multi-engine design 1

[dh114]

Greetings,I am considering a new aircraft design which would possibly go back to a 4 engine design. I have two questions:
1. Does anyone know of any avialable data on twin verses four engine configurations? I am interested in any comparistive analysis that may be published.

2. Through history, has anyone every looked at the effects of staggering the engines and thereby over-lapping the propellers?

Thanks

 

[Sparweb]

Given the number of other wild combinations they tried, I bet NACA tried this at one point.

http://naca.larc.nasa.gov

Steven Fahey, CET
"Simplicate, and add more lightness" - Bill Stout

 

[dh114]

Thanks for the reply Sparweb. I have done some research in the naca documents but I have not found anything yet. The search capability seems to me to be a bit clunky for identifying work done with 4+ engine applications where the engines were staggered to allow the props to be close over even overlap.

I also have not found a lot of comparative analysis on 2 verses 4 engines for specific airframes. That actually surprises me and makes me somewhat suspicious of my search methods.

 

[Sparweb]

Tip for searching the NACA database: Open the "all citations" page, and use your browser's search <ctrl>-F to find the word you want.

You can also search NASA's STI database, which can be set to include NACA reports (kept at Langley).

Any comparison must include reliability or it's not complete. Twice as many engines means twice as likely to have one fail, if you ask me.



Steven Fahey, CET
"Simplicate, and add more lightness" - Bill Stout

 

[joekm]

My gut tells me that overlapping the propellors is not a good idea. If one is allowed to pass in front of the other, it could generate some nasty forces on the prop blades depending on thier actual separation. Even if they are synchronized such that they don't overlap, you still have a reduced disk area as compared to two separate props. That implies that you will have to generate more energy to develop the same amount of thrust since you would have to develop more air velocity to get the same delta mV.

...anyway, just my intial thought on the subject.

Joe

--
Joseph K. Mooney
Director, Airframe Structures - FAA DER
Delta Engineering Corporation

 

[Wittenborn]

I agree with you, joekm.

In addition, one would need to think about the vibration and fatigue that would go into the blades. That said, if you look at the KMAX helicopter, the rotor blades mesh into one another, and is considered "more efficient" than the conventional configuration.

In your search, you may want to look for information on the Rolls Royce Griffon engine. This engine had dual output shafts, which allowed for couter-rotating propellers, one behind the other. This configuration reduced (or eliminated) the swirl in the propwash. Look for more information on this, as a Reno Air Racer, Miss Ashley II used this engine.

Good luck!

Regards,
Grant
Aerospace Engineer

 

[joekm]

Just a couple of comments on Wittenborn's post:

The KMAX is an example of an intermeshing rotor design. It is not the only such example. One thing about intermeshing designs is that, it's kinda like an eggbeater, you really don't have one blade pass in front of another one. My suspicion is that going with an intermeshing set-up allows them to run an insanely high power to weight ratio without running into rotor torque issues. This allows the relatively small KMAX to have the impressive lifting capability that it has. As far as efficiency, I don't know, but it wouldn't necessarily be more efficient just because it's intermeshing.

As far as Ms. Ashley II, it's a similar story. Most likely, this is done to deal with prop torque given the nasty power to weight ratio of these machines. Spacing on the fore and aft prop-blades will effect loads but remember, this is a Reno racer, it only has to hold together long enough to finish the race and land safely. I'm trying to remember, but I'm not even sure that Ms. Ashley qualifies in the Unlimited Gold class (with such notables as "Rare Bear" and "Dreadnought" - both single prop aircraft I might add).

--
Joseph K. Mooney
Director, Airframe Structures - FAA DER
Delta Engineering Corporation

 

[Wittenborn]

Some very good points joekm.

I suppose there are different takes on what makes a propeller system more "efficient" than another. By removing the swirl from the propellers, you reduce the drag over the body behind the propeller. However I suppose this would not make the propeller itself more efficent.

Miss Ashley II did qualify in the Unlimited Gold class several years back, but was not able to compete with the Bear and the other big names. Sadly, the airplane and pilot were lost in 1999 in a Reno crash.

Regards,
Grant
Aerospace Engineer

 

[dh114]

Thanks Joekm and wittenborn for your input. Sorry about the delayed response, I replied earlier but it did not get posted for some reason.

I agree with you Joekm it seems such a design would generate "nasty forces" perhaps an issue of uneven loading as well. I agree with all of the possible pitfalls you all mentioned. Can you think of any benefits (specific to thrust and power) that might exist? Such as, if the props are counterrotaing, could an environment be created whereby the front prop is loading the back prop in the overlapping disc zone?

I am surprised that I have not found data evaluating the effect of overlapping the prop disc by horizontally staggering the engines.

Can the penalty of an overall reduced disc be at least neutralized allowing a 4 engine design where the engines can be located closer together reducing the necessary wing structure and length?

Thanks guys

Bill

 

[habu2]

Sorry for this digression, but the mention of counter-rotating props brought to mind the Tupolev 'Bear' series and this thought prompted me to ask: Does anyone know of readily available sources on Soviet/Russo aeronautics? It seems to me that the Soviets gambled on innovative/creative concepts to overcome deficits in their relative technology and manufacturing; moreover, without the drag of economics, these ideas could be pursued further than might be permitted in the West. While their concepts and methods may be archaic or moot today, I'm sure their data might still be of some interest, especially in non-high performance applications.