rtfm Posted April 1, 2014 Share Posted April 1, 2014 Gentlemen, I have been working at this design for a few years now, progressing from rough sketches of improbable concepts, to what I now have - via Raymer, Roncz, Whitney and others. And lots of help from experienced friends and designers. Lately, I have discovered X-Plane as a great visual concept tool, and have been using it as a sanity check on my design formulae. Interestingly, I've discovered that there are very few design concepts which satisfy both the spreadsheets/formulae and X-Plane. What I now have ticks all the design boxes I can find, AND flies brilliantly in X-Plane. I'm a happy camper. Proposed specs: Seating: Single seat Power: 50hp Valley Engineering twin (116lbs, including redrive, full oil, starter etc) Configuration: Tractor, tandem wing (larger wing in front), tricycle undercarriage Performance: Stall 43kts, Max S&L 135kts Controls: Elevators/ailerons on both wings, flaps on both wings Construction: Glassfibre/3-D Core foam sandwich Current status: I am putting the finishing touches to the plug (completing the wing recesses), and will hopefully be doing final prep for the molds within a week or so. Regards, Duncan 5 Link to comment Share on other sites More sharing options...
Bob Llewellyn Posted April 21, 2014 Share Posted April 21, 2014 Gentlemen,I have been working at this design for a few years now, progressing from rough sketches of improbable concepts, to what I now have - via Raymer, Roncz, Whitney and others. And lots of help from experienced friends and designers. Lately, I have discovered X-Plane as a great visual concept tool, and have been using it as a sanity check on my design formulae. Interestingly, I've discovered that there are very few design concepts which satisfy both the spreadsheets/formulae and X-Plane. What I now have ticks all the design boxes I can find, AND flies brilliantly in X-Plane. I'm a happy camper. Proposed specs: Seating: Single seat Power: 50hp Valley Engineering twin (116lbs, including redrive, full oil, starter etc) Configuration: Tractor, tandem wing (larger wing in front), tricycle undercarriage Performance: Stall 43kts, Max S&L 135kts Controls: Elevators/ailerons on both wings, flaps on both wings Construction: Glassfibre/3-D Core foam sandwich Current status: I am putting the finishing touches to the plug (completing the wing recesses), and will hopefully be doing final prep for the molds within a week or so. Regards, Duncan That's a big speed range - how much should it weigh, and what are you using for a prop? 1 Link to comment Share on other sites More sharing options...
rtfm Posted April 21, 2014 Author Share Posted April 21, 2014 Hi I estimate the MAUW will be in the region of 560lbs (256kg). Valley Engineering supplied a prop with the engine, but to be honest, I don’t know what its specs are, besides being two-bladed, 5ft diam, wooden. In X-Plane, I’m using a 2.5ft radius 2 blade with the following specs: http://1drv.ms/1i7Ofg6 I’m not much of a prop person, and I was planning on getting someone who knows what he’s talking about to select a prop for me (or confirm the V.E. supplied prop is OK) closer to the time. If you use X-Plane, you might want to grab a copy of the model and fly it. Feedback invited, of course. You can find the model here: http://1drv.ms/1h2LaSI Link to comment Share on other sites More sharing options...
Bob Llewellyn Posted April 21, 2014 Share Posted April 21, 2014 HiI estimate the MAUW will be in the region of 560lbs (256kg). Valley Engineering supplied a prop with the engine, but to be honest, I don’t know what its specs are, besides being two-bladed, 5ft diam, wooden. In X-Plane, I’m using a 2.5ft radius 2 blade with the following specs: http://1drv.ms/1i7Ofg6 I’m not much of a prop person, and I was planning on getting someone who knows what he’s talking about to select a prop for me (or confirm the V.E. supplied prop is OK) closer to the time. If you use X-Plane, you might want to grab a copy of the model and fly it. Feedback invited, of course. You can find the model here: http://1drv.ms/1h2LaSI The deal with fixed pitch props is that they have a pretty limited speed range; it can be extended by having the inboard part of the blades stalled at low fowards speed, but this requires either a bigger donk (Spitfire 1 is an example) or a very long takeoff roll! Even the humble Piper Cherokee, which barely manages twice its stall speed at high cruise, was offered with a "climb" and a "cruise" prop. Because of the thick shank, most wooden props are stalled near the blade roots at low speed, which contributes to the perceived vibrations and poor cooling flow; but the reduced anti-rotational "lift" of the blades allows the engine to develop more power early in the TO run. The classic helical twist prop shape - c.f. Fred Weick - is very much of the "extend the speed range by stalling the shank" school. From this aspect, use of a smaller diameter prop gives a higher static speed through the prop, so for the same speed ratio, the smaller prop has a higher top speed. If you let me know the prop RPM at the engine's full power, I can run you a couple of performance curve estimates (they will vary with twist distribution and planform, but not hugely). Link to comment Share on other sites More sharing options...
rtfm Posted April 21, 2014 Author Share Posted April 21, 2014 Hi, Very kind of you to offer to help. The engine red lines at 3600 rpm, and is fitted with a 1.6 to 1 PSRU. Engine is rated at 50hp max. Does this help? Duncan Link to comment Share on other sites More sharing options...
Bob Llewellyn Posted April 21, 2014 Share Posted April 21, 2014 Hi,Very kind of you to offer to help. The engine red lines at 3600 rpm, and is fitted with a 1.6 to 1 PSRU. Engine is rated at 50hp max. Does this help? Duncan sure thing - I'll have a go later tonight, after I've welded myself to the scenery (or set fire to my overalls!) and convinced the kid to sleep :o) 1 Link to comment Share on other sites More sharing options...
Bob Llewellyn Posted April 21, 2014 Share Posted April 21, 2014 Hi,Very kind of you to offer to help. The engine red lines at 3600 rpm, and is fitted with a 1.6 to 1 PSRU. Engine is rated at 50hp max. Does this help? Duncan Hi,Very kind of you to offer to help. The engine red lines at 3600 rpm, and is fitted with a 1.6 to 1 PSRU. Engine is rated at 50hp max. Does this help? Duncan Sorry I was distracted, and only ran one prop - I normally start with a constant-chord helical twist, and it's in the attachment. I should be able to run a couple more tomorrow, and a couple more the next day - by which time you may be shopping for a variable pitch prop that'll hold its blades!! Cheers, Bob. Prop #1.pdf Prop #1.pdf Prop #1.pdf Link to comment Share on other sites More sharing options...
rtfm Posted April 21, 2014 Author Share Posted April 21, 2014 Hi, Props are really a bit of an art, aren't they? They don't seem to make sense... Duncan Link to comment Share on other sites More sharing options...
Bob Llewellyn Posted April 21, 2014 Share Posted April 21, 2014 Hi,Props are really a bit of an art, aren't they? They don't seem to make sense... Duncan They're fair cows... the physics is NOT as simple as it looks. I've spent a lot of time developing prop analysis tools (and, I'm pleased to say, I've won an argument with Hartzell about the performance of one of their own props!)... Anyway, once you've got a set of charts showing the effects of various geometry changes, you'll have a better idea of how to pick one, at least.. Link to comment Share on other sites More sharing options...
Bob Llewellyn Posted April 22, 2014 Share Posted April 22, 2014 Hi,Props are really a bit of an art, aren't they? They don't seem to make sense... Duncan Find attached the effects of increasing the advance by 10", with no other change. Stay tuned... Prop #1&2.pdf Prop #1&2.pdf Prop #1&2.pdf Link to comment Share on other sites More sharing options...
Bob Llewellyn Posted April 22, 2014 Share Posted April 22, 2014 Hi,Props are really a bit of an art, aren't they? They don't seem to make sense... Duncan Now find attached, the effects of tapering the outboard 1/3 of the blade, from 4" to 3"... Prop #1-3.pdf Prop #1-3.pdf Prop #1-3.pdf Link to comment Share on other sites More sharing options...
Bob Llewellyn Posted April 22, 2014 Share Posted April 22, 2014 Hi,Props are really a bit of an art, aren't they? They don't seem to make sense... Duncan Getting back to the constant chord versions, here's what happens when you reduce the root advance and increase the tip advance... it's getting a lot more useable, although the first part of the takeoff will still be pretty lethargic. This is probably it for today, but we still have to look at effects of spinner size, tapered tips on the reduced-twist form, changing the thickness distribution, optimising the twist distribution, fine-tuning the chord distribution, and fine-tuning the taper (or planform).... Prop #1-4.pdf Prop #1-4.pdf Prop #1-4.pdf Link to comment Share on other sites More sharing options...
David Isaac Posted April 22, 2014 Share Posted April 22, 2014 Good on you Bob for helping Duncan. This stuff fascinates me, doesn't mean I fully understand it, but the physic is really interesting. 1 Link to comment Share on other sites More sharing options...
Bob Llewellyn Posted April 22, 2014 Share Posted April 22, 2014 Good on you Bob for helping Duncan. This stuff fascinates me, doesn't mean I fully understand it, but the physic is really interesting. No worries... That's because you're not mad enough ... it rots your brain... makes you think politicians are silly and the media is shallow... 2 Link to comment Share on other sites More sharing options...
Bob Llewellyn Posted April 22, 2014 Share Posted April 22, 2014 Hi,Props are really a bit of an art, aren't they? They don't seem to make sense... Duncan ....and here's the one I wasn't going to put up today... the sub-helical twist with tapered tips and a tad more advance... shows that efficiency and thrust don't go hand-in-hand... Prop #1-5.pdf Prop #1-5.pdf Prop #1-5.pdf Link to comment Share on other sites More sharing options...
Bob Llewellyn Posted April 23, 2014 Share Posted April 23, 2014 Hi,Props are really a bit of an art, aren't they? They don't seem to make sense... Duncan ...and here's prop #5, with the twist on the outer half of the blade(s) fudged a little - not optimally, but in a more betterer direction. Note the ugly little dip around 40kts - that's due to a stall instability (a chunk of the blade is very close to stall, so a very small change in local inflow causes it to stall... and then unstall by 50kts). If you've ever had a prop that buzzed at a certain speed, or whined, or hummed, it was the same sort of thing. I could micro-twist that behaviour out of it, but there are bigger improvements to be had. Since the compressibility margin is good (it gets nowhere near sonic choking at any speed), increasing the section thickness outboard should improve the stalling characteristics, which should give the highest thrust yet at ~40kt, and may improve the sub-40kt curve... ps I've dropped the first coupla props, the graph was just getting too crowded... Prop #3-6.pdf Prop #3-6.pdf Prop #3-6.pdf Link to comment Share on other sites More sharing options...
rtfm Posted April 23, 2014 Author Share Posted April 23, 2014 Hi, Gosh, I turn my back at return to see you hard at it. Good on you... I have to just sit back and gaze in wonder at all this. Fascinating, but completely beyond me. You mentioned spinner size once or twice. I favour a large spinner (currently looking at 14.5") but this is not set in stone. It just looks nice. Some design targets: Stall 43kts Cruise 135kts My engine produces 50hp @ 3600rpm, and is fitted with a 1.6:1 PSRU. I trust these specs line up OK with your prop tinkering? Regards, Duncan Link to comment Share on other sites More sharing options...
Bob Llewellyn Posted April 23, 2014 Share Posted April 23, 2014 Hi,Gosh, I turn my back at return to see you hard at it. Good on you... I have to just sit back and gaze in wonder at all this. Fascinating, but completely beyond me. You mentioned spinner size once or twice. I favour a large spinner (currently looking at 14.5") but this is not set in stone. It just looks nice. Some design targets: Stall 43kts Cruise 135kts My engine produces 50hp @ 3600rpm, and is fitted with a 1.6:1 PSRU. I trust these specs line up OK with your prop tinkering? Regards, Duncan Well, I've been studying props, it's nice to get a chance to use it... of course, when I've got it all working to my satisfaction, I plan to charge huge sums of money for advice...... or start making props! This lot use those donk specs; as you can see, the rising line of the aircraft drag will cross the descending line of the prop thrust; and when you push the thrust out to higher speeds, you lose takeoff performance. I'm also trying to make a point about how many variables are significant... the analytical technique I use is pretty accurate; so it should be able to let you work out what's the best you can expect from a fixed pitch prop. If you get a prop that won't deliver, you should then know to change the prop, not the aeroplane... ps When I get to the end, I hope to produce a less confusing summary for you (and me!). If you'd care to let me know your wing area, aspect ratio, tail areas, approx fuselage surface area, and wheel size, I can fold this prop data into a polar and give some estimated performance figures, including TOR / TOD, by the methods I've found to work. Cheers, Bob Link to comment Share on other sites More sharing options...
Bob Llewellyn Posted April 23, 2014 Share Posted April 23, 2014 Hi,Props are really a bit of an art, aren't they? They don't seem to make sense... Duncan and here's the previous one (Prop 6) with a 1% increase in thickness to chord ratio. Makes quite a difference... Prop #4-7.pdf Prop #4-7.pdf Prop #4-7.pdf Link to comment Share on other sites More sharing options...
rtfm Posted April 23, 2014 Author Share Posted April 23, 2014 Hi Bob, Again, this is terribly kind of you. Thank you. MAUW: 576lbs Wing area (front wing): 63.08ft^2 AR: 7.02 Tail area: 34.05ft^2 AR: 8.53 Fuse surface area: 68ft^2 (more or less) Wheel size (front): 9.6" diameter, 4.3" wide, Mains: 13" diameter, 5" wide Sorry to be working in Imperial units, but my design software is all in feet and inches. Duncan Link to comment Share on other sites More sharing options...
Bob Llewellyn Posted April 23, 2014 Share Posted April 23, 2014 I think of it as marketting... or venting? Thanks to NATO, I too work in Imperious units! Link to comment Share on other sites More sharing options...
Bob Llewellyn Posted April 23, 2014 Share Posted April 23, 2014 Hi,Props are really a bit of an art, aren't they? They don't seem to make sense... Duncan Here's prop #7 with the outboard taper made a bit more abrupt (still 3" tip chord), and an extra 1% thickness. Prop #5-8.pdf Prop #5-8.pdf Prop #5-8.pdf Link to comment Share on other sites More sharing options...
rtfm Posted April 23, 2014 Author Share Posted April 23, 2014 Hi, That 20kt dip is a bit weird isn't it? To someone like me, that just doesn't make sense... Yes, the higher end of things is probably where I'll be aiming to optimise. The plane is very small and very light, so I'm not overly concerned with optimising for the climb. I reckon it'll get there soon enough. Do you tjhink we'll be able to get away with a fixed pitch prop? Duncan Link to comment Share on other sites More sharing options...
Bob Llewellyn Posted April 23, 2014 Share Posted April 23, 2014 Hi Bob,Again, this is terribly kind of you. Thank you. MAUW: 576lbs Wing area (front wing): 63.08ft^2 AR: 7.02 Tail area: 34.05ft^2 AR: 8.53 Fuse surface area: 68ft^2 (more or less) Wheel size (front): 9.6" diameter, 4.3" wide, Mains: 13" diameter, 5" wide Sorry to be working in Imperial units, but my design software is all in feet and inches. Duncan Ok, here's a first shot at the probable performance. I've assumed spatted and faired U/C, no unwanted separation anywhere, Prop #8, and a 48lb load on the 34 sq. ft HS... looks like you'll get impressive TO and 110kt IAS cruise, which suggests we could stop working on the TO end of the prop performance, and try to improve the top end. Cheers! Rough Performance Estimates #1.pdf Rough Performance Estimates #1.pdf Rough Performance Estimates #1.pdf Link to comment Share on other sites More sharing options...
Bob Llewellyn Posted April 23, 2014 Share Posted April 23, 2014 Hi,That 20kt dip is a bit weird isn't it? To someone like me, that just doesn't make sense... Yes, the higher end of things is probably where I'll be aiming to optimise. The plane is very small and very light, so I'm not overly concerned with optimising for the climb. I reckon it'll get there soon enough. Do you tjhink we'll be able to get away with a fixed pitch prop? Duncan Well, the curves show 110kts indicated on Prop #8, and bags of TO performance, so I'd say 120kts indicated is within reach of a 2-blader... if the diameter is reduced, 130kts may be possible, fixed pitch. At this end of town, it's all about drag; you've eliminated induced drag pretty well, but in my experience things like separation at control surface hinges, cooling drag, exhaust "plume" drag and wingroot-fuselage interference still have the potential to stuff things. From the sounds of your research, maybe you've got all that under control too... Note that 120kts indicated will, at 5,000ft on a warm day, get you over the ground at close to 135 kts... Link to comment Share on other sites More sharing options...
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