View Full Version : Question on Drag and Design
11-12-2005, 06:39 PM
I'm new to this forum - hope I am in the right place....
I am on may way back into the flying game and looking for a gyro.
I have been out of the flying game for a few years but I notice something that puzzles me a little about modern design. I must be missing something here, since it doesn’t seem to be a question on any of the forums I have read recently – perhaps it is all so old and understood by the rest of you experienced folks that I am just out of the “loop” so-to-speak.
Given that drag is always a problem anytime we move through any medium – even something a light and “thin” as air – why don’t gyro manufacturers enclose the back of their machines. As a sailor, a glider pilot, ultralight flyer and a previous owner (21 years ago – sheesh, am I that old?) of a Bensen, and a son and brother of pilots, including helicopters, everything I know seems to suggest that the back end is as important as the front end for drag. One doesn’t see a glider with anything but a very smooth line from nose to tail, likewise for everything else except gyros and some ultralights (I used to have a Teradactyl that took me across Canada and it wasn’t that streamlined – but should have been!).
I realize it is difficult in an open machine, but so many now have enclosures but still ignore the back end. Most modern gyros are pushers, wouldn’t it also provide smoother air flow to the propeller? It is difficult to tell from the pictures I have, and I didn’t take much notice of these details when I flew a RAF 2000 last year, but at 80 miles per hour wouldn’t even streamlining the mast make a difference? Even the simple exercise of sticking one’s small hand out the window of a car at 60 mph tells me that. What am I not understanding?
If anyone has a quick answer to this, I’d appreciate your efforts
Thanks so much,
11-13-2005, 12:23 AM
Welcome to the forum.
Good observation. Gyros are ment to fly very slow without stalling. We fly between 20 mph and 60 mph. In the most treachrous and hostile enviroments in the world.Thats the fun part and makes them the safest in general aviation.
There are faster ones with better aerodynamics.
Air Command Pakistan
11-13-2005, 02:09 AM
I'll take a stab at it although I'm definately a Noob here.
There are several reasons for it: many are ultralights and have little, if any, weight to spare.
I also tend to believe that there are inherent dangers in streamlining gyros in that it's extremely easy to wind up with a large area forward of the mast (as viewed from the side) that is difficult to offset with a corresponding area aft of the mast. This could make for tricky handling in crosswinds.
That leaves aft streamlining as you suggest. It's a good idea I believe. I'm working on a seat cover that streamlines to the mast behind it which may provide some small benefit aerodynamically and also gives a small compartment for 2-cycle oil, rotor blade ties, and a few wrenches.
What ideas do you have?
11-13-2005, 11:44 AM
You are correct that bluff body drag behind most gyros is huge.
One of the problems is making the tradeoffs between weight and complexity and reward.
Take a peek at the south end of an RAF or SparrowHawk. You'll see that from the prop hub (~6 inches in diameter) and the engine/radiator/pre-rotator (a rough oval area of about 40" X 30 inches) there is only about 6 inches longitudinally.
The closure angle would be very abrupt. It's real hard to get air to stay attached to an angle bigger than 7 degrees or so.
I'm not saying it would not help, I'm saying, thus far, the reward has not been worth the downside of weight, etc. to the manufactuers.
I do believe that a few percentage points of drag reduction could be gained by fairing masts, landing gears, etc.
11-15-2005, 11:19 AM
Thank youall for your input. I don't see nything that might suggest a major stability problem, though one would have to be csareful in the design of it. It seems to me that just enclosing an area that already has a substantial longitudinal surface wouldn't affect it too much. Need to do some measuring of the specific craft I guess to see how sharp the angles would have to be.
One of the things I like about the gyro is its slow flight capability, but I'm not sure that is a reason for not improving the top end. I assume (perhaps wrongly) that the manufacturers have thought of this.....not a new concept I'm sure! My ultralight only had about half the prop running in relatively free air and you could see the distortion in the prop even at half throttle on the ground ....a lot of extra strain on things? - although the day I had a look it was blowing at 40 mph (whic was why I was on the ground AND near the machine!).
I think I will make some cautious approaches to engine coulings connected to the cabin in whatever I decide to get. I don't think it has to be heavy to be effective. At thi point I'm thinking sparrowhawk as I want a twoplace capacity machine and would have to fly a lot in the cold and rain...my ultralight trip across the country cured me of those sadistic tendencies in open copckpits.
one point to make is this, keep in mind that a pound of weight is a po-
und of weight but a pound of drag is a monster that multiplies with speed
11-15-2005, 12:37 PM
Like this. It can be done!
11-15-2005, 01:58 PM
Mike is this gyro flying yet?
11-15-2005, 06:00 PM
It has flown in ground affect. Dad and Dick will hopefully be test flying it this winter. Dick decided to do some modifications to the transmission which he anticipated he would have completed when they come down.
You are absolutely right about the penalties incurred by not making the effort to reduce drag on gyros. But I simply can't accept some of the opinions expressed so far in this thread - that streamlining the airframe is either too complex or too heavy. And shrugging off attempts to speed up gyros by so doing as futile because they were "meant to fly slowly". This last one is so far wrong I don't quite know how to respond to it.
The fact of the matter is that gyros are designed today more out of historical accident than by conscious choices being made. It is just easier to go with what has gone before, to stick to the tried and true than to design something new. How else do you explain gyros being designed and built today which are practically clones of pre-war designs? Couple this with the oft-quoted mantra that somehow simplicity is the answer to everything, and you've got a formula for atrophy. This is one of the main reasons why we are still stuck with draggy, slow, inefficient gyro designs. Simplicity is a valid design concept, but as your only design mantra, it sucks. Complexity isn't evil. If it were, we'd never fit complex reciprocating engines, or even think of turbo charging engines, or use constant speed props, or use hydraulic undercarriages, independent brakes, steerable nosewheels... the list goes on. Complexity doesn't have to be unsafe, either - we live with complexity all around us, and in most cases, complexity is extremely reliable. We've just come to revere simplicity as the holy grail, and it isn't. Only a healthy dose of complexity is going to produce a gyro which can proudly challenge fixed wing aircraft both in popularity and utility.
Put quite simply, gyros do not have to dawdle in the air. "Low and slow" might be fun for a while, but palls very quickly when there is any actual distance to cover. Every gyronaut I've come across yearns for more speed. But mention aerodynamic pods and you get the "complexity" argument, or the "adverse yaw" argument. Mention powered rotors and you hear a chorus of "too complex", "keep it simple". And don't even bother bringing up the subject of wings on a gyro...
Now before you rush off to write harsh words in defence of your low and slow gyro, please remember that it is all a matter of taste. For some folk, low and slow is cool, and precisely what they want out of a gyro. So be it. My point is just that it doesn't HAVE to be this way. Am I a successful gyro designer who can speak with authority on these issues? No - I'm a dreamer, just like you are. But a dreamer who knows that a fast, efficient, 100Kts gyro just can't be that difficult to design. And why so many gyros are flying around with no attempt to reduce drag by streamlining either the cabin or the engine area is quite beyond me. No reason for it.
That realy looks like a very nice machine. Have you got a side on pic, or one of whole machine?
11-16-2005, 06:29 AM
Try this Sam:
L.F.I.N.O. Concept Aircraft (http://www.rotorflightdynamicsinc.com/lfino.html)
11-16-2005, 12:16 PM
I'm building a gyro so I can fly low 'n' slow, but I could fly low 'n' slow over more interesting places if I didn't have to trailer to get there. A little extra speed and range would be nice in such cases.
11-16-2005, 04:18 PM
a dreamer who knows that a fast, efficient, 100Kts gyro just can't be that difficult to design.
I believe most folks who know me will accept that I am sincere when I say:
"I look forward to seeing your first sucessful design. I hope you do not find it difficult."
Best of luck.
I *think* you're putting me in my place, but you're so polite, I can't be sure... :-)
My first design is currently taking shape in the workshop. I'll be posting some photos this weekend, if I can find the time to do so. And as for the difficulty factor - Mmmm... some aspects are proving terribly difficult, others not so. But I forge on, boldly venturing into unchartered territory with the boundless optimism only the inexperienced enjoy.
11-16-2005, 04:33 PM
As I said Duncan,
I sincerely wish you the very best. I hope you do not find it as difficult as many of us have.
When it flies, and meets your design spec, you'll be the proudest person on the planet.
11-16-2005, 04:52 PM
Duncan, the design work is not the problem. The streamlining is not a problem. The basic problem is money (or lack of). To build the ultimate 100 kt cruise gyro takes many design considerations that will take it well out of the reach of the average gyronaut. I suspect that Dick DeGraw's Gyrino (?) wouldn't even cruise at 100 kts and that is indeed a design masterpiece and out of the reach of even most engineers. This sort of complexity cost takes it up to the level of many light aircraft so it makes it pretty hard to justify.
The knowledge and the ability is out there, question is are you prepared to pay for it.
Like Jim, I seriously wish you well with your project and hope that you can achieve your dream.
11-17-2005, 12:56 PM
Anyone that saw the latest episode of "Mythbusters" on Discovery now knows exactly what the dual seating pusher gyros need: a tailgate! With two idential pick-ups they tried tried the tail gate up or down for better gas mileage. Turns out, with the tailgate up there is a detached bubble in the bed that forces wind over the tailgate. With the tailgate down, or off, the detached bubble doesn't exist and bluff body drag is increased. The question becomes: How do you either get the bubble back or break up the drag? Well, I used to have a GMC S-15 Jimmy. Mine didn't have the neat looking little spoiler over the back window. The back window was always dirty. A friend had the Chevy S-10 Blazer with the spoiler and said he never had a problem with a dirty back window, so I bought an after-market spoiler and my window stayed cleaner and, here's the kicker, I gained about 2mpg. There's one for you engineer types. Add drag to reduce drag? I understand what happened but I don't have the skill to explain it.
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