flyrotary@lancaironline.net Mailing List Archive

Great summary on aircraft vs Auto racing engine differences Lynn and an interesting discussion on manifolds in general.

There is not enough appreciation of these differences. The original comment that started this thread (about my short runner carburetor manifold) has to be evaluated in this context. In a race car, that manifold would be a disaster but it was an unqualified success in the airplane. Compare it to the stock manifold. The entire short runner intake system with carbs weighed 9 pounds vs about 45 for the stock system. And, it was redundant. It could tolerate the complete failure of any one of its three carburetors and keep flying. The engine (1988 NA 13B) with short runners made 160 HP vs 146 in the car (granted, much of the increase was from exhaust system changes) As installed, it was the first auto engine conversion to equal or better the power to weight ratio of the Lycoming engine that it replaced (as far as I know).

Regarding the suitability of short runners on motorcycles, the short runners do not preclude good performance at low rpm. The Yamaha XS 1100 was legendary for it's low end torque. Ever ride a Harley Davidson? Stump pulling torque. Ever see long runner intakes on one? Could these engines make even more torque with a long intake runner manifold? Of course they could, but the point is that good low rpm performance is not impossible with short runners.

(Also really funny that we are calling 6000 rpm "low" : )

There is another important factor that is frequently missed when comparing rotary and piston engine manifolds but that is a discussion for another day.

Tracy (loading plane for S n' F)

Subject: [FlyRotary] Re: Very short intake manifold length

In a message dated 3/31/2006 6:31:49 A.M. Eastern Standard Time, Hennie.vanRooyen@kumbaresources.com writes:

"That 173.3 HP from the dyno sheet is from an engine with a very short intake tract. It is tuned for best power at 9,400 RPM. That was 244.9 HP. It is not tuned for 6,500 RPM. In use there is no reason to be below 7,800 RPM and I watch the tape from each session to alert the driver is he is dragging the Revs that low in some situations. We will change final drive ratios or hold off an upshift to a different location to avoid going below 7,800 RPM (229.4 HP)."

Hi Lynn,

I'm sure I'm going to be flamed for this question, but I'll reveal my ignorance by asking anyway. (I'm still a baby at this, but growing stronger every day)

If the rotary engine can be tuned for higher output at higher RPM, why are they operated in the lower RPM range for aircraft applications even though we use PSRU's? Is it for fuel consumption, heat generation and reliability only or should I ought to know something else?

Hennie

It sounds like you know those answers.

Drag (friction) increases at the square of velocity. So, the higher you rev any engine the more difficult the efficiency problem in BSFC. Even if the engine is not loaded, some of that goes on. Loaded is worse of course.

So the race engine has the big HP numbers and is in use for 45 minutes. But the duty cycle is actually pretty low. That is the amount of time spent at WOT is always less than the owner wants it to be. I heard a famous driver complain to the engine man about power after the second engine change. The frustrated engine man asked the driver if he was going around the corners flat out? Of course not said the driver. Well then, talk to the chassis guys not me.

Anyway, in the aircraft application, the duty cycle is close to 100% Get in the plane. Open throttle to 100%. fly to Sun&Fun (Zephyr Hills is better) reduce throttle, land, shut down. So almost 100% full throttle. The rotary is not as efficient as the piston engine, so for each HP produced it takes more fuel measured in pounds per HP hour. Brake specific fuel consumption.

So the piston engines are about 28% efficient. Recovers only 28% of the total heat from the fuel is converted to work. In the rotary about 26% efficient.

That means that there is one big bunch of heat to get rid of, when that unused heat appears as coolant heat, oil heat, and the big one is exhaust heat.

For example, it is normal to end up with between 160 to 180 HP in the airplane roll, and to the man, there is a temp watch on all flying, very bad when local air temps are high. The heat load can be adjusted with the throttle to reduce the total BTU load on the cool system, (But what fun is that). So I can build you a 250 HP engine, but you cannot cool it. But you could use it for takeoff and hope that the airspeed goes up faster than the water temp.

I have the biggest radiator Griffin makes and on scalding hot days, the water and oil is at 190 degrees. When 180 water and 150 to 160 oil would be great.

Tracy needs to make a three speed reduction unit, so the rotor heads can leave Lakeland in the vertical.

Another problem is the wear rates go up with speed and fuel consumption, and since reliability is a selling point, rapid wear is nothing to look for.

The more popular higher reduction lets the swept volume go up, so the engine gets to act like a bigger engine. Yes it does. Since it is burning more fuel per each revolution of the prop, and the number of CCs involved per revolution is higher.

Another factor is the short stroke, low mechanical advantage of the short stroke is overcome by running at higher RPM than a piston engine to produce a specific HP number.

So direct driving a prop at 2,750 would get you maybe 50 HP. Only 25 or so is required to go 70 MPH with a slick car so for cars it isn't a big deal. But not of any value for airplanes.

Lynn E. Hanover