Scaled Jets (F-16)

Discussion in 'Aircraft Design / Aerodynamics / New Technology' started by Acrojet, Jun 6, 2016.

Help Support HomeBuiltAirplanes Forum by donating:

  1. Jun 7, 2016 #41

    autoreply

    autoreply

    autoreply

    Moderator

    Joined:
    Jul 8, 2009
    Messages:
    10,732
    Likes Received:
    2,542
    Location:
    Rotterdam, Netherlands
    That sounds like something is seriously wrong.
    CFRP has less than half the density of alu and cutting a few corners, roughly the same strength. It is way stiffer though, so since most aerospace structures are buckling-constrained, NOT stress constrained, carbon will end up much lighter. Practical factors, like joint design lower this a bit, but I think I can design any GA-sized alu plane at an equivalent weight for glass fiber and considerably lighter in carbon. For most airframes that'd be half the weight in carbon.

    But with your application it gets worse. Flutter is a bloody complex subject and you're well into flutter-territory. Flutter is roughly driven by speed, airframe geometry, part weights and CG and airframe stiffness. Now we have CFRP that's twice as stiff and half the weight.

    So, the conclusion about that material comparison worries me, because it indicates underlying issues.

    A further positive point; such a scale F16 might be heaps easier to manufacture in composites. It will also save you lots of time engineering (because the structure is so much stiffer, most vibrations modes won't have to be checked).

    Notably full-scale foam molds, temporarily covered in Stretchelon or similar are a great way to make cost-effective molds for a single airframe.

    With respect, unless a full modal analysis is done, that incorporates the non-linearities (like the LERX effects and the agressively swept stabilators), I don't think you've seen more than a fraction of the engineering that has to take place before any sane test pilot would take it up for a test flight.

    I agree with remarks about the market. As long as you're building it for yourself and can cover the cost, that's fine. You might get a small part of your cost and time back in selling a few, but I think the realistic market is tiny to non-existent.

    Last remark, is there something (Questair Venture, Extra 500) that has a landing gear you could take over? Huge time saver and will be a considerable part of the cost.
     
    BoKu and ultralajt like this.
  2. Jun 7, 2016 #42

    autoreply

    autoreply

    autoreply

    Moderator

    Joined:
    Jul 8, 2009
    Messages:
    10,732
    Likes Received:
    2,542
    Location:
    Rotterdam, Netherlands
    Out of curiosity, what was the driver to scale down the F16, or rather, what made you choose the F16?

    I've always loved the F5 (and the F104), the F5 with two big model engines (AMT Lynx for example) might make a nice mini-jet where there's a bit of a bigger market for engines, so you're not dependent on one project failing or succeeding)
     
  3. Jun 7, 2016 #43

    Acrojet

    Acrojet

    Acrojet

    Well-Known Member

    Joined:
    Jun 6, 2016
    Messages:
    53
    Likes Received:
    51
    Location:
    Gardiner, Maine USA
    It's (F-16) always been one of my favorite aircraft. It's got some crazy looks to it. I've thought down range (into the future) too and I can see other "scaled jets" - I think I just found my company name... "Scaled Jets" in the future, that's if it all goes well.

    I also like the F-18, A-4 Skyhawk, just to name a few.....
    Are we going into business together? ;-)


    Peter
     
    Last edited: Jun 7, 2016
  4. Jun 7, 2016 #44

    Acrojet

    Acrojet

    Acrojet

    Well-Known Member

    Joined:
    Jun 6, 2016
    Messages:
    53
    Likes Received:
    51
    Location:
    Gardiner, Maine USA

    We will run the numbers again, but it turned out lighter in metal than in carbon fiber, unless of coarse the laminate and epoxy laminate structures were calculated with the wrong values. I almost wonder if that was the case, I know there were many variables when composites were run. Carbon fiber vs glass, honey comb vs foam vs balsa cores, graphite, carbon, glass...etc. I will have him run a different model. I know he's has more quantifiable data for metal construction and has leaned that way. I didn't question his data, only because I knew of other aircraft that were tried in composite vs metal that were heavier every time vs the metal ones they were designed to replace. Now that could have been fabrication error too, but I've seen it two different times so I always assumed they were heavier for the "same" structure. I could be that there were over engineered spars too. I'm truly not well versed in the composites and certainly not on the engineering side for sure. I just tell my guy what I need or want and we go from there. Having worked with metals with my BD-5J that's where I'm most comfortable. I also checked into the composite plug-mold-part manufacture and it's very expensive to make and fabricate. I can build up a metal aircraft easier and quicker and for less money, at least that's where we are driving. But I will check and see what was plugged for variables......

    Gear- I actually already thought about "already existing" gear that I could Frankenstein to the f-16. The closest I could find where the gear from the Swearingen SX-300 until I looked further, it just didn't look right. I found a company here in the US that can machine the gear from a scale RC F-16 up to 60% for a really good price. They will even be powdered coated white after machining. They can supply the entire gear and retract mechanism too. I just need to match the tire sizes with existing standard ones that are available.

    I think the F-5 twin sounds like a cool one. Remember the F-20 tiger shark that never made it past testing. It was an F-5 with one motor. Pretty cool

    Peter
     
  5. Jun 7, 2016 #45

    Doggzilla

    Doggzilla

    Doggzilla

    Well-Known Member HBA Supporter

    Joined:
    Jun 2, 2013
    Messages:
    1,965
    Likes Received:
    414
    Location:
    Everywhere USA
    I think what people might be missing here is some scale. A lot of people assume that small jet engines have kept up with large ones, but that's simply not the case.

    What Acro is aiming for is similar to an Williams FJ, but those engines are 3 times the size. The FJ has quickly become dated, so its not unattainable to match it by any standard.

    The FJ turbines have pressure ratios of about 13, not even a third of what modern engines have reached.

    The small engines we use have pressure ratios of about 3-4, absolutely abysmal. They also have fuel consumption about 4 times higher.

    Any sort of decent effort to produce a new small engine would easily exceed what we have, and the efficiency Acro is aiming for requires a fraction of what commercial engines have reached since the FJ came out.

    Its attempting to match a 24 year old engine with a third the pressure ratio of modern designs. That is far from unreasonable.

    There is lots of room to meet or exceed the requirements.

    Anything above 10:1 and below 1.0 SFC would make a huge impact on light aircraft. So even a "failure" would provide an engine that could still be used on a wide variety of aircraft.

    Its a reasonable goal with lots of room to work with before its considered a failure, and just as much room to exceed the requirements.

    We are just used to dealing with these little turbines that are single stage, sub par materials, and have abysmal pressure ratios. It should be easy to greatly exceed them with any sort of reasonable effort.

    If Acro reached even half the thrust for the fuel consumption, it would greatly exceed anything we currently have available for light aircraft. The engines we currently have are too small with massive fuel consumption, or too large and cost as much as a house.
     
    BobbyZ, flyvulcan and Acrojet like this.
  6. Jun 7, 2016 #46

    bmcj

    bmcj

    bmcj

    Well-Known Member HBA Supporter

    Joined:
    Apr 10, 2007
    Messages:
    12,973
    Likes Received:
    4,917
    Location:
    Fresno, California
    I agree with Autoreply... something is amiss in the calculation if CF is coming out heavier than aluminum for structural components.

    Nothing wrong with the F-16... great plane. I suspect your glide (compared to the BD5J) might not be as strong as you hope based on span loading or just plain ol' aspect ratio. Stability shouldn't be a problem though; just as others have said, run the CG forward enough to obtain a favorable static margin. The short aft body may give you some control sensitivity issues though (short-coupled).

    Personally, I always thought a scaled F-15 would be a blast (followed by a scaled SR-71 of course, but who hasn't thought about that one?).

    Anyone for a two seat tandem scaled Space Shuttle? :gig:
     
    BoKu, Battler Britton and Acrojet like this.
  7. Jun 7, 2016 #47

    Lucrum

    Lucrum

    Lucrum

    Well-Known Member

    Joined:
    Jun 10, 2008
    Messages:
    956
    Likes Received:
    189
    Location:
    Canton, GA
    Incidentally my T-58 has a compressor pressure ratio of 8.4:1 and I don't expect a sfc of less than 0.85
     
    Doggzilla likes this.
  8. Jun 7, 2016 #48

    Doggzilla

    Doggzilla

    Doggzilla

    Well-Known Member HBA Supporter

    Joined:
    Jun 2, 2013
    Messages:
    1,965
    Likes Received:
    414
    Location:
    Everywhere USA
    Burt Rutan did a demonstration back in the day to compare the strength of fiberglass VS aluminum of the same weight, and the fiberglass won by a pretty clear margin.

    [video=youtube;7eUt0YnNF3o]https://www.youtube.com/watch?v=7eUt0YnNF3o[/video]

    The main reason is that composites dont have stress concentrations at rivets, and the foam core stops buckling. Try hitting a coleman cooler with a baseball bat or a sledge hammer. An aluminum version would crumple like a soda can, while the foam reinforced plastic just ignores the blows.

    I wasnt really sure about composites until I read up on the Rutan Quickie and compared it to other similar aluminum aircraft like the Hummel Bird. Composites allow a much stronger aircraft to be built in a fraction of the time, and have much higher performance.
     
    Last edited by a moderator: Mar 20, 2019
  9. Jun 7, 2016 #49

    Acrojet

    Acrojet

    Acrojet

    Well-Known Member

    Joined:
    Jun 6, 2016
    Messages:
    53
    Likes Received:
    51
    Location:
    Gardiner, Maine USA

    I will have him check again, but it was a total fabrication in CF vs aluminum.

    At heavier weights (2000lbs) the glide will be substantially lower obviously. But at a more typical weight it won't be as good as the BD-5 jet but not all that bad considering.

    Short aft Body - may I suggest the BD-5 for you. It too has an all flying tail and is very sensitive but not uncontrollable.
    A lot of my design is based on what I know about the BD-5J. Metal construction, all flying tail, short coupled. Heavy taoke off weights and light landing weights, realitivley high wing loading. This F-16 should be very similar in a lot of ways.

    Peter
     
    bmcj likes this.
  10. Jun 7, 2016 #50

    StarJar

    StarJar

    StarJar

    Well-Known Member

    Joined:
    Sep 6, 2011
    Messages:
    1,723
    Likes Received:
    403
    Location:
    El Centro, California, USA
    Here's a guy who is not a graphic artist, a Silicone Valley exec, or a college senior, who seems to have some realistic design sense, in addition to thousands of applicable flight hours and experience.
    The question, IMO, is can he shed the various species of airplane designer fleas that exist on planet earth. Sometimes that's the biggest muscle building experience, and one will always find it here. If you know what I mean.
     
  11. Jun 7, 2016 #51

    Lucrum

    Lucrum

    Lucrum

    Well-Known Member

    Joined:
    Jun 10, 2008
    Messages:
    956
    Likes Received:
    189
    Location:
    Canton, GA
    Same here, not the least of which is the mid wing and blended fuselage

    I've estimated an empty weight of 810 lbs and with just me and full fuel a gross weight of 1760 lbs for my project
    Calculated an L/D max of 11.5:1 at 106 KTS
    Estimated climb rate of 6500 fpm with full fuel The X-Plane model (FWIW) confirms this
    I decided against a trailing edge flap The X-Plane model didn't behave well when they were extended
    I'm going with full span ailerons which should give me a roll rate in excess of 360 deg/sec at Va
     
    Acrojet likes this.
  12. Jun 7, 2016 #52

    Doggzilla

    Doggzilla

    Doggzilla

    Well-Known Member HBA Supporter

    Joined:
    Jun 2, 2013
    Messages:
    1,965
    Likes Received:
    414
    Location:
    Everywhere USA
    There is nothing wrong with aluminum, and if thats what you have to make it out of to get something working and tangible for people to see, then thats what you have to do.

    If your guy is uncomfortable with composites, it might just be better to leave him be, as asking him to go over it again could result in some rustled feathers.

    You already know aluminum works, and are comfortable with it, then there is no reason not to use it. I actually was personally thinking about making one out of Aluminum because a shimmering polished Falcon would make a huge impression on those around it, which is something that is badly needed in the light aircraft market.
     
    Jon Ferguson and Acrojet like this.
  13. Jun 7, 2016 #53

    rv6ejguy

    rv6ejguy

    rv6ejguy

    Well-Known Member

    Joined:
    Jun 26, 2012
    Messages:
    3,749
    Likes Received:
    2,792
    Location:
    Calgary, Alberta, Canada
    It's a cool and interesting project technically and I hope it works as planned but as has been said countless times here before:

    1. Why not wait until the engine runs and the plane flies before getting too excited about specs, figures and costs?

    2. When you put out your project here on HBA for feedback/ scrutiny before step one is complete, expect some feedback, suggestions and/or reality checks.

    I like the small company approach on the engine development (along with lots of testing prior to flight) to keep the engine costs in check and the self funded part plus the realization that this is a relatively limited market. If you get 100% of a small market, life/ business can still be good.

    Excited to see the project develop from here. This ranks up there with the UL-39, LP-1, Bugatti 100-P replica and the PJ-II as far as difficulty and breaking new ground in many areas goes. Power to you Acrojet. Hope you keep us updated.
     
    Acrojet, BoKu and Matt G. like this.
  14. Jun 7, 2016 #54

    autoreply

    autoreply

    autoreply

    Moderator

    Joined:
    Jul 8, 2009
    Messages:
    10,732
    Likes Received:
    2,542
    Location:
    Rotterdam, Netherlands
    As much as I'd like to, I love what I'm doing now, though it's certainly tempting ;)


    It's all too common. Many composite planes copy an aluminium airframe built-up into carbon or glass. In the industry it's known as "black aluminium". Seeing typical metal structural like ribs, stiffeners ought to raise the hairs on the neck of a composite specialist. Wrong thinking for the structural material.
    I always opened my "introduction to composites" lectures with a few slides showing a castle, the Eiffel Tower and a modern sailplane. Great examples of adequate structure for that specific material. A castle in steel blocks or CFRP blocks however would be ridiculous.
    Plenty of composite planes that were designed like a metal one and thus were miserable. The Learfan comes to mind. Even the Cirrus (SR20/22) might be a good example. If I walk around during at typical GA trade-show I cringe about most planes. Layer after layer of thick glass, heaps of epoxy over that and than a few more layers for good measure...

    Further, with glass fiber, it's hard to come out significantly lighter than alu. Buckling drives most aircraft structures, so your figure of merit for say fuselage panels is stiffness cubed over density. Glass has the same ratio as aluminium, so unless you optimize the structure it's unlikely to be lighter than alu. Carbon on the other hand has ten times that ratio. This link (last column) gives a good overview: https://en.wikipedia.org/wiki/Specific_modulus#Approximate_specific_stiffness_for_various_materials

    Being comfortable with a material is a huge plus. It typically takes a few thousand hours before you're fully comfortable in one and that's true for both fabrication and engineering.

    Molds don't have to be expensive and the prices of CNC machining are quickly coming down too. You could for example mill a plug for the F16 wing, pull a mold and pay cubic feet of cash. Or hotwire/CNC-cut a foam female mold, lay in a sheet of mylar and be done with it for 10, 20 or 50 times less cost. Same for the fuselage.
    I started thinking whether you had an "escape plan" if your engine doesn't turn out the way you hoped it would. Hence the question, a very light twinjet would touch the peak of the drone market and avoid being chained to one engine.
     
    BoKu likes this.
  15. Jun 7, 2016 #55

    Acrojet

    Acrojet

    Acrojet

    Well-Known Member

    Joined:
    Jun 6, 2016
    Messages:
    53
    Likes Received:
    51
    Location:
    Gardiner, Maine USA

    Those numbers are pretty close to what we have. Good to know..... Are you going with an all flying stab?

    Peter
     
  16. Jun 7, 2016 #56

    Lucrum

    Lucrum

    Lucrum

    Well-Known Member

    Joined:
    Jun 10, 2008
    Messages:
    956
    Likes Received:
    189
    Location:
    Canton, GA
    Yes I am At present plan on a spring loaded / trim system along the lines of the Cirrus SR-22

    edit: I think I mentioned a wingspan of 18' originally It's actually 20'
     
    Acrojet likes this.
  17. Jun 7, 2016 #57

    Himat

    Himat

    Himat

    Well-Known Member

    Joined:
    May 5, 2011
    Messages:
    2,809
    Likes Received:
    653
    Location:
    Norway
    AIDC F-CK-1 Ching-kuo, twin engines and almost the f-16 look.
     
  18. Jun 7, 2016 #58

    Matt G.

    Matt G.

    Matt G.

    Well-Known Member

    Joined:
    Nov 16, 2011
    Messages:
    1,241
    Likes Received:
    468
    Location:
    Kansas, USA
    I'm fully aware of that. I wasn't sure the OP was, as he posted a picture that looked like a perfectly reduced scale F-16 to my eye, and as you know, we've had quite a few threads on here lately where it's apparent the designer doesn't know what he doesn't know. The OP seems to have done his homework in this area at least. The nice thing about RC airplanes is the structure can be made so light that mass distribution to get the CG in the right place is pretty easy. May not be as easy with a man-carrying aircraft, but with a really light engine and the pilot's feet all the way to the front of the nose, glider-style, it's probably within the realm of possibility without making the airplane look ridiculous, although I still am having trouble picturing A) where the passenger is going to fit without making the fuselage longer than scale and B) how the horizontal tail isn't going to have to be radically larger (based on my previous experience with a scale design), but he's apparently run the numbers and I haven't, so...
     
    Acrojet likes this.
  19. Jun 7, 2016 #59

    Himat

    Himat

    Himat

    Well-Known Member

    Joined:
    May 5, 2011
    Messages:
    2,809
    Likes Received:
    653
    Location:
    Norway
    If I remember right from reading about the F-16, only the first production blocks where marginally stable. Along the production run a larger horizontal tail was fitted, making the aircraft stable aerodynamic. Next, the tail size required is speed dependant and increases with speed.
     
  20. Jun 7, 2016 #60

    bmcj

    bmcj

    bmcj

    Well-Known Member HBA Supporter

    Joined:
    Apr 10, 2007
    Messages:
    12,973
    Likes Received:
    4,917
    Location:
    Fresno, California
    The designers of the F-16 had a couple of factors thay had to design to that Peter (Acrojet) will not have to deal with. First of all, the F-16 CG was moved aft and the tail sized for exceptional maneuverability (for combat, made flyable by redundant computer control). Second, control surfaces had to be designed to be effective in transonic and supersonic flight (this is probably a major reason for choosing a full flying stab). Being subsonic and hand flown allows peter to design a little more conventionally.
     
    Acrojet, Matt G. and StarJar like this.

Share This Page



arrow_white