Propeller thrust...before and after

[Will Crisp]

Greetings!

Has anyone measured propeller thrust on a test stand (without obstructions) and then on their gyro and documented the difference in thrust? Just curious as to how much power is lost with the propeller sitting behind the cockpit.

Thanks in advance! :)

[rehler]

I would think that there would be no difference if they were both sitting on the ground, as the air would come to the blades from the sides as much as the front. So the test would be of little value. But in flight while moving through the air where the seat would block the air it would be different.

[Will Crisp]

:) Hi Ken,

Any guesstimations on what percentage of thrust might be lost in flight mode? Common sense suggests the ideal placement of the propeller would be without obstructions; in an undisturbed air flow. I can't help but think a fair amount of efficiency is lost with the propeller positioned behind the cockpit. But perhaps I'm sadly mistaken.

I realize there is no cowling around the blades, but is there a Bernoulli's effect, pulling air around the cockpit?

Do the tractor designs make better use of their thrust, or is it like comparing apples and oranges?

Thanks for batting the idea around a bit! :)

[Doug Riley]

Will: You can't say categorically that either a pusher or a tractor arrangement will always be most efficient. Sure, the pusher suffers from turbulent inflow -- but the tractor suffers from blockage of the (much higher speed) OUTflow. Accelerated air coming off the tractor prop strikes the fuselage and creates drag. That's why it is more important to have a somewhat-enclosed fuselage on a tractor aircraft than on a pusher, even a slow one.

If you can streamline and taper the portion of the airframe that's upstream of your pusher prop, you can minimize the turbulent-inflow problem, while at the same time avoiding the "draggy bits in the slipstream" problem of the tractor. That was the philosophy behind a couple of Molt Taylor's airplanes. He used a long driveshaft leading from the engine back to a pusher prop located aft of the tail feathers.

Obviously you can't precisely copy Taylor's approach on a gyro because of the presence of the aft-tilted rotor. We can do better than we generally have so far, though. People who streamline pusher-gyro frames seem always to neglect the need for a nice tapered cowl leading over the engine to the prop, followed through by a good spinner on the aft side of the prop. The typical enclosed gyro has a graceful pod around the occupant(s), but the pod terminates abruptly in a blank wall ahead of the engine. Such designs make a lot of prop noise, if nothing else.

The old Skylark gyro of the early 70's (sort of a pre-Dominator, although created in California) was one exception. It was basically open-frame, except that it had very nice streamlining from the pilot's seat aft over the engine and on through the large spinner. Maybe one of the guys who collect digital pictures can post one of this craft. It made the cover of the PRA mag at least once.

[Cobra]

Why would a spinner help on a pusher prop- there can't be a lot of air moving from that blocked area anyway.

Id guess you would get more benefit from streamlining the landing gear and mast and reducing weight elsewhere.

[Doug Riley]

The spinner won't do much for thrust, but it will reduce drag. The frontal area of the engine immediately ahead of the prop represents more or less another blank wall perpendicular to the airflow -- an area that will create parasite drag in the form of a partial vacuum and turbulence downstream. It's best to continue the lines of the cowl right through the prop and aft to a point, which necessarily involves a spinner.

If you could complete all your cowl-tapering ahead of the prop, that would be even better, but would require putting the prop on a long shaft protruding aft of the engine -- a troublesome and heavy setup that's not worth it.

[Cobra]

I agree with your comments regarding the engine enclosure, the bit I dont understand is how do you get drag in the middle of the prop wash with all of the vortex flows and turbulence involved?

[Will Crisp]

Doug,

Thank you for the enlightenment! :) I now understand "the drag of the fuselage behind the tractor prop vs. the turbulent inflow of a pusher prop issue"' a bit better. I'm sure more insight will follow, I think this through.

I, too, have thought of the idea of a longer drive shaft for the gyro propeller, but not sure if you could take it back far enough to be beneficial.

I did a web search for Molt Taylor's design and found some interesting information. Thanks for the lead. I was impressed with the 5 minute conversion from plane to car!

I also found this: http://www.aerocar.com/ The up to date version of Molt's idea.

"The AEROCAR 2000 is a modern and high performance version of Molt’s 1950’s AEROCAR. It is under construction at this time in my shop in Black Forest, Colorado. Many things are different since 1949! The opportunities are new and demanding. The most significant enabling factors for a practical new flying car are:

1. The ADVANCED GENERAL AVIATION TRANSPORT EXPERIMENTS. This is a joint venture of government and private industry created by NASA and the FAA to revitalize general aviation.
2. The Global Positioning Satellite ( GPS ) system of 24 geosyncronus satellites is in place for flight and ground navigation.
3. Lotus Cars of Norwich, Hethel, England, introduced the light weight Lotus Elise sports car in 1996."

It is interesting to read of a modern day flying car concept! (have you got your ears on, Victor?)

I could not find any information on the Skylark Gyro. Perhaps someone else would post. I would love to see this craft as well.

I have read on the forum about a spinner making things more aerodynamic (perhaps it was you) and I made a note to use a spinner on my future project. It just makes sense.

As usual, thanks!! :) :)

[Doug Riley]

Cobra, the idea is to eliminate some of that vortexing, specifically within the cylinder of air in the center of the propwash. If the air is swirling into that central area from farther out, it's a symptom of the existence of a low-pressure area there. Note that a turbojet engine typically will have a "pusher spinner" near its exhaust, to guide the airflow in a somewhat orderly manner in the "shadow" of the center of the turbine disk.

Without the spinner, you'll have a more severe partial vacuum downstream of the center of the prop. Low-pressure areas behind the craft are what you'd like to minimize. They literally try to suck your aircraft back, not unlike pulling your foot out of gooey mud. The spinner fills up this potential vacuum area with solid material, out to a point where more normal streamlined flow can place along the spinner's surface.

[Cobra]

Thanks Doug- that makes sense. I wonder if a spinner might affect the balance/control of big tail surfaces like the Sparrowhawk...

Now that you mention it, I have noticed that the high-speed pusher gyroplanes (Hawk and Cartercopter) both use large spinners.

[Doug Riley]

I dunno about helping the tail surfaces. I think that adding just a spinner, while keeping a pod with a big back wall perpendicular to the airflow, might be shooting spitballs at the proverbial elephant.

It's tough to make a graceful tapered cowl from a side-by-side cockpit to a pusher engine. Everything from the McCulloch J2 to the Cessna Skymaster to the Seabee presents the same problem; they all come out a little awkward and bumpy and they all taper more abruptly than you'd really like. None of them is the ideal teardrop shape. Molt Taylor's setup is the theoretician's dream (but purchased at the cost of a driveshaft and all the headaches that go with it).

However, we can at least do better than the present RAF and similar cockpits. A better taper would almost certainly reduce noise and improve performance.

[RHerron]

I dunno about helping the tail surfaces. I think that adding just a spinner, while keeping a pod with a big back wall perpendicular to the airflow, might be shooting spitballs at the proverbial elephant.

It's tough to make a graceful tapered cowl from a side-by-side cockpit to a pusher engine. Everything from the McCulloch J2 to the Cessna Skymaster to the Seabee presents the same problem; they all come out a little awkward and bumpy and they all taper more abruptly than you'd really like. None of them is the ideal teardrop shape. Molt Taylor's setup is the theoretician's dream (but purchased at the cost of a driveshaft and all the headaches that go with it).

However, we can at least do better than the present RAF and similar cockpits. A better taper would almost certainly reduce noise and improve performance.

Right on, Doug.

The McCulloch J-2 is pretty well thought out and cowled as well as can be expected but the air still can't get to the prop sufficiently and it is very noisy. Too much "stuff" in the way and not much way to improve it. It has a big spinner already.

[Doug Riley]

Ron (sigh) at least old Drago gave it a good try. Seabees and Skymasters -- even Variezes -- are noisy, too.

Going by memory, the taper of an ideal streamline shape that produces the least drag for a given frontal area is around 3:1. We usually don't have enough room for a taper that gradual on a pusher gyro having a piston engine and a prop. The engine provides too much frontal area and the prop has to be close to the rotor for clearance reasons, so we're stuck with an abrupt taper.

The McCulloch J-2's streamlining is still better than the typical homebuilt gyro with cabin, though, IMHO. You'd expect its performance to suffer if you took off the various aft cowls and panels and the spinner and flew about with a bare butt as most of us homebuilders do.

[jos]

I have been told of a push pull aircraft with identical engines front and rear to have best performance in one engine operation by the pusher.
If I am not wrong it was wing commander Wallis who told me.

JOS

[Dean_Dolph]

I have posted the picture of Don DuBusschere's Skylark II before but I couldn't find it so had to go back and make a quick and dirty copy (no enhancement!) from the cover of the 1970 Sept/Oct issue of Pop Flying (what Rotorcraft used to be called!). The black/white picture is from the July/Aug issue.

Don has been called 'only' a designer by some, which I thought was strange since the airplane's first flight was 'only' by two bicycle manufacturers! You notice the dropped keel and probably CLT. This was 30+ years ago so I guess the 'designer' was just lucky instead of knowledgeable and innovative. The one thing he hadn't figured out yet was the need for a horz stab.

[Victor Duarte]

Dean, that's a little nugget ! very interesting.
I searched the web for this gyro but nothing..
I dont think he was lucky, i think he intentionnally aligned the cog of the pilot with the engine and prop (as teached to every beginner). and, yes, some innovations came from "bicycle manufacturers" ..
Have you a more detailed view or link please ?
Thanks in advance.

[Vance]

Wow! I really like Don's Skylark II. It has wonderfull design integrety. I especialy like the brace on the tail. Thank you for sharing that Dean. Thank you, Vance

[chuter]

Very Cool! There appears to be a rod going from the center of the prop back to the tail; anybody know what that's for? :o

And what's the stuff at the top of the mast? :confused:

[Dean_Dolph]

Very Cool! There appears to be a rod going from the center of the prop back to the tail; anybody know what that's for? :o

And what's the stuff at the top of the mast? :confused:You are seeing that because there isn't enough detail in the picture. That rod is a counter balance on the rudder and really doesn't touch the prop although it looks like it. I can't answer your question about the top of the mast.

[Dean_Dolph]

Victor, I'm sorry but that is all the info I have on the Skylark. I saw it at my first PRA Convention in Hemet CA. and didn't have a good camera at the time to take close up pictures.

[Doug Riley]

Hey, that's it. Thanks for not making me root through the attic for my copy of that PRA mag.

I think Don Debusschere was aiming for drag reduction. He was using a VW engine, which is not exactly a ball of fire unless hotrodded to death. Don (correctly, I think) saw more benefit to streamlining behind the pilot and over the engine than in front of the pilot. This is easier and more effective to do if you raise the pilot's seat.

Skylark II was a one-off machine. Chuck Beaty reported on the other forum that someone in Florida bought it and flew it there for awhile. Chuck commented that it had a bad "sideways hop" problem on landing -- a common issue when you pivot your main axles from a point way up on the frame. Happens on Dominators, too, though perhaps not to the same extent as on SkylarkII.

Too bad there aren't molds for that enclosure.

[Dean_Dolph]

Here is a picture of Hank Hinchmann's H1 Racer after he installed a full enclosure. Chuck B. indicated he tried to acquire one of these enclosures from Hank without success. To have Chuck bless a design has to be gratifying!

The pictures I have posted of this machine and of Skylark II are both single place machines. It is likely that the aerodynamics of these machines are considerably better than open frames with smoother air flow to the prop.

I suspect that Mark Givens tandem 'Predator', which I believe he sold, (see 1st & 2nd pictures! The H1 pictures were supposed to be first but I had problems with the upload!) also has improved prop air feed to the prop although the side tanks probably interfere. Mark's design is one that I would like to see worked with (full enclosure/move side tanks & etc.) and developed commercially. This machine is on my top ten favorites list! The workmanship and detail have to be seen to be appreciated.

It will take some innovation to improve the airflow to the prop on an enclosed side by side.

I read that a prop spinner provides 'recovery'. Anyone know what that means and does it just apply to either a tractor or a pusher?

[Dean_Dolph]

........I think Don Debusschere was aiming for drag reduction. He was using a VW engine, which is not exactly a ball of fire unless hotrodded to death. Don (correctly, I think) saw more benefit to streamlining behind the pilot and over the engine than in front of the pilot. This is easier and more effective to do if you raise the pilot's seat.Yeah, Don impressed me at the PRA Convention members meeting that year. Either the VW was quick or the streamlining was a big help because Don & Ken Brock were playing air tag and Don didn't appear to have any problem staying up with the hot Mac of Ken's.Too bad there aren't molds for that enclosure. It seems to almost be a crime against nature to have some of these designs disappear without preservation.

[Doug Riley]

Dean, Mark Givens's machine looks to be a drop-keel Barnett tandem. It retains the big flat wall behind the cockpit seen on regular Barnetts, which unfortunately is a nasty spoiler of flow. Hinchman and Debusschere solve this problem by running a streamlined cowling over the engine. They also reduce frontal and side area by raising the pilot's legs to a semi-prone seating position -- a standard tactic to minimize drag on sport aircraft, including sailplanes.

It's tough to do a complete enclosure around a tandem without running into the infamous Air & Space 18A problem: unstable slip-roll coupling. However, if you get the occupants to sit in a semi-prone position and locate the cabin to create CLT, you can both minimize the overall size of the enclosure and get it high enough so that it's less likely to cause instability. Of course, leaving the area ahead of the seats unfaired like the Skylark minimizes cabin side area, too (besides improving the view).

A large dorsal fin enclosing the mast (as seen on both the Air & Space and the McC. J-2) can help a bit in this regard. Such fins help to bring the center of pressure of the cabin's side area up to or above the aircraft's CG. This can convert the slip-roll coupling from unstable to stable (i.e. it introduces an effect similiar to wing dihedral).

I haven't heard the term "recovery" applied to spinners, but would guess it means minimizing the creation of a low-pressure area behind the engine on a pusher. A pusher spinner fills in the space that would otherwise have a slight vacuum and create drag.

[Dean_Dolph]

Dean, Mark Givens's machine looks to be a drop-keel Barnett tandem. Yeah, there certainly are signs of the Barnett influence. However, if I remember correctly, Mark commented on Rotorcraft.Com that it wasn't just a modified Barnett but an entirely new design........It retains the big flat wall behind the cockpit seen on regular Barnetts, which unfortunately is a nasty spoiler of flow...........I agree that there is a lot that could be done to clean up this design aerodynamically and should have included that in my comments about making a commercial project out of it. I think that it is doable by taking the steps you have mentioned. Even though I love the looks, I do wonder about a semi-reclining seating position during an extended flight. And this design seems to be a fit for long cross countrys. If there wasn't a large weight penalty then this design begs for a retractable front landing gear ala Rutan's.

Pure of heart and intent but it looks like a thread hijack has occured! Sorry 'bout that!