Somebody please enlighten this student pilot, wannabe plane owner.

I don't get the fact that airframes seem to carry a gross weight rating that is not coupled to the power of the engine.

My mission is the back country. Eventually on floats (amphibs actually, as there is no float base near me). Ultimately I'd like to have an honest 4 place + camping gear airplane. My budget has started around the $100k mark.

I started looking at 182s, then 180s and 185s, most recently the Murphy Moose.

I've found an older 180 on amphibs with Horton STOL, wing extensions and an injected 470. Owner says he never paid much attention to allowable gross weight until the plane told him he had too much stuff in it. Says him, the wife, two big dogs and as much baggage as would fit would almost always be manageable off the water, even easier off pavement. I know there is an up-gross STC for the 182 but I don't know if that is a paper shuffle or an actual change to the aircraft.

There are also a couple Murphy Moose out there, first one I looked at was 400 hp, second was 360 (both Russian radial engines) and now I see another one for quite a bit cheaper with an O-540 which is probably around 260hp, but they all advertise almost 1200lbs usable. The owner of the 400hp version told me he would only use full power to get up on step, then he would pull it back a bit because the plane didn't really need that much power.

Is gross weight more a load (g-force) calculation than a lift (what the plane can actually do) calculation? Is the 540 powered Moose capable of hauling 1100 lbs given a long enough takeoff run?

MTV and others can explain this better than I can, but my experience has indicated a diminishing return. I have flown O-540 powered Pawnee and Call Air, 235 to 260 hp. The extra power allows more load, but not much. The difference in the Piper and Call Air wing, while equal in area, is greater than the 25 hp difference. And any height of ground effect higher than six inches will negotiate the help of either a good wing or extra power.

Horsepower, wingspan, gross weight, wing area are all having an effect on performance.

For amateur built aircraft have a look at Transport Canada Airworthiness Manual Chapter 549.107 and 109 which have all the pertinent formulas. It's online.
It gives you an idea what HP you need for a specific gross weight to get minimum climb rate.


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I usually do not like to post unsupportable information such as he said, rumor has it, etc. But, I once asked a CFI the question, "What determines Gross weight?" He claimed that "IT" was basically related to the load that the landing gear can withstand on landing. I have read that some of the larger planes have a 'Ramp' weight etc. I have heard that it includes a vertical rate of descent and or g-load.


A few minutes later now I can tell you that there are multiple gross weights. PLURAL: A Quick count stopped around ten gross weights for a larger commercial aircraft.

SOURCE: Structural Loads Analysis: Theory and Practice Commercial Aircraft:

Chapter 14 - page 233 By Ted L. Lomax

https://books.google.com/books?id=ltx92 ... es&f=false

Don't know if link will go direct - could not copy even a small sample.

Somewhere in the book may be the horsepower correlation / ratio / formula etc.

Hope that helps

Chris C

Design gross weight of a single engine general aviation has many factors. However one of the major criteria is load factor. That's why wings and horizontal stabilizers get static loaded with sandbags to verify that they are able to withstand a certain g-load for the category of airplane. As mentioned before you also need the appropriate HP to get the airplane in the air.
The amateur built section of the appropriate FAA or Transport Canada regulations have a lot of useful info and formulas in this regard.

For a certified aircraft you really don't know what criteria derived the max gross weight unless you have the design and test data.

The OP asked if there is a correlation of HP to gross weight.

There is, which is the the minimum rate of climb regulation, which has to be 1180' in 3 minutes (Canada).


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Gross weight is largely dictated by the airframe.

How well the plane will perform at gross weight is largely determined by the power.

For example almost all Maules nowadays have a 2,500 pound gross weight.

Loaded to 2,500 pounds high and/or hot my 180 will perform like an anemic diabetic pig with a prostate condition whereas a 235-260 HP will display much better health.

Of course this does not apply to landing, in which case a Maule is a Maule.

albravo wrote:
Is gross weight more a load (g-force) calculation than a lift (what the plane can actually do) calculation?

It depends. Simply put, on SEL airplanes gross weight is determined by either the max load the airframe can support (static load test) or by climb rate (FAA required minimum climb rate at gross). The lesser of these two weights is the max gross allowable.

For example the Luscombe model 8 airframe has been proven to 1700lbs but it was only ever certified up to 1400lbs because that's all it could carry and still meet the minimum climb rate with the factory engine options. As lager engines were installed the gross weight was increased from 1130lbs to 1400lbs.

Being a experimental the Moose doesn't have to meet the minimum climb rate requirement (at least in the US) so its gross weight is a function of what its airframe will support and your desired performance.

Another example: The typical gross weight set for the Bearhawk is 2500lbs but some guys set it to 2700lbs. I flew a 180hp Bearhawk with a 2500lb gross/1250lb useful and was completely satisfied with the performance. Someone else probly would have not been happy with the performance at all. My BH will have 210hp, 2700gross and I'm hoping a 1350useful. If the performance at gross weight is equivalent to how my Luscombe flew at gross weight then I'll be happy. Most other Bearhawk owners would not be happy with this performance standard.

Negate . It hate auto correct.

The Cessna 180 was introduced in 1953: 225 hp, 1550 empty & 2550 gross weight.
56 went to 230 hp.
57 still 230hp but gross wt bumped to 2650.
1964 still 230hp, gross weight bumped again to 2800.

Don't know the official empty wts for the later models but most everyone will admit that they got heavier as time went on. I think Cessna bumped the gross up to maintain a reasonable useful load. Don't know if the airframe was ever beefed up much if any for the added gross. FWIW

Hoo, boy.....this topic can be a very complex can of worms. For CERTIFICATED aircraft, there are many factors that can dictate (or limit) maximum gross weight. As others have noted, climb rate and load factors are high on that list, but consider that, for example, engine cooling can limit climb rate.....not because the airplane can't climb at a higher rate, but rather, the engine cannot remain within required temperature limits while sustaining that minimum climb rate. And, bear in mind that this testing has to be done as corrected to maximum operating temps.....

Also, landing gear drop tests are required at maximum landing weight, which, for light aircraft, is typically the same as max takeoff weight, so that can limit GW.

Etc.

When it comes to experimental aircraft, the maximum gross weight is whatever the builder sets it at. And, that may or may not have been based on empirical testing, or any testing at all. I assume that all the kit manufacturers do some at least empirical testing, if not genuine stress testing on their airplanes, but if a builder buys one of those kits, he is not bound by the recommendations of the kit manufacturer.

To the OP: If your mission is indeed four people and gear, on amphibious floats, the MINIMUM airplane I'd be looking at would be a Continental powered Cessna 206F or G. Even the 206 on amphibious floats can be very limited in useful load. It is possible to lighten the airframe up some by reducing the weight of the interior "decor" massively, and some other ways, but amphibious floats are HEAVY.

The Murphy Moose on amphibious floats in my limited experience (a few hours in a Vedenyev powered version) doesn't have enough vertical fin and rudder for the float application. Performance was great on water, but we weren't heavily loaded.

In any case, what you are asking for (your mission profile) is a LOT for an amphibious airplane. And, if you also need good takeoff performance, think deHavilland Beaver rather than Cessna. At that loading you are describing (and remember that FUEL is also a fundamental part of your useful load), ANY Cessna amphib is going to be barely able to carry that kind of load, perform well and go much distance. In your description of your mission profile, you didn't describe what sort of range you would be considering. Remember that, by definition, amphibious floated airplanes are pretty slow, and because of drag and weight, they burn a lot more gas typically than their wheel equipped brethren. So, don't forget fuel loads.

Bigger engines offer better takeoff performance, which is vital on floats, but they also burn more fuel. Granted, if you have the discipline to pull the power way back on that big engine in cruise, you can burn the same or close to the same fuel per hour as the smaller engine......if you have the discipline. I rarely demonstrate that kind of self control.....

I don't know what the status of Murphy Aircraft is. The Moose seems like a pretty good airplane. I would stay away from the Vendenyev engines for the most part, for a lot of reasons. But, they are much cheaper than a good Lycoming......and they make power. If you are willing to tolerate some of the "unique features" of those Russian engines, they do work.

But, I'd be doing a LOT of research before I pulled the trigger on any one aircraft type.

MTV

Some thoughts come to mind I'd like to share.

First, others have answered your question with more specifics. My own experience has been that airplanes which are essentially equal except for power can't carry much more load, but they climb at a higher rate to a higher altitude. Good examples are 172 airframes, which (depending on model) came factory equipped with 145 hp Continentals, or 150 hp Lycomings, or 160 hp Lycomings, or 180 hp Lycomings, or 195 hp Continentals. There's also an STC to bring the 195 hp Continental up to 210 hp. Although factory gross weights have varied from a low of 2100 lbs up to 2500 lbs, the fact is that if you load a 195 hp XP to 2500 lbs, it's a dog. At 2300 lbs, it's reasonable, but so is a 180 hp version. Both will climb better and higher than any of the lower horsepower versions, but only if you keep the gross weight down.

It's similar with 182s, which (depending on model) came from the factory with 225 hp, 230 hp, and 235 hp. The gross weights varied more dramatically, from 2550 lbs. to 3100 lbs. There are STCs allowing engines as large as 325 hp, but so far as I recall, that doesn't affect the allowable gross weight--just makes them climb a whole lot better.

So there are correlations with gross weight and horsepower, but from a performance standpoint, it's more a correlation between actual flying weight and power. Load lightly, good performance from even a low powered airplane. Load the same airplane heavily, and its performance will suffer. Stick a big engine on the front, and it's performance will improve.

Like Mike said, add amphibious floats to any airplane, and its practical usable weight goes down a lot. Just add straight floats, and the difference is dramatic. My example for that is my own airplane, and the one I took my SES training in. Mine's a 1963 P172D with a 180 hp Lycoming, CS prop. The SES trainer was a 1964 172D with the identical conversion, 180 hp Lycoming, CS prop, on straight floats. If I compare my airplane's performance to the SES trainer at sea level, I can haul a full gross load and climb at a better rate than the SES trainer with only 2 aboard.

We're all students to one degree or another, but if you really are well short of your checkride, you have a long way to go before you'll be comfortable in any high performance amphibious float plane. Between now and then, you're mission is likely to change several times. I think most of us misread our missions while we were still students, or even for the first couple hundred hours after passing the ride. Even then, our missions can change dramatically over a short time.

So while it's great to dream, realize that what you're dreaming about now may not be remotely what you decide to do down the road. My suggestion is that you pass your checkride and then get really good at flying whatever airplane you're able to rent. Learn to set it down smoothly within 50' of the spot you intend, smack on the centerline of the strip (even one without a centerline painted on it), every single time. Do that alone and with a full load.

Once you've done that, then start thinking about how to afford the airplane of your dreams. My suggestion there is to have a budget at least 1 1/2 times the purchase price, plus the ability to maintain it every year, from the annuals that only cost $1000 to the annuals that will set you back $8000 or more.

Lastly, don't be satisfied with mediocrity. Always strive for better and more--IR, commercial, an aerobatics course, etc. As I said, we're all students to one degree or another; none of us is so good at flying that we can't be better, and learn more.

Cary

albravo's post is bringing aviation's version of gearheads out of the woodwork. As such, for you good folks in the know, how about a formula and explanation of the correlation of hp and rate of climb?

Back when the CH-701 kits came out it predated ELSA and had a MTOW of 1100 lbs, thus a real light a/c. Climb and take off runs are impressive when the mass is so low. For 80 hp 1100 fpm, 60' take off roll. For 100 hp 1200 fpm and 50' roll for take off. I've seen (2) 914 powered CH-701 on Barnstormers in the last year and wonder if anyone can crunch the numbers for the 115 hp 914? And for grins and giggles, maybe the numbers for 140 hp (Zipper big bore)?

I do understand quite well the correlation of mass, hp, and acceleration. For 40 years of hot rodding, I was stuffing massive amounts of hp into cars like 5000 lb 55 Ford and lighter weight Corvettes at 3400 lbs. The correlation really hit home on my last rod, a 1923 Ford Track-T that tips the scales at just 1790 lbs. Can I expect about the same performance difference when weight and hp is juggled in a very light plane as I did in a very light car? I don't want DA at Tahoe in the Summer to be a limiting factor, and if the hp/weight for a car correlates in performance change the same as it would be applied to a/c, a turbo 914 with Zipper slugs and jugs and MTOW at 1100 lbs might be the aerial equivalent to my Track-T.

Pusher has twice listed resource documents you can try. One is online.

I listed one but it is near $70 +

Found one more on EAA or Homebuilder (?) and that will be it for me:

Ramer's Home builders book. All posting members rated it highly.

Chris C

deckofficer wrote:albravo's post is bringing aviation's version of gearheads out of the woodwork. As such, for you good folks in the know, how about a formula and explanation of the correlation of hp and rate of climb?

Back when the CH-701 kits came out it predated ELSA and had a MTOW of 1100 lbs, thus a real light a/c. Climb and take off runs are impressive when the mass is so low. For 80 hp 1100 fpm, 60' take off roll. For 100 hp 1200 fpm and 50' roll for take off. I've seen (2) 914 powered CH-701 on Barnstormers in the last year and wonder if anyone can crunch the numbers for the 115 hp 914? And for grins and giggles, maybe the numbers for 140 hp (Zipper big bore)?

I do understand quite well the correlation of mass, hp, and acceleration. For 40 years of hot rodding, I was stuffing massive amounts of hp into cars like 5000 lb 55 Ford and lighter weight Corvettes at 3400 lbs. The correlation really hit home on my last rod, a 1923 Ford Track-T that tips the scales at just 1790 lbs. Can I expect about the same performance difference when weight and hp is juggled in a very light plane as I did in a very light car? I don't want DA at Tahoe in the Summer to be a limiting factor, and if the hp/weight for a car correlates in performance change the same as it would be applied to a/c, a turbo 914 with Zipper slugs and jugs and MTOW at 1100 lbs might be the aerial equivalent to my Track-T.

To an extent, but probably not as dramatic as the car. Again I'll use my airplane as an example, since I have no experience with LSAs. I have supposedly 180 hp at sea level, and because of some hot-rodding tweaks the engine builder made, probably 170 hp at our altitude here of 5000' on a standard day. If I fly light, just me and less than half tanks, my airplane's performance on a 5000' DA day (like a nice early Spring evening or morning flight, for instance) is phenomenal, without much effort--pretty easy to leave the ground in less than a thousand feet, climb out at 750 fpm or more. Fill the tanks and load the rest of the airplane to gross (which is effectively a difference of about 900 lbs., bringing it up to 2350 lbs.), and now I'm rolling about 1 1/2 times as far and climbing out at about 500 fpm or less.

But now add in the DA issues. That "phenomenal" performance is gone. At a 9500' DA (Lake Tahoe on an 85 F day) and the light load, it's very nearly the same as the 5000' DA and a heavy load. At a 9500' DA and the heavy load, it's twice the ground roll (yeah, 3000') and climb out at less than 250 fpm.

My view is that DA is a greater limiting factor than many pilots realize. It not only saps power at the rate of about 3% per thousand feet of DA (so that that 114 hp engine at sea level is only putting out about 85 hp at 9500' DA), but it also materially affects lift. It also requires a significantly and noticeable increase in ground speed to achieve the necessary indicated airspeed for a safe lift off and climb out--at 80 knots indicated, for instance, that's a 95 knot (110 mph) ground speed.

The day I flew to Leadville some years ago, I was light--me, dog, full tanks on take off, and a survival kit. I burned off about 1/4 tanks, about 88 lbs. When we left, the DA was 12,100'. Leaning on the ground, the mixture control looked like it was within half an inch of shut-down. We rolled about 2/3 of the 6400' runway to achieve lift off speed of 75 mph indicated (90 mph TAS), stayed in low ground effect until reaching 90 IAS (105 TAS), and then climbed out at the extraordinary rate of about 150 fpm. Had the airplane been much heavier, I would have had to wait until much later in the day for the evening cool air or stayed overnight waiting for the cool morning air.

Does that help?

Cary

Lots of great, detailed information here. Glad to see such a high quality of content! Good going guys.

I like to think about the performance as a question of "excess" power, for a rough rule of thumb.

For example, if you need 90hp to keep a given 2500lb aeroplane flying, then any power above 90hp is all added performance. So a 150hp engine has 60hp of "excess performance" at sea level, at ISA. Whereas at 10,000ft, it has basically zero excess performance leftover - it's only making about 90hp, and the plane will not be able to climb much.

But a 260hp engine has 170hp of excess performance at sea level, ISA (260hp-90hp). So in that example, the gross hp difference between the 150 and 260 engine is about 70% more power overall. But the excess power difference is about 280% greater in the 260hp, compared to the 150 (170 / 60). Assuming a similar airframe in each case.

But - to the point that has already been made - there are diminishing returns. As you compare planes with higher gross horsepower to one another, the difference is less. For instance, compare a 400hp engine to the 260hp. The 400hp is about 60% more total power again (about the same increase in gross power as going from 150 -> 260). But this time, there is only 80% more excess power. Compared to the previous 280% increase in excess power from a 150hp -> 260hp. So the difference between 260 -> 400 engines is less remarkable, than the 150 -> 260.

So our conclusion is, the first hundred horsepower above 90hp (in this example) makes a big, noticeable difference. But as you go bigger, the difference is less obvious. Again - I'm assuming the same weight and similar airframe. It's not scientific, just given an indication of how the plane might feel - performance wise.

I hope that helps answer your original question!

Albravo-

I would echo MTV's comments on choices for an honest 4 place amphib equipped plane. I am guessing the useful loads that many are discussing above are on wheels.. I have a 185 with wing extensions / 3525 gw (1500 useful on wheels) It could barely do your mission with partial tanks, but it would be a dog.. Especially if you are planning to fly where it is hot and/or high.

Subtract the gear and add the floats and you can easily subtract 400-500lbs from the useful loads.. My gear legs with tires weigh about 120lbs. Aerocet 3400 amphibs weigh 650lbs. Net add to empty weight is a substantial 530lbs!!!

Think 206 or beaver.. If straight floats are an option, consider it.. The Aerocet 3500L's weigh in at 448lbs... An extra human and then some..

I am not firmilar with the moose, but more power would definitely be better.

IMO stick to the book weights especially if you are hauling people..

SI.

I've asked a number of questions on this forum and this, by far, has been the most instructive. Thanks.

I think i understand the difference between gross weight and performance. If you can get it up in the air, the gross weight is safe. If you can't, add more HP. I'm hearing "Your mission is possible, just add a few hundred thousand dollars." I think I should lower my expectations rather than increase my budget. I can't imagine a Beaver or decent 206 for $100k

To Cary, I don't think the mission goals will change, they have been the same for 30 years, I've just done it with backpacks, boats, bikes, trucks and ATVs. It just might take a few more trips to get 4 people and gear into the spots I want to go. Thankfully, where I live is close enough to where I want to be, three trips won't be a showstopper.

MTV, I'd like to get you back up in a Moose. I'm definitely relying on other people's opinions, but I've heard the Moose is a lot like a Beaver. Same wing, I believe.

Again, thanks all. I was travelling for the last couple days, slow to acknowledge the posts.

More questions soon to follow.

Allan

The gross weight of an aircraft is determined by the culmination of factors including flight performance and strength of the aircraft structure.

However as designee's what dictates gross weight and what we have to demonstrate compliance to is FAR Part 23. All of the "stuff" that people are mentioning, i.e. climb, structure, etc. are all spelled out in the regulations.

You can take a look yourself:
http://www.ecfr.gov/cgi-bin/text-idx?SI ... ain_02.tpl

Now it depends on what type of aircraft you are designing. Is it going to be a low and slow small horsepower draggy airplane with a low Vne or a high horsepower fast slick high Vne aircraft. Both could have the same gross weight but be limited by two different variables due to their design. It could be the climb gradient FAR for low and slow and structural limit loads at Vne for fast and slick.

With everything there are trade-offs pending what the aircraft is however if you look over the regulations you will see how we get to what we call "certified" and how that plays into dictating the gross weight of the airplane you fly.

albravo wrote: ..... I can't imagine a Beaver or decent 206 for $100k .....

A 206 is doable in that price range from what I've noticed in my everyday window shopping sessions online (Controller, Barnstormers, TAP). Or you could get like a Beaver wing... and then save up another 100k and get the fuselage, then another 100k for everything else...

-Asa

asadarnell wrote:

albravo wrote: ..... I can't imagine a Beaver or decent 206 for $100k .....

A 206 is doable in that price range from what I've noticed in my everyday window shopping sessions online (Controller, Barnstormers, TAP). Or you could get like a Beaver wing... and then save up another 100k and get the fuselage, then another 100k for everything else...

-Asa

Yes, you may find a 206 for around $100 K. Then you can plan an additional $118 K for a set of Wipline amphibious floats (installed). And, that's assuming you found a 206 with a float kit for that price. Otherwise, the float conversion would be substantially costlier.

A couple months ago, I saw a really nice amphib Beaver for only $740 K.

My comment on the Moose related to it's handling (barely) of a (relatively light) crosswind on wheels. That is often a challenge on amphibs anyway.

MTV