HP & the Eagle

[Bob S.]

THIS from the UltraVair Forum. I thought it interesting and informative to all and wanted to share with permission granted by Oscar...

----- Original Message -----
From: Oscar Zuniga
To: UltraVair
Sent: Sunday, February 23, 2014 11:12 PM
Subject: RE: [ultravair2] HP/XL

 

Gil;
 
I just couldn't let this post pass without a few comments on the philosophy of flight, especially of ultralight flight where we're asking every ounce of the engine and airframe to work to its limits to get us in the air. 

 >one builder of a Legal Eagle with a VW engine producing only 29 HP turning some pretty low performance numbers.
 
There are really too many variables involved for you to take this as absolute information.  The same ultralight vehicle will perform very differently depending on density altitude, and the performance results are dependent on who measured them, how they were measured, and under what conditions.  Unless the test pilot was being very rigorous and using very accurate calibrated instruments, the results can be quite subjective, so don't let this test series or this example put you off.  And in any case, how did he know that the engine was producing 29HP?
 
>his plane was heavy, about 20 pounds over weight and he weighs in around 225.
 
Let's look at wing loading and power loading for this configuration.  The Legal Eagle has a listed wing area of 107 sq.ft. and at legal ultralight weight with an FAA-standard pilot weight of 170 lbs. and 5 gallons of fuel, the wing loading will be about 4.24 lbs./sq.ft.  If you increase the vehicle's empty weight by 20 lbs. and the pilot's weight by 55 lbs., the wing loading increases to 4.94 lbs./sq.ft., which means he's asking the wing to lift 16% more than it was designed to do.  The power loading for the standard configuration is 15.13 lbs./HP but it increases to 18.24 lbs./HP in the configuration that he flew it... a 20% increase in what he's asking the engine to do.  Is it surprising that the performance numbers were not stellar?
 
Somebody else mentioned that they would be happy if their Legal Eagle would perform as good as a lowly Cessna 150.  Let's look at the numbers for a 150: with a gross weight of roughly 1600 lbs. (varies between models and variants) and an engine of 100 HP for the O-200, that's 16 lbs./HP.  The 225 lb. gentleman with the 20 lb. "fat" ultralight is asking his VW to do 14% better than a C-150 (18.24 lbs./HP for his ultralight compared to 16 for the Cessna).  Despite promises from Washington DC, there is still no such thing as a free lunch.
 
Let's examine this gentleman's exact same configuration except now with a 35 HP Ultravair.  The power loading is now just a shade over 15 lbs./HP, which is better than the Legal Eagle design and better than the C-150.  If you knock off the illegal extra 20 lbs. of airframe weight and put an FAA-standard 170 lb. pilot in the vehicle, you have right at 13 lbs./HP power loading.  That's better than a 230HP Skylane ;o) 
 
Ultralights have some very, very stringent construction and performance limitations but if you operate them within those limitations, they can be safe and exciting.  Just don't ask them to do more than the laws of physics will permit, and recognize that every pound and horsepower matters.   

NOW if we can get that pilot down below 225................ .

[bill utt]

Thanks Bob, Very informative!

[hvacrbert]

really puts things into perspective for me. i was honestly having second thoughts about building an xl after reading about the perceived lack of power. if i understand this my proposed plane 252 me 180 and 5 gal of fuel 30 total of 462 lb. divided by 30 hp for the milholland 1/2 vw is 15.4 lb per hp. that is 6 tenths of a lb. better than a 150

[Sam Buchanan]

really puts things into perspective for me. i was honestly having second thoughts about building an xl after reading about the perceived lack of power. if i understand this my proposed plane 252 me 180 and 5 gal of fuel 30 total of 462 lb. divided by 30 hp for the milholland 1/2 vw is 15.4 lb per hp. that is 6 tenths of a lb. better than a 150

30 hp might get you and your XL around the pattern at low density altitude but you will be much, MUCH happier with a 37 hp engine.

[Tom H]

The response by Oscar Zuniga shown earlier in this post was to a post that I made regarding our LEU on this site under the topic of "Jim's XL build".  I think Gil may have shared the post with an Ultravair group, where Oscar commented, then that post was brought back into this site.  Interesting.

Just to clarify for Eagle builders, my point was our relatively small displacement half VW (90.5 x 69) put out 29 hp at 3400 rpm per wood bar dyno test.  With the prop we had, full throttle take-off rpm was 3200 rpm, which would correlate to even less hp, just guessing, maybe 24-25 hp.  So, with this combination, our heavy LEU was not a stellar performer. We were looking into modifying with a longer stroke crank, which we guessed would increase our hp by 5 or 6 hp, which we thought would be significant.

There are many variables, including engine, prop and weight, which influence how an airplane performs.  In this case, how it climbs.  We could have changed the prop to increase the take-off rpm, thus getting more hp out of the engine, possibly improving climb.  Should have reduced weight, but we were thinking about engine mods as our plan of attack.

I believe most real happy LEU flyers are using larger displacement engines, like I said, producing mid 30 to mid 40 hp.

Again, this was based on experience on the LEU, not the XL.  XL may be different.

[Sam Buchanan]

really puts things into perspective for me. i was honestly having second thoughts about building an xl after reading about the perceived lack of power. if i understand this my proposed plane 252 me 180 and 5 gal of fuel 30 total of 462 lb. divided by 30 hp for the milholland 1/2 vw is 15.4 lb per hp. that is 6 tenths of a lb. better than a 150

30 hp might get you and your XL around the pattern at low density altitude but you will be much, MUCH happier with a 37 hp engine.

I should have added that in addition to wing loading, airframe drag must be considered when estimating performance. The Eagles are very draggy and a lot of power is consumed just overcoming drag. In my opinion, comparing wing loading with a certificated aircraft may be of little value in estimating performance.

[grdev]

Tom, i 'm building a 1/3 Corvair, so I did share some of your numbers with the Corvair guys.  I didn't want to build an underpowered engine.  So for everybody thinks the 1/3 Corvair and XL is a good match up for the money.

[Tom H]

Sounds good, Gil.  Hope you can get it done and do some testing to answer a lot of questions.  Or, more likely, generate a bunch more questions!

[grdev]

There is a lot of talk now on the Vair group about how much power loss the engine suffers because of the oil pump and mag drives.  It will be interesting to see some real numbers on the engines output.