MyGeekShow

Hey guys,

So I built the glider I designed in MyGeekShow episode 2, and it flew great! Except finding the perfect center of gravity seemed more difficult than I thought.

Next time I’m going to shift the wing further back to allow for easier CG adjustment, add more weight so drag doesn’t overcome its momentum as easily, and make some kind of shock absorber to the nose so it can survive more than a few crashes.

What do you think? What do you do to find the center of gravity?

[youtube]http://www.youtube.com/watch?v=CNO7D9InWQo[/youtube]

AmishWarlord

My Feedback: (5)

Hi, I think your having trouble because your "moments" are off.

First off let me say that anything can be made to fly. They proved that one year at Joe Nall by bolting a chain saw engine to a sheet of plywood. The thing is can the design inherently fly well and efficiently.

Here is a list of 100 or so sail planes. Look at how they are shaped. http://www.nesail.com/categories.php...35e286d50c521a

As you see most of them have high aspect ratio wings. The span of the wing is much greater than it's cord. Also the the fuselage is about 1/2 the length of the wing on them.

Your plane looks pretty much square in the video, meaning the length of the tail is as long at the span of the wing. This is giving you too much leverage and making your CG adjustments difficult because even if it's right on your plane will still be too sensitive to control inputs.

Try redesigning a new plane with these moments

wing span to cord 9.25 to 1

wing span to fuselage length 1.9 to 1

horizontal stab span 1/4 of wing span

vertical stab span 1/5 of wing span

Go by what you know works "mainstream designs" then play around with different designs.

Here is good info on CG's

http://members.multimania.co.uk/A75Church/tips3.htm


PS your glider looks almost exactly the same as my fist scratch built design did. No one gave me good advice on how to find the CG either. "Just go by what the plans say" LOL I designed it myself! Anyway have fun learning about aerodynamics and if you need any help let me know.

MyGeekShow

@AmishWarlord,

AmishWarlord

My Feedback: (5)

No prob, I'm going to subscribe to your You Tube channel so I can see your next design.

All Day Dan

My Feedback: (5)

Use this for the CG. Dan.
http://www.geistware.com/rcmodeling/cg_calc.htm#mac

Lnewqban

Good video, MGS.

The devil is in the little details.

You don't need more weight but less air resistance and more lift to reach farer; paper airplanes fly well.

There is more than balance into a good flight, dynamic stability for each axis is also needed.

http://adamone.rchomepage.com/index5.htm

http://www.free-online-private-pilot...in_flight.html

http://www.av8n.com/how/htm/

http://www.paperplane.org/Aerodynamics/paero.htm

These articles will explain to you how to build, trim and balance a free flight model:

http://f4bscale.worldonline.co.uk/hand.htm

http://www.pensacolafreeflight.org/page5/page5.html

I would turn that wing upside down.
The way it is now, creates turbulence on top and structural weakness, which are against your goals.

AmishWarlord

My Feedback: (5)

check out this site also

http://airfieldmodels.com/index.html

BMatthews

A bunch of points for you to consider;


[ul][*] First, a model does not need weight to "carry momentum" to fly successfully. It only needs to be set up with the right balance point and wing to tail angle to generate stable flight. Once that is set it'll do just fine at whatever weight it comes in at. An example of this can be seen if you look up "indoor rubber model" in YouTube and find the world champ duration stuff which typically weigh only a little over a gram and a half and fly on rubber motors that would look at home as elasctic in the waist band of most pants. They fly thanks to proper trim and owe nothing at all to "carrying momentum" for their success. And with gliders the lighter it is the longer it'll stay up. For a glider altitude is the fuel in the tank. A heavier model will use up the "fuel" faster than a light model.[*] Having said this a model that is TOO light will be tossed around too much by the turbulent conditions found outdoors. So yeah, there's a good range of weight for a model to make it fly a bit faster and deal with outdoor conditions more easily. But your foamy glider is a LONG way from being too light for mild to medium conditions.[*] The tail area you used for the horizontal stabilizer is too small. Generally you want the tail area to be between 15% and 20% of the wing's area when used with a "normal" fuselage lenth such as you used.[*] The wing is best set up with some dihedral. As you get your model to fly for longer the lack of dihedral will bite you in the backside and the model will tend to be less than stable in the roll axis and will tend to want to tighten any turns and go into a spiral dive. An easy fix would be to cut the wings about mid way out from the center and jack up the wing tips by around 5 to 7 degrees from the joints to the tips. Glue the joint with more hot glue and then add a lower side paper joint reinforcement applied using a thin spreadable glue such as carpenters glue.[*] Speaking of strengthening you'll find that your doubled layer of foam is going to be too weak and flexible once you get to trying out any towing. Something to act as a wing spar would be a good idea for at least the center section. Maybe a fiberglass or carbon arrow shaft set into the top layer of foam at around the 1/3 wing chord point.[*] To be stable in the pitch axis you want the tail to have a small amount of negative angle to the wing. Since you've got rubber bands holding the tail on I'd first suggest you make a new set of tail feathers with the new size of stabilizer. And when you mount the new tail back onto the fuselage use a shim at the rear to provide about a 1 to 1.5 degree negative angle. Alternately you could put a shim of one layer of foamboard under the leading edge of the wing to do the same thing. [*] With the new tail and wing to tail angle difference you can then play with your nose weight shifting it back and forth looking for a stable and flat glide. Once you get it into the ball park where it flies in a stable glide you can alter the glide speed while still keeping a flat flight path by small shifts of the nose weight fore and aft. You can do this between the limits where with the nose weight back it begins an oscillating hunting slow flight where it is just shy of a stall and flying so fast that it would be best considered as a dive. [*] Use the online CG calculator with the stability margin set to 5% for a free flight model of this sort. Oddly enough you don't want a free flight model to be TOO stable. It just needs to be slightly positively stable for best flying. And if you do the calculations for your old stabilizer and the new suggested larger one I think you'll easily see how it alters the CG location to be farther back as the tail becomes bigger. A good flying model will be where the stabilizer is big enough to put the desireable CG at around 30% of the wing. [*] You can get this same effect by altering the shim under the leading edge of the wing or under the trailing edge of the stabilizer to alter the wing to stabilizer angle as well. On real planes this is done with the elevator. The sensitivity to the balance point is also why passenger and cargo planes also take great care to place the passengers and freight loads evenly in the fuselage to avoid a shifted balance point that is beyond the capability of the plane to deal with.
[/ul]

Good luck with the Mark II version...

MyGeekShow

@BMattews

Well, I'd say that is about as difinitive as you can get! Thank you! I think we all need to remember these points when we are considering design and construction of our airplanes. I'll make sure MkII finds its way to this post.

Again, excellent information, and thanks for watching

MyGeekShow

BMatthews

As far as it goes the video and the steps used are superb other than for a few points.

Glueing. Glue works best when it's used at the points of maximum stress. For example for attaching the tail the glue would be best used in two lines along the outside edges of the fuselage and then the tail placed and pressed down while it's still as hot as possible. Glue at the tail in the middle between the fuselage sides does very little for holding it down since the stresses are always going to be concentrated at the extreme edges. Adding the final exterior beads isn't a bad idea though. This trick works with hot glue and foam. However if you "graduate" to wood models exterior glueing does very little but add weight.

Have you tried out the modified wing to stabilizer angles by adding a shim under the leading edge of the wing and playing with the balance point yet? The "glide" you presented in the video has more in common with a lawn dart than a flying glider. Adding a shim to raise the leading edge by about 1/6 to 3/16 and then playing with the balance point a little will produce an actual flat slowly descending glide path once trimmed out.

AmishWarlord

My Feedback: (5)

Looking better. The scooter trash is giving you sage advice.

That tail still belongs on a lawn dart or jet rather than a glider. Keep slow, slim and eloquent in mind on your designs.

Ah, lawn darts banned because of one man's campaign on Washington to get the evil things banned. Mean wile ponies are still on the market maiming and killing little kids. If you call Washington about it their like "Dude shut up, we're not going to ban ponies, OK".

MyGeekShow

@BMattews

Good advice on glueing, I'm still learning what is really adding value structurally, and what is unnecessary. I'll keep that tip in mind.

I finally understand what you shim idea, unfortuantly I already built the improved glider for Episode 6, but I will try that in future versions for sure. Again, thank you for the tip!

MyGeekShow

@AmishWarlord

Slow, slim, and eloquent... describes me perfectly!

Will keep in mind.

dreadnaut

My Feedback: (5)

I noticed a bit of flexing in the wing at launch. You have gotten a lot of good aerodynamic advice so far. Here comes some structural engineering 101.

the figures below show diagrams involving I-Beams. The top figure gives the nomenclature used in the Structural Engineering discipline. One difference you will note is that in aircraft, the flange is often called the spar.

The second figure shows what happens to the beam when a force is applied. The beam is subjected to three kinds of stress. Two are shown here and are called compressive and tensile stresses.

To explain the third tipe of stress, you will first need to study the stress diagram in the bottom figure. What the diagram says is that the further you get from the centre, the greater the stress becomes. Another important way to think about it is that the stress is increasing. This changing compressive and tensile stress produces a third type of stress, called shear stress.

To further understand shear stress, think about what happens when you take a pack of playing cards and bend them as if to shuffle them. Notice that the faces of the cards slide against each other. The stress that resists this sliding is shear stress.

The second drawing shows the typical construction of a wooden aircraft spar. Notice that the grain direction is chosen to take advantage of the fact that wood has better tensile and compressive strength in the direction of the grain, and better shear strength in the direction perpendicular to the grain. In aircraft spars, the web is often referred to as the shear web.

Hopefully this will help you redesign the wing for better stiffness. I know that you are using blue foam, but here's the thing. . . that stuff has a grain too!

If you take a fairly large sheet of it and flex it, then turn it 90 degrees and flex it again, you will notice that it is a little stiffer in one direction than in the other. Make the flanges in the stiffer direction.

MyGeekShow

@dreadnaut

Thank you for the Structual Engineering summery, a spar will be making it on the next version of my glider (on Episode 6, this Saturday) and hopefully the wing won't see so much flexing.

About the foam flexing "angle" I also noticed this! It really does have different flexing properties depending on the side (finished, unfinished) and direction (with the "waves", against the "waves"). Never thought to use that to my advantage...

Thanks for watching, and the tips!

BMatthews

Your efforts so far remind me hugely of my own more than a few decades ago when I first got started in model buiding and flying. But back then we didn't have any FFF for cheap material. Instead I used sheet balsa and built a huge number of 4 to 16 inch back yard and school yard gliders of all sorts from conventional layouts to deltas to flying wings both swept and "plank" and flying saucers and canards and many other combinations of those types. Working with all of those and finding out the little trimming discoveries was like a big light bulb coming on over my head each time. And USUALLY it resulted in a gain of both model performance and building understanding. And, of course, sometimes learning what didn't work was just as important as learning what did.

Have fun and enjoy your series on this building. And keep an open mind on what we're telling you and what you want to try. And most of all have fun with your gliders.

MyGeekShow

Thank you for all your comments! I took your recommendations and built a better glider. It flies MUCH better than the previous version. Check it out:<meta charset="utf-8" />

Lnewqban

Congratulations; much better now.

I would add more dihedral in both axes, longitudinal and transversal (decalage and wingtip dihedral) in order to improve stability.

I still would turn that wing upside down, improving the coefficient of lift and reducing unnecessary turbulence on top.

Check the links of post #6 above.

MyGeekShow

Thanks! Your recommondations helped. I'll be doing some dihedral testing this summer, and will reach out to you then as my knowledge in that area is limited.

Yeah, that wing might not be the best. Its just really easy to build : ) and for now, I go through so many models its the best one. I'll be testing wings in the future.

Thanks for the advice and for watching!

ARUP

Check out 'discus launching' of sailplanes!

MyGeekShow

@ARUP

Yeah,AmishWarlord just showed that to me, its pretty cool! I might have to give that a try! Thank you for the tip.

dreadnaut

My Feedback: (5)

Now there is tendency of the plane to roll to the right. It looks like this roll starts at launch when you give it a tiny little twist at release. Putting a little dihedral, as BM mentioned earlier. would tend to right the model.

Dihedral does a couple of things. The figure below shows an exaggerated angle so that the effects can be seen more easily. The lifting force of the wing is represented bay an arrow called a Vector. In this case the units of "length" can be expressed as a force, in ounces, or grams. Notice that the force is acting at right angles to he surface of the wing. As the airplane rolls to one side, the sideways component of the vector on the high wing, causes a tendency for the plane to turn, while the vertical component shrinks relative to the vertical component of the lower wing. This creates a tendency for the craft to right itself.

BMatthews

All the changes you made have helped but there's still some ways to go on your road to knowlegde and a TRULY successful glider. Here's the next set of observations.

[ul][*] As noted already by dreadnaut the lack of dihedral is what is making the model veer off of your straight flight path. Even some 6 inch stubs on the wing tips angled up at around 30 degrees would aid in avoiding that sideways slide that is occuring. The dihedral angle doesn't need to be actually at the middle line. In fact a perfect spot to put it on your present wing would be at the ends of the dowel spars. Just cut the wing there and sand a 10 to 15 degree angle into each piece and glue it back together with some 5 minute epoxy such that both sides are the same. [*] The model is still flying like a "lawn dart". That is, the flight path is describing what is basically a ballistic path. You need to add some angular difference between the wing and the stabilizer to achieve pitch stability. And since you're using the 1/4 inch thick blue foam doing so is actually pretty easy to do on your present setup. The diagram below shows how.[*] NORMALLY raising the aspect ratio aids in producing a better glide. But in this case you increased the aspect ratio at the expense of a reduction in wing area. That raised the wing loading and so any gain you got from the higher aspect ratio will have been squashed by the rise in the wing loading over what it COULD have been. There's another aspect as well. At the size of your model the scaling effect comes into play. This effect is described in aerodynamic circles as the Reynolds number. It's a way of comparing various sizes of wings at various airspeeds. But basically a model glider with a narrow wing chord will have a very low Reynolds number. Higher numbers tend to fly better so in the case of your glider making the wing more narrow actually hurt the performance due to reducing the Reynolds number it operates at as well as increasing the wing loading.
[/ul]

Note that with the slices cut or sanded off the stabilizer the model may want to stall. To counter act the stall move the nose weight slightly ahead or use a slighly heaver weight. The idea is to achieve a consistent flight path instead of the steadiliy increasing nose drop that ends in a dive like it does now. Having the tail at a slight negative angle to the wing in cojunction with the nose weight produces a balance of forces that is speed regulating. The nose weight is fixed and wants to make the model dive much like it is now. The tail is lifting less than the wing due to the slight negative angle with respect to the wing. But lift is speed sensitive. So the faster the model flies, such as when the nose drops and the model goes into a shallow dive, the more the wing lifts and the tail lifts less which lifts the nose until the lift forces are balancing the nose weight forces at a fixed airspeed.

MyGeekShow

Once again, you both give me more to think about and tempt me to build another glider to test/prove these concepts... Thank you for the tips!