Electric Stunt > Gettin all AMP'ed up!

Electric Airframes

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Ron King:
Frank Carlisle wrote this in the motor thread:

"BUT...................I have been thinking about the plane. So far we've all been converting I.C. models to electric and to me it seems we should rethink the plane.
I'm thinking that since we don't have to tame or hold the engine back we should build a model with less drag. It doesn't have to be vibration proofed as much and the structure doesn't need to be so beefy.
Getting an electric motor to behave like an I.C. engine in an airframe that's had 50 plus years of refinement to perform with an engine rather than a motor is in my opinion what the drawback is to electric at this time."

I would move his post if I knew how, but I'm new to this job so please bear with me.

There are several factors that influence airframe design for CLPA - powerplant being one. Another factor to consider is the environment: we fly our CLPA models within a confined hemisphere. In some aspects our models act like real airplanes; in others they act different. Real airplane designers don't worry about a constant centrifugal force, nor do they worry about a constant yaw - or that tether attached to the wingtip.

Mike Palko's Silencer was the first airframe I saw that was designed for electric propulsion. Bob Hunt and I took existing airframes and converted them for our first attempts.

My latest effort was designed for electrics and it has very little hardwood. There are no plywood doublers, formers, or maple motor mounts in the fuselage. The wing is built normally because of the hemispherical conditions and every effort was made to select the lightest wood possible for each and every part of the plane. But if you look at my model, it is really just a bigger Nobler. Those classic design "numbers" have been proved over time.

Frank may have a good point about drag reduction. I haven't considered that as much as weight reduction. The batteries are still our biggest concern.

Anyone else have an opinion?

Ron

frank carlisle:
Ron,
Sorry for making you work extra. It's good that you started this thread over here. I'm anxious to see where it goes.
One thing I've noticed with the electric R/C pattern planes is that they weigh a lot more and don't seem to have such a large wing to compensate. Of course they have the whole sky to turn in and we have about 70 feet.
Also the airfoils are thin compared to ours.
Am I correct in my assumption that a large part of the thick airfoil on clpa planes is to hold back the engine?

Dick Fowler:
FC – “Am I correct in my assumption that a large part of the thick airfoil on clpa planes is to hold back the engine?”


I think that’s probably true. The general trend today seems to be to bolt a bunch of horsepower on the nose. Back in “the old days”, we flew some rather large airplanes on Fox .35 engines. Today these same airplanes are flown with .40 to .46 size engines.

I noticed this when I returned after about 35 year layoff. The horsepower thing along with thick airfoils and more drag in the current designs.

The way I see this is the increased drag along with higher horsepower creates a model that flies in a narrower speed band than did the old stuff. The drag keeps the speed variations down. This would be a good thing for the electrics too.

Remember how we use to fly faster with the old stuff. Wingovers were a classic example of trading kinetic for potential energy. You had to fly fast enough to carry some speed over the top of maneuvers like a wingover to have any kind of tension overhead. I probably lost most of my airplanes due to loss of line tension aka control in the overhead eights. I can remember being on my knees more than once while flying the second eight. Today, overhead line tension is not a concern even at the slower speeds.

More to the point of this thread, with properly balanced props, the need for lateral rigidity in the nose goes way down for electrics. Most of the weight savings can be had up front but as Ron pointed out there are still the same aerodynamic stresses present on the airplane regardless of type of power source.

I think I still like the idea of putting the batteries on or close to the CG. Just one less mass moment to deal with. I do wonder though if this would translate into a pitch sensitive, twitchy airplane if the nose moment of inertia was reduced too much.

There also problems associated with a model being too light. (I hope Sparky doesn’t read this part! j1) So I don’t think we can go nuts with weight reduction.

RC Storick:

--- Quote from: Dick Fowler on August 16, 2006, 06:19:41 PM ---There also problems associated with a model being too light. (I hope Sparky doesn’t read this part! j1) So I don’t think we can go nuts with weight reduction.

--- End quote ---

I see everything!

Ron King:

--- Quote from: Dick Fowler on August 16, 2006, 06:19:41 PM ---I noticed this when I returned after about 35 year layoff. The horsepower thing along with thick airfoils and more drag in the current designs.

The way I see this is the increased drag along with higher horsepower creates a model that flies in a narrower speed band than did the old stuff. The drag keeps the speed variations down. This would be a good thing for the electrics too.

Remember how we use to fly faster with the old stuff. Wingovers were a classic example of trading kinetic for potential energy. You had to fly fast enough to carry some speed over the top of maneuvers like a wingover to have any kind of tension overhead. I probably lost most of my airplanes due to loss of line tension aka control in the overhead eights. I can remember being on my knees more than once while flying the second eight. Today, overhead line tension is not a concern even at the slower speeds.

More to the point of this thread, with properly balanced props, the need for lateral rigidity in the nose goes way down for electrics. Most of the weight savings can be had up front but as Ron pointed out there are still the same aerodynamic stresses present on the airplane regardless of type of power source.

I think I still like the idea of putting the batteries on or close to the CG. Just one less mass moment to deal with. I do wonder though if this would translate into a pitch sensitive, twitchy airplane if the nose moment of inertia was reduced too much.

There also problems associated with a model being too light. (I hope Sparky doesn’t read this part! j1) So I don’t think we can go nuts with weight reduction.


--- End quote ---

Dick,

You raise quite a few interesting ideas. I'll give you some of mine and let's see how they match up.

I was told the same things you were when I came back to CLPA. Many folks use a fat airfoil to control airspeed. The other reason for a fat airfoil is it gives you a fat leading edge. This keeps the airflow attached to the surface longer and increases the angle of attack available before the wing stalls. I prefer a slightly thinner airfoil (Hunt/Werwage theory) and let the pipe control the engine on my IC ships. My first big electric ship used the Gieseke Nobler airfoil - slightly thicker that the regular Nobler. The jury is still out on this.

Moving the batteries back to the CG does create a different handling airplane. My latest creation turned like a combat ship and scared me to death the first couple of flights.  "Twitchy" was the only printable word I used after the first flight. ~^

There is a reason we have longer nose moments on CLPA ships and it's not just to balance the rear end. We also want smooth turning machines and I'm going to have to do some work on mine before it smooths out the turn to my satisfaction.

Randi Gifford watched my Shocker Cardinal at the FCM contest last week and told me that some people would kill for a turn like that.   <=   That plane can, and will, do a five foot radius turn. The problem with such a tight turn is it also kills airspeed. I got into trouble twice with the gusty wind and had to bail on a couple maneuvers. The cure is BETTER PILOT SKILLS. The pilot will have to learn to control the handle a little better in the wind.

In my modeling life, I have seen a few planes that were too light - very few. I think weight reduction is great until it impacts the structural rigidity of the airframe. For electric power (with its reduced vibration), we can build lighter structures. I have been using X-bracing and other engineering tricks in my planes and think I can go a lot farther.

We still have to deal with those doggone heavy batteries. You build something that's as light as a feather, then push a 4200mah battery pack inside and it breaks the scale.  Ugh. ''

Take care,

Ron

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