Personal Design

Homebuilt Aircraft & Kit Plane Forum

Help Support Homebuilt Aircraft & Kit Plane Forum:

Topaz

Super Moderator
Staff member
Log Member
Joined
Jul 29, 2005
Messages
14,752
Location
Orange County, California
Hello Topaz...
...Chances are good that you may be right. With my limited knowlege and experience it may be 57 steps missed, but, I'm doing the best that I can with what I have and, I'm not designing the new and improved shuttle. I do however understand that either one can kill you if done incorrectly so I am proceeding with intended speed and caution....
Do understand that I didn't mean any kind of offense. Of course you're learning. I'm an amateur designer just like you, and good lord knows that I'm still a raw newbie in many areas, particularly structures. I'm sure that Orion, experienced professional that he is, would say that he's learning new things every day. This is a simply huge endeavor, designing an airplane and doing it in such a way that there's some assurance that it will fly safe and well, and will hold together when flown as it's expected to be flown. I don't speak for anyone else here (although I know that they probably feel the same) that if we're expressing concerns is simply because you're one of our little family here, and we want to see you succeed and be safe. I suppose my comments do fall under the heading of criticism, but they're intended to be constructive criticism - I really like your design and I very much hope to see it in the air!

I've been around here for about three years now, off and on, and in my own case and in many others that I've seen, questions and design issues usually follow a fairly 'common' progression when a new person comes along wanting (seriously) to design their own airplane. It's also pretty common, as Orion has alluded, to see someone get frustrated with the volume and pace (and iterative nature) of the work, and start to convince themselves that they don't need to really do this or that part of the process, so that they can just "get it done!" In the course of the posts that I've seen you make here, there were some holes in the 'usual process'. And based upon some of the questions you've asked here and privately, it suggests that you're at (or should be at) a certain point in the overall design process. To then hear you talk about a step much farther along in the process raises a little red flag - "Why is he talking about that already? It doesn't seem like he's done 'X' yet." It's not that you're deficient in some way, it's just that it seems from the information at hand like you should be "here", but you're already talking about being "there" in the process of designing your airplane. Part of that information at hand is simply some of the questions you've been asking - about things that should already be 'nailed down' before you can do a real loads analysis, for example.

The process is long and it's sometimes frustrating, especially for people new to it. As you said, sometimes a change at Step 3 means you do have to go all the way back to Step 1 and start parts of the work over. Part of that is a function of the relative inexperience of guys like you or me. Orion or some other professional engineer has already done that sort of thing so many times that they've developed the experience to know how to "get it largely right" earlier in the process and not have to "go back" so many times - and yet I'd be willing to bet that some level of iteration like this is simply a part of the design process at any level of experience. Good, flexible design tools are probably critical for making the iteration process as pain-free as possible. And it also explains why "conventional" designs are so prevalent in the world - there are a lot more "rule of thumb" guidelines to at least get the design in the ballpark, so that the real analysis proceeds more smoothly.

Don't be discouraged! One of the most difficult things about learning to design an airplane, from my own experience, is that there are few learning tools that can show you simply the steps in the process - "Where do I start, and how do I get from Point A to Point B?" Raymer comes the closest yet that I've seen, and even he is hindered by the broad nature of the process. I fervently wish, since I'm studying it now, that someone would do something similar for structural work. Bruhn, thorough as it is, is a classic college text - long on theory and very, very short on process. That may be more appropriate for structures. I don't know enough to make a judgement on the subject yet.

You're doing a great job, and just need to go back and fill in some of the details before you move on to the more advanced stuff. My personal suggestion is that you go back to either of Raymer's books (and especially the larger one, if you're willing), and do for your airplane the type of conceptual work that you see for the 'homebuilt' (aerobatic) example at the back of Aircraft Design, A Conceptual Approach. To my mind, that sort of basic work on stability and control, performance, and weights has to be completed in its entirety before you move on to loads work. Otherwise, there will be so many iterative 'loops' as you finally do fill in the 'early' numbers, that it will become incredibly frustrating.
 
Last edited:

Gray Out

Well-Known Member
Joined
Jul 12, 2008
Messages
206
Hello Topaz,

I agree with everything being said by you and Orion and I thank you for your kind words. I will go back to the book and work it as you have suggested. Jumping around has caused me to go back to several iterations quite a few times like you pointed out.

Thanks
 

Mac790

Well-Known Member
Joined
Jun 22, 2008
Messages
1,529
Location
Poznan, Poland
Hello Gray

I think there is one more thing which you need to consider. Maybe I'm wrong, but personaly I've never seen tapered wings, with the aft/back spar parallel to the main spar (in your renderings it's almost parallel atachment 3), and I've seen a lot of wings. Normaly aft/back spar is parallel to the trailing edge. Check out attachment 1 Nexus Mustang, 2 Laser Z2300. Plus isn't it better to have constant chord, ailerons and flaps? Most plane have them constant.

Seb
 

Attachments

Topaz

Super Moderator
Staff member
Log Member
Joined
Jul 29, 2005
Messages
14,752
Location
Orange County, California
Most spars I've seen (at least in sportplanes and gliders) follow a constant percentage of the chord, rather than be parallel with either the leading or trailing edge.
 

addaon

Well-Known Member
Lifetime Supporter
Joined
Feb 24, 2008
Messages
1,697
Location
San Jose, CA
I thought that the primary reason for this was just to make the top and bottom edges of the spar straight instead of curved...
 

Mac790

Well-Known Member
Joined
Jun 22, 2008
Messages
1,529
Location
Poznan, Poland
Most spars I've seen (at least in sportplanes and gliders) follow a constant percentage of the chord, rather than be parallel with either the leading or trailing edge.
Probably you are right, I've never measured it . But visualy it seems parallel or close to it. I don't think that Gray's aft spar follow a constant percentege of the chord. But of course like I wrote in my previous post maybe I'm wrong.

Seb
 

Topaz

Super Moderator
Staff member
Log Member
Joined
Jul 29, 2005
Messages
14,752
Location
Orange County, California
...I don't think that Gray's aft spar follow a constant percentege of the chord. But of course like I wrote in my previous post maybe I'm wrong.

Seb
As far as I can see, you're right. They seem to both be perpendicular to the long axis of the airplane, from tip to tip. It can be done that way. It's more work.
 

Mac790

Well-Known Member
Joined
Jun 22, 2008
Messages
1,529
Location
Poznan, Poland
Oops It seems I was wrong. I just checked it out in Raymer's book, Gray probably has ailerons 0.20-0.30of the chord, because his ailerons are less than half-span, that's why it looks so big, but still it looks big special if you compare it with for example Zodiac 650 (attachment), do you know any homebuilt airplane with so big ailerons and flaps.

Seb
 

Attachments

Gray Out

Well-Known Member
Joined
Jul 12, 2008
Messages
206
Hello to all,

I think there is one more thing which you need to consider. Maybe I'm wrong, but personaly I've never seen tapered wings, with the aft/back spar parallel to the main spar (in your renderings it's almost parallel atachment 3), and I've seen a lot of wings. Normaly aft/back spar is parallel to the trailing edge. Check out attachment 1 Nexus Mustang, 2 Laser Z2300. Plus isn't it better to have constant chord, ailerons and flaps? Most plane have them constant.
Mac, thanks for the pics. All spars follow the symmetrical shape of the leading and trailing edge taper. I say symmetrical because the tip is centered. If the trailing or leading edge were perpendicular to the center line, then it would still be a .5 taper but not symmetrical. The chord is constant for the taper. In other words, they are the same percent of chord at the tip as it is at the root for their respective lengths. I doubled checked it to be sure.

Most spars I've seen (at least in sportplanes and gliders) follow a constant percentage of the chord, rather than be parallel with either the leading or trailing edge.
This is true of the Peregrine as well. If the wing were a Hershey bar, then the chord would look perpendicular to the center-line. If the leading edge were perpendicular to the center-line for the same taper, the trailing edge would be sloped as would the same chord and vice versa for the leading edge. The chord is constant from root to tip.

I thought that the primary reason for this was just to make the top and bottom edges of the spar straight instead of curved...
Looking at the area of transition a little closer, it appears that there is still a small vertical arc of space between the spar and the curve for the foil. I’m not sure how this would be handled, (wedge filler?) but I suspect that the gap would be greater the wider the spar becomes chord-wise. Obviously the more intimate the contact the better transfer of loads.

Probably you are right, I've never measured it. But visualy it seems parallel or close to it. I don't think that Gray's aft spar follow a constant percentege of the chord. But of course like I wrote in my previous post maybe I'm wrong.
Their appearance of parallel is probably due to their proximity to each other, but as you can see, the extremes are at the leading and trailing edge and the only true perpendicular span would be at absolute dead center of the foil. Everything else would diverge proportionately at the root and converge the same proportion at the tip and unless the tip or root distances are kept the same, they cannot be parallel, but then the chord be different.

As far as I can see, you're right. They seem to both be perpendicular to the long axis of the airplane, from tip to tip. It can be done that way. It's more work.
It only appears that way. They are not parallel. There can only be 1 true perpendicular span (chord-wise center line) and two parallel sides (root and tip) in an isosceles trapezoid as this wing plan form denotes.

Oops It seems I was wrong. I just checked it out in Raymer's book, Gray probably has ailerons 0.20-0.30of the chord, because his ailerons are less than half-span, that's why it looks so big, but still it looks big special if you compare it with for example Zodiac 650 (attachment), do you know any homebuilt airplane with so big ailerons and flaps.
Good observation. If you remember from prior posts, there were certain recommendations made that I incorporated for the first cut. Longer span flaps for higher landing CL and wider chords for the ailerons due to tip losses and their already reduced span. There are different ways I looked at it when I was studying it to try to find patterns. The chord percent is approximately 35% consistently for both aileron and flap from root to tip. The flap/aileron areas calculated differently as a percent of the wing area. The flap area is 22.6% of wing area and the aileron area is 12.76% of wing area. I don't remember what the flap area was to the "flapped" area, but I could look it up if you want to know it. I attached a better picture which shows that the spars are not truly parallel to themselves of the leading/trailing edges as denoted by their distances to each other at the root and tips. I hope this helps and hope my explanations didn’t create any more confusion.

Thanks
 

Attachments

Mac790

Well-Known Member
Joined
Jun 22, 2008
Messages
1,529
Location
Poznan, Poland
Hello Gray.

Sorry for confusions, and a little mess about my posts yesterday, I was probably too tired or too drunk:beer:, when I wrote that in most planes rear spar is parallel to the trailing edge.

Seb

btw Jake maybe it's a good idea to create a poll for a sillies post of the year, I think I'll have a chance for the first place.:roll:
 

Gray Out

Well-Known Member
Joined
Jul 12, 2008
Messages
206
Check out that page, you will like it.
Seb
Hello Mac,

I checked it out. It is very nice indeed. Too bad the bird broke a wing but at least everyone was safe and the problem was found. Hard way to learn the lesson, but she sure does look really nice.

I have been recalculating all over again and find that I am very very close to the weight ratio for composite. It appears that I can get the design built in composite glass/foam for only 10% greater weight.

I'm attaching partial findings based on my design calculations (the red entries) and the comparisons to Raymers book (black entries and picture cutouts) so that I can keep track of what I did and did not do instead of second doubting myself. I started from the first page in the book and will do the entire book before it is all over. I did a lot more, but this is the part I want you to check out. The design calculates to 1.19 for a constant and as you can see Raymer's constant is .99.

It looks like this persistence to recalculate the design turned out to be a favorable push because we are once again back to composites and this time, I am sticking to my guns. Composite all the way for the Peregrine.;)

I will post some more as I calculate more. The more I look at the results, the more I see that the Peregrine will be FAST even if the LSA restricts the velocity to 120 knots. Stand by...this isn't over by a long shot.

Thanks
 

Attachments

AVI

Well-Known Member
Joined
Feb 25, 2003
Messages
525
Location
Banff, AB
Gray

This time I did something for you:), It took me about 3h I'm hoping you will like it. I dont remember what type of aiirfoil you use, I chose 65-215 (maybe not the best but this one I have).
Just in case, if you decide to build your wings out of composites I did for you a little presentation (simplified version how to build composite wings). Thanks to these Poliwagen plans, at least I have the idea how to make it.

Description
pic 1 "wood mould" before assembly
pic 2 after assembly
pic 3 first skin (note I did'n make sandwich, its simplified version)
pic 4 main and rear spar (simplified)
pic 5 ribs
pic 6 top skin

I chose carbon because I dont have fiberglass texture in my software.

Orion wrote at least 2-3 times what type of material is usually use for sandwich skin construction, you need to find it, same about skin (number of layers).
Everything what you need to calculate is spar (spar caps and shear web), do you have/know all loads (airload, shear load etc).

btw You will find more information in these Poliwagen plans check them out.

Seb

It's only simplified version so I didnt pay to much care for details.
Seb,
I've often tossed around in my head the possibility of fabricating a fuselage mold in this manner. Any suggestions?

Terrific graphics, BTW !!!
Alex
 

Mac790

Well-Known Member
Joined
Jun 22, 2008
Messages
1,529
Location
Poznan, Poland
Seb,
I've often tossed around in my head the possibility of fabricating a fuselage mold in this manner. Any suggestions?

Terrific graphics, BTW !!!
Alex
Alex,

In my opinion that method should be good for the wings construction (skins), personally I don't see any reason why it shouldn't, but it won't work for the fuselage unless you are trying to make something like Davis DA-2. Otherwise you could use mixed technique, plywood sheets for less complex curvature and foam blocks for more complex curvature, next you could cover it with a 2/3 layers of glass add some putty, hmm it should work.

If you are going to make only one fuselage probably the best idea is to make something similar to what Duncan or Rom did.
If you are going to make more fuselages the only way for it is to make a real molds, here is a nice thread about a new Berkut/clone fuselage, with some pic
A-Solution - Canard Zone
and of course I really like this one (I gave you that link a year ago or something, but just in case) Àññîöèàöèÿ Ýêñïåðèìåíòàëüíîé Àâèàöèè - Intruder

But I believe there are two possibilities,
first decide what type of shapes you are going to build, and after that look for proper technique, real molds, like Rom, or let's call it "Vision-aircraft" technique.
Second option is, first choose method next try to design your fuselage for it, you won't build a really nice curved fuselage with "Vision" method.

Seb
 
Top