Air Flow Aft Of Cabin

[Therapy]

Speaking to modern side-by-side machines with enclosed cockpit, how does the aft air flow feeding the prop affect overall performance, stability, safety and/or airworthyness.

Just how much is the difference overall?

I have followed some threads here talking about stabilizers and center of thrust and can get pretty lost in the technical aspects.

The Argon (my favorite) is blunt for sure. At another extreme I see the Niki Cruiser.

 

[Aerofoam]

Speaking to modern side-by-side machines with enclosed cockpit, how does the aft air flow feeding the prop affect overall performance, stability, safety and/or airworthyness.

Just how much is the difference overall?

I have followed some threads here talking about stabilizers and center of thrust and can get pretty lost in the technical aspects.

The Argon (my favorite) is blunt for sure. At another extreme I see the Niki Cruiser.

View attachment 1163197

The aft section of that design was actually the first thing I noticed, it is blunt enough to restrict flow at speed, but I am thinking it may have been intentional to create a low pressure area to help bring airflow though the engine area for cooling....Since these aircraft are neither efficient, or high speed, it seems to be an acceptable trade off.
The aft flow is actually very important, if it is really bad, like most of our open frame machines, it is creating a turbulent wake that can be wider than the frontal area of the aircraft and it effects the flow through the prop.
Our open frame machines are not very fast, so it matters less. Airflow at 45 mph. around a "Draggy" object does not have a huge effect.
When you increase the speed to over 75 mph. the drag will be far more profound. most of our single seat open gyros seem to cruise in the 50 to 65 range and are not expected to be efficient.

 

[Therapy]

The aft section of that design was actually the first thing I noticed, it is blunt enough to restrict flow at speed, but I am thinking it may have been intentional to create a low pressure area to help bring airflow though the engine area for cooling....Since these aircraft are neither efficient, or high speed, it seems to be an acceptable trade off.
The aft flow is actually very important, if it is really bad, like most of our open frame machines, it is creating a turbulent wake that can be wider than the frontal area of the aircraft and it effects the flow through the prop.
Our open frame machines are not very fast, so it matters less. Airflow at 45 mph. around a "Draggy" object does not have a huge effect.
When you increase the speed to over 75 mph. the drag will be far more profound. most of our single seat open gyros seem to cruise in the 50 to 65 range and are not expected to be efficient.

I have seen/heard some of these do have engine cooling problems.
And cruise speeds (75% power) are ~100mph.

 

[Abid]

Speaking to modern side-by-side machines with enclosed cockpit, how does the aft air flow feeding the prop affect overall performance, stability, safety and/or airworthyness.

Just how much is the difference overall?

I have followed some threads here talking about stabilizers and center of thrust and can get pretty lost in the technical aspects.

The Argon (my favorite) is blunt for sure. At another extreme I see the Niki Cruiser.

View attachment 1163197

View attachment 1163198

Niki Cruiser does not really work. I think they stopped production of it. Too novel to use a tube through the center of prop hub to support the tail. Required modification of engine gearbox and so on. Too much novelty and not needed. I am a fan of keeping things simple if possible.

I had a brief flight in I believe the first GTL that came through Florida with Raul. I had some trouble relating center line of runway on landing or takeoff to align with center of the windshield because of a wide windshield with no mark in front to focus on but other than that it flew ok. It definitely used more power for the same speed than say a tandem like the AR-1. Something like the Calidus is more efficient than the AR-1. At the end of the day the difference may amount to burning 5.3 GPH versus 6.0 GPH at 100 mph. There is no going around that wider frontal area creates more drag. It just does and in this case it creates more drag than even an open cockpit AR-1 with same engine with approximately the same weight with similarly sized rotor disc. Whether that is because of the blunt aft side, I cannot say for sure without further testing. It is better than the standard Argon which is shorter and the disturbed air from the front does not get a long enough path to connect back as much as it does on the longer body I am guessing. It is still a pretty blunt inward path for the air to follow. But it is a gyroplane. Realistically you are going to cruise at 100 mph max unless you got a lot of $$ to keep getting gas every 2 hours. I did not do a proper fuel flow test on it but it definitely used more engine power for same cruise speed than I see in say AR-1 with same 915 engine. No doubt about it.

 

[WaspAir]

Shortly before the company folded, Aero Resources, inheritor of the McCulloch J-2 manufacturing, was working to extend the hub to move the prop half a foot further aft, improve flow, and increase efficiency. It was an easy way to deal with this issue without much change to tooling or molds, engine mounts, etc.

 

[Doug Riley]

The drag of a 3-dimensional body depends on an array of factors. A very rough estimate of the effect of streamlining the front vs. the back of a gyro body pod is to look at the drag of a bullet as compared to that of an airfoil with the same frontal area. The bullet has about seven times the drag of the airfoil (bullet Cd 0.3; airfoil Cd 0.045).

A gyro pod isn't going to be streamlined to a perfect airfoil, so the real-world ratio likely won't be that dramatic. OTOH, this comparison doesn't account for extra inefficiency (and noise) caused by the prop spinning in "dirty" air. Pusher aircraft are notorious for prop growl.

Decades ago, the Springers, sellers of a blunt-backed body pod for the Bensen gyro, came up with some real-world body-drag numbers. At Bensen airspeeds (say 55 mph), the completely open-frame Bensen had about 60 lb. of body drag, while the Springer-bodied version had about half that. No telling how much more improvement was possible with a long, tapered aft section added to the blunt-backed body pod.

I imagine that a good deal of the increase in cruise speeds enjoyed by podded gyros is due to body (parasite) drag reduction. There's more money still on the table, though, to reward more complete streamlining, back through a conical prop spinner.

The typical gyro rotor is most efficient around 100 mph, but you need to reduce body drag to keep the total drag within reason.

 

[MikeBoyette]

The Dominator pod was modeled after the F-16 nose cone.

 

[Aerofoam]

Many years ago, I was modifying a small production RC wing (Multiplex) which could do 70mph straight and level.
It had a sort of lower fin strake/keel to land on which terminated as a fin with a 1/2" wide flat 90deg square trailing edge right in front of the propeller.
This made a lot of noise and restricted the intake, so I sawed off the extra offending foam to about an 8 to 1 taper to a pointed trailing edge.
With no other modifications, it could then do 83mph. That's about an 18% increase from shaving off 2 very small slices of foam..........
Of course I put a brushless 3 phase motor in it and then it did 160mph....
This was all just a quick experiment to see how well that wing design would perform at higher speeds because it flew amazing well right out of the box. I actually got a letter from Multiplex thanking me for experimenting with their design. Don't know if they ever used that info for anything, they dropped that model and replaced it with something called the "Fun Jet" which was a plank wing design....

 

[Doug Riley]

Mike, the Dom. nosecone is fine as far as it goes. My Dom. would do 100 or more (I heeded the warnings about the windshield bowing in). It did require extra throttle to get to those speeds, though, indicating that parasite drag was kicking in with a vengeance (since the rotor likely had LESS drag at 100 than at 60).

From the streamlining viewpoint, that Dom. would have been even better if the fairing extended all the way back and tapered gradually at the trailing edge.

As far as Boyette projects go, the LFINO was one of the best examples of this aft-tapering I have seen on a gyro. A nearly perfect teardrop, tapering through a spinner. Nice.

 

[MikeBoyette]

Yes Doug it was he worked very hard to get that shape. He carved it out of urethane foam for over 3 months. The problem it had was despite having two rabbit radiators it kept overheating. We put a fan on each one. That didn’t fix it. We even cut openings in the cabin near the nose wheel. Dad called them gazinta holes meaning air goes into it. Still overheated. It was suspected there were air pockets in the system but at 1200lbs it’s hard to stand it on its head to burp it. It was sold with the cooling problem being the only known deficiency unsolved.

 

[Jean Claude]

Proper cooling of a rear engine also requires serious work on internal aerodynamics.
- Well-placed air intake for maximum intake airspeed,
- Diffuser to avoid internal separation at high speeds
- Airproof partitions to constrain air flow to pass in radiators instead bypassing,
- Outlet nozzle to re-accelerate and discharge hot air at the same speed as the local external flow.

In this conditions, the cooling is efficient and the low drag

 

[Abid]

I have not seen diffusers used so much on engine compartments. For cooling air intake. Burt Rutan has pusher engines in his designs that were completely enclosed.

 

[Jean Claude]

 

[Abid]

View attachment 1163221

I guess my idea of a diffuser was a bit different than that. Directing the air is certainly standard practice. I imagined multiple vented intakes

 

[Therapy]

Proper cooling of a rear engine also requires serious work on internal aerodynamics.
- Well-placed air intake for maximum intake airspeed,
- Diffuser to avoid internal separation at high speeds
- Airproof partitions to constrain air flow to pass in radiators instead bypassing,
- Outlet nozzle to re-accelerate and discharge hot air at the same speed as the local external flow.

In this conditions, the cooling is efficient and the low drag

Yes. This is another concern. How many modern manufactureres flow test this stuff?