strongly negative angle of attack. It's still producing positive lift due to the camber but it's now running with a far higher drag coefficient than when it's flying with a neutral or sligtly positive angle of attack. When you compare this to a sport model with a symetrical airfoil the wing is actually producing less drag at high speeds such as when you firewall the throttle and then go into a slight dive to regain ground from downwind.

What you're seeing with a power model is not "penetration" due to the mass of the model but rather the difference in maximum attainable airspeed in level flight or at a reasonable descent angle due to the design. This maximum attainable airspeed
is actually what you're seeing as penetration on windy days.

Apples - pick your power.
Oranges - pick your pitch.


A heavier airplane (with all other variables remaining the same) needs to EITHER increase the AOA or increase the speed to maintain level flight. Just because the plane is heavier, doesn't mean it will automatically "fly slower" as stated earlier. WE get to decide this with one of our primary flight controls - either change the pitch or change the power. And yes, sometimes both are needed. Full scale pilots learn this early on. Pitch and power, pitch and power, pitch and power!

To say that a heavier model flies slower than a lighter one because it has to fly at a higher AOA and thus has more induced drag is too simplistic and one dimensional, to say the least! Where to begin!

Kurt

Then you don't agree with his statement.

I'm having a difficult time wraping my mind around this. All other things equal : airframe, power, level flight how is the heavier plane not slower?

On one hand you say all things equal then you say change the power to get the heavier plane to fly faster. Changing the power isn't all other things equal.

Of course if you have a heavier airplane and the power is also variable(more power than the lighter plane) it could possibly fly faster, but I don't think that's what is being discussed.

Are we talking about maximums and minimums?


Kurt

How about this question:

You are flying a .40 stick trimmed for level flight and 1/2 throttle hands off.

Magically, the plane is instantly a 1/2 pound heavier. Without touching the TX, what will the plane do?

Kurt

Note to all who have posted.

I appreciate the input (and the support) however it appears that we have yet to even come close to anything which might qualify as ANY sort of explanation of the phenomenon.

While I find company in misunderstanding comforting, that isn't quite what I had in mind.
Beauty's definition is best stated as "in the eye of the beholder".

I find I'm not even that close on the subject of penetration. It seems I have lots of company. [X(]

Penetration-once again
air resistance will change the speed.
You have to minimize air resistance to maximize penetration
NOTE any disturbance of equalized air pressure will cause lift or drag.
this can reduce penetration
In a vacuum , there is no "penetration" problem as all objects will accelerate at the same rate.
How YOU choose to do this is up to you

Then you might have something else in mind. Buzzard bait and BMatthews both explained what many consider it to be. They'd both qualify as "ANY" for sure.

I guess it depends on what most guys at the field would see a model do and say, "look at that sucker penetrate". In every case I can remember something like those words, they were marveling at a model that was flying back upwind better than other models had that day.

When it was a glider, it was one that was what you'd call a fast one, and usually carrying ballast. Gliders get their speed from energy that comes from their weight. They turn their potential energy into forward motion. More forward from more energy from more weight.

Nowadays I don't get to fly gliders but have heard comments about penetration. They were usually mumbled by guys who were sitting out the breeze because they had basically slow airplanes that had little chance to hurry back against the wind due to their lack of speed and abundance of drag.

Two things especially benefit "penetration", lower drag and higher speed. Of course, you usually see both in a single model. But with a glider, you can give it more potential for the higher speed deal.

Perhaps you'd describe again what you'd point out as penetration if you observed it?

There is another performance that might come under penetration, or at least add to a modeler's judgement of a model's penetration. Maybe it's what you're after.

Some models fly with less reaction to gusts. We've all seen models that're jostled about by the wind. And seen others that seem oblivious or at least are better than the bouncy birds. Wing loading is one reason. Drop a feather and a bowling ball, and you get to see a difference in penetration for sure. Moreover, it there are thermals and the wind is gusting, you'll see intermittent differences in the penetration. Unless of course, they drop in a vacuum. (Have they vacuumed around that tower since it was built? )

I have to extend my apologies to all.

I looked at the last poster, which was then Bozarth . Same as when I last looked at the thread yesterday.
I assumed it was the same post. . Nope.
I was thinking of his post #6. []

"Ass-u-me" that word in action again. I need to read and absorb the posts I hadn't read.

I am reminded of the old saw "Please engage brain prior to placing mouth in motion."
Guilty.

While waiting for TallPaul, you can chew the excellent information given by Dick, daRock and BMatthews, which is a beautiful summary of myths and facts around penetration.

Initially it'll lower the nose and descend in a shallow dive to accellerate back to the trimmed airspeed. But that trimmed airspeed will now occur at some amount of descent angle. The extra 1/2 lb requires some additional energy to hold it up. That energy has to come from either the power of the engine or from converting the potential energy of altitude over to kinetic energy by "gliding" downward at a rate that produces the additional energy needed to support the extra weight.

To return to level flight one of two things can happen. One option is the engine can be throttled up until the extra power allows the model to return to level flight at a higher airspeed that allows the wing to generate the lift at the original angle of attack. The angle of attack for this particular level trim being set by the elevator. The second option is to ease back on the elevator to return to a now slower level flight speed at a higher than previous angle of attack. This second one works because we're making the wing work at a higher lift coefficient. The conservation of energy is still working on this since you've traded some of the airspeed for higher lift so now more of the engine's power is being used by the wing for lifting and less is going towards fighting parasitic drag. What suffers in this elevator trim option is the flying speed.

You were quite correct when you countered the idea that a heavy model isn't slower but a heavy model IS more draggy than a light one. When the person that said a heavy model is "slower" this is likely what they meant.

If nothing is changed, the airplane has to fly faster to generate enough lift to fly the additional 1/2 lb.

But "if nothing is changed" it won't fly flaster.

Kurt

After reading and absorbing all the posts (that is the big change from my last post).
I have a much better understanding of "penetration".
Thanks to all.

Note: I just returned from flying and handling the variable wind conditions addressed here.
For fun and added challenge during the last two years I have switched from Glow/Gas to electric.
Interesting to learn the new parameters of flying.

I also have gone a LOT smaller, 500 to 700 squares with an AUW of 60oz.

The 'penetration' question came from flying two designs by E-flite under "heavy" wind conditions. Heavy = I was the only one flying.
One design was the Patternish Diamante (spent the best part of 49 years in that specialty).
The other was the Eratix. A boxy 3D design with light wing loading (12.3oz on 700 sq. in.)

Power systems were identical I.E. E-flite 32, A123 x 2300 x5S,CC Phopenix 60, even the weight for the two models turned out the same, 60oz.
In strong and variable crosswinds I expected the much faster Diamante to settle in for landing much better than the Eratix.
Not so !
The Eratix was a joy with which to do touch and go's. Even with the pronounced and variable crosswinds.
The Diamante was a hand full. Yes it could do them, but with a lot of effort from the pilot.

Airborne the opposite was true. (Pattern maneuvers). There the Diamante was at home while the Eratix struggled. Airborne the winds were not variable but steady.

That prompted the thought process which produced the question.

I have identical setups of the two models ansd the results are -strangly enough-
identical.
For those new to all this :
The very thick wing on the ERATIX is, as are all very thick wings - fairly "numb to small changes in pitch.

This is where the slow speed maneuvering (read that -landing) character of the two models is noticeable different

You can easily assume the three point attitude with the fat wing -as the change in angle does not dramically alter lift
Some think that a thick wing has more lift
Nope
It has less
The "high lift" is actually a more tolerant response to AOA
I have only two airfoils
one is round like a cylinder
the other is flat like a piece of paper
depending on what I want to do - I flatten the fat one till I get what I want
The series n number of them is not of any interest to me on these small aircraft.

Kurt, you're getting hung up on the sematics of what folks are typing. I'm sure that Jim meant "if nothing ELSE is changed.....". Just read between the lines a little, OK?

Onewasp, so many things come into play when you're looking at model performance and steadiness in the wind. There's roll and pitch damping, mass distribution, pitch stability margin and likely a few others. Roll and pitch damping and mass distribution plays a big part in turbulent wind behavior. A model with heavier extremities will be displaced less and slower by wind direction and velocity changes than a lightly built model. Such a model will look more "stable" in turbulent conditions. But once accelerated into a displacing velocity it'll be harder to get it under control and back to where you want it to be. Like on landing approches where you're working the path to conform to a rather narrow approach corridor to land where you want and not where the model thinks it should be. When you're coming into a touchdown a lighter model will kick around more but at the same time it takes a lot less to stop it and less control input to correct it. It'll LOOK nervous as heck but if you sort of average out the "palsy" it seems to have it'll actually be easier to keep it in the approach corridor. Being lighter and able to respond to power changes more easily, or having a higher power to weight ratio in the case of your weight matched examples, the "lighter" model can also deal with the airspeed changes brought on by the wind velocity gradient in that last 10 feet and deal with any sudden turbulence velocity issues with less drastic inputs.

The pitch stability margin comes into play as well. Models trimmed closer to being neutrally stable will respond with much less nose rising and dropping to the sudden localized airspeed changes. Again the flight like pundits will nod knowingly and say "that baby sure don't balloon much.... ". This tendency to "balloon" is strictly due to a higher pitch stability margin. With a higher margin the model is going to react more strongly to speed varitions. This makes for a fine behaviour in a traner where the student just dove it as it'll return to level flight more quickly. However it's a bad thing for aerobatic models or those flown in blustery turbulent conditions. But your pattern and 3D models will both be excellent on this count.

I'm not sure I'd class these charactaristics as "penetration" since it's more about cutting through turbulence. Although like so many abused terms I've heard many a flight line pundit describe a heavy model flying in rough conditions as "penetrating well". To the bystanders on the ground this may well be the case. But to the pilot the model seems stable enough until something more major hits it and then he needs to deal with the effects in a much stronger manner than the minor jiggles that a lighter model undergoes in the same turbulence and where only smaller or no corrections are typically required. The heavier model in particular will often feel "sluggish" if the disturbance also suddenly reduced the airspeed.

When it comes down to it I'd rather have a lighter model that kicks around more but that is more controllable for correcting and is more tolerant of the often sudden airspeed changes that occur on such days. And I often do so. I actually love "storm" flying in such conditions as long as it's not some snazzy new scale model or something where I have far too many hours involved or where it's too new to know how it'll behave. But the ragged old sporty model? BRING IT ON BABY ! ! ! !

Does that sort of describe your findings?

BMatthews,
At some point we have to be technically specific and accurate (at least I like to be). Too many Old Wives Tales floating around on how and why planes fly.

Kurt



.......... edited to correct keyword so quote would show as such