Lucrum
Well-Known Member
At Jake's suggestion I'm going to chronicle my Mazda 20B engine rebuild, some of which having already been posted elsewhere on the site, as well as the test stand testing of an engine/fan/shaft combination for possible use in a DF of my own design.
Background
I've been intrigued by the idea of a jet look a like DF for over 25 years. I've heard/read opinions on the subject ranging from "forgetaboutit" to done correctly with realistic goals it can probably be done. In fact not long after joining this site I all but gave up on the idea, after reading opinions of some of the more informed members here. Ultimately though I decided I just wasn't going to be content until I tried it myself. 1) I personally need to do something a little different to justify all the effort involved in building. 2) I won't be really happy unless I end up with something that has relatively "high performance". Obviously I'm going to have to do something at least somewhat if not significantly different than the few other similar attempts or else I will surely fail as they did.
The only successful DF jet look a like airframes that I know of are RC models. I believe one of the favorite pro DF arguments is that "if the RC guys can do it so can I". Although that argument has it's limits. Well in my research I determined that "low" performance RC DF's have weight to power ratios starting at about 5 lbs per hp. "High" performance RC DF models have ratios of around 1 lb per hp, which is obviously not realistically achievable in a full scale maned aircraft. I figure to be successful I need to be in that weight to power range. With a large enough, high power to weight ratio engine I have calculated (using mostly EAA spreadsheets) that I should be able to build an airframe that is in the upper end of the RC DF weight to power ratios. I'll post more details latter in the log, but I've rebuilt a Mazda 20B with turbo that should easily loaf along at 450 HP. I'll have to accept somewhat higher stall speeds and big fuel bills, but that's not atypical for higher performance designs.
2) PROPULSIVE EFFICIENCY
I am certainly no aeronautical engineer, but but from my research I've gathered that one of the draw backs to DF aircraft is the relatively narrow speed range of relatively good propulsive efficiency. The RC guys routinely choke the fan exit area to achieve higher speeds. And a guy named Raspet did some DF research years ago indicating a larger duct exit area to fan area significantly improves low speed DF performance. So I thought, what if I could do both. I plan to incorporate a non rotating telescoping cone aft of the fan hub that when retracted forward yields an duct exit area greater than fan area and when extended aft yields a duct exit area less than fan area. Something of a technical risk I'll concede, but I believe it's worth a try.
3) FAN
Most if not all of the relatively few full size DF projects I know of incorporated what were probably more shrouded propellers than ducted fans, even if it met the technical definition of a DF. I have nearly completed a 30 inch diameter high blade density 15 blade fan. Assuming I'll need every ounce of thrust I can get to achieve my primary performance goal the blade angles are optimized for 250 knots at S.L. and W.O.T or 6500 RPM. Meaning an efficiency penalty at lower power settings and speeds but again a price I came to the conclusion I was going to have to pay.
Three Major Improvements Over Previous Attempts
1) POWER
The only successful DF jet look a like airframes that I know of are RC models. I believe one of the favorite pro DF arguments is that "if the RC guys can do it so can I". Although that argument has it's limits. Well in my research I determined that "low" performance RC DF's have weight to power ratios starting at about 5 lbs per hp. "High" performance RC DF models have ratios of around 1 lb per hp, which is obviously not realistically achievable in a full scale maned aircraft. I figure to be successful I need to be in that weight to power range. With a large enough, high power to weight ratio engine I have calculated (using mostly EAA spreadsheets) that I should be able to build an airframe that is in the upper end of the RC DF weight to power ratios. I'll post more details latter in the log, but I've rebuilt a Mazda 20B with turbo that should easily loaf along at 450 HP. I'll have to accept somewhat higher stall speeds and big fuel bills, but that's not atypical for higher performance designs.
2) PROPULSIVE EFFICIENCY
I am certainly no aeronautical engineer, but but from my research I've gathered that one of the draw backs to DF aircraft is the relatively narrow speed range of relatively good propulsive efficiency. The RC guys routinely choke the fan exit area to achieve higher speeds. And a guy named Raspet did some DF research years ago indicating a larger duct exit area to fan area significantly improves low speed DF performance. So I thought, what if I could do both. I plan to incorporate a non rotating telescoping cone aft of the fan hub that when retracted forward yields an duct exit area greater than fan area and when extended aft yields a duct exit area less than fan area. Something of a technical risk I'll concede, but I believe it's worth a try.
3) FAN
Most if not all of the relatively few full size DF projects I know of incorporated what were probably more shrouded propellers than ducted fans, even if it met the technical definition of a DF. I have nearly completed a 30 inch diameter high blade density 15 blade fan. Assuming I'll need every ounce of thrust I can get to achieve my primary performance goal the blade angles are optimized for 250 knots at S.L. and W.O.T or 6500 RPM. Meaning an efficiency penalty at lower power settings and speeds but again a price I came to the conclusion I was going to have to pay.
Test Stand
Originally conceived as a single seater I've added a very small rear seat to accommodate my 4'8" 104 LB wife.Having read most of the threads here on torsional resonance I concluded that given my very limited engineering skills I needed to have a engine/shaft/fan running before committing to an airframe. This way if I run into vibration or resonance issues I cannot readily correct I can abandon the DF concept for a more conventional pusher prop design and still use the same engine, which is where most of my expense will be prior to starting on an airframe. I have done some very basic calculations suggesting I at least have a gamblers chance of success. But I simply don't have the knowledge or skills to predict that the torsional resonance monster won't devour me. Hence the testing I intend to do prior to beginning airframe construction. To help put the odds in my favor the core of the fan blades are wood ,as well as the fan hub and shaft fly wheel coupling. Which I'm told tends to absorb vibration better than other common blade materials.
Basic Airframe Construction Materials and Methods
Methods and materials won't change much even if I have to abandon the DF project.
I plan on and have done most of the sizing for a one piece spar of laminated spruce, web and caps. There will be four wood ribs in each wing in an otherwise "solid" foam wing with plywood skin and a single layer of glass to protect the wood. HZ and VT tail construction will be essentially the same. The majority of the airframe will also be wood formers/stringers with a ply skin protected with a single layer of glass.
I plan on and have done most of the sizing for a one piece spar of laminated spruce, web and caps. There will be four wood ribs in each wing in an otherwise "solid" foam wing with plywood skin and a single layer of glass to protect the wood. HZ and VT tail construction will be essentially the same. The majority of the airframe will also be wood formers/stringers with a ply skin protected with a single layer of glass.
Airframe Configuration
IF I end up being able to proceed with the DF, the basic layout will approximately resemble an F16. It is however NOT a scaled down replica. The latest iteration is for a 23' airframe, 18' wing span, 73.8 ft^2 wing area, HSNLF0213 airfoil and NACA 0012 for both tail feathers, 1160 LB empty weight and Vs of 72 KTS. I'm assuming a propulsive efficiency of 60% for my upper speed range performance estimates.
Engine Rebuild
I've been working on it for just over a year and a half. Much longer than I would have thought. I reasoned that the rotary would be the best overall match for a DF because of the power to weight ratio of the engine, the smoother power stroke of the rotary and the favorable RPM range for an appropriately sized fan. I completely tore down the engine. Replaced all gaskets, rotor seals and springs, rotor bearings, main bearings oil pump and chain, water pump, new 76 mm turbo, aftermarket external waist gate, FPR and Tracy Crooks EC3. I currently plan on dual radiators, oil coolers and inter coolers all mounted in the strakes. I haven't been able to get the engine started yet. I've been trouble shooting engine wiring for a month now. Recently discovered my FPR was plumbed incorrectly, discovered a couple of fuel and oil leaks and well the list of details that need to be addressed just seems to go on and on.
I'll follow up with some photos soon.
I'll follow up with some photos soon.
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