

Jan 28, 2006, 07:47 PM  

Quote:
Welcome to the electric world of RC. Funny thing... when you stop and think about it... it's easier to configure an electric airplane than a gas plane, when it comes to "will it will fly or not, and to what degree". That's because we talk in terms of watts per pound. What's watts per pound (you say, not)? It's the best universal unit used to describe the performance of ANY plane. That's right, any plane. You can universally apply watts per pound to a model or a full scale plane, or anywhere inbetween and it works to characterize the flight performance. When I was talking about 32 oz. and a Speed 400 motor, I made some assumptions that we eguys all know, but maybe you don't. A speed 400 is good for about 100 watts input with an output at or around 50 watts at this level of power. It's not very efficient (loss of around 50% of the energy used). With a plane getting 50 watts (give or take a bit) to the propeller, and a weight of 32 ounces, it's easy see (for us Egeeks) that the flight performance will be lackluster. Here's the basic rules to watts per pound: (watts is current x voltage) 20 watts per pound: able to sustain flight 40 watts per pound: able to climb and perform mild aerobatics. 60 watts per pound: able to ROG and fly with reserve in performance compared to the scale plane. 80 watts per pound: able to perform aggressive acrobatics. 100 watts per pound: able to perform extreme aerobatics. 120+watts per pound: able to hover (just) and to fly at any attitude in capable hands. 140 watts per pound  the sky's the limit. With the stock configuration allowing for (probably) 110 watts input and about 60 watts output, you can see that the flight wattage is 30 watts per pound at 32 ounces (the upper quoted and most likely the real weight of the airplane). At 30 watts per pound, it's 'just' going to fly. That's ok, if it's a calm day, winds right down the runway, wide open spaces (no other planes to dodge in the sky), skilled pilot, etc. There are little if any reserves if needed. Now, to keep things in perspective... 120+ watts per pound (like I want to do) can also have its short comings. Most notably, Airframe stress. To fly at these higher potential power ratings, one must be aware that there is a throttle and know how to use it. Power is cool, but not if you bust up a plane every time you go fly. That's what I meant by saying "it won’t do for me". That comment won't apply to everyone, just others who know what I'm talking about. Now, all of that (the formalities) aside, I think the plane will fly with the stock power setup. But keep in mind that it will probably need a skilled pilot that understands what it's like to fly an underpowered airplane. Honestly, I think you (assuming you are an accomplished gas pilot) will not be happy with the stock plane. And, I can say that without even seeing the plane at hand. Numbers don't lie. 30 watts per pound ='s a disappointing flight for one who's used to power in reserve. Add 19oz per sq. ft with 30 watts per pound and it's not getting any better. The structure looks ok and the basic loading numbers look acceptable if you are used to hitting the numbers on most landings. All that's needed is more power. My suggestion is that you follow this tread to see how different motor/prop/battery combinations pan out. When all the data is in, you will see that basic performance levels will all hinge around the Watts per Pound. (Darn..., do I sound like a KnowItALL? Sorry, I'm really not (as my peers will soon demonstrate)... Gary  

Jan 29, 2006, 07:09 AM  

HP vs Watts
Gary, thanks for the lesson on "watts per pound". If the plane does perform as you predict at 30 watts per pound, I would be highly disappointed. My nitro powered Lancair models have plenty of power as I use an OS 91 four stroke in each. The full scale planes are designed to "go fast" and the models should at least be capable of moderate speeds. One of the full scale Lancairs was flown at over 300 mph. An under powered Lancair just won't cut it (IMO). Also, because I fly off grass fields, I need a little extra power for those days when the grass is a little high.
I will definately take your advise and wait to see the responses from those who fly this bird with the stock equipment. BTW After reading your message, I converted the BHP output from my nitro engine (1.6 BHP) to watts (1 HP = 746 watts). Since my plane weights about 9 ponuds, this gives me about 132 watts per pound. This puts me at the upper range of your scale. Is this correct or do I need to make some other adjustments. 
Jan 30, 2006, 04:11 PM  

Quote:
Rerun the numbers and see if the watts match the listed flight characteristics. For fun, fullscale Cessna 172 with 180 BHP engine. 2500 pounds, 134,280 watts. That's 53.712 watts per pound. I did this formula for a B747 and it worked out to about 60 watts per pound if I recall correctly. I don't have the numbers in front of me at this time. The watts per pound even works out well for indoor airplanes that only weigh an ounce or two. I wonder what an F14 or F16 would work out to? Gary  

Jan 30, 2006, 04:29 PM  

Quote:
No I don't think Watt/lb can tell you the whole picture. Remember that sailplanes has ABSOLUTELY NO POWER so they are always 0 watt/lb. Still I have seen sailplanes perform incredible aerobatics and flys faster than many powered plane. The other end of the extreme are the helicopters. In order to hover and take off vertically all helicopters have more than 1:1 thrust to weight ratio and their power to weight ratio are generally much better compared to plane in the similar size. However almost all helicopters flys slower than comparable size airplanes. How an airplane performs involves much more factors than simply Watt/lb. For example, you mentioned that you want speed. However even if you get over 140 Watt/lb but you are using a large low pitch prop on a high gear ratio, you may hover or go straight up but still you are not getting the speed you want. In fact you can be flying MUCH SLOWER than a plane with only 30 Watt/lb but properly propped to fly at high speed. Power and speed are related to some extent, but their relation is much more complicated than a simple "equality sign". 

Jan 30, 2006, 04:37 PM  

Quote:
It all depends on throttle management. Some assumptions can be made. A Speed 400's max current should not be over 15 amps. On 8x1800 nickel cells, these batteries are going to choke down to about a volt per cell. 8 volts at 15 amps is 120 watts input. (VxA=W) 1.600 amp hour (1600ma) (most likely usable capacity of 1800ma pack) divided by the current (15 amps) gives a full throttle discharge in .106 hours. .106 x 60 = 6.36 minutes (call it 6 minutes). Using the watts per pound, we can approximate the flight times based on flying style performance. So, if all you want to do is puttputt around the pattern (say, 20 watts per pound) the battery voltage at the lower current will come up to about 1.1 volts per cell, or 8.8 volts. At 32 ounces we would need 40 watts (at the prop) to fly around in a slow fashion. The motors efficiency will probably come back up to the 60% range. So, motor input watts would be about 65 watts (65 x 60% = 39 watts). Now we know we need 65 watts on 8.8 volts. That's 65 divided by 8.8, or 7.4 amps. Again, 1.6 amp hours divided by 7.4 amps ='s .216 hours to discharge the battery. .216 x 60 minutes = 12.96 minutes (call it 13 minutes) So, the flight times should range from 6 minutes to 13 minutes. These are the theory numbers, but the real flight times might be 1015% more or less. Gary  

Jan 30, 2006, 05:40 PM  

Thanks for all the replies.
Shschon, I am well aware that these are theoretical assumptions and performance may vary depending on a bunch of factors including airplane aerodynamics, airfoil shape, fuse shape, prop pitch and size, and host of other factors. I am only looking for "round" numbers as we are not planning a trip to Mars. The figures that Gary presented will put you in the ball park and that is all I need. Gary, my four stroke engines are the OS 91 Surpass II which bost a whoping 1.6 BHP at 11,000 rpm. I prop the engine and mixture to run around 900010,000 rpm on the ground so I am probably turning somewhere near 11,000 rpm in the air. As such, your charted wattsperpound performance is close to what I am seeing. Again, thanks for the "yardstick" to use as a general gauge for estimating airplane performance. 
Jan 30, 2006, 07:17 PM  

Quote:


Jan 30, 2006, 07:57 PM  

shschon,
OK, here's an experiment in the WPP (watts per pound) modeling: Sailplane: 5 pound, sink rate 100ft per minute Question: What's the WPP added to the airframe to maintain level flight (enough 'lift' to sustain constant level flight)? Fact: Horsepower is expressed as 33000 foot pounds per minute The WPP needed is: 100 ft x 5 pound = 500 foot pounds per minute 500 / 33000 = .01515 horsepower 746 (1 HP in watts) * .01515 = 11.3019 WPP, and that's the exact MINIMUM WPP needed. Now, for a sailplane with a good aspect ratio and an excellent sink rate this number is right on. Now add extra sink rates for maneuvering in the same level of lift and more watts per pound will be need. Say, 15 WPP or so. 20 WPP is a good number for sport planes. I think the WPP IS an excellent way to characterize aircraft performance. The comment that a sailplane flies with '0' watts per pound is some what false. Now airplanes can fly at '0' watts per pound, but they all are falling in flight. Be it a Cessna, B747, Eplane or sailplane. If it's not falling, it's getting energy from something. Rising air for the sailplane and motor thrust for a power plane. Do you know why solor flight is so marginal, at best? Do the math. Gary  
Jan 30, 2006, 10:08 PM  

I can't resist. I know this is getting a fair bit offtopic..., but I've always wondered... what the watts per pound's are needede for 'excellent' lift (yes, I compete in sailplane contests ).
Ok, I talked to a friend tonight that carries a RAM altitude recorder on his sailplane. We both agree that the average 'good' lift situation hovers around 500 feet per minute. So, lets see how many watts per pound (WPP) is needed to climb at this rate. The basic model I described before will suffice for this example. Sailplane: 5 pounds, Sink rate 100 feet per minute, climb at 500 feet per minute. WPP needed for 500 ft per minute is: 600 ft per minute (500 climb + 100 feet per minute sink), x 5 pounds ='s 3,000 foot pounds per minute. 3000 / 33000 ='s .0909 HP 746 (1 HP in watts) * .0909 ='s 67.8114 WPP Wow! There is a lot of energy in lifting air! 67.8114 WPP's is a lot of energy! Don’t' believe me? Put your hand in the way of a prop running 339 watts (67.8 WPP x 5 pounds). Gary  
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