mhm21

So I will start this off with a more formal question than is implied by the thread title. From an efficiency (aka endurance) perspective what is the most efficient engine-aircraft combination. Now I know that I'm going to get the usual "it depends" answer so I'll elaborate. We are in the process of designing an experimental UAV for Boise State University and are having some difficulty in determining the best engine size(s) for our aircraft. We would greatly like to use a twin engine setup for enhanced reliability (which is why I'm posting here and not on the UAV forum) and greater power. Currently the design will be in the ballpark of 12-15 lbs with a 100" wing span and approximately 1030 sq in wing area for a wing loading between 25 and 30 oz/sq ft. Obviously these will change depending on the outcome of the engine recommendations.

So from what limited knowledge I have on the subject, I seem to recall that it is always desirable (more efficient) to swing a large prop slower as opposed to a small propeller at ten million rpm. This would seem to point to using a larger engine running at a low throttle setting. However, I would also assume that an engine running slower is not operating in it's maximum power or efficiency range which would then point to using a smaller engine with a bigger prop? AHH! Help! Im stuck in the endless trade-off design loop.

I also need some help in determining what size range of engines I should be looking at. At first glance the plane seems to be sized for engines in the 46 range, but I'm not sure. Thanks!!!

boingram

A couple of thoughts....the twin engine design has a reliability trade off that has to allow for the managability of the airframe in an engine out situation.

Singles are so reliable that it would be worth considering. Gas is much more efficient, but battery capacity for the engine is a limiting factor unless you use a magneto engine.

Large prop, small diameter better for the ap...key to efficiency is matching this combo to the engine and airframe.

Bo

cobi.p

RCV 91 CD .

go to RCV site and read .

cheers - cobi

tevans55

I am not sure I am entirely qualified to answer your questions but I will try to help. Your question of efficiency (aka endurance) seems to indicate you are looking for lowest cost per mile. If that is correct I would suggest a gas engine because the cost for a gallon of fuel is 1/4 to 1/5 as expensive as glow fuel. If you are considering a twin engine aircraft I would suggest a design that is a tandem (push/pull) type aircraft. If you lose an engine on a twin however, your endurance will be greatly affected by having to prevent a yaw condition if you can continue to fly at all. A tandem setup will simply result in deminished power but you won't have to fight a yaw effect.

If you are considering a twin because of reliability I would go with a single engine setup. Most of the gas engines are extremely reliable when adjusted properly. They are also very efficient. I recently helped a friend setup his Hanger 9 Piper Cub (100" wingspan) with a Zenoah gas engine. I think he flew that plane close to 45 minutes at 1/4 throttle on a single tank of gas. If you build light (use light wieght materials in your design) you should be able to construct a high lift wing structure that will give you substantial endurance at a relatively low cost per mile flown.

Hope this helps a little!

Villa

Both efficiency and endurance need more explanation. Those words can mean a lot of things. What exactly is the requirement? I read of an R/C plane that flew from Canada to Ireland on one gallon of fuel. I believe the weight was around 12 pounds. That was about 2-3 years ago.

Wayne22

My Feedback: (2)

Check what Meynard Hill used to cross the atlantic with....

http://www.sciencenewsforkids.org/ar...7/Feature1.asp

There are other similar articles you can find through Google...

Scar

My Feedback: (3)

Wow. You really should go to the UAV forum.

Efficiency? Choose a single engine. Half the friction.
Reliability? Choose a single engine. Half the opportunity for failure.
Endurance? How far are you going? Choose gasoline (Spark ignited) for a lighter fuel load.

If you really get 25 to 30 oz/sq. ft on a hundred inch wingspan, I believe a G23 would pull it just fine (once it's in the air.) From the meager details you've given, that's the best I can come up with.

Or, check with the people who've done it & get their opinion.

Good luck,
Dave Olson