The Razorback F1 Construction thread

[rtfm]

Hi folks,

 

I have posted extensively on the HBA web site regarding the little Razorback airplane I have designed and am now about to start building. I thought I'd use this forum as my primary one, since it is Aussie, after all.

 

I am in the process of moving out of my old workshop into new (much larger) premises, and so everything is in a state of suspended animation till that job is completed. But I have the stand all ready, the templates on file, and very soon now, I'll be forking out for new foam, and some CNC cutting of the templates.

 

I promise to post lots of build photos.

 

Basically, I'll be building a plug, from which I will take molds, and then use these molds to make the actual fuselage and wings. I could have built a single one-off plane, but where's the fun in that? If she flies as well as the spread sheets suggest, there may well be others who might want to build one. So I went with the molds.

 

Till next time

 

Regards,

 

Duncan

 

 

[JimG]

Yep , I have been following the HBA thread from almost day one .

 

I've got to say your dedication and enthusiasm is certainly inspiring .

 

I hope all goes well and you see it though cos I'm looking forward to seeing the finished product.

 

Close call with 'Thunder Engines', so glad you didn't sent him a deposit.

 

I'm interested to see how the big valley twin goes although i think if it went for cowl considerations the BMW would have been beaut.

 

All the best...!

 

cheers

 

JimG

 

 

[fly_tornado]

have you built an RC model of it yet?

 

 

[rtfm]

Hi fly_tornado,

 

No I haven't. I have often thought I'd do so, but never got round to it. What I AM doing, though, is I've created an X-Plane model, and have spent some time trying to fly it in the sim. I have no sim flying experience, so it is easier said than done. No spatial awareness, no control feedback. All I do is crash, mostly...

 

Do you have any RC building/flying experience?

 

Duncan

 

 

[rtfm]

Close call with 'Thunder Engines', so glad you didn't sent him a deposit.

I'm interested to see how the big valley twin goes although i think if it went for cowl considerations the BMW would have been beaut.

Hi Jim,

Yes, it has been a long often disappointing road so far. Although I have evolved the design considerably from when I first started, my MAIN issue has been with construction. I have no background in building anything - apart from two or three foam surfboards back in the early 60's. And even then, my father did most of the work. I simply didn't have the necessary skills. Fortunately I now have Alan Clarke as my mentor, and he has all the building smarts I lack.

 

I was promised a brand new engine so long as I specified the Thunder engine as standard on any kit I produced. Too good to be true, unfortunately.

 

And yes, if I hadn't already bought the Valley Engineering Big Twin, I'd fork out the extra and get the BMW 1200. With that engine, the little Razorback would truly go like the proverbial bat outta hell. But at 74kg, it would be a challenge to get the CG right...

 

Cheers,

 

Duncan

 

 

[fly_tornado]

I would build a few RC models. scale them up in size each time.

 

 

[rtfm]

Hi,

 

I have the templates in DWG format, so I was thinking of scaling them down (10% maybe) and checking to see if my construction method works, and to have something to play with in the shop while waiting for resin to cure. I have no RC experience, however, so to embark on a proper RC project would be too big a distraction I think.

 

Regards,

 

Duncan

 

 

[fly_tornado]

not at all, you gain valuable insight into how your plane flies without risking your life or your test pilot's life.

 

 

[Old Koreelah]

I'd have to support FT on this one, Duncan. A quick and painless way to quickly develop valuable skills.

 

I did pretty much what you're planning to do. Spent years researching and designing. That was the most exciting part. Did quite a bit of travelling and got lots of help from many people, including four of this country's best designers. A CASA engineer told me that every second shed in Australia had a part- finished plane in it!

 

Spent a few years building the fuselage and tail plane, and part of one wing. Then realised that I'd be old before it was flying. Bought a completed aircraft that I could fly and make improvements to instead.

 

Wisdom comes from experience

 

Experience comes from making mistakes

 

 

[damkia]

http://www.homebuiltairplanes.com/forums/aircraft-design-aerodynamics-new-technology/4204-razorback-construction-thread.html

 

For those not in the know...

 

 

[fly_tornado]

I would like to see it flying even if its only a RC model. With that short fuselage it looks like it could be very pitch sensitive.

 

 

[rtfm]

Hi,

 

I get this every time...

 

A short-coupled aircraft is a function of the relative distance between the wing and the stab. Not the actual distance. If you scale down an RV so that the actual distance between the wing and the stab decreases, it doesn't change the relative distance between them, which is why RC models of RV's are just as pitch-stable as full sized ones.

 

Here's a re-post from HBA (with some edits to remain current):

 

From time to time, the subject of an aircraft being "short coupled" comes up, both here on this forum, and in hangars just about everywhere. I used to believe that 'short coupledness' was simply in the eye of the beholder, but someone on this forum suggested an empirical measurement of this, and then followed it up by undertaking a trade study of as many planes as he could find.The Couple-Index (CI) is a dynamic stability index, and is calculated thus:

 

Tail moment arm divided by average wing chord

 

A trade comparison reveals that any ratio between 2.5 to 3.5 can be considered acceptable. Lower than 2.5, and the plane can be considered "short-coupled" My Razorback for example has a tail moment arm of 8.61ft, and a MAC of 2.86ft, giving a CI of 3.01, so absolute size doesn't matter.

In the flight sim, as corroboration, the Razorback flies with no pitch sensitivity at all.

 

Regards,

 

Duncan

 

 

[djpacro]

Tail volume coefficient is directly related to stability.

 

 

[rtfm]

Ah, yes. I was assuming reasonable values for Cvt and Cht. I have selected 0.85 and 0.065 respectively. But while the tail volume coefficients chosen certainly affect the dynamic stability of the plane, these do not affect the short-coupledness of the plane. That is another matter.

 

Regards,

 

Duncan

 

 

[Head in the clouds]

Tail volume coefficient is directly related to stability.

I would have thought tail volume had more to do with controllability than stability, and that stability was more a function of the static margin isn't it?

 

 

[djpacro]

Tail volume coefficient is a major factor in static margin.

 

 

[Head in the clouds]

Tail volume coefficient is a major factor in static margin.

Then I must be missing something DJ. As far as I understood it the only relationship between them is that the greater the horizontal tail volume coefficient (Vh) the greater the static margin can be (negative value, but ahead of the CG for positive stability) and still have full controllability up to and through the stall.

 

The formula for Vh is -

 

Vh = Sh x Lh / Sw x m.a.c. where Sh = horizontal tail area, Lh = distance from tail's aerodynamic center to the aircraft CG, Sw = main wing area and m.a.c = the mean aerodynamic chord.

 

So the formula makes no mention of static margin (-h). As I understand it the greater the static margin the greater the static stability will be but as the static stability increases (due to increased static margin) then a commensurate increase of tail volume is required for controllability, to overcome the increased inherent stability. So wouldn't that mean that a plane's static margin is a major factor in the size of the tail rather than the other way around?

 

Here there's a layman's text on tail volume coefficient that I found useful in understanding it but it's not a formal aeronautical engineering publication so perhaps it's flawed?

 

 

[djpacro]

You should be able to download this free from the NASA server: NASA Contractor Report 1975 Riding and Handling Qualities of Light Aircraft - A Review and Analysis.

 

Short-coupled by reference to simple geometry is misleading - true meaning is aircraft behaviour conducive to pilot induced oscillations. See page 192 for desirable range of longitudinal short period frequency and damping. See also page 131 for stick-free static margins.

 

Cmq on page 63 is important - seems to me that consideration of the value of this would be useful in determining whether something is short-coupled - refer equation near the bottom of the page.

 

Some good notes also at flightlab.net - refer info on longitudinal static stability and dynamic stability - easy reading for pilots.

 

Stick fixed neutral point is a direct function of tail volume coefficient. And h = neutral point minus cg position.

 

 

[rtfm]

Hi djpacro.

 

You said:

 

Short-coupled by reference to simple geometry is misleading - true meaning is aircraft behaviour conducive to pilot induced oscillations

You've got the cart well and truly before the horse, mate. In designing a plane, one doesn't have the luxury of flying it first to check its PIO tendencies. One has to look at "simple geometry" and try to predict whether or not it will exhibit excessive PIO tendencies or not. So one aims for a generous horizontal tail volume coefficient. And one shoots for a healthy static margin. And aims for a healthy Couple Index. One does it all, and then possibly fly it in X-Plane as a sanity check. And if you have a mate who's into RC, you might get to see it fly as a RC.

In short, do it all. But you can't fly it first.

 

But a nice report - I've read it before. Thanks for reminding me of it again.

 

Cheers mate

 

Duncan

 

 

[Head in the clouds]

I guess there's not been a lot of progress on the Razorback just lately because, as I let the cat out of the bag earlier about Duncan and his lovely wife competing in the Queensland State Ballroom Championships, they've been practicing three hours every day.

 

Anyway I just heard it paid off with a Third and a First Placing to add to their Win in the Australian National Championships a couple of months ago.

 

Congratulations again Duncan and Glenda! You guys are unstoppable. Well done!

 

Now get back to work on the plane ...

 

 

[flyvulcan]

Congrats Duncan, and as HITC has suggested, now get back to the building!

 

 

[Yenn]

If you are having trouble flying the X Plane model check how accurate your model is and find out why. I modelled the Corby and it flies very much correctly in X Plane, except that I can't control it on take off. I need rudder pedals. Once in the air it copies the real thing.