Discussion thread: mcrae0104 project

Discussion in 'Aircraft Design / Aerodynamics / New Technology' started by mcrae0104, Nov 27, 2017.

Help Support HomeBuiltAirplanes Forum by donating:

  1. Nov 27, 2017 #1

    mcrae0104

    mcrae0104

    mcrae0104

    Armchair Mafia Conspirator HBA Supporter

    Joined:
    Oct 27, 2009
    Messages:
    2,959
    Likes Received:
    1,953
    Location:
    BDU, BJC
  2. Nov 27, 2017 #2

    Topaz

    Topaz

    Topaz

    Super Moderator

    Joined:
    Jul 30, 2005
    Messages:
    13,623
    Likes Received:
    5,275
    Location:
    Orange County, California
    Oh good, I'll repeat publicly what I just sent you via PM: I showed one of your earlier images of your project's current iteration to my friend from Boeing. His jaw dropped a bit, as your project is extremely similar to his own, right down to the "family resemblance" to the Polen Special. His is probably going to run behind an O-320 or 360, but in all other respects is very much like your own. Which, given where he works and the programs he's worked upon, I'd take as the compliment it is.

    Nice job.
     
  3. Nov 27, 2017 #3

    wsimpso1

    wsimpso1

    wsimpso1

    Well-Known Member

    Joined:
    Oct 19, 2003
    Messages:
    5,772
    Likes Received:
    3,000
    Location:
    Saline Michigan
    Looks like it should be a fun project!
     
  4. Nov 27, 2017 #4

    wsimpso1

    wsimpso1

    wsimpso1

    Well-Known Member

    Joined:
    Oct 19, 2003
    Messages:
    5,772
    Likes Received:
    3,000
    Location:
    Saline Michigan
    I would look really hard at the Tailwind wings for rib details, rib spacing, skinning, etc. Spar should still end up being built up and tailored to shear and bending moment. Fuselage design work will be interesting, but with only a single seat canopy cut, it will not be a tough nut to scheme out. Look hard at the Falco for details.

    Billski
     
  5. Nov 27, 2017 #5

    Winginit

    Winginit

    Winginit

    Well-Known Member

    Joined:
    Jun 13, 2016
    Messages:
    811
    Likes Received:
    242
    Location:
    x
    Here is a place where you might find some pertinent info on the questions you have about building a fast airplane from wood. As you can see, the GP-5 bears a resemblence to your design, so there might be some analitical data available, or at least a finished plane to compare your design to. The GP-4 has a news letter, so it might be a way to interface with some other wood builders for a similar design.

    http://springfieldaviation.com/Newsletters/GP4BFN56.pdf

    GP5.jpg
     
    Last edited: Nov 27, 2017
  6. Nov 27, 2017 #6

    plncraze

    plncraze

    plncraze

    Well-Known Member HBA Supporter

    Joined:
    May 12, 2006
    Messages:
    1,556
    Likes Received:
    321
    One of the early issues of Contact! Magazine had an issue devoted entirely to the the GP-4. Reprints are available through their website.
     
  7. Nov 27, 2017 #7

    Autodidact

    Autodidact

    Autodidact

    Well-Known Member

    Joined:
    Oct 21, 2009
    Messages:
    4,513
    Likes Received:
    799
    Location:
    Oklahoma
    It would be a desirable aircraft no matter what it was made of, but wood just seems to trigger emotions in people - it does to me.
     
  8. Nov 27, 2017 #8

    BJC

    BJC

    BJC

    Well-Known Member

    Joined:
    Oct 7, 2013
    Messages:
    9,486
    Likes Received:
    6,240
    Location:
    97FL, Florida, USA
    I like it.

    Where is the fuel, and what type of tanks?

    Fixed conventional landing gear?

    A comment on landing speed: there are good reasons for “lower” landing speeds, but don’t think that higher landing speeds require much more that getting some minimal experience with higher pattern and approach speeds.

    Looking forward to following your progress.


    BJC
     
  9. Nov 27, 2017 #9

    TarDevil

    TarDevil

    TarDevil

    Well-Known Member

    Joined:
    Jun 30, 2010
    Messages:
    278
    Likes Received:
    132
    Location:
    Coastal North Carolina/USA
    I'm looking forward to this!
     
  10. Nov 28, 2017 #10

    mcrae0104

    mcrae0104

    mcrae0104

    Armchair Mafia Conspirator HBA Supporter

    Joined:
    Oct 27, 2009
    Messages:
    2,959
    Likes Received:
    1,953
    Location:
    BDU, BJC
    You guys are going to be able to provide feedback more quickly than I can react! But even if I can't respond to each of you directly or quickly, thank you for chiming in.

    Thanks for the positive reinforcement, Topaz (and others). I have also considered a 320 or 360. One angle is this: I have a friend with a 285 hp Bonanza. I thought it would be interesting to see if I could match or exceed its performance with an airframe of my design and an "alternative" engine built by me (except for useful load, of course). There is much more discussion to be had on the topic of engine selection, although I want to sidestep it for now in order to start at the beginning, as it were.

    Thank you for these suggestions. I have been reading back articles from the Sequioa newsletter; I'll add the GP-4 materials to that list. Perhaps someday I'll tackle something like the GP-5 with a V-8... there is a certain visceral appeal, but I'm not ready yet.

    Before answering those questions, I will just note that the process of design is very much a process. There have been many decisions I thought I new the answer to from the outset, but there is a certain open-mindedness that is required until things begin to settle into place. You make educated guesses, check how the whole thing works out, then make adjustments. So I will tell you what I think right now but it is all subject to refinement.

    I know that opinions on fuel location can be a sensitive subject, so don't rake me over the coals, please! I am aware of the tradeoffs involved in safety, fuel management, and W&B--and the final configuration may be different. For now, the fuel will be located in the fuselage in two tanks. The main tank is between the firewall and instrument panel. This tank will be in the upper 50% of the fuselage, with a forward baggage compartment below this. The main will be about 23 gallons. The second tank will be an auxiliary tank located behind the cockpit below the baggage shelf, with a capacity of 10-12 gallons. Another (simpler) option is to carry all the fuel up front in one tank, with all baggage behind the cockpit. A fuselage cutout for baggage access would certainly be easier behind the cockpit (or better yet, with all access from inside the cockpit). W&B scenarios will help inform this decision.

    I have not given any thought to the tank material but I am open to welded aluminum, composites, or possibly an off-the-shelf fuel cell if a suitable size can be found. The fuel & baggage have not been run through W&B but I suspect it will be OK after I run the numbers. Depending how favorable the W&B is, it may also be possible to have a larger aux tank for extended operation in normal or utility category.

    Conventional (taildragger). Most likely fixed, but undecided. The performance numbers above assume a well-faired, fixed gear. There are a lot of options to evaluate in the family of fixed gear before I even consider retracts. No doubt most would counsel against retracts.

    Noted, and thank you for the advice from a more seasoned pilot's perspective. I primarily had reduced kinetic energy in mind, although I also do not want a plane that is "too hot." The stall speed is just shy of a Bonanza--much higher than LSA territory, where I started out. When I start getting the wings and flaps sorted out, I will know this number with more certainty. Provided that the gear remains fixed and the fuel remains in the fuselage, it should not be a major problem to reduce wing area should I decide to.
     
    Topaz and BJC like this.
  11. Nov 28, 2017 #11

    mcrae0104

    mcrae0104

    mcrae0104

    Armchair Mafia Conspirator HBA Supporter

    Joined:
    Oct 27, 2009
    Messages:
    2,959
    Likes Received:
    1,953
    Location:
    BDU, BJC
    I'm transcribing Billski's input from some other threads so that all the information related to this design is in one place. If you've already been through this, just skip along...

    This is all good stuff for would-be engineers to pay attention to.

    Billski, while I've thanked you privately, I should do so again here. I hope others reading this can see what a great quick-and-dirty outline of the structural design process this is that you have laid out. I can't respond to every comment you've generously made, except to say that this student is eagerly paying attention!

    On the last bit regarding fuselage/wing aero interaction: Yes, your point is understood. I've seen Lednicer's articles on CFD and the RV-6 in particular. Dr. Frati also discusses this in The Glider, which I just found the other day. NAA got so much right on the P-51 layout (esp. compared to its contemporaries) that it is staggering.

    It's probably not evident from the images I've posted so far, but these are the conditions I've designed with precisely your point in mind:
    • The fuselage sides are very nearly vertical. The lower half transitions from elliptical at the spinner to what you see in the image below by the time it reaches the wing. Despite the dihedral, it's 87°, which I hope will be close enough to satisfy the air.
    • The fuselage sides get most of their expansion out of the way before the leading edge of the wing, but they do continue to expand slightly until max width right at the TE. I've been careful to limit the rate of change of the curvature to prevent separation. (We've discussed the "poor man's area rule" in other threads; while it wasn't a design driver for me, I may do a quick analysis of the cross-sectional area just to see how closely I've followed it.)
    • An expanding-radius fillet will be provided, beginning all the way at the leading edge, growing to about 2" at the spar and roughly 8" at the TE. The wing may not even need it but I like the way it looks and I'm a glutton for punishment.
    • The walls of the fuselage finally "let go" and really begin converging about 20" aft of the TE (just about even with the pilot's shoulders). The upper longerons undergo about 8° of change over a length of about 10".
    02.jpg
     
    wsimpso1 likes this.
  12. Nov 29, 2017 #12

    proppastie

    proppastie

    proppastie

    Well-Known Member

    Joined:
    Feb 19, 2012
    Messages:
    3,699
    Likes Received:
    1,074
    Location:
    NJ
    Fantastic build log, I am not doing much aero for my bird, but to have all the real world numbers laid out as an example is great. Thank you very much. I hope you will do the same treatment for the stress.
     
    mcrae0104 likes this.
  13. Nov 30, 2017 #13

    Chris Young

    Chris Young

    Chris Young

    Well-Known Member

    Joined:
    Feb 19, 2013
    Messages:
    88
    Likes Received:
    29
    Location:
    France
    Hello,

    Just a quick question : if I understand correctly and it is now a single-seater, why is there so much room between the pilot and the engine ? This would increase the airframe weight and can cause problem with CoG : the overall CoG when loaded is the same in flight condition as the CoG with the pilot and engine closer together and closer to the spar, but when the pilot exits the airplane, it is now loaded very much forward and possibly forward of the landing gear which would cause a noseover. I had designed a two-seater like that, that would have needed a counterweight installed before the pilot could come out. That was common among the Caudron racers of the 30s but would not be very welcome for a touring aircraft.

    To still have the canopy aft enough for "inverse pressure gradient matching", you could have the seat reclined a bit more.

    Cheers,
    Chris
     
  14. Nov 30, 2017 #14

    wsimpso1

    wsimpso1

    wsimpso1

    Well-Known Member

    Joined:
    Oct 19, 2003
    Messages:
    5,772
    Likes Received:
    3,000
    Location:
    Saline Michigan
    Mcrae0104,

    I am glad you find my descriptions useful. You are too kind in your praise, as I laid out a huge amount of work for you. While I have applied this stuff to my fiberglass bird, I have never applied this stuff to wood. Your turn to figure out which points never get close to mattering, and which do.

    Aren't Dave Lednicer's articles great? He was the one who pointed out to me (personally) about how the most important thing for fuselage aerodynamics is to let the wing work like the fuselage is not even there. He also pointed out that tail drag can get big (same reasons) if you do not separate the thickest parts of the vertical and horizontal tails.

    Some people will disagree on the need for that sort of fuselage shape, and certainly the sailplane guys take a different path, but without tailoring the airfoil close to the root and doing extensive CFD, yours is the reliable path to low drag.

    I too am curious about the logic behind longitudinal layout. In such a small airplane, I would expect that the CG and controllability study would confirm your configuration or drive changes. People and bags don't move much in flight, but fuel forward and aft means that you might have to burn from both to keep CG within the controllable range. You might be wise to check out how bad your CG gets if you have drawn from the forward tank and then find the aft tank won't feed and a safe airport is a ways off forcing you to further draw down the forward tank. Worst case analysis is forward tank empty, aft tank full. That will, at minimum, set aft and forward baggage relationships. In my case, fuel is in the wing and centered at about 37% MAC, so it matters little, but people and most of my baggage is aft of CG, so to get to a workable range, I ended up with more tail volume than most would want in order to be able to flare at the forward CG end (in my case, light pilot, no pax, no bags, min fuel) envisioned. I think that you should run the range of what can be put in each position, and then run the permutations. That may place the limits on how much you can put where, or drive other changes.

    Billski
     
  15. Dec 1, 2017 #15

    mcrae0104

    mcrae0104

    mcrae0104

    Armchair Mafia Conspirator HBA Supporter

    Joined:
    Oct 27, 2009
    Messages:
    2,959
    Likes Received:
    1,953
    Location:
    BDU, BJC
    Chris, thank you for this post. I really appreciated it.

    The room is there because I like the way it looks with a longer nose and the canopy pushed back. You are correct; it does have a weight penalty. It's romantic appeal, not functional rationalism. (Well, really, a compromise between the two that suits my taste.)

    I am really glad you pointed this out, because I had not considered this loading case. I ran the numbers, and the CG falls behind the main gear. (Checked w/ and w/o fuel, baggage, etc. and the various combinations thereof).

    Wow, what a cool airplane that I had never heard of before. That is a plane with style.

    side-view-of-blue.jpg

    Yes. I actually need to build a mockup of the cockpit at some point to see exactly what I find most comfortable, which may be more reclined. The planes I am accustomed to do not have supine seating, and even on long drives I tend to find myself most comfortable more upright (but sometimes wishing I could stretch my legs a bit more). In my cockpit I would like to have enough room above the pedals to stretch out when needed. This is one advantage of not having the fuel tank directly above the pedals.

    Hmm, I had missed that. One more thing to investigate.

    Yes--right on point. The numbers won't lie. One thing that is not immediately obvious in the drawing is that the forward tank location actually plays a relatively minor role in CG shift. Relatively speaking, the "forward" tank is not as much of a factor as it would be if the pilot were positioned closer to the CG. I'm glad you pointed out controllability, too, since this scheme does increase the polar moment of inertia.
     
  16. Dec 1, 2017 #16

    BBerson

    BBerson

    BBerson

    Well-Known Member HBA Supporter

    Joined:
    Dec 16, 2007
    Messages:
    11,824
    Likes Received:
    2,242
    Location:
    Port Townsend WA
    I assumed sitting behind the wing was for a straight down view behind the wing.
     
  17. Dec 1, 2017 #17

    Tiger Tim

    Tiger Tim

    Tiger Tim

    Well-Known Member

    Joined:
    Apr 26, 2013
    Messages:
    2,844
    Likes Received:
    1,586
    Location:
    Thunder Bay
    I thought it was to stick a big fuel tank right over the CG, with "because it looks cool" as a close second.
     
  18. Dec 1, 2017 #18

    BBerson

    BBerson

    BBerson

    Well-Known Member HBA Supporter

    Joined:
    Dec 16, 2007
    Messages:
    11,824
    Likes Received:
    2,242
    Location:
    Port Townsend WA
    Agplanes have big tanks right over the CG. Puts the pilot in a good spot for crashes. They look cool too.
     
  19. Dec 4, 2017 #19

    mcrae0104

    mcrae0104

    mcrae0104

    Armchair Mafia Conspirator HBA Supporter

    Joined:
    Oct 27, 2009
    Messages:
    2,959
    Likes Received:
    1,953
    Location:
    BDU, BJC
    Static stability

    Does anyone have guidance on what goal I should have for static margin? Currently, I have an estimated 6.5%-10% depending on the useful load cases (subject to better weight estimation and more accurate calculation of the power-on, stick free neutral point).

    Raymer suggests:
    • 12%-20%: nice-flying, stable design
    • 8%-12%: sportier handling
    • less than 8%: serious aerobatic plane for expert pilots
    Also he mentions in the "big book" that transport aircraft may be lower than GA aircraft, in the range of 5%-10%.

    I found this paper that outlines the increase in static margin afforded to Lancairs with the Mk II tail (increase from 8% to 16.5%). This paper includes static margin figures for a few other planes:
    [​IMG]

    Finally, I did some searching around HBA for any discussion of longitudinal stability. I ran across various ranges, such as 5%-8% or 7%-12%.

    So... what say you? I am particularly interested in answers from those who have run through this process (*cough, cough* Topaz? Billski?) or who have more data points on existing aircraft.
     

    Attached Files:

    • 01.JPG
      01.JPG
      File size:
      25.8 KB
      Views:
      109
  20. Dec 4, 2017 #20

    jac

    jac

    jac

    Well-Known Member

    Joined:
    May 18, 2013
    Messages:
    74
    Likes Received:
    9
    Location:
    Gore, Southland New Zealand
    Don't know if you have read of this aircraft in NZ. Couple of other page / links in reference to it @ top LH corner. I see owner/builder passed away several years ago. http://homepages.paradise.net.nz/~tuff/articles.htm
    With regard to rearward cockpit placement, since pilot weight is a given ( in most cases! ) its probably better to have fuel, gear and variable weight items like baggage on the CG
     
    Last edited: Dec 4, 2017
    mcrae0104 likes this.

Share This Page

arrow_white