Retractable Mains?

[BUD ONEAL]

Best question Why? seems to be a solution to a non existing probrem. It was tried and proven not to be of any use. (except for the blade builders)

 

[Alan_Cheatham]

The old NACA did extensive wind tunnel testing on various forms of strut and landing gear fairings, very informative and recommended reading to someone wanting to apply streamlining to a gyroplane. They can be found for download on the NASA Technical Reports Server (NTRS).

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[Mark E]

You'll add weight and complexity for little total drag improvement, I'll bet. Think about how much drag that rotor system makes, and how much drag your wheels make, and you might agree with me.

I could be completely wrong, but I'd thought any drag reduction on the body (pod/seat/frame/undercarriage) should be doubly rewarded .... for the thing to fly straight and level the drag above the thrust point would be the same as the drag below ... so the angle of the rotor disk would be held so the the disk drag equalled the body drag...

Reduced body drag ...= reduce rotor disk drag (lesser blade angle of attack)? .... = faster machine eace:

 

[Illini85]

I certainly am not the authority on this topic but it seems to me that the gyros flying characteristics that we all love so much is attributable to the AoA of the rotor disk. There is a sweet spot in the range of the AoA that is very safe. Outside that range is very distinctly dangerous as a flattened rotor disk at speed makes handling increasingly more challenging.

I can't see where speed will ever be the domain of gyro's except a hybrid like the Cartercopter that can provision two lifting philosophies and transition between them.

This is just me bumping this thread hoping the dialog goes on because I would like to hear more from you guys.

 

[Alan_Cheatham]

I could be completely wrong, but I'd thought any drag reduction on the body (pod/seat/frame/undercarriage) should be doubly rewarded .... for the thing to fly straight and level the drag above the thrust point would be the same as the drag below ... so the angle of the rotor disk would be held so the the disk drag equalled the body drag...

Reduced body drag ...= reduce rotor disk drag (lesser blade angle of attack)? .... = faster machine eace:

You have several misconceptions here.

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[Alan_Cheatham]

I certainly am not the authority on this topic but it seems to me that the gyros flying characteristics that we all love so much is attributable to the AoA of the rotor disk......................

I can't see where speed will ever be the domain of gyro's....................

For level flight and a given flight speed the rotor will need to fly at a certain AOA to create just enough lift to support the aircrafts weight, no more and no less. Just like an airplane, at low speed straight and level flight the rotor needs to operate at high AOA, at higher speeds at a lower AOA.

At progressively higher speeds and flatter rotor AOA the fuselage will tilt progressively nose down due to rotor thrust line changes unless counter balanced by a force from the horizontal stabilizer. For the greatest drag reduction it is usually desirable to align the fuselage with the slipstream so an adjustable trim tab on the H.S. would be desirable.

Most people usually equate streamlining with flying at higher speeds but it's greatest benefit for gyroplanes is likely to be less fuel burn, less engine strain and greater range.

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[Illini85]

For level flight and a given flight speed the rotor will need to fly at a certain AOA to create just enough lift to support the aircrafts weight, no more and no less. Just like an airplane, at low speed straight and level flight the rotor needs to operate at high AOA, at higher speeds at a lower AOA.

At higher speeds and flatter rotor AOA the fuselage will tilt progressively nose down due to rotor thrust line changes unless counter balanced by a force from the horizontal stabilizer. For the greatest drag reduction it is usually desirable to align the fuselage with the slipstream so an adjustable trim tab on the H.S. would be desirable.

Most people usually equate streamlining with flying at higher speeds but it's greatest benefit for gyroplanes is likely to be less fuel burn, less engine strain and greater range.

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Thanks Alan I concur.

At high speed and low AoA isn't there risk of say a downdraft effectively reversing airflow through the rotor and causing a dramatic lose of rrpm? Not that air speed is necessarily an issue in a downdraft environment but it would be a situation where the AoA would be more likely to be perpendicular to said change in airflow plus it might be a factor in reaction time for any corrective inputs. I'm just trying to think through this stuff. I appreciate any input to help mold my understanding.

 

[Mark E]

I think the bensen gained about 10 mph with the gear folded, a gyro with a pod would likely gain more if you could keep the weight down. I once took the gear off a challenger and flew it off a sled to get it home and it was over 20 mph faster.
Norm

These are pretty remarkable improvements ... 10 to 20 % ... be they considered as speed or efficiency gains, this seems worthwhile.

 

[Barron 64]

These are pretty remarkable improvements ... 10 to 20 % ... be they considered as speed or efficiency gains, this seems worthwhile.

Certainly interesting.

 

[WaspAir]

There's reason for skepticism about big performance gains on a gyro solely from retractable main gear. Without any hard data to work from, I suggest a little reality check by analogy.

I have manuals for the 1979 Cessna 172N fixed gear and the 1980 Cessna 172RG retractable. Aside from the gear, the airframes on these two are very, very similar, and about as comparable as you can get. Both aircraft are already pretty clean from an L/D standpoint compared to the typical gyro (in the 8 or 9 to 1 range at best glide speed) so the landing gear looks like a good target for improvement. Operating speeds for both are higher than typical for gyros, and drag reduction could pay some dividends in that regime.

At 75% power, recommended lean mixture, max gross weight (which is a bit higher as should be expected for the more complex RG), and 2000 feet altitude at standard temperature, the RG tops out only about 15 knots faster than its fixed gear brother. BUT 75% power isn't the same thing on these two. The RG has an O-360 rated at 20 more horsepower that the O-320 in the N model, and a controllable prop to convert that power to thrust. If we credit part of that speed gain to the extra horsepower and prop efficiency, it doesn't leave a great deal to attribute to the gear alone. If you put the smaller engine and fixed prop of an N on an RG, and accepted the weight penalty of the retracting gear, how much faster would it be than a standard N model?

For drag reduction / performance improvement, I think there's probably lower hanging fruit to go for on most gyros than folding the gear.

 

[Alan_Cheatham]

There's reason for skepticism about big performance gains on a gyro solely from retractable main gear.

I agree.

Aircraft design is a compromise and it is up to the aircraft designer to weigh the pros and cons of all the variables to get the desired outcome.

But drag reduction does increase performance, a good example being Woodstock, and while a far cry from an efficiently streamlined aircraft increased efforts at drag reduction did increase flight performance.

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[Doug Riley]

Reducing airframe drag, or moving the center of airframe drag up or down, has absolutely no effect on an autogyro rotor in level flight. The rotor provides lift. Lift opposes gravity. Gravity is a vertical force.

Drag, OTOH, in level flight is a horizontal force. Engine thrust opposes drag. Therefore, a drag reduction affects only the amount of engine power needed to maintain level flight. The rotor doesn't know or care.

People often get confused about the effect (or lack of it) of trim issues on the rotor.

Think about a long dirigible. If the weights aboard that big balloon aren't distributed properly, it won't fly level; it'll fly arse-up or arse-down. You can move weights fore or aft to get it level. By bringing it into trim, however, you won't make it rise. It will weigh the same as it did before and will need the same amount of gas to stay level.

Same with a rotor. If you move the center of drag up, the gyro will fly more nose-up, but as long as its weight stays the same, the rotor won't work any less hard. In physics terms, a force does no work (and hence requires no energy to produce) unless it accelerates some mass. (This concept is hard to swallow because pushing on a rock that doesn't move sure FEELS like work to us. But in physics terms, you're doing no work on the rock.)

In descending flight, the frame drag acts somewhat upward (partly opposing gravity) so a draggy airframe does then help the rotor do its job.

A rotor does not become unstable at low disk angles of attack. Poorly-designed airframes do, however. Well-designed airframes (and well-designed entire gyros) feel "stiffer" and more solid at higher airspeeds. Unstable ones get squirrelly.

 

[Mark E]

Thanks Doug, for the neat explanation. I appreciate you putting in your time to explain it so clearly.

 

[Vance]

Misguided Fantasies?

Misguided Fantasies?

Reducing airframe drag, or moving the center of airframe drag up or down, has absolutely no effect on an autogyro rotor in level flight. The rotor provides lift. Lift opposes gravity. Gravity is a vertical force.

Drag, OTOH, in level flight is a horizontal force. Engine thrust opposes drag. Therefore, a drag reduction affects only the amount of engine power needed to maintain level flight. The rotor doesn't know or care.

People often get confused about the effect (or lack of it) of trim issues on the rotor.

Think about a long dirigible. If the weights aboard that big balloon aren't distributed properly, it won't fly level; it'll fly arse-up or arse-down. You can move weights fore or aft to get it level. By bringing it into trim, however, you won't make it rise. It will weigh the same as it did before and will need the same amount of gas to stay level.

Same with a rotor. If you move the center of drag up, the gyro will fly more nose-up, but as long as its weight stays the same, the rotor won't work any less hard. In physics terms, a force does no work (and hence requires no energy to produce) unless it accelerates some mass. (This concept is hard to swallow because pushing on a rock that doesn't move sure FEELS like work to us. But in physics terms, you're doing no work on the rock.)

In descending flight, the frame drag acts somewhat upward (partly opposing gravity) so a draggy airframe does then help the rotor do its job.

A rotor does not become unstable at low disk angles of attack. Poorly-designed airframes do, however. Well-designed airframes (and well-designed entire gyros) feel "stiffer" and more solid at higher airspeeds. Unstable ones get squirrelly.

Hello Doug,

I apologize for my confusion, I am not clear on the point you are making.

The reason for my inquiry is we are trying to reduce the parasitic drag of the gyroplane we are building, Mariah Gale by 30% compared to The Predator, the gyroplane I am flying now. The Predator has almost two square feet of landing gear and over a square foot of external fuel tanks. The rotor mast and the connection to the keel on The Predator may not be the best shape for low drag. We hope to improve the shape and size of these, clean up the details and add a cowl to achieve this 30% reduction in parasitic drag. Based on my experience at Bonneville this drag reduction is not an unreasonable goal and more is possible.

My first supposition is that the reduction in parasitic drag will allow a higher cruising speed and that combined with the lower disk angle we will be able to fly further on the same amount of fuel.

My second supposition is that the reduction in parasitic drag will leave more power left over to climb and she will climb faster.

Are these misguided fantasies?

To quantify my fantasies The Predator seems to be happy flying at 70kts (80 miles per hour) on 120 horsepower (75% of 160 horsepower). Her consumption actually suggests I am using closer to 104 horsepower (65%) at this speed according to the consumption numbers from Lycoming. At gross weight the Predator seems to have a top speed of just over 90kts (104 miles per hour).

At gross weight, 1400 pounds, the Predator climbs at around 650 feet per minute.

My fantasy is that Mariah Gale will fly at 85kts (98 miles per hour) on the same 120 horsepower with a top speed closer to 100kts (115 miles per hour) and climb at 850 feet per minute at the same weight.

Are these unreasonable numbers?

We are building fixed gear for Mariah Gale so these questions are a little off topic for retractable gear but on the topic of drag reduction for gyroplanes.

Thank you, Vance

 

[Doug Riley]

Vance, reducing drag certainly reduces the engine power consumed at any given airspeed. Whethre your numbers relaistic will depend on a bunch of things that I don't know, including the L/D characteristics of your particular rotor.

The fallacy I was addressing was the common notion that the rotor has to work harder to hold either the nose or the tail up if the aircraft has a center of drag that's not aligned with the CG. This is the twin fallacy to the notion that a HTL gyro with no effective H-stab works the rotor harder, because the rotor has to hold the nose up. My point was that the rotor's thrust can be used as a trimming device without the need for MORE rotor thrust.

Rotor-thrust-based trimming is accomplished by moving the LOCATION of the rotor thrustline relative to the CG. This, in turn, is NOT done by changing the rotor's disk angle of attack. Instead, the dangle angle of the frame changes to locate the rotor thrustline either ahead of or behind th eCG, whichever is need for equilibrium. The rotor's disk angle to the horizon is, and remains, whatever is needed to make one "G" worth of lift.

Example: A given rotor makes 500 lb. of lift at an angle of ten degrees to the air at 50 mph. On a gyro with perfect CLT and a drag center precisely aligned with the CG, the aircraft will fly level with the rotor's thrustline passing straight through the CG.

If you add floats, they will move the center of drag down below the CG. Say the new drag is 100 lb., located a foot below the CG, at your 50 MPH. The frame drag now creates a nose-down moment about the CG of 100 ft.-lb. to keep things simple, assume you took enough weight off the frame that gross weight with floats is the same as gross with wheels.

If you have no effective HS, the rotor must keep the craft from nosing over in flight. Does that mean the rotor disk AOA has to increase? No. Doing that would make the gyro climb and/or slow down. Instead, what happens is that the frame rotates nose-down, while the rotor maintain the same old ten degrees of disk AOA. Nose-down rotation of the frame causes the rotor thrustline now to pass ahead of the CG. Rotor thrust is the same as it was, but that same force now acts on lever arm (its length is the new distance forward from the CG to the rotor thrustline). As result, the same thrust now creates nose-up moment, where it used to create none.

 

[Vance]

Confused on a higher level.

Confused on a higher level.

Thank you Doug,

I thought I was in possession of all the fallacious fantasies and somehow I missed that one so I didn’t understand your response.

I am always concerned that I am going down a dark path based on unreasonable conjecture particularly when I am working on something so large and complex that I know so little about.

The higher cruising speed and reduced fuel usage are not critical items to measure the success of Mariah Gale. The extra fuel, 38 gallons compared to 22, will allow her close to double the range with an hour reserve making cross country travel more manageable. Having a place other than Ed’s lap for luggage will also be a very positive thing.

If she flies as well as The Predator we will be very pleased and satisfied with our efforts.

Thank you for sharing your considerable knowledge and communication skills.

Thank you, Vance

 

[Aviomania]

Vance, reducing drag certainly reduces the engine power consumed at any given airspeed. Whethre your numbers relaistic will depend on a bunch of things that I don't know, including the L/D characteristics of your particular rotor.

The fallacy I was addressing was the common notion that the rotor has to work harder to hold either the nose or the tail up if the aircraft has a center of drag that's not aligned with the CG. This is the twin fallacy to the notion that a HTL gyro with no effective H-stab works the rotor harder, because the rotor has to hold the nose up. My point was that the rotor's thrust can be used as a trimming device without the need for MORE rotor thrust.

Rotor-thrust-based trimming is accomplished by moving the LOCATION of the rotor thrustline relative to the CG. This, in turn, is NOT done by changing the rotor's disk angle of attack. Instead, the dangle angle of the frame changes to locate the rotor thrustline either ahead of or behind th eCG, whichever is need for equilibrium. The rotor's disk angle to the horizon is, and remains, whatever is needed to make one "G" worth of lift.

Example: A given rotor makes 500 lb. of lift at an angle of ten degrees to the air at 50 mph. On a gyro with perfect CLT and a drag center precisely aligned with the CG, the aircraft will fly level with the rotor's thrustline passing straight through the CG.

If you add floats, they will move the center of drag down below the CG. Say the new drag is 100 lb., located a foot below the CG, at your 50 MPH. The frame drag now creates a nose-down moment about the CG of 100 ft.-lb. to keep things simple, assume you took enough weight off the frame that gross weight with floats is the same as gross with wheels.

If you have no effective HS, the rotor must keep the craft from nosing over in flight. Does that mean the rotor disk AOA has to increase? No. Doing that would make the gyro climb and/or slow down. Instead, what happens is that the frame rotates nose-down, while the rotor maintain the same old ten degrees of disk AOA. Nose-down rotation of the frame causes the rotor thrustline now to pass ahead of the CG. Rotor thrust is the same as it was, but that same force now acts on lever arm (its length is the new distance forward from the CG to the rotor thrustline). As result, the same thrust now creates nose-up moment, where it used to create none.

In real life the rotor WILL have to produce more lift because the nose down frame has the engine thrust pointing downwards so a component of the engine thrust adds as an artificial weight. ( the same as a downloaded stab)...... unless you have an adjustable engine mount and keep the engine parallel to the direction of flight.

 

[Doug Riley]

Yes, unless the engine mounts are shimmed so that the propeller points in the correct direction.

A downloaded H-stab also produces an additional load on both the rotor and the engine.

Neither of these effects, however, has anything to do with the basic rule of physics: A force of a given magnitude will produce zero moment, or any given amount or direction of moment, about a point -- depending only on the relative locations of the point and the line-of-action of the force.

This is the principle behind the old notion that you can move Planet Earth with a long enough crowbar.

 

[Alan_Cheatham]

Vance,

If you haven't done so already I would suggest reading several NACA reports that can be downloaded from the NASA Technical Server, these contain good information on various strut and landing gear fairings and are very applicable to gyro drag reduction.

http://ntrs.nasa.gov/search.jsp?Ns=Loaded-Date|1&N=0

Of primary interest:
Report #

485 The Drag of Airplane Wheels, Wheel Fairings, and Landing Gears part I
518 The Drag of Airplane Wheels, Wheel Fairings, and Landing Gears part II
(NonRetractable and Partly Retractable Landing Gears)
522 The Drag of Airplane Wheels, Wheel Fairings, and Landing Gears part III
788 Drag Determination of the Forward Component of the Tricycle Landing
Gear
468 The Interference Between Struts in Various Combinations

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[WaspAir]

Think about a long dirigible. If the weights aboard that big balloon aren't distributed properly, it won't fly level; it'll fly arse-up or arse-down. You can move weights fore or aft to get it level. By bringing it into trim, however, you won't make it rise. It will weigh the same as it did before and will need the same amount of gas to stay level.

Now that the real point has been made and discussed, and my comments are unlikely to confuse the main topic, I can safely toss in my wise-guy nit-picking about airship operations. A dirigible can actually make lift from from tilting the nose up, not as a buoyancy effect, but because the shape of the envelope works like a symmetrical airfoil with enormous chord, and a nose up attitude provides a positive angle of attack. With forward motion you can generate lift from the shape. It doesn't have to be much to be helpful, because the lifting gas will take care of most of the load.

The Zeppelin NT is usually loaded so that it has a net weight of 300-800 pounds (so that it's not quite lighter than air), which is overcome by thrust and this airfoil effect. Loading weight is adjusted with fuel, lead, and/or jettisonable water carried in the cabin structure. For trimming, there are air chambers (ballonets) fore and aft in the envelope that can be pumped up or vented to the surrounding atmosphere to adjust longitudinal balance, so separate trimming weights don't have to be carried or moved around.