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- This topic has 25 replies, 9 voices, and was last updated 4 years, 10 months ago by Scott Lapraik.
03/27/2018 at 4:18 am #81727
I am starting the build of a Starduster 900. I have seen notes that contain recommendations for modifying the design for more modern engines (eg. K&B 6.5) and one recommendation has me confused. It says to “thin the tailplane by 1/8″ (8%)”. The plan shows a thickness of 3/4″, which is exactly 8% of the chord (9-3/8″). Was the tailplane 7/8″ thick at one time, or was it always 3/4″ and I should thin it to 5/8″, which would then be 6.66%? Any help would be appreciated.03/29/2018 at 1:31 pm #81732Rudy KluiberParticipant
Go back about five pages on this forum and you will see the same comment about thinning the stab.
I believe this advice originated with Sal himself.
Rudy03/29/2018 at 5:06 pm #81733
I’ve looked at that string of comments but it doesn’t address my dilemma. Everyone agrees that the tailplane should be thinned, but no-one is very specific about what the actual thickness should be. Where I am confused is that the comment I have read from Sal Taibi says “thin tailplane by 1/8″ (8%)”. I interpret this to mean that I should take 1/8″ off the thickness and this will leave me with a section that is 8%. The plans I have (from Sal’s Competition Models kit) show a tailplane that is 3/4″ thick and 9-3/8″ chord – that is exactly 8%! Should I shave 1/8″ off, to make it 5/8″ thick, which would be 6.66%? Or was the original 7/8″ thick, which would be 9.33%, and somehow the plan I have is correct at 8%?03/30/2018 at 7:16 am #81734James TaylorParticipant
Roy, If you haven’t built the fuselage on your 900 yet,many of us over the years added an extra 3to 5 inches of length to the fuselage. Among other stability benefits it helped mine groove into a nice power pattern. hope this helps.03/30/2018 at 8:55 am #81735
Thanks James, that is very helpful. I had thought that it looked very short-coupled, and as I haven’t built the fuselage yet, only the engine mount/pressure bladder box, I will definitely make this modification. Do you make the tailplane any smaller, or thinner, when making this mod? How thick is your tailplane – 3/4″ or 5/8″? I assume that you might move the engine forward a bit too, to keep the CG in the right place. With the longer tail moment, what is the right CG position – do you use the Hanford formula to determine it? A longer nose would make it easier to hold while in the starter – which has been a concern of mine, being more used to the pylon layout. My wing and tail have full-depth I-beam spars, and geodetic ribs, as Dan Berry recommended some time ago.
Thanks again for the help.
Roy03/31/2018 at 7:23 pm #81740Dean McGinnesParticipant
Where does one find this “Hanford Formula”?04/01/2018 at 6:32 am #81741
Hi Dean, the Hanford formula is contained in an article in the 44th NFFS Symposium Proceedings (2011). I have found this formula to be really useful to give me a good starting point for trimming. It seems to work well whether the section is flat or undercambered – I don’t have experience with high-thrust vs pylon but I suspect that it will work just as well. I can email you a scan if you send me your address – its too big to fit within the forum limit.04/01/2018 at 7:06 am #81742
Unless you have massive holes in the bottom of your fuselages, you don’t have the complete Hanford trimming method.
Denny04/01/2018 at 7:17 am #81743
Thanks Denny, I’ll keep that in mind! Should the holes be pre-formed, or are they generated over time as a result of DT’ing onto narrow objects, like fenceposts. I know Brad Bane did that one time – but he put the hole into the crankcase of his Johnson 29R.
Roy04/07/2018 at 11:33 am #81760Scott LapraikParticipant
Can someone let me know what the issue is with the original stab profile. I’ve been given an completed Starduster 900 with the promise to get it flying again!! Thks, Scott04/07/2018 at 1:49 pm #81761
There are some building notes written by Sal Taibi, recommending some changes if the 900 is to be flown with one of the more powerful 40’s (such as the K&B 6.5). One of the recommendations was that “the tailplane should be thinned by 1/8″ (8%).” I found this to be confusing because it doesn’t specify what actual dimension it should be thinned to. The reference to 8% implies (to me) that it should be thinned by 1/8″ to a thickness of 8%. The thickness shown on my plan is 3/4″, which is exactly 8% of the 9-3/8″ chord. If I thin that by 1/8″, to 5/8″ then the thickness will be 6.66%. All the plans I have come across, have the thickness at 3/4″ (8%) so I am left wondering if I should thin to 5/8″ or whether what I have is correct.04/10/2018 at 9:13 am #81764SIMON BLAKEParticipant
For a stab that size, 3/4″ is pretty thin so I wouldn’t reduce it anymore. It would be difficult to make it stiff enough.04/13/2018 at 8:46 am #81770
I have a retired B-C Y-Bar that has a barn door stab…38 1/2 x 9 3/4…11/16 thick. Now that is powered by an OS .35 not a K&B 6.5. Just checked it…still very flat…Might have to re-visit that one. Always flew pretty good. Just scaled it too big.
Denny04/13/2018 at 11:16 am #81792
That is a big tailplane! At 11/16″ with a 9-3/4″ chord it is at 7%. My SD900 is 5/8″ thick at 9-3/8″ chord, which is 6.66%. I decided in the end to thin it down from the 3/4″ shown on the plan (8%) – I couldn’t get any really definitive advice on the subject, except to say it should be thinned by 1/8″, but from what actual thickness, or to what actual thickness, has been unclear. I have built the wing and tail with geodetic ribs and full depth spars, they’re not covered yet but the tailplane appears to be quite stiff torsionally and in bending, even before covering.
Roy04/14/2018 at 5:08 am #81795
I was going to suggest adding a few diagonals too. You should be fine. Rudy has his ongoing Starduster contest. Bring it to Muncie!
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