Thread Tools
This thread is privately moderated by maguro, who may elect to delete unwanted replies.
Feb 15, 2012, 10:00 AM
Registered User
Dickeroo's Avatar
Quote:
Originally Posted by Cap_n_Dave
Dickeroo, could you explain your reasoning with regards to your statement, quoted above?
The Mythbusters did a study on pick-up trucks and fuel efficiency. Was it more effective to drive with the tailgate down or up? It turned out that it was more efficient fuel to drive with the tailgate up. They also found that the trapped vortex provided a small amount of forward push due to the air rotation. I don't think that physics is going to change its mind when it comes to the KF airfoil. But this is just my opinion, not fact.

If you have ever stood under the eaves of a roof while it is raining slightly and noticed how the water kicks backward after leaving the roof? Same principle.
Sign up now
to remove ads between posts
Feb 15, 2012, 10:39 AM
Jack
jackerbes's Avatar
Quote:
Originally Posted by Dickeroo
"personally im not a huge fan of laboratory testing, either for aerodynamics or radio. imo nothing beats the real world for gathering reliable information."

I would agree with that statement. Plenty of inventions have been successfully tested in the lab only to fail in the real world. With a KF airfoil test in either a computer or wind tunnel test, it is not likely that any forward pushing air pressure directly behind the step would be revealed unless the testing was specifically designed to do so. Computers are not set up for that aspect of testing.

The KF airfoil has been shown to be less than stellar when it comes to gliding. However, it does more than hold its own in many other areas. My guess is that the KF can carry a much heavier payload than the thin Drela airfoil to which it is always compared. So, it really depends on what the purpose is. Gliding or payload.

If I'm off base with the statements above anyone is more than welcome to straighten me out.
I certainly have to agree that testing in the field is the most fun. I have little or no interest in owning a wind tunnel.

And I think maybe your supposition on the cargo carrying capacity is correct. If it is, it would be of little attraction or value to sailplanes as they more value the lower drag of their thinner wings and more evolved foils and have little interest in adding cargo weight to their planes.

But us knuckle dragging KF fliers that fly primarily motor gliders and that are also satisfied with doing that at less than the level of world class competitive gliding, the KF winged glider is always going to be a good choice.

I know that I have strapped bits and pieces like cameras, Eagle Tree eLoggers, and stuff like that into and onto some of my builds and it does not seem to detract from the flying traits any great amount. And, when I had to tape/Velcro/rubber band the stuff onto the outside (on the fuselage or on top of the wing) the small addition to the drag was cheerfully accepted and away it flew. I am smart enough to honor the CG needs...

On the data collecting thing, when I started building the Big Blu 96 I looked for info on glider performance and made this page my standard for comparison on glider performance and I also chose sink rates as the primary data that I would measure.

Performance Analysis of a Family of R/C Thermal Sailplanes - http://www.rc-soar.com/tech/perfanal.htm

That uses the Selig-Donovan SD7037 foil for it's theoretical comparisons and describes the three classes of gliders as follows:

"..Models which achieve sink rates of 1.0 ft/sec are true floaters, slow, graceful and able to work very light thermals. These planes typically thermal at 20 ft/sec and have wing loadings of about 7 oz/ft2..."

"..Models with minimum sink rates of 1.25 ft/sec are the mid-range of quality thermal sailplanes, thermalling at 25 ft/sec with wing loadings about 10 oz/ft2..."

"..Models which achieve a minimum sink rate of 1.5 ft/sec favor speed above light air thermalling performance, circling at about 30 ft/sec with wing loadings of 16 oz/ft2..."

So there you have it, I consider good gliders can have wing loadings of 7 to 16 oz/ft2 (21.4 to 48.8 grams/dm2) and sink rates of 1 to 1.5 feet per second (60 to 90 FPM and 18.3 to 27.4 meters per minute).

Then I decided to try to see where Big Blu compares to those three theoretical classes gliders. I used my Eagle Tree eLogger V3 and altitude micro sensor to collect data on a series of climbs. The elogger was resting on the top of the fuselage just in front of the wing as there was not room for it inside the fuselage. The weight and loading for the flights was:

AUW: 1505 grams/53 oz.
Wing Loading: 9.4 oz./sq. ft. or 28.5 grams/dm2

The sensors had added about 54 grams to the AUW and are included in the 1505 grams.

I mentally patted myself on the back for having produced a build that had wing loading that put Big Blu in the same range that was discussed in the study.

The day I did the tests was clear and the winds were light. I did not think that thermal lift would play much of a part in the sink rates. I later noted that I occasionally got few seconds of slowed descents at lower altitudes but no real increases in altitude.

I've posted images of the tracks from the five flights in this post over on the Big Blu 96 thread so I won't repost them here:

https://www.rcgroups.com/forums/show...2&postcount=39
.
The bottom line of my tests

Highest descent rate - 701 feet in 3.33' = 210.5 FPM or 3.5 FPS

Lowest descent rate - 908 foot descent in 5.5' = 164 FPM or 2.8 FPS

So, based on those tests, I consider the KFm9 at that wing loading to be capable of about 180 FPM or 3 FPS.

The wing and fuselage were Dow 2" blueboard and FFF covered with colored packing tape.There are some techniques that that have not been explored yet that will lower the drag on a wing like that. Things like the leading edge thinning and tip shaping as used in viking60's Dancer series of gliders.

For those not familiar with him, Bruce Stenulson, who posts here as viking60 is one of the true pioneers of using and improving the Kline-Fogelman airfoils. And particularly so in pursuing lowering the drag of the KF airfoil in gliders.

So at this point my analysis is that Kline-Fogelman winged gliders can have sink rates that are 35% to 53% of the best high performance gliders.

JetPlaneFlyer did a well described and objective comparison between a ELF discus launch glider (DLG) and a KFm3 winged clone of that glider last year. That is all detailed in this thread:

https://www.rcgroups.com/forums/show....php?t=1542598

His conclusion was that the KFm3 clone had about 75% of the performance potential (based on flight times) of the ELF. So that gives us another datapoint for making comparisons.

A major factor in glider performance is the "thermalling ability" of gliders. That is nothing that is very well explained or described, I decided that it describes the ability of the glider to travel and explore, in a mostly horizontal plane, the air around it to look for thermal columns.

Thermal ability is mentioned in the study as being in the range of 20 to 30 feet per second for that study. I suspect that thermal ability is another area where the KF winged gliders will be inferior to better gliders. But it is also going to be harder to compare as it will be very subjective. The pilot's ability will be a major factor as will the flight conditions during the test. I'm putting that one on hold for now.

So we owe it to ourselves to all get some eLoggers or other data collecting gadgets and start getting some data points here. As an example, the Eagle Tree Altimeter MicrosensorTM V4 is a 4 gram device that can be flown alone and it will blink out the highest altitude of the last flight on a LED after landing. Using just that and a stopwatch, you can easily measure and evaluate sink rates.

To record the track data and see the descents graphed, you would need the eLogger too of course.

Jack
Feb 15, 2012, 11:05 AM
Registered User
Dickeroo's Avatar
Jack...

You're amazing. You packed a lot of info in a rather short space. It is concise and loaded with good info. Thank you for being so thorough in providing a lot of info to chew on. One of the things that makes me so happy is to see how the KF involves so many people coming from so many different points of view. And, sharing their information freely. We all benefit from this type of collaboration. And, the creative aspects give us an opportunity where you try many different things. This is such a great learning curve for all of us and something we would not acquire if we purchased a completed model or kit where someone else did all the thinking. Hands on is always the best because its information that we get to "own" and it comes from personal first-hand experience.

Again, thank you Jack for all that you do.

– Dick
Last edited by Dickeroo; Feb 15, 2012 at 11:45 AM.
Feb 15, 2012, 11:30 AM
Jack
jackerbes's Avatar
No, you are the one that is amazing!

You have brought so much joy to so many that is boggle the mind!

And, or course, there are wonderfulness awards due for Tony, Bruce, Roger, Dave, (insert your name here please) and all the other plankowners in this small and insignificant corner of the wonderful world of RC flying!

Jack
Last edited by jackerbes; Feb 15, 2012 at 12:20 PM.
Feb 15, 2012, 11:47 AM
Registered User
Dickeroo's Avatar
Quote:
Originally Posted by jackerbes
No, you are the one that is amazing!

You have brought so much joy to so many that is boggle the mind!

And, or course, there are wonderfulness awards due for Tony, Bruce, Roger, and all the other plankowners in this small and insignificant corner of the wonderful world of RC flying!

Jack
And, I would add Dave Reap's name to the list as well. Are we having fun, or what?
Feb 15, 2012, 12:20 PM
Jack
jackerbes's Avatar
Done! And, YES, we are!

Jack
Feb 15, 2012, 02:05 PM
treefinder
springer's Avatar
Good Stuff, Jack, we don't even have to do the research ourselves! Interesting this quote:
"..Models which achieve sink rates of 1.0 ft/sec are true floaters, slow, graceful and able to work very light thermals. These planes typically thermal at 20 ft/sec and have wing loadings of about 7 oz/ft2..."

Perhaps that explains why I find my OSG more fun than my Gentle Lady, the OSG comes in at 3.5oz/sqft.

Dave: if you can just get the overall measurements of the wind tunnel for me I'd be grateful. Still just playing with the ideas. The measuring mechanisms for lift and drag are going to be the tricky ones, I'm sure, but if I get the overall size right, so no serious wall effects, and straight flow, It'll be fun to play with.
Feb 15, 2012, 04:41 PM
RC beginner
will do. when i get home this weekend. i do recall the fans suck. if they blow its ng. and the bank of tubes were important to minimize turbulence.
Feb 15, 2012, 05:56 PM
treefinder
springer's Avatar
That itself is good to know, I was gonna put fan on inlet!
Feb 16, 2012, 08:04 AM
gpw
gpw
“There’s no place like Foam”
gpw's Avatar
Shouldn’t the fan (wind) speeds reflect our normal flying speeds ... and what about the size of the test airfoils ... the larger airfoils having the advantage , the air remaining the same density and all (scale effect) ... ?
Latest blog entry: Lost plans
Feb 16, 2012, 08:23 AM
RC beginner
Quote:
Originally Posted by gpw
Shouldn’t the fan (wind) speeds reflect our normal flying speeds ... and what about the size of the test airfoils ... the larger airfoils having the advantage , the air remaining the same density and all (scale effect) ... ?
since models range from 2-3mph for super slow walking speed lrf floaters to 200mph dynamic soaring its hard to say just what is meant by "normal". the wind tunnel my friend built can certainly handle the low range with its variable speed fan. it goes down to virtually zero. im not sure how fast it can go but i think way beyond that of most parkflyers and foamies before they fall apart.

its true about reynolds numbers being influenced by scale. all i can say is the paper airfoils in the box were made from a sheet of copy paper folded back so were about 3-4" chord. not too far from most foamies. not as big as some of some of those huge gliders and scale warbirds but definitely bigger than itty bitty bnf um and other micros. the paper foils were just about in the middle for most hobby wings i think.

as i said this kind of testing does not excite me as much as actually flying wings at the park to see how they perform. but i must admit the discussion has piqued my interest to the point of considering setting up that tunnel. biggest problem is finding a place to do it.
Feb 16, 2012, 10:22 AM
treefinder
springer's Avatar
It's funny what gets us going isn't it? my thought are like yours, Glen; use airfoil that are actual foam slices out of typical planes I've built, like the 30's racers (simple KFM2 @ 7" chord); bbp51 (DT foam KFM3 @ 6" chord); erc. initial thought is to use a two speed box fan (because I have it, and think it generates speeds in the range those planes fly. (i'll have to check speeds w/wind meter). still not sure how big the cross section needs to be. twenty inches square would be convenient, tho... But ideally no need for any scaling of any factors.
Feb 16, 2012, 11:29 AM
Jack
jackerbes's Avatar
Quote:
Originally Posted by springer
Good Stuff, Jack, we don't even have to do the research ourselves! Interesting this quote:
"..Models which achieve sink rates of 1.0 ft/sec are true floaters, slow, graceful and able to work very light thermals. These planes typically thermal at 20 ft/sec and have wing loadings of about 7 oz/ft2..."

Perhaps that explains why I find my OSG more fun than my Gentle Lady, the OSG comes in at 3.5oz/sqft...<snip>
Yep, and all the joys of flying are there. When the winds pick up it can get to the point where there is not enough penetration into the wind to maintain both forward flight and altitude. So we call it a day or watch it get blown away downwind. And that is also where the merits of higher performance gliders start to come to the front. Some of those big megabuck glass slippers have tubes in the wings and will take bars of metal for ballast. And not in small amounts, it can be like a kilogram or or two in some cases. The weight is added on the CG line, the plane is balanced the same, starts driving into the wind, and you're still able to fly.

It is a little like the BB 33 with the KFm3 wing compared to the UC wing, heavier, cleaner entry at the wing, better penetration, flies in higher winds.

Jack
Feb 16, 2012, 12:38 PM
Registered User
Don Stackhouse's Avatar
"Thermalling ability" is the plane's ability to gain altitude by circling in thermals. There are a number of factors involved, such as spiral stability, airspeed at which the plane achieves minimum sink rate (and therefore how tight a turn radius it can sustain), and maneuverability, but overall the most important factor is its minimum sink rate.

For model sailplanes (regardless of whether for sport or competition), it's necessary to have a sink rate that's less than the speed of the rising air in typical thermals in that area. This varies a bit depending on what part of the country you're in. For example, Texas and parts of California tend to have stronger lift than here in the midwest. We joke about "Texas brick-lifters", while here we have to have planes that can sustain altitude on a "gopher fart".

Quote:
...A major factor in glider performance is the "thermalling ability" of gliders. That is nothing that is very well explained or described, I decided that it describes the ability of the glider to travel and explore, in a mostly horizontal plane, the air around it to look for thermal columns....
No. That's what's commonly referred to as "range", "searching ability", or "ranging ability". That is mainly a function of L/D max and the speed at which L/D max occurs. In general, it's an advantage to have a high speed for L/D max, but a low speed for minimum sink rate. Those are obviously contradictory requirements.

Back to thermalling ability and min sink performance - At L/D max, the plane's induced drag (the drag that's a by-product of making lift, mainly a function of weight, the square of the wingspan, and how effectively the plane uses that span) and parasite drag (all other drag that is not "induced") are exactly equal.

Min sink (which is at or near the best thermalling speeds) occurs at an airspeed a little below best L/D, so the induced drag is more than half the total drag. The profile drag of the airfoil (a form of parasite drag) tends to be a lesser player in that, unless the profile drag is really bad. Yes, that's right, the airfoil drag generally does not have a big effect on thermalling ability unless the airfoil is really bad, i.e.: has a really poor L/D. Sink rate is a function of how steep the glide path is (L/D) and how fast the plane is travelling down that glide path (airspeed). If the glide angle is very poor, you would need an extremely slow airspeed to make up for the steeper glide slope enough to keep the sink rate low. An extreme example would be a dandelion seed; its L/D is effectively zero, but it has a vanishingly small airspeed on that vertical glide slope, so its thermalling ability is outstanding. OTOH, its searching ability and its penetration are nonexistent, but for its "mission profile" (drifting with the wind as far as possible) that is not a problem.

In order to sustain altitude, a plane that has a min sink of 3.0 FPS will need a minumum thermal twice as strong as one that has a 1.5 FPS min sink rate, and three times as strong as one that has a 1.0 FPS min sink rate. That's not too difficult to do in slope lift with a moderately strong wind, but thermals that will sustain a 3.0 FPS min sink rate glider are relatively uncommon, especially in this part of the country. I know this firsthand. The full-scale Schweitzer 2-22E that I flew for my glider pilot's license had a min sink rate of 3 FPS, and "cold slides" were pretty much the norm around here. Just maintaining altitude was a struggle, and actually gaining altitude was relatively rare, especially with the extra weight of an instructor on board. The Schweitzer 2-22 (no "E") I occasionally flew was a little better in this regard, mainly because it was significantly lighter. However, many folks insisted on calling them "gliders", not "sailplanes". A sailplane is a glider with enough thermalling ability to sustain its flight significantly on typical-strength thermals, and those folks were strongly of the opinion that a 3.0 min sink rate did not qualify as a "sailplane".

Getting a decent min sink rate is really not all that difficult. If Jack could get his 3.0 FPS sink rate model down to one-fourth the weight and wing loading, it would have one-half the thermalling speed (because lift is proportional to the square of the airspeed), and one-half the sink rate, or 1.5 FPS. That's not great thermalling performance, but acceptable in most parts of the country. However, that would put it closer to the characteristics of a dandelion seed, good sink rate but no range or penetration. A successful sailplane really needs to have both.

There seems to be a misconception on this thread that the higher performance sailplanes you're comparing yourselves to are all $2k "glass slippers". This is not true. There are many excellent wooden sailplanes that cost well under $150, quite a few of those under $100 and even some airplanes that could do well in a club-level contest for under $50. There are also "foamies" such as the Radian that don't have the penetration of the better wooden designs, but do offer adequate range and have good thermalling ability.
Last edited by Don Stackhouse; Feb 16, 2012 at 01:37 PM.
Feb 16, 2012, 01:08 PM
Registered User
greyflight390's Avatar
Don Stackhouse that is one funny line. "while here we have to have planes that can sustain altitude on a "gopher fart".


Quick Reply
Message:

Thread Tools

Similar Threads
Category Thread Thread Starter Forum Replies Last Post
Discussion ** Kline-Fogleman (KFm) Airfoils - Building/Flying Discussion ** jackerbes Foamies (Scratchbuilt) 10233 Nov 07, 2019 04:38 PM
Discussion ** Kline-Fogleman Airfoiled Flying Wing ** Tony65x55 Foamies (Scratchbuilt) 3960 Jul 20, 2018 06:01 AM
Cool Here is my KFm-5 DLG GLider (Kline-Fogleman) dougmontgomery Foamies (Scratchbuilt) 151 Apr 21, 2014 09:08 AM
Video Kline Fogleman Airfoil on a flying wing Tony65x55 Electric Plane Talk 3 Jan 30, 2009 08:37 PM
Idea Per Dick Kline, Kline-Fogleman test dougmontgomery Hand Launch 49 Apr 13, 2007 02:13 AM