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

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! ) So I don’t think we can go nuts with weight reduction.

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! ) So I don’t think we can go nuts with weight reduction.

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

Ron, I think we are in agreement in most respects.

Some thoughts on the "twitchy" problems when the nose MOI is reduced. Most of today's stuff has much larger stab/elevator areas when compared to the old designs. Probably helps overcome the fact that we now put more iron on the nose.

It might be interesting to move the batteries back on to the CG and reduce the area and or throw of both flaps and stab/elevator. With less mass up front, the required pitch forces needed to turn drop rather quickly. Wouldn't this still achieve good turn characteristics without creating as much induced drag.... hence the airspeed would not suffer as much on hard corners. Now before the purists jump on me and say that stall is a function of AOA and not speed, I would think a light plane with less MOI up front would fly through the corners as opposed to sink.

Dick,

Your thoughts echo those of Bob Hunt. His Genesis Extreme had very small flaps and was designed for the mass weight at (or just in front of) the CG. I agree and my latest had much smaller flaps, too.

It is painful to admit you wasted many months of effort, but my Alouette-E suffers most from a short tail moment. I shortened the nose, but I should have learned from my original IC Alouette and made a longer tail. The IC powered version has a much longer tail moment than the norm and flies like it's on rails.

C'est la vie. I'll either build a new one this winter - or do the Al Rabe thing and cut off the whole tail and build a new one.  Either way, it's a lot of work.

Ron

Sounds interesting. I'm curious as to why you would want a low aspect ratio wing.

Thanks Frank, kind of thought you were looking to reduce drag. You would be better off with a high aspect ratio wing planform. Gliders are a classic example of getting as much lift and as little drag as possible using high aspect ratio wings.

 

Got any video of the 5 ft. radius turns Ron?

You can build too light.  It takes a certain amount of weight and speed to keep the lines properly tight.  A plane that is too light will lose line tension in tight turns, allowing the lines to flop the plane around.

Reducing the MOI of the plane by moving the batteries back towards the balance point can only help.  But like anything else, you have to balance the change with something else, maybe less control movement.  And with the batteries way back, you have to balance the plane, either by making the nose longer or adding nose weight.  The MOI is very dependent on the nose moment.  It is not hard to increase the MOI by using a lighter motor on a longer nose.  Take a look at Wild Bill's half A designs.  He had a very long nose and it took a 30%+ stab to control it.  If you want to minimize the MOI, mount the motor and battery as close together as possible, and then adjust the nose length for balance.

"In my opinion, there is no such thing as too light, just to twisty and flimsy. using weight to produce line tension will become an anachronism. Side force generation will permit solid 5.7 second laps. A prediction that you read here first!"

Interesting... use a lifting fuselage (airfoil shape)? Ducted fan? Certainly could use some sort of lift overhead instead of depending on CF. Engine and rudder offset provide this however the Gurus feel that the drag isn't a fair trade. Probably not when you depend on speed to keep it out there.

Frank, a low aspect ratio wing, down in the 4-4.5 range, does handle maneuvers better.  fewer upsets, yaws, etc. from gusts and the lines whipping around.  The problem is the lower aspect wing builds up drag quicker in maneuvers, unless the weight is kept in line.

So, if you want to go the low aspect route, you have to keep the weight in proportion to the span.  A lot of guys are flying 60 oz, 60 in. planes with and AR of 5.2 or so.  If you build the same 700 sq.in. area on 56 in. span you'll need to keep the weight down below 55 oz. or so.  At AR 4 the span will be 53 in. and the weight will have to be kept below 52 oz, probably down below 50.  That gets really hard to do with conventional construction.

You might take a look at really low aspect ratios, like the saucer, or the ARUP.  Supposedly a well-rounded very low aspect wing(~1) does a surprisingly good job keeping the lift and drag in line.

I wonder if you have considered Tom Morris' Millenium wing which incorporates a 1/16" spar?
Very light.