engine requirements?

[speed]

I know this is going to be an open ended question. What attributes/specifications make an engine good for aviation ?

If we had no limits what would be the perfect engine. RPM vs HorsePower ? I would think you could map any rpm range via gears but is that impractical? I assume we have to do that for these auto conversions, similar to thge torque converter & transmission on cars.

I will ask more intelligent questions as I advance my understanding. This I hope will generate a flow of comments that will at least target my knowledge search to the important stuff.

thank you in advance.

[asav8tor]

Since the turboprops MUST be geared, the engine/airframe/prop designer can pick any prop speed they wish. It seems many of them turn slower than us piston guys. More like 1,000 rpm. Part of that could be they must use a bigger disk and have to turn slower to maintain subsonic prop tip speed. On the other end little props on model airplanes can turn 30,000 rpm.

I'm guessing someone will chime in and explain the best compromise speed for a 150-250 hp/100-200mph airplane is.................. 2,700 RPM.

[rtry9a]

IMHO, the desired engine attributes are: durability, reliability, high power to weight, low cost (initial and future overhauls), economical operation (miles per dollar), low vibration, simplicity (in installation and operation), and low noise.

Ideal rpm: probably close to the engines torque peak/ best bsfc; that speed varies with engine type and design. With piston engines, the torque peak generally falls below the hp peak rpm, not so with rotary engines where the torque/ hp rise linearly with rpm. With air cooling, heat generation is also a major consideration that could limit operational power settings. FWIW, I believe the best bsfc in the Mazda 13B rotary is near 6000 rpm, where internal friction is minimized by centrifugal rotor forces.

I believe a fixed prop should be picked to give the best cruise speed near that ideal torque/engine speed, and still provide acceptable climb rates. Most of the available geared PSRU units fall around 2.2 to 2.8 to 1 ratios- linked to the common automotive planetary gear sets available. Id stay away from the belted reduction units with RV-level powerplants.

[John Clark]

Originally Posted by asav8tor

Since the turboprops MUST be geared, the engine/airframe/prop designer can pick any prop speed they wish. It seems many of them turn slower than us piston guys. More like 1,000 rpm. Part of that could be they must use a bigger disk and have to turn slower to maintain subsonic prop tip speed. On the other end little props on model airplanes can turn 30,000 rpm.

I'm guessing someone will chime in and explain the best compromise speed for a 150-250 hp/100-200mph airplane is.................. 2,700 RPM.

The simple bottom line in this discussion is that it is all a compromise. Various engines make power at various RPMs. Most classic Lycoming/Continental flat opposed engines were engineered to run at speeds that will allow efficient propeller performance without a gear reduction. There will always be a compromise between prop speed (tip speed limits) and prop diameter (ground clearance and weight,) and the aerodynamic realities.

I'm going to get flamed, but the reality that I see is although the "modern" automotive engines are very attractive, by the time your get the prop speed issue taken care of (reduction gears/belt) and the cooling issues addressed (engineering a cooling system and getting it in the airplane) you could have been flying a long time with something traditional. (I have) Now having said that, I would consider an auto-based engine if someone engineers a really bullet proof reduction system and there is an airframe available designed to accept a liquid cooling system.

John Clark
RV8 N18U "Sunshine"
KSBA

[rtry9a]

I do NOT understand the implied negative comments regarding planetary gear reduction drives- Ive not seen a failure specified in all my research- they are as bulletproof as any component, far more so than known failure points like crankshafts and valves (and empty gas tanks). Drive belts can stretch and break. The downside is the extra weight and a small reduction in drive efficiency- but not enough to counter other advantages that many alternative engines provide. The weight is countered by the necessity of having a heavy crankshafts, rods, and pistons in conventional big bore (high displacement) air cooled engines.

If you ask me, the big advantage of the Lycs these days involves ease of installation and possibly, holding resale value. The costs, particularly rebuilds and parts, and some operational characteristics like noise & vibration favor the alternative engines, with a few exceptions. Performance differences seem minor depending on specific apps.

[Ola]

If science could make a battery that came close to gasoline in terms of watthour-to-weight, the electric motor would be perfect for aircraft engines. It's as simple as you get. Start up checklist: "Switch ON" End of list. Push lever forward to go, pull back to stop. "Switch OFF" whenever you want. The torque curve is amazingly flat, you could turn it at any practical speed without the need for gears. Silky smooth and very quiet. Shock cooling? Hah! Dive as fast as you dare and let the windmilling prop recharge your battery!

The downside is that charging time will probably never come close to the 2-3 minutes it takes to fill a tank even if discharge times were the same. Tolerable compared to the benefits of simplicity, but that's seen from the dreamworld perspective set in the first sentence...I discussed this on another forum a few days ago and did some non-scientific math. It showed that current lithium-ion batteries are 26 times heavier than gasoline per watt-hour of stored energy. Battery tech needs a breakthrough...

[John Clark]

Originally Posted by rtry9a

I do NOT understand the implied negative comments regarding planetary gear reduction drives- Ive not seen a failure specified in all my research- they are as bulletproof as any component, far more so than known failure points like crankshafts and valves (and empty gas tanks).

Run a search on DanH (Dan Horton) He is the resident expert on the vibration issues with a redrive. Some very interesting reads.

John Clark
RV8 N18U "Sunshine"
KSBA

[Jconard]

Ideal rpm: probably close to the engines torque peak/ best bsfc; that speed varies with engine type and design. With piston engines, the torque peak generally falls below the hp peak rpm, not so with rotary engines where the torque/ hp rise linearly with rpm. With air cooling, heat generation is also a major consideration that could limit operational power settings. FWIW, I believe the best bsfc in the Mazda 13B rotary is near 6000 rpm, where internal friction is minimized by centrifugal rotor forces.

You should really read the previous discussions on this...it has all been hashed out before.

1. All engines are air cooled in the end...with a rad it is just that an intermediary transfer is used...coolant...which means two additional heat transfers 1. engine to water, and 2. water to rad, before the air can cool the rad. Each transfer is less than 100% efficient which means that by the time it gets to the rad you have some additional cooling needs. When the air interacts with the rad, it is at a much lower delta T, which means a greater volume of air is required to achieve the same cooling in terms of heat energy removed.

2. Weight has been hashed again and again, the "heavy" cranks and such in an aircraft engine do not result in heavier packages...as noted many times the car engine will result in a hundred pounds or more installed weight. This is being generous.

3. There is no documented fuel burn/ performance advantage, and in the RV class of aircraft, just the opposite.

However, you can expect smoother operation, and easier starting.

Back to the original question, to me it is all about speed/fuel burn, and the most simple installation possible. Simple, not because I am afraid of instalation and the work, but rather because the simple solution ought always be preferred over the complex, from a reliability standpoint.

[captainron]

Originally Posted by rtry9a

I do NOT understand the implied negative comments regarding planetary gear reduction drives- Ive not seen a failure specified in all my research-

I have not seen ANY negative comments in this thread regarding planetary gear reduction drives. Other than mentions of turboprops, of which the two popular brands both have planetary reductions, you are alone in bringing up this subject! The cobbled together spur gear or belt-drive PSRU's that are common on car engine conversions are, I believe, what most posters are refering to here.

[Dave Cole]

A consideration of importance to me is access to parts and service while traveling. Although the builder of an experimental aircraft can receive a repairman?s certificate and perform any required repair work, there will inevitably be times when they need local assistance...tools, shop access, replacement parts, etc....when away from home.

If you have a Lycoming engine, that assistance will be available almost anywhere.

Where will you find help to fix your auto conversion powerplant when you are a thousand miles from home?